Dissertations / Theses on the topic 'Space and Solar Physics'

This thesis is dedicated to the theory of solar radar experiments. The Sun exhibits a variety of interesting and complicated physical phenomena, examined mainly through analysis of its radiation. Active solar probing by radar provides an alternative possibility to study the Sun. This concept was tested originally in the 1960's by solar radar experiments at El Campo, Texas, but due to an insufficient level of technology at that time the experimental results were of a poor quality and thus difficult to interpret. Recently, the space weather program has stimulated interest in this topic. New experimental proposals require further development of the theory of solar radar experiments to meet the current knowledge about the Sun and the modern level of technology.

Three important elements of solar radar experiments are addressed in this thesis: i) generation of wave turbulence and radiation in the solar corona, ii) propagation of the radar signal to the reflection point, and iii) reflection (scattering) of the incident radar signal from the Sun.

It is believed that the radio emission of solar type II and III bursts occurs due to conversion of Langmuir waves, generated by electron beams, into electromagnetic radiation (plasma emission mechanism). The radar signal propagating through the emission source region can get scattered by the Langmuir turbulence and finally deliver the observer insights of the physics of this turbulence. Such process of scattering is considered in this thesis in the weak turbulence limit by means of the wave-kinetic theory. Scattering frequency shifts, scattering cross-sections, efficiency of scattering (the coefficient of absorption due to scattering), optical depths, and the spectra of the scattered signal are estimated.

Type II solar radio bursts are known to be associated with the electron beams accelerated by interplanetary shocks. From their dynamic spectra the properties of the shocks and regions in the vicinity of the shock are usually inferred by assuming a plasma emission mechanism. In situ observations of the source region of type II burst, presented in this thesis, suggest that an additional emission mechanism may be present. This mechanism is related to energetic particles crossing the shock front, known in electrodynamics as transition radiation.

Plasma density fluctuations are known to scatter radio waves and thus broadening their angular dispersion. In the thesis this process is studied in the solar wind and terrestrial electron and ion foreshocks on the basis of in situ observations of density fluctuations. It is shown that the angular broadening of the radar signal is negligible in this regions.

The results of this thesis can be applied for the preparation of future solar radar experiments and interpretation of experimental data.

Advances in technology and instrumentation open new windows for observing astrophysical objects. The first half of my dissertation involves the development of atomic layer deposition (ALD) coatings to create high reflectivity UV mirrors for future satellite astronomical telescopes. Aluminum (Al) has intrinsic reflectance greater than 80% from 90 ? 2,000 nm, but develops a native aluminum oxide (Al2O3) layer upon exposure to air that readily absorbs light below 250 nm. Thus, Al based UV mirrors must be protected by a transmissive overcoat. Traditionally, metal-fluoride overcoats such as MgF2 and LiF are used to mitigate oxidation but with caveats. We utilize a new metal fluoride (AlF3) to protect Al mirrors deposited by ALD. ALD allows for precise thickness control, conformal and near stoichiometric thin films. We prove that depositing ultra-thin (~3 nm) ALD ALF3 to protect Al mirrors after removing the native oxide layer via atomic layer etching (ALE) enhances the reflectance near 90 nm from ~5% to ~30%. X-ray detector technology with high readout rates are necessary for the relatively bright Sun, particularly during large flares. The hot plasma in the solar corona generates X-rays, which yield information on the physical conditions of the plasma. The second half of my dissertation includes detector testing, characterization and solar science with the Miniature X-ray Solar Spectrometer (MinXSS) CubeSats. The MinXSS CubeSats employ Silicon Drift Diode (SDD) detectors called X123, which generate full sun spectrally resolved (~0.15 FWHM at 5.9 keV) measurements of the sparsely measured, 0.5 ? 12 keV range. The absolute radiometric calibration of the MinXSS instrument suite was performed at the National Institute for Standards and Technology (NIST) Synchrotron Ultraviolet Radiation Facility (SURF) and spectral resolution determined from radioactive sources. I used MinXSS along with data from the Geostationary Operational Environmental Satellites (GOES), Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), Hinode X-ray Telescope (XRT), Hinode Extreme Ultraviolet Imaging Spectrometer (EIS) and Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) to study the solar corona. This resulted in new insights on the coronal temperature distribution and elemental abundance variations for quiescence, active regions and during solar flares.

付記する学位プログラム名: 京都大学大学院思修館
京都大学
新制・課程博士
博士(総合学術)
甲第23343号
総総博第16号
京都大学大学院総合生存学館総合生存学専攻
(主査)教授 山敷 庸亮, 教授 寶 馨, 准教授 浅井 歩
学位規則第4条第1項該当
Doctor of Philosophy
Kyoto University
DFAM

Magnetic reconnection is a universal process occurring at boundaries between magnetized plasmas, where changes in the topology of the magnetic field lead to the transport of charged particles across the boundaries and to the conversion of electromagnetic energy into kinetic and thermal energy of the particles. Reconnection occurs in laboratory plasmas, in solar system plasmas and it is considered to play a key role in many other space environments such as magnetized stars and accretion disks around stars and planets under formation. Magnetic reconnection is a multi-scale plasma process where the small spatial and temporal scales are strongly coupled to the large scales. Reconnection is initiated rapidly in small regions by microphysical processes but it affects very large volumes of space for long times. The best laboratory to experimentally study magnetic reconnection at different scales is the near-Earth space, the so-called Geospace, where Cluster spacecraft in situ measurements are available. The European Space Agency Cluster mission is composed of four-spacecraft flying in a formation and this allows, for the first time, simultaneous four-point measurements at different scales, thanks to the changeable spacecraft separation. In this thesis Cluster observations of magnetic reconnection in Geospace are presented both at large and at small scales.

At large temporal (a few hours) and spatial (several thousands km) scales, both fluid and kinetic evidence of reconnection is provided. The evidence consist of ions accelerated and transmitted across the Earth’s magnetopause. The observations show that component reconnection occurs at the magnetopause and that reconnection is continuous in time.

The microphysics of reconnection is investigated at smaller temporal (a few ion gyroperiods) and spatial (a few ion gyroradii) scales. Two regions are important for the microphysics: the X-region, around the X-line, where reconnection is initiated and the separatrix region, away from the X-line, where most of the energy conversion occurs. Observations of a separatrix region at the magnetopause are shown and the microphysics is described in detail. The separatrix region is shown to be highly structured and dynamic even away from the X-line.

Finally the discovery of magnetic reconnection in turbulent plasma is presented by showing, for the first time, in situ evidence of reconnection in a thin current sheet found in the turbulent plasma downstream of the quasi-parallel Earth’s bow shock. It is shown that turbulent reconnection is fast and that electromagnetic energy is converted into heating and acceleration of particles in turbulent plasma. It is also shown that reconnecting current sheets are abundant in turbulent plasma and that reconnection can be an efficient energy dissipation mechanism.

Magnetized planets, such as Earth, are strongly influenced by the solar wind. The Sun is very dynamic, releasing varying amounts of energy, resulting in a fluctuating energy and momentum exchange between the solar wind and planetary magnetospheres. The efficiency of this coupling is thought to be controlled by magnetic reconnection occurring at the boundary between solar wind and planetary magnetic fields. One of the main tasks in space physics research is to increase the understanding of this coupling between the Sun and other solar system bodies. Perhaps the most important aspect regards the transfer of energy from the solar wind to the terrestrial magnetosphere as this is the main source for driving plasma processes in the magnetosphere-ionosphere system. This may also have a direct practical influence on our life here on Earth as it is responsible for Space Weather effects. In this thesis I investigate both the global scale of the varying solar-terrestrial coupling and local phenomena in more detail. I use mainly the European Space Agency Cluster mission which provide unprecedented three-dimensional observations via its formation of four identical spacecraft. The Cluster data are complimented with observations from a broad range of instruments both onboard spacecraft and from groundbased magnetometers and radars.

A period of very strong solar driving in late October 2003 is investigated. We show that some of the strongest substorms in the history of magnetic recordings were triggered by pressure pulses impacting a quasi-stable magnetosphere. We make for the first time direct estimates of the local energy flow into the magnetotail using Cluster measurements. Observational estimates suggest a good energy balance between the magnetosphere-ionosphere system while empirical proxies seem to suffer from over/under estimations during such extreme conditions.

Another period of extreme interplanetary conditions give rise to accelerated flows along the magnetopause which could account for an enhanced energy coupling between the solar wind and the magnetosphere. We discuss whether such conditions could explain the simultaneous observation of a large auroral spiral across the polar cap.

Contrary to extreme conditions the energy conversion across the dayside magnetopause has been estimated during an extended period of steady interplanetary conditions. A new method to determine the rate at which reconnection occurs is described that utilizes the magnitude of the local energy conversion from Cluster. The observations show a varying reconnection rate which support the previous interpretation that reconnection is continuous but its rate is modulated.

Finally, we compare local energy estimates from Cluster with a global magnetohydrodynamic simulation. The results show that the observations are reliably reproduced by the model and may be used to validate and scale global magnetohydrodynamic models.

The Sun is the main source of all kind of solar energetic particles in the Solar System, electrons, protons and ions with energies from few keV to several GeV. These particles are released from the solar corona and spread through the interplanetary space, the heliosphere, influenced by the interplanetary magnetic field and arriving to the Earth and interacting with the terrestrial magnetosphere. The effects of SEP interactions with space-based devices, manned missions and the Earth atmosphere are encompassed by what is known as space weather. This thesis describes the work we performed on this field, that can be divided in three parts: i) observational studies of solar energetic particles carried out using data coming from space-based missions such as STEREO and Helios, as well as tools like SEPEM server; ii) the development of tools and particle instrument modelling in order to use of them with pre-existing models to be used in the simulation of solar events; iii) solar energetic particle event simulations making use of transport models, either adapting tools previously developed by our group, as SEPInversion, or creating new software capable of carrying out full inversions of events, that is, taking into account the angular response and the energetic response of the particle instrument. These tools developed during this work have allow us to study and characterise the radiation conditions in the inner heliosphere applying modelling techniques never used done before. We also explore some of the applications of these tools. We developed a study about the radial dependence of electron peak intensities and anisotropy, we simulate observations of EPD/EPT instrument on board Solar Orbiter using Helios data and finally we studied the expected cumulated fluence and the fluence spectra computed using SEPEM for Solar Orbiter mission. In conclusion, the obtained results as well as the developed tools will be very useful for the study and interpretation of the future scientific data coming from Parker Solar Probe, Solar Orbiter and BepiColombo.
El Sol és la principal font de partícules que podem trobar al medi interplanetari del sistema solar, i els esdeveniments solars de partícules energètiques són la principal font de radiació dins de l'heliosfera. L'estudi i predicció d'aquest tipus d'esdeveniments i les seves causes i conseqüències ha esdevingut una àrea d'especial interès per la seva importància enfront dels perills que suposa aquesta radiació per a les telecomunicacions i la salut durant missions espacials tripulades. En aquesta tesi exposem el treball que hem desenvolupat en aquest camp, dividit en 3 àmbits diferents: i) estudi observacional d'esdeveniments de partícules fent servir dades observacionals de missions espacials com STEREO i Helios, i eines com SEPEM; ii) desenvolupament d'eines i modalització d'instruments de partícules per fer-los servir conjuntament amb els models preexistents per la simulació d'esdeveniments; iii) simulació d'esdeveniments de partícules mitjançant models de transport, tant adaptant eines prèviament desenvolupades pel nostre grup, com SEPInversion, com nou programari capaç de realitzar inversions totals, es a dir, tenint en compte la resposta angular i energètica dels instruments. Les eines desenvolupades ens han permès estudiar les condicions de radiació a l'heliosfera interior com no s'havia fet fins ara. Els resultats obtinguts així com aquestes eines seran molt útils per a l'estudi i interpretació de les dades científiques provinents de les futures missions espacials com Parker Solar Probe o Solar Orbiter. A més a més, les eines desenvolupades ens permetran fer un ús efectiu d'aquestes dades tan aviat com estiguin disponibles.

Les petits corps du système solaire sont les vestiges des planétésimaux du disque primordial. Leur étude peut améliorer notre connaissance des conditions présentes dans la nébuleuse solaire, et des processus thermiques et physiques qui ont eu lieu pendant les premières phases de vie du système solaire. Pendant mon doctorat, je me suis concentré sur la caractérisation physique des astéroïdes (2867) Steins et (21) Lutetia, cibles de la mission spatiale Esa-Rosetta, et des petits corps du système solaire externe : Centaures et Objets transneptuniens (OTN). J'ai effectué des observations photométriques et spectroscopiques dans le domaine du visible de Steins et Lutetia, en utilisant le Telescopio Nazionale Galileo, à la suite de la réduction, l'analyse, et l'interprétation des données obtenues, j'ai amélioré la connaissance physique des deux objets avant le survol de Rosetta. Je présente aussi les résultats que j'ai obtenus en participant à un programme d'observation conduit à l'Observatoire européen austral sur les OTN et les centaures : j'ai participé à l'interprétation des spectres obtenus en utilisant des modèles de transfert radiatif ; j'ai obtenu la classification taxonomique des objets observés sur la base des couleurs photométriques, et j'ai fait une analyse statistique utilisant la littérature disponible, à la recherche de corrélations entre taxonomie et dynamique ; à la suite de l'interprétation de courbes de lumière, j'ai rassemblé des informations sur la rotation, la forme, et la densité des objets analysés, et j'ai étudié la statistique de densité des petits corps du système solaire externe en combinant ces nouveaux résultats avec des données de littérature
The minor bodies of the Solar System are the remnants of the primordial planetesimal population, their investigation can hence improve our knowledge about the environment conditions in the solar nebula, and the thermal and physical processes that took place in the early phases of the Solar System. During my Phd, I focused on the physical characterization of the asteroids (2867) Steins and (21) Lutetia, targets of the Esa-Rosetta space mission, and of the minor bodies of the outer Solar System (Centaurs et Trans-Neptunian Objects, TNOS). I performed visible photometric and spectroscopic observations of Steins and Lutetia using the Telescopio Nazionale Galileo, from the reduction, and the analysis, and the interpretation of the obtained data, I improved the physical knowledge of both the objects prior to the Rosetta fly-by. In this thesis I also present the results I obtained as part of large programme performed at the European Southern Observatory on Centaurs and the Trans-Neptunian Objects. I contributed to the interpretation of the obtained spectra using radiative transfer models. On the basis of the obtained photometric colors, I derived the taxonomic classification of the observed objects, and I performed a statistical analysis using also the whole available literature looking for correlations between taxonomy and dynamics. From the interpretation of the light curves I gathered information about the rotation, shape and density of the objects under analysis, and I investigated the density statistics of the small bodies of the outer Solar System combining these new results with literature data

Coronal Mass Ejections (CMEs) are considered to be one of the most energetic events in the heliosphere. Capable of inducing geomagnetic storms on Earth that can cause damage to electronics, a pillar which the modern society we live in leans heavily upon. Being able to accurately predict the arrival of CMEs would present us with the ability to issue timely warnings to authorities and commercial actors, allowing for protective measures to be put in place minimizing the damage. In this study the predicted arrival times and speeds from the Drag Based Model (DBM) and Drag Based Ensemble Model (DBEM) were compared to observational data from a set of 12 events containing fast, Earth-directed Halo CMEs and their corresponding shocks. Although DBM was developed to model CME propagation, varying some parameters allow it to be used for estimating shock/sheath arrival. The results presented in this study indicate that on average DBM performs best when the drag-parameter γ is in the range 0.2 ≤ γ ≤ 0.3. However the variability in the results show that determining a universal value of γ for fast CMEs does not increase the consistency in the model's performance. For completeness, further investigation is needed to account for not only halo CMEs. This will allow to test broader range of variation in the DBEM input parameters.

This paper examines the neutral coma of comet 67P/Churyumov–Gerasimenko by using measurements of charged particles (water ions) and tracing them back to their place of ionisation. The measurements were taken from Rosetta’s Ion Composition Analyser. The simulations made use of an existing program which traces particles forward, which was changed to trace particles backwards, with new conditions for terminating the simulation. Two types of simulations were made. The first type is referred to as ”one-day simulations”. In these, simulations are made using data from a single occasion, with nine occasions studied per selected day. The days were selected so that the spacecraft was in different positions in relation to the comet. The second is referred to as the ”full-hemisphere” simulation. In this simulation, data from all usable days are used to produce an image of the hemisphere facing the Sun. The full-hemisphere simulation suffers from lack of simultaneous measurements, and indeed it is impossible to obtain in-situ measurements at all positions at once. Both simulations could be improved using more precise models, which could not be done within the allotted time of this work.

Present-day Mars is a cold and dry planet with a thin CO2-dominated atmosphere comprising only a few ­­­mbar pressure at low altitudes. However, the Martian surface is marked with valley networks, hydrated mineral clays, carbonates and the remains of deltas and meandering rivers, i.e. traces of an active hydrological cycle present early in the planet's geological history. A strong greenhouse effect, and thus a thicker atmosphere, would have been required to sustain a sufficiently warm environment, particularly under the weaker luminosity of the early Sun. The fate of this early atmosphere is currently unknown. While several mechanisms can remove atmospheric mass over time, a prominent hypothesis suggests that the lack of an intrinsic Earth-like global magnetic dipole has allowed the solar wind to erode the early Martian atmosphere by imparting energy to the planet's ionosphere which subsequently flows out as ion escape, over time depleting the greenhouse gasses and collapsing the ancient hydrological cycle. Previous studies have found insignificant ion escape rates under present-day conditions, however, the young Sun emitted significantly stronger solar wind and photoionizing radiation flux compared to the present. The geological record establishes the time of collapse of the hydrological cycle, estimated to have occurred in the mid-late Hesperian period (~3.3 billion years ago) at latest. To constrain the amount of atmosphere lost through ion escape since, we use the extensive database of ion flux measurements from the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) particles package on the Mars Express orbiter (2004-present) to quantify the ion escape rate dependence on upstream solar wind and solar radiation conditions. The Martian ion escape rate is shown to be insensitive to solar wind parameters with a weak inverse dependence on solar wind dynamic pressure, and linearly dependent on solar ionizing photon flux, indicating efficient screening of the bulk ionosphere by the induced magnetic fields. The impact of an extreme coronal mass ejection is studied and found to have no significant effect on the ion escape rate. Instead, intense solar wind is shown to only increase the escaping energy flux, i.e. total power of escaping ions, without increasing the rate by accelerating already escaping ions. The orientation of the strongest magnetized crustal fields are shown to modulate the ion escape rate, though to have no significant time-averaged effect. We also study the influence of solar wind and solar radiation on the major Martian plasma boundaries and discuss factors that might limit the ion escape rate, including solar wind-ion escape coupling, which is found to be ≲1% and decreasing with increased solar wind dynamic pressure. The significant escape rate dependencies found are extrapolated back in time, considering the evolution of solar wind and ionizing radiation, and shown to account for only 4.8 ± 1.1 mbar equivalent surface pressure loss since the time of collapse of the Martian hydrosphere in the Hesperian, with ~6 mbar as an upper estimate. Extended to the late Noachian period (3.9 billion years ago), the found dependencies can only account for ≲10 mbar removed through ion escape, an insignificant amount compared to the ≳1 bar surface pressure required to sustain a warm climate on early Mars.

We present results on the ionospheric structure around Titan observed during numerous deep (<1000 km) flybys by the Cassini spacecraft. Our results are based on measurements by the radio and plasma wave science instrument, in particular the Langmuir probe. In addition, data from the magnetometer and electron spectrometer have contributed. The ionosphere of Titan is created when the atmosphere of the moon becomes ionised. There are several mechanisms that contribute to this, the most important of which are considered to be photoionisation by EUV from the Sun with associated photoelectron ionisation, and particle impact ionisation by electrons and ions from Saturn’s corotating magnetosphere. We investigate the influence of the solar zenith angle on the electron number density at the ionospheric peak. The results show on average four times more plasma on the dayside compared to the nightside, with typical densities of 2500 – 3500 cm-3 and 400 – 1000 cm-3, respectively. In a complementary study, we make a case study of a nightside flyby and show that the altitude structure of the deep ionosphere is reproducible by a simple electron impact ionisation model. Taken together, this leads to the conclusion that solar photons are the main ionisation source of the dayside ionosphere. However, magnetospheric particle precipitation also contributes and can explain the electron densities seen on the nightside. As Titan does not exhibit any large intrinsic magnetic field, the fact that it is embedded in the magnetosphere of Saturn means that the Kronian field drapes around the moon and gives rise to an induced magnetosphere. We show that there are currents of the order of 10 – 100 nA m-2 flowing in the ionosphere of the moon. Associated with the currents are perpendicular electric fields ranging from 0.5 to 3 µV m-1. Finally, we investigate measurements obtained during T70, the deepest Titan flyby performed to date. We show that there is a substantial amount of negative ions present below an altitude of 900 km. This confirms previous result by the electron spectrometer, showing negative ions at higher altitudes in Titan’s ionosphere.

This report is the result of a conducted Minor Field Study (MFS), to the greatestextent funded by the Swedish International Development Cooperation Agency(SIDA), in an attempt to design a system for evaluating smaller solar power systems indeveloping countries. The study was to the greater part conducted in Nairobi, Kenyain close collaboration with the University of Nairobi. The aim was to develop asystem that would use easily available components and keep the costs to a minimum,yet deliver adequate performance. The system would measure certain parameters of asolar power system and also relevant environmental data in order to evaluate theperformance of the system. Due to the specific competence of the collaboratinggroup at the University of Nairobi, a Kinetis Freescale K64-microcontroller with anARM-Cortex processor was selected as the core of the design. Components wereselected, schematics were drawn, a circuit board was designed and manufactured andsoftware was written. After 12 weeks a somewhat satisfying proof-of-concept wasreached at the end of the field study in Kenya. The project however proved howdifficult it is to go from first idea to a functional proof-of-concept during a limitedtimeframe, and also in an East-African country. The final proof-of-concept was testedat Mpala Research Centre in Kenya and despite containing some flaws proved that itwould indeed be possible to design a working system on the principles discussed inthis report. The system is open-source, so anyone may use and modify it.

Fossil fuels are, by their very nature, finite resources. There are, however, numerous renewable energy sources that should be taken advantage of. One of the most abundant is also the most difficult to produce on Earthsolar energy. This thesis explores the feasibility of a space-based solar power satellite. The thesis focuses specifically on the satellite design as opposed to the end-to-end design to include the ground segment. It explores the potential orbits for such a satellite to operate from and ultimately concludes that a geostationary orbit is the only logical location for an operational orbit. This thesis also focuses on two segments of the spacecraft the solar array and the power transmission payload. The solar array area was calculated using the current best theoretical solar cells and assumed a 1 GW transmission power. Finally, this thesis explored which transmission payload to recommend for an operational system, concluding that a laser system is the most efficient use of space and weight. The final portion of this thesis was to examine the business case. Based on the design in this thesis, space-based solar power cannot compete with fossil fuels and likely will not for the foreseeable future.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2004.
Includes bibliographical references (p. 59).
In this thesis, I discuss the Brans-Dicke theory of gravitation, the Higgs mechanism, and the relevance of these two theories to the bridging of cosmology and particle physics. Although the former theory is in the field of cosmology while the latter is in particle physics, their similarities are impressive. Both attempt to understand the origins of mass. Even more striking is the mathematics involved in each of these theories. The Brans-Dicke theory and the Higgs mechanism both introduce a new scalar field that is coupled to matter in the universe. Although these theories were formulated around the same time in the early 1960s, are so similar in motivation and method, and became quite popular in their own respective fields, they remained relatively unknown outside of their field for quite some time. In this thesis, I have summarized both the Brans-Dicke theory and the Higgs mechanism. Then, I have analyzed the number of articles citing the Brans-Dicke and Higgs papers to understand when particle physics and cosmology first began integrating. To extend this further, I have looked at how many articles in 1961, 1971, 1981, and 1991 can be categorized as both particle physics and cosmology. In conclusion, we see that the two fields were slow to build common ground, although this has improved since the 1980s. By the 1990s, collaboration between particle physics and cosmology had greatly increased, most likely because of attempts to unify gravity with the other three forces.
by Shefali Bharat Oza.
S.B.

The performance of three types of passive solar feature has been studied; fifteen Roof-Space Collectors on an estate of low energy houses at the Milton Keynes Energy Park, 101m2 of Thermosyphoning Air Panels at a county primary school in Nazeing, Essex, and three Thermosyphon Solar Water Heaters installed on a group of three terraced cottages at Cranfield, Bedfordshire. Each of these passive solar features was monitored intensively for at least one heating season using dedicated data-acquisition systems. The maximum specific annual solar contributions to the auxiliary space/water heating systems were 128 kWh/M2 , 78 kWh/M2' and 104 kWh/M2 respectively. The corresponding payback periods were 25,37 & 21 years respectively, on replication.

This thesis deals with the device physics of organic solar cells. Organic photovoltaics (OPV) is a field of applied research which has been growing rapidly in the last decade leading to a current record value of power-conversion efficiency of 10 percent. One major reason for this boom is a potentially low-cost production of solar modules on flexible (polymer) substrate. Furthermore, new application are expected by flexible or semitransparent organic solar cells. That is why several OPV startup companies were launched in the last decade. Organic solar cells consist of hydrocarbon compounds, deposited as ultrathin layers (some tens of nm) on a substrate. Absorption of light leads to molecular excited states (excitons) which are strongly bound due to the weak interactions and low dielectric constant in a molecular solid. The excitons have to be split into positive and negative charges, which are subsequently collected at different electrodes. An effective dissociation of excitons is provided by a heterojunction of two molecules with different frontier orbital energies, such that the electron is transfered to the (electron) acceptor and the positive charge (hole) remains on the donor molecule. This junction can be realized by two distinct layers forming a planar heterojunction or by an intermixed film of donor and acceptor, resulting in a bulk heterojunction. Electrodes are attached to the absorber to collect the charges by providing an ohmic contact in the optimum case. This work focuses on the electrical processes in organic solar cells developing and employing a one-dimensional drift-diffusion model. The electrical model developed here is combined with an optical model and covers the diffusion of excitons, their separation, and the subsequent transport of charges. In contrast to inorganics, charge-carrier mobilities are low in the investigated materials and charge transport is strongly affected by energy barriers at the electrodes. The current-voltage characteristics (J-V curve) of a solar cell reflect the electrical processes in the device. Therefore, the J-V curve is selected as means of comparison between systematic series of simulation and experimental data. This mainly qualitative approach allows for an identification of dominating processes and provides microscopic explanations. One crucial issue, as already mentioned, is the contact between absorber layer and electrode. Energy barriers lead to a reduction of the power-conversion efficiency due to a decrease in the open-circuit voltage or the fill factor by S-shaped J-V curve (S-kink), which are often observed for organic solar cells. It is shown by a systematic study that the introduction of deliberate barriers for charge-carrier extraction and injection can cause such S-kinks. It is explained by simulated electrical-field profiles why also injection barriers lead to a reduction of the probability for charge-carrier extraction. A pile-up of charge carriers at an extraction barrier is confirmed by measurements of transient photocurrents. In flat heterojunction solar cells an additional reason for S-kinks is found in an imbalance of electron and hole mobilities. Due to the variety of reasons for S-kinks, methods and criteria for a distinction are proposed. These include J-V measurements at different temperatures and of samples with varied layer thicknesses. Most of the studies of this this work are based on experimental data of solar cells comprisiing the donor dye zinc phthalocyanine and the acceptor fullerene C60. It is observed that the open-circuit voltage of these devices depends on the mixing ratio of ZnPc:C60. A comparison of experimental and simulation data indicates that the reason is a changed donor-acceptor energy gap caused by a shift of the ionization potential of ZnPc. A spatial gradient in the mixing ratio of a bulk heterojunction is also investigated as a donor(acceptor)-rich mixture at the hole(electron)-collecting contact is supposed to assist charge extraction. This effect is not observed, but a reduction of charge-carrier losses at the “wrong” electrode which is seen at an increase in the open-circuit voltage. The most important intrinsic loss mechanism of a solar cell is bulk recombination which is treated at the example of ZnPc:C60 devices in the last part of this work. An examination of the dependence of the open-circuit voltage on illumination intensity shows that the dominating recombination mechanism shifts from trap-assisted to direct recombination for higher intensities. A variation of the absorption profile within the blend layer shows that the probability of charge-carrier extraction depends on the locus of charge-carrier generation. This results in a fill factor dependent on the absorption profile. The reason is an imbalance in charge-carrier mobilities which can be influenced by the mixing ratio. The work is completed by a simulation study of the influence of charge-carrier mobilities and different recombination processes on the J-V curve and an identification of a photoshunt dominating the experimental linear photocurrent-voltage characteristics in reverse bias
Diese Dissertation beschäftigt sich mit der Physik organischer Solarzellen. Die organische Photovoltaik ist ein Forschungsgebiet, dem in den letzten zehn Jahren enorme Aufmerksamkeit zu Teil wurde. Der Grund liegt darin, dass diese neuartigen Solarzellen, deren aktueller Rekordwirkungsgrad bei 10 Prozent liegt, ein Potential für eine kostengünstige Produktion auf flexiblem (Polymer)substrat aufweisen und aufgrund ihrer Vielfältigkeit neue Anwendungsbereiche für die Photovoltaik erschließen. Organische Solarzellen bestehen aus ultradünnen (einige 10 nm) Schichten aus Kohlenwasserstoffverbindungen. Damit der photovoltaische Effekt genutzt werden kann, müssen die durch Licht angeregten Molekülzustände zu freien Ladungsträgern führen, wobei positive und negative Ladung an unterschiedlichen Kontakten extrahiert werden. Für eine effektive Trennung dieser stark gebundenden lokalisierten angeregten Zustände (Exzitonen) ist eine Grenzfläche zwischen Molekülen mit unterschiedlichen Energieniveaus der Grenzorbitale erforderlich, sodass ein Elektron auf einem Akzeptor- und eine positive Ladung auf einem Donatormolekül entstehen. Diese Grenzschicht kann als planarer Heteroübergang durch zwei getrennte Schichten oder als Volumen-Heteroübergang in einer Mischschicht realisiert werden. Die Absorberschichten werden durch Elektroden kontaktiert, wobei es für effiziente Solarzellen erforderlich ist, dass diese einen ohmschen Kontakt ausbilden, da ansonsten Verluste zu erwarten sind. Diese Arbeit behandelt im Besonderen die elektrischen Prozesse einer organischen Solarzelle. Dafür wird ein eindimensionales Drift-Diffusionsmodell entwickelt, das den Transport von Exzitonen, deren Trennung an einer Grenzfläche und die Ladungsträgerdynamik beschreibt. Abgesehen von den Exzitonen gilt als weitere Besonderheit einer organischen Solarzelle, dass sie aus amorphen, intrinsischen und sehr schlecht leitfähigen Absorberschichten besteht. Elektrische Effekte sind an der Strom-Spannungskennlinie (I-U ) sichtbar, die in dieser Arbeit als Hauptvergleichspunkt zwischen experimentellen Solarzellendaten und den Simulationsergebnissen dient. Durch einen weitgehend qualitativen Vergleich können dominierende Prozesse bestimmt und mikroskopische Erklärungen gefunden werden. Ein wichtiger Punkt ist der schon erwähnte Kontakt zwischen Absorberschicht und Elektrode. Dort auftretende Energiebarrieren führen zu einem Einbruch im Solarzellenwirkungsgrad, der sich durch eine Verringerung der Leerlaufspanung und/oder S-förmigen Kennlinien (S-Knick) bemerkbar macht. Anhand einer systematischen Studie der Grenzfläche Lochleiter/Donator wird gezeigt, dass Energiebarrieren sowohl für die Ladungsträgerextraktion als auch für die -injektion zu S-Knicken führen können. Insbesondere die Tatsache, dass Injektionsbarrieren sich auch negativ auf den Photostrom auswirken, wird anhand von simulierten Ladungsträger- und elektrischen Feldprofilen erklärt. Das Aufstauen von Ladungsträgern an Extraktionsbarrieren wird durch Messungen transienter Photoströme bestätigt. Da S-Knicke in organischen Solarzellen im Allgemeinen häufig beobachtet werden, werden weitere Methoden vorgeschlagen, die die Identifikation der Ursachen ermöglichen. Dazu zählen I-U Messungen in Abhängigkeit von Temperatur und Schichtdicken. Als eine weitere Ursache von S-Knicken werden unausgeglichene Ladungsträgerbeweglichkeiten in einer Solarzelle mit flachem Übergang identifiziert und von den Barrierefällen unterschieden. Weiterer Forschungsgegenstand dieser Arbeit sind Mischschichtsolarzellen aus dem Donator-Farbstoff Zink-Phthalozyanin ZnPc und dem Akzeptor Fulleren C60. Dort wird beobachtet, dass die Leerlaufspannung vom Mischverhältnis abhängt. Ein Vergleich von Experiment und Simulation zeigt, dass sich das Ionisationspotenzial von ZnPc und dadurch die effektive Energielücke des Mischsystems ändern. Zusätzlich zu homogenen Mischschichten werden Solarzellen untersucht, die einen Gradienten im Mischungsverhältnis aufweisen. Die Vermutung liegt nahe, dass ein hoher Donatorgehalt am Löcherkontakt und ein hoher Akzeptorgehalt nahe des Elektronenkontakts die Ladungsträgerextraktion begünstigen. Dieser Effekt ist in dem hier untersuchten System allerdings vergleichsweise irrelevant gegenüber der Tatsache, dass der Gradient das Abfließen bzw. die Rekombination von Ladungsträgern am “falschen” Kontakt reduziert und somit die Leerlaufspannung erhöht. Der wichtigste intrinsische Verlustmechanismus einer Solarzelle ist die Rekombination von Ladungsträgern. Diese wird im letzten Teil der Arbeit anhand der ZnPc:C60 Solarzelle behandelt. Messungen der Leerlaufspannung in Abhängigkeit von der Beleuchtungsintensität zeigen, dass sich der dominierende Rekombinationsprozess mit zunehmender Intensität von Störstellenrekombination zu direkter Rekombination von freien Ladungsträgern verschiebt. Eine gezielte Variation des Absorptionsprofils in der Absorberschicht zeigt, dass die Ladungsträgerextraktionswahrscheinlickeit vom Ort der Ladungsträgergeneration abhängt. Dieser Effekt wird hervorgerufen durch unausgeglichene Elektronen- und Löcherbeweglichkeiten und äußert sich im Füllfaktor. Weitere Simulationsergebnisse bezüglich des Einflusses von Ladungsträgerbeweglichkeiten und verschiedener Rekombinationsmechanismen auf die I-U Kennlinie und die experimentelle Identifikation eines Photoshunts, der den Photostrom in Rückwärtsrichtung unter Beleuchtung dominiert, runden die Arbeit ab

Two types of solar cell AlGaAs-GaAs structures which are heteroface and triple heterojunction are investigated in this study. A complete theoretical study including optimisation for the optical properties ( transmission and reflection) of the heteroface Alo.sGao.2As- GaAs space solar cell is presented. The grid shadow and window layer effects, angle of incidence and the effects of the layer design parameters for AR-coating and window layer on the optical properties are considered in the calculations. A new structure for space solar cell which consists of double heterojunction AlGaAs­GaAs structure with GaAs/AlGaAs heterojunction back surface field (triple heterojunction(TIIJ))-to enhance the performance of the existed double heterojunction solar cell- is proposed. The analytical model for this TIU cell is presented as a function of all the cell's design parameters ( such as _layers doping, thicknesses, etc). The calculated results for this structure is compared with the experimental results for the previous double heterojunction structure. The effects of the design parameters of all layers including the AR-coating on the cell's output performance and the optimisation conditions are studied as well. The techniques of the light trapping and the photon recycling( which are gocxl for space solar cells) are applied for the THJ thin film AlGaAs-GaAs structure to improve further the efficiency . The change of the optimisation conditions due to the usage of these two techniques is also discussed.

This thesis explores solar variability and particularly the consistency and accuracy of its measurement over the period 1945-2015. Automated algorithms have been implemented to measure to measure sunspot number, area and classification and compared with manual reduction techniques. Parameters developed were then used to determine X-ray associations with regions. Automated Ha flare analysis was also developed.

Lightning, a familiar phenomenon on Earth, may also occur at other times and locations in our solar system. It has been suggested as a mechanism for forming chondrules, millimeter-sized beads of glassy silicate found in primitive meteorites formed in the early solar system 4.5 billion years ago. It has also been detected in Voyager images of Jupiter, and there is evidence that it may occur on other planets as well, including Venus, Saturn and Neptune. The mechanism believed to produce lightning discharges on Earth, and possibly other planets, is charge production by collisions of ice particles, followed by gravitational separation of oppositely-charged large and small particles. This work examines the possibility of the occurrence of lightning discharges in the atmospheres of Jupiter and Neptune as well as in the protoplanetary nebula (PPN) of the early solar system by modeling charge separation and growth of the electric field. The model is also applied to the Earth as a test of its predictive power. It is found that the model can reproduce the correct timescale, particle charge and electric field magnitude seen in terrestrial lightning. The model also predicts lightning on Jupiter at the 3-5 bar level provided that the local water abundance is greater than the solar value. This is a much higher abundance than measured by the Galileo probe into Jupiter's atmosphere, which suggests that the water content measured by the probe does not apply to the entire planet. An application of the model to Neptune's water and NH₄SH clouds finds that lightning is unlikely in these clouds due to the large electric field required for electrical breakdown. Lightning may be possible in the overlying H₂S-NH₃ cloud provided that these substances can undergo collisional charge exchange with a magnitude at least 1% of that found in water ice. In the protoplanetary nebula, it appears that large-scale precipitation-induced lightning could not have occurred, due to the small mass density, low temperature and high electrical conductivity of the surroundings. This is a robust conclusion that does not depend sensitively on the values of the parameters involved.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2002.
Includes bibliographical references (p. 41).
We attempt to locate magnetic holes from several years of data collected from WIND and ACE spacecraft and use them to study some statistical properties of the magnetic holes.
by Paisa Seeluangsawat.
S.B.

Cryovolcanism has played an important role in the geological histories of many icy satellites. Cryovolcanism, like the more familiar silicate volcanism on Earth and the other terrestrial planets, takes many forms. Many individual cryovolcanic landforms are morphologically surprisingly similar to terrestrial volcanic landforms and can be explained with similar mechanisms. However, assemblages of cryovolcanic, tectonic, and impact structures form unique and varied landscapes quite alien in their collective expression. Many variables can affect the cryovolcanic style of a satellite but none more so than cryolava composition. This work considers the compositional variable in considerable detail. This work summarises existing knowledge of phase equilibria and physical properties of cosmochemically relevant unary, binary, and multi-component chemical systems, and where published knowledge was found lacking, the author presents his own measurements of the physical chemistry of volatile mixtures. The author then takes the reader on a brief tour of cryovolcanic landscapes, and applies knowledge of the physical chemistry of volatile mixtures to problems of cryovolcanological interest. Aqueous cryolavas may range in composition from salt-water brines to cryogenic ammonia-water-rich multi-component solutions possibly involving methanol, ammonium sulfide, alkali chlorides, and many other potential components. Cryomagmatic distillation can greatly accentuate the importance of trace and minor constituents of icy satellites. The viscosities, densities, and other physical properties of these liquids vary considerably and depend sensitively on their exact compositions. These properties affect everything from cryovolcanic eruptive styles and landforms, to the way cryovolcanic crusts respond to tectonic stresses. It is believed that the compositional variable is directly or indirectly implicated in a wide variety of geomorphic aspects of contrast among the icy satellites. Thus, even though we can not as yet confidently attribute any specific morphology to a specific composition (for lack of in situ compositional analyses), there appears to be a powerful link between the composition of the ices originally accreted by a satellite and its subsequent interior evolution and exterior geomorphic appearance.

Organic solar cells have the potential to be portable power sources that are light-weight, flexible, and inexpensive. However, the highest power conversion efficiency for organic solar cells to date is ~8%, and most high-efficiency solar cells have an area of less than 1 cm². This thesis advances the field of organic solar cells by studying the physics and engineering of the devices to understand the reverse saturation current, which is related to efficiency, and the effects of area scaling. The most commonly accepted models to describe the physics of organic photovoltaic devices are reviewed and applied to planar heterojunction solar cells based on pentacene / C60 as a model system. The equivalent circuit model developed for inorganic solar cells is shown to work well to describe the behavior of organic devices and parameterize their current-voltage characteristics with five parameters. Changes in the parameters with different material combinations or device structures are analyzed to better understand the operation of the presented organic solar cells. A one-dimensional diffusion model for the behavior of excitons and treatment of the organic layers as planes is demonstrated to adequately model the external quantum efficiency and photocurrent in pentacene / C60 solar cells. The origin of the open-circuit voltage is studied using cells with different electrodes and different donor materials. While changing the electrodes does not affect open-circuit voltage, it is greatly modified by changes in the donor. Tests with additional semiconductors show the change in open-circuit voltage is not consistent from donor to donor as the acceptor is varied, suggesting a more complex relation than just the difference in energy levels. Study of the temperature dependence of the equivalent circuit parameters shows that the reverse saturation current, which has a significant role in determining the open-circuit voltage, has a thermally activated behavior. From this behavior, the reverse saturation current is related back to charge transfer at the donor / acceptor heterojunction to suggest that both the effective energy barrier presented by the energy levels and the electronic coupling are important in determining the reverse saturation current and open-circuit voltage. This marks a shift from just considering a built-in voltage or the energy levels to also considering the electronic coupling of the donor and acceptor materials. Temperature-dependent performance characteristics are also used to show key differences between organic and inorganic devices. Finally, the effect of area scaling is explored with pentacene / C60 solar cells having areas of 0.11, 7, and 36.4 cm². Analysis with the equivalent circuit model shows that performance decreases as area increases because of an increasing series resistance presented by the transparent electrode. A metal grid, to provide low resistance pathways for current, fabricated on top of the transparent electrode is proposed to reduce the effective resistance. The grid is unique in that it is placed between the electrode and the semiconductor layer and must be passivated to prevent shorts through the thin semiconductor to the back metal electrode. Analysis of the grid predicts greatly reduced series resistance, and experimental results show reduced resistance and improved performance for the 7 cm² and 36.4 cm² devices when including the grid.

This Ph.D work reports the studies of photovoltaic devices produced by solution processable methods. Two material systems are of interest: one is based on organic semiconductors, and another on organic/inorganic hybrid composites. Specifically, organic photovoltaic (OPV) devices are made using photoactive materials consisted of a -conjugated polymer [Poly(3-hexylthiophene), or P3HT] and fullerene derivative [phenyl-C60-butric acid methyl ester, or PCBM] in a bulk heterojunction (BHJ) structure of donor/acceptor network. On the other hand, hybrid photovoltaic (HPV) devices are made from blend of quantum dots and -conjugated polymers. The QD material presented here are of the lead sulfide (PbS), and lead selenide (PbSe), whereas the polymers are either P3HT or Poly(3-dodecyl thienylene vinylene) (PTV)with controlled regio-regularity. For OPV devices, two different device geometries are investigated, namely, the conventional or normal structure where indium tin oxide (ITO) is used an anode, and a metal cathode is fabricated by thermal vapor deposition (TVD). In this geometry, thin layer (about 30~35nm) of poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is deposited from aqueous solution onto ITO as hole transport layer (HTL). The second geometry, called the inverted structure, uses ITO as the cathode of the device. A thin layer of cesium carbonate (Cs2CO3) (about 1~2nm) is applied over the ITO and functions as electron transport layer (ETL), thereby decreasing the work function of ITO and allowing it to function as the cathode. In this case, PEDOT:PSS is mixed with 5vol.% of dimethylsulfoxide to increase conductivity for serving as anode. Two solution processable methods, spin-coating and spray processes were investigated, and a detailed study of nanomorphology influence under different annealing conditions, different solvents and thickness are reported. The main contribution of this Ph.D. work was the development and implementation of a layer-by-layer (LBL) all-spray solution-processable technique to fabricate large-scale OPV arrays, with more than 30% transmission in the visible to near IR range. Comparing with traditional laboratory OPV fabrication based on spin-coating and using metal as cathode contact, which greatly limits transparency of solar cells and posts difficulty for large scale manufacturing, this LBL spray process solves these two problems simultaneously. This technology eliminates the need for high-vacuum, high temperature, low rate and high-cost manufacturing associated with current silicon and in-organic thin film photovoltaic products. Furthermore, this technology could be used on any type of substrate including cloth and plastic. Single cell OPV with active area of 4mm2 was used as preliminary test device to obtain fabrication parameters for multi-cell OPV arrays. Three different sizes of OPV arrays were fabricated and tested under various illumination conditions. Starting from a 4" x 4" array with 50 cells in series connection 4" x 4" substrate consisting of 50 cells with total active area of 30cm2, a scaled up 1' x 1' array was fabricated as a proof of concept, and whose results are reported. Scaled down arrays, called micro arrays, are also presented in this work. OPV micro array has the potential application in DC power supplies for electrostatic Microelectromechanical systems (MEMS) devices. The first generation micro array consists of 20 small (1mm2) solar cells connected in series for a total device area of approximately 2.2cm2. The 2nd generation micro array with 60 cells shares the same size substrates and single cell active area as the first generation. However, the 2nd generation micro array cell has a new design with reduced series resistance and improved cell occupancy by 3 fold. Infrared quantum dots (QD) such as PbS and PbSe have potential in photovoltaic applications. These solution processable quantum dots with tunable electronic properties offer very attractive approach for expanding spectral sensitivity of -conjugated polymers to infrared region of solar spectrum. However, these QDs often have defects originated from either incomplete surface passivation or imperfections in the quantum Dots. The electronic levels of defects often are within the bandgap of the semiconductor. These in-gap states are of great importance since they affect the final destiny of excitons. Continuous wave photoinduced absorption spectroscopy has proven to be a convenient and successful technique to study long-lived photoexcitations of in-gap states. Part of this Ph.D work was the investigation of a peculiar gap state found in films of PbS QDs. This gap state bears confinement dependence, with a lifetime about 2μs. A detailed analysis of the Stokes shift, temperature dependence of PL, absorption and photoinduced absorption reveals the unconventional GS is a new state of a trapped exciton in a QD film. This gap state is directly relevant to exciton dissociation and carrier extraction in this class of semiconductor quantum dots. As synthesized PbSe and PbS quantum dots usually have bulky ligands such as oleic acids or TOPO (trioctylphosphine oxide). This capping layer is necessary to prevent nanocrystals from coalescence, however, the bulky ligands hinder charge extraction from and charge transport through the nanocrystals, as well as exciton dissociation at the nanocrystal/polymer interface. Common ways to manipulate ligands include ligand wash and ligand exchange in solution, and ligand removal on films. Through this Ph.D. work, a novel method using electric field to manipulate quantum dots ligands for interface of quantum dots and polymer, which possibly could facilitate charge extraction from the quantum dots and charge transfer between quantum dots and polymers, without the need of harmful chemicals. Over four orders improvement of photoconductivity at zero bias and more than six orders improvement at 5V reverse bias in a sandwich structure quantum dots photovoltaic device, and more than 5x improve in film smoothness. After thorough fundamental study on QD optoelectronic properties, hybrid photovoltaic (HPV) device was fabricated using a blend solution of PbS QDs and P3HT. Two different solution processes are used to form the QD/polymer active layer, one is the traditional spin coating method, and another is the spray technique developed in this Ph.D. Work. Different film morphology was observed with these two methods. Although the film is slightly rougher in the case with sprayed QD/polymer active layer, the phase segregation is more distinct and with smaller domain, which is beneficial for charge transport.

The steady, supersonic outflow from the Sun we call the solar wind was first posited in the 1950s and initial theories rightly linked the acceleration of the wind to the existence of the million-degree solar corona. Still today, the wind acceleration mechanisms and the coronal heating processes remain unsolved challenges in solar physics. In this work, I seek to answer a portion of the mystery by focusing on a particular acceleration process: Alfven waves launched by the motion of magnetic field footpoints in the photosphere. The entire corona is threaded with magnetic loops and flux tubes that open up into the heliosphere. I have sought a better understanding of the role these magnetic fields play in determining solar wind properties in open flux tubes. After an introduction of relevant material, I discuss my parameter study of magnetic field profiles and the statistical understanding we can draw from the resulting steady-state wind. In the chapter following, I describe how I extended this work to consider time dependence in the turbulent heating by Alfven waves in three dimensional simulations. The bursty nature of this heating led to a natural next step that expands my work to include not only the theoretical, but also a project to analyze observations of small network jets in the chromosphere and transition region, and the underlying photospheric magnetic field that forms thresholds in jet production. In summary, this work takes a broad look at the extent to which Alfven-wave-driven turbulent heating can explain measured solar wind properties and other observed phenomena.
Astronomy

A Gallium Arsenide (GaAs) Solar Concentrator Array (SCA) was constructed using the recently developed fresnel lens technology. The parts used were a mixture of space qualified components and readily available off-the-shelf items. THe objective of the test to verify that the SCA would substantially increase the array's power output and reject the high thermal radiation it would encounter in space. It was found that the 3 by 3 GaAs SCA power output increased from 29.88 mW to 433.71 mW. The thermal analysis and the steady state cell operating temperature calculation showed that the SCA's thermal design would keep the GaAs solar cells at a steady state temperature of 50.7 C. However, because of the low intensity level of the light source available for use the cell operating temperature was only 27 C. This was considered to be inconclusive in determining the adequacy of the thermal design

This work describes computer simulations of the behaviour of plasmas similar to those observed in the near Earth environment The work can be split into three main threads Firstly we have developed a set of algorithms to allow the implementation of particle type simulation models on parallel computer architectures ranging from small workstation clusters to massively parallel supercomputers These algorithms allow large simulations with many particles to be performed We address the problems of e cient use of available computational resources and the scaling of algorithms as computers get larger Secondly we use a parallel implementation of a two dimensional hybrid simulation code with periodic boundaries to explore the evolution of ion beam distributions similar to those observed upstream of the Earth s bow shock We follow the evolution of the resonant instabilities of these cool tenuous proton beams both isotropic and anisotropic in temperature into the non linear regime We examine the waves generated their e ects on the ion distribution function the phenomenon of gyrophase bunching and describe the life cycles of two dimensional magnetic features including oblique propagating shocklets We suggest that such two dimensional structures may play a role in the saturation of beam instabilities Coherence lengths of the waves are calculated We see some evidence of anisotropy driven mirror waves late on in these simulations Thirdly we explore the nature of parametric instabilities in two dimensions We examine the role of parametric or wave wave instabilities in the late evolution of beam instability generated waves We nd little evidence of any parametric instability in this case The two dimensional evolution of a wave known to be unstable to one dimensional parametric instability is described We nd that in this case the instability develops in a manner similar to that found in one dimensional simulations although with some angular broadening in wavevector space There is some evidence of anisotropy driven instabilities later in the simulation

La quête de la nouvelle physique est un défi impliquant à la fois la recherche de particules de matière noire dans les halos galactiques, et celle, aux collisonneurs, de particules dont l’existence est prédite par des théories au-delà du Modèle Standard, telles que la supersymétrie. Alors que les contraintes expérimentales sur ces particules s’intensifient, il devient capital de combiner les limites provenant de ces deux volets afin de guider la suite des recherches. Pour ce faire, il est indispensable d’évaluer et de tenir compte correctement des incertitudes astrophysiques, cosmologiques et nucléaires, pourtant souvent ignorées. La première partie de cette thèse est dédiée à l’étude de ces incertitudes et leur impact sur les contraintes obtenues en supersymétrie, ainsi que la complémentarité entre les contraintes des collisionneurs et de matière noire pour la recherche de nouvelle physique. La deuxième partie est consacrée au développement d’outils de calculs pour les détections directe et indirecte de matière noire, conçus afin de prendre correctement en compte les incertitudes astrophysiques et nucléaires, et à leur implémentation dans le code public SuperIso Relic. Enfin la troisième partie du travail concerne l’étude des implications cosmologiques d’une éventuelle découverte de nouvelles particules aux collisionneurs. Nous avons montré qu’il serait possible de tester les hypothèses du modèle cosmologique standard et d’obtenir des informations sur les propriétés de l’Univers primordial à une époque observationnellement inaccessible
The quest for new physics is a challenging task which involves, on the one hand, the search for dark matter particles from space, and on the other hand, the search at colliders for particles predicted by theories beyond the Standard Model, such as supersymmetry. With the experimental constraints on new particles getting stronger, it becomes crucial to combine the limits from both sectors in order to guide future searches. To this end, it is essential to estimate and take into account correctly the astrophysical, nuclear and cosmological uncertainties, which are most often ignored. The first part of this thesis is dedicated to the study of such uncertainties and to their impact on the constraints applied on supersymmetry. Moreover, we investigate the interplay between the constraints from colliders and dark matter searches in some detail. The second part concerns the development and the implementation in the public code SuperIso Relic of numerical tools for the calculation of direct and indirect dark matter detection constraints which were designed specifically to take correctly into account astrophysical and nuclear uncertainties. Finally, in the third part of this work, we consider the cosmological implications of a hypothetical discovery of new particles at colliders. We show that it would be possible to test the assumptions of the standard cosmological model and to obtain information on the properties of the primordial Universe at an epoch which is beyond observational reach

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Physics, 2003.
Includes bibliographical references (leaves 77-78).
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Perfect crystal real space imaging has limitations in its resolution imposed by position sensitive detectors. The disadvantage, its limited resolution, of a position-sensitive detector can be overcome by replacing the conventional detector with an area detector and moving to reciprocal space. Reciprocal space imaging is proposed in this thesis with the state-of-the-art neutron interferometry at National Institute of Standard and Technology. An aluminum wedge produces various phase gradients and a specially designed sample is introduced as a test subject. Superposition of the waves from the sample beam path and the gradient wedge beam path creates an interferogram that suggests an inhomogeneous phase distribution. The result shows the existence of spatially encoded phase gradients, even though imaging was unsuccessful. A next generation design of reciprocal space imaging is proposed in the conclusion.
by Changwoo Do.
S.M.

This thesis demonstrates a measurement of a plasma fluctuation velocity-space cross-correlation matrix using laser induced fluorescence. The plasma fluctuation eigenmode structure on the ion velocity distribution function can be empirically determined through singular value decomposition from this measurement. This decomposition also gives the relative strengths of the modes as a function of frequency. Symmetry properties of the matrix quantify systematic error. The relation between the eigenmodes and plasma kinetic fluctuation modes is explored. A generalized wave admittance is calculated for these eigenmodes. Since the measurement is a localized technique, it may be applied to plasmas in which a single point measurement is possible, multipoint measurements may be difficult, and a velocity sensitive measurement technique is available.

This is a thesis which contributes to research in two different fields: theoretical physics and physics education research. The common link between these two research areas is that both involve explorations of abstract physics and mathematical representations, but from different perspectives. The first part of this thesis is situated in theoretical physics. Here a cosmological scenario is explored where a de Sitter phase is replaced with a phase described with a scale factor a(t) ~ tq, where 1/3<1. This scenario could be viewed as an inflationary toy model, and is shown to open up the possibility of an information paradox. This potential paradox is resolved even in the worst case scenario by showing that the time scales involved for such a paradox to occur is of the order of magnitude of the recurrence time for the de Sitter space. The second part of this thesis is situated in physics education research. A number of learning situations that are experienced as abstract by students are explored: probability in one dimensional quantum tunnelling; the mindsets that students adopt towards understanding physics equations used in typical teaching scenarios; and what students focus on when presented with physics equations. The results for the quantum scattering study are four phenomenographic categories of description, for the mind sets study, six epistemological components of mindsets and for the focus on physics equations study, three foci creating five levels of increasing complexity of ways of experiencing physics equations.  Pedagogical implications of these results are discussed.

Solar radiation is the main driving force for the Earth’s weather and climate. It is also the prime source for renewable energy technologies.Solar energy technologies can satisfy the current energy demand and at the same time reduce anthropogenic greenhouse gas emissions. It follows from scientific and engineering research within the field that accurate terrestrial solar radiation data and derived models can improve the detection of longterm climate change, the validation of Earth radiation budget estimates, and the deployment of solar energy systems. But, accurate assessment of solar energy at the Earth’s surface is difficult due to spatial, temporal and angular variability. These variations emphasise the need for localised solar radiation measurements and models.

In light of the above, this work has aimed at improving the quality of radiation data at two specific locations; one in Trondheim, Norway and the other in Durban, South Africa. This has entailed the development of an automatic data logging system, and frequent radiometer calibrations with traceability to the World Radiometric Reference. In addition, a radiometer called the Direct-Diffuse Solar Radiometer has been developed for the mapping of sky radiance. Supplementary work has been done to re-affirm the need for quality ground-based data.

The work done is twofold, the first on the measurements of solar irradiance and the second on the implementation of solar radiation models. So, the presentation of the work is divided into two parts. Chapters 1 to 5 are included in Part A under the heading “Measurement of Solar Irradiance”. Chapters 6 is given in Part B, “Data Modelling”.

Chapter 1 describes the concept of solar irradiance, discusses the components of solar radiation and introduces the radiometers in common use for the measurements of solar irradiance. Motivations for the development of the Direct-Diffuse Solar Radiometer are also included.

Chapter 2 describes the radiation mapping at the two sites, with particular emphasis on methods implemented for higher accuracy sampling, data reliability and continuity.

Regular radiometer calibration is necessary to ensure the data quality. Therefore the methodology for and results from calibration of the site radiometers are given in Chapter 3.

The acquired direct and global data for the sites are presented in Chapter 4. Diurnal 1-minute variations are discussed along with estimates of monthly and annual trends, with particular emphasis on atmospheric transmittance and the clearness index.

The last chapter in Part A, i.e. Chapter 5, presents the work done on the Direct-Diffuse Solar Radiometer (DDSR-1B). The instrument is designed for measuring sky irradiance as well as direct solar irradiance. The DDSR-1B is a prototype ground based instrument which is mounted on a remotely controlled solar tracker/sky positioner. The Chapter discusses the DDSR- 1B’s design, control, calibration and field tests in detail.

Chapter 6 in Part B considers 2 available software products that can generate estimates of direct normal solar irradiances for any ground location. This chapter also concludes the work for this thesis by reemphasising the need for localised solar radiation measurements and models.

Finally, Chapter 7 collects the major conclusions given through the course of this text.

Thesis: S.B., Massachusetts Institute of Technology, Department of Physics, 2017.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 62-66).
The fabrication of inorganic semiconductor devices, such as silicon solar cells and photodetectors, is despite significant research still expensive and energy intensive. Especially for solar cells, high upfront costs are a limiting factor for large-scale fabrication and implementation. Organic semiconductors have attracted increasing attention over the last decade for their optoelectronic properties and potential applications in electronics and photonics. The ease of processing, light weight and low cost of organic semiconductors provides the opportunity for inexpensive, large-scale fabrication. In addition, the mechanical flexibility and bandgap tunability of organic materials enables the fabrication of flexible and semi-transparent organic solar cells and photodetectors that could be installed on any surface and integrated into windows and displays. This thesis demonstrates visibly transparent solar cells and photodetectors, fabricated using the organic materials tetraphenyldibenzoperiflanthene (DBP) and chloroaluminum phthalocyanine (CIAlPc). We fabricated our devices through spin-coating, thermal evaporation and sputtering and characterized the performance of our devices by measuring the current-voltage behavior, external quantum efficiency and visible transparency. Our devices demonstrate an average visible transparency of around 56%, power conversion efficiencies between 0.53% and 1.36% and specific detectivities on the order of 1011 to 1012 Jones. These values are comparable to results found in literature. The primary goal of this work is to highlight the potential of organic materials for transparent electronics and to provide a detailed overview of the fabrication and characterization techniques needed to advance research in this area.
by Anna Jungbluth.
S.B.

A new method of calculating accurate visible radiance values in the solar aureole region is derived in this study. The method interpolates between the radiance values output by a Gauss-Seidel flat atmosphere radiative transfer model using the radiative transfer equation itself. The method thereby produces accurate radiance values for arbitrary directions with only a slight increase in the required computational effort. The radiance and diffuse irradiance are then calculated for a variety of specified model atmospheres. The impact of polarization and the vertical stratification of aerosol particles is also investigated under various atmospheric conditions, and found to be relatively small under most realistic conditions. It is also found that some care must be taken when choosing the maximum aerosol radius at which to truncate the aerosol size distribution employed in the model atmosphere, to ensure that one does not exclude significant contributors to the scattered radiance field.

In the last few years, there has been a considerable increase of low cost space mission. Almost every small satellite has appendages like Solar Panels OR Antennas. These appendages have large inertial forces with low structural rigidity and mass to size ratio. These appendages are in stowed position in launcher to accommodate the whole satellite in the Launcher. However after launching these appendages are deployed to obtain the required power and the pointing of antennas. Their power producing capability is directly related to their area of exposure to sun, that is why these panels are available in different sizes and configurations. The requirement for the deployment mechanism for these appendages need to be cost effective, compact, lighter in weight and simple. It is entirely dependent on the structural designer to create such a robust mechanism that conforms to the requirements of the mission. In order to deploy the solar panels, it is mandatory that some sort of actuating system is employed. In this regard, we are aiming to use the torsional spring to induce the required torque for the deployment. There has been a considerable amount of research work [1-4] for a robust deployment mechanism to deploy the solar panel reliably without compromising on the overall cost and strength of the system. To realize the practical utility of Torsion Springs [5] for solar panel deployment, with the deficiency of locking after deployment and hold down mechanism when stowed. We propose a simple actuation scheme of designing the mechanism using Spiral Torsional spring for small satellite with its Zero 'g' testing and mitigation of expected Risks.

Water is important in the solar nebula both because it is extremely abundant and because it condenses out at 5 AU, allowing all three phases of H₂O to play a role in the composition and evolution of the solar system. In this work, a thorough examination of the inward radial drift of ice particles from 5 AU is undertaken. Drift model results are then linked to the outward diffusion of vapor, in one overall model which is numerically evolved over the lifetime of the nebula. Results of the model indicate that while the inner nebula is generally depleted in water vapor, there is a zone in which the vapor is enhanced by ∼40-100%, depending on the choice of ice grain growth mechanisms and rates. This enhancement peaks in the region from 0.1-2 AU and gradually drops off out to 5 AU. Conversely, ice abundance is enhanced over 3-5 AU. Representative hot (early) and cool (later) conditions during the quiescent phase of nebular evolution are examined. Additionally, the effect of the radial dependence of water depletion on nebular chemistry is quantified using a chemical equilibrium code that computes abundances of nebular elements and major molecular C, N, S, etc. species over a range of temperatures. In particular, changes in the local C/O ratio and organics abundance due to the radially dependent decrease in oxygen fugacity are tracked and plotted. Generally, the diffusion-drift model results in a more complex water distribution than previous models, with both radial and temporal variations in the C/O ratio which produce both relatively oxidizing and reducing nebular conditions across 1-5 AU. Depending on the value assumed for the solar C/O ratio, modest to significant enhancements of CH₄ and other organics abundances are produced in the inner nebula. These results coupled with the revised ice distribution may explain the radial signatures of hydration detections and darkening in asteroids, and perhaps the oxidation states of enstatite chondrites. The results also indicate that the inner nebula could have supplied organics and water to the terrestrial planets, as well as possibly to Europa and beyond, via outward mixing processes.

This thesis presents a study of hybrid solar cells, specifically looking at various methods which can be employed in order to increase the power conversion efficiency of these devices. The experiments and results contained herein also present a very accurate picture of how rapidly the field of hybrid solar cells has progressed within the past three years. Chapters 1 and 2 present the background and motivation for the investigations undertaken, as well as the relevant theory underpinning solar cell operation. Chapter 2 also gives a brief review of the literature pertinent to the main types of devices investigated in this thesis; dye-sensitised solar cells, semiconductor sensitized solar cells and perovskite solar cells. Descriptions of the synthetic procedures, as well as the details of device fabrication and any measurement techniques used are outlined in Chapter 3. The first set of experimental results is presented in Chapter 4. This chapter outlines the synthesis of mesoporous single crystals (MSCs) of anatase TiO₂ as well as an investigation of its electronic properties. Having shown that this material has superior electronic properties to the conventionally used nanoparticle films, they were then integrated into low temperature processed dye-sensitised solar cells and achieved power conversion efficiencies of > 3%, exhibiting electron transport rates which were orders of magnitude higher than those obtained for the high temperature processed control films. Chapter 5 further investigates the use of MSCs in photovoltaic devices, this time utilising a more strongly absorbing inorganic sensitiser, Sb₂S₃. Utilising the readily tunable pore size of MSCs, these Sb₂S₃ devices showed an increase in voltage and fill factor which can be attributed to a decrease in recombination within these devices. This chapter also presents the use of Sb₂S₃ in the meso-superstructured configuration. This device architecture showed consistently higher voltages suggesting that in this architecture, charge transport occurs through the absorber and not the mesoporous scaffold. Chapters 6 and 7 focus on the use of hybrid organic-inorganic perovskites in photovoltaic devices. In Chapter 6 the mixed halide, lead-based perovskite, CH₃NH₃PbI_3-xCl_x is employed in a planar heterojunction device architecture. The effects of Lewis base passivation on this material are investigated by determining the photoluminescence (PL) lifetimes and quantum efficiencies of treated and untreated films. It is found that passivating films of this material using Lewis bases causes an increase in the PLQE at low fluences as well as increasing the PL lifetime. By globally fitting these results to a model the trap densities are extracted and it is found that using these surface treatments decreases the trap density of the perovskite films. Finally, these treatments are used in complete solar cells resulting in increased power conversion efficiencies and an improvement in the stabilised power output of the devices. Chapter 7 describes the materials synthesis and characterisation of the tin-based perovskite CH₃NH₃SnI₃ and presents the first operational, lead-free perovskite solar cell. The work presented in this thesis describes significant advances in the field of hybrid solar cells, specifically with regards to improvements made to the nanostructured electrode, and the development and implementation of more highly absorbing sensitizers. The improvements discussed here will prove to be quite important in the drive towards exploiting solar power as a clean, affordable source of energy.

Cadmium telluride is an attractive material for solar cell applications because of its near optimum bandgap and high absorption coefficient. This thesis presents the results of a study into the use of CdTe for solar cells. Three types of cell have been investigated, namely; CdS/CdTe devices fabricated by the vacuum evaporation of CdS onto either: (a) single crystal p-CdTe substrates or (b) p-CdTe thin films, and (c) p-Cu(_2)Te/n-CdTe devices made by a chemiplating process onto single crystal n-CdTe. The effects of substrate polishing and preparation on the performance of CdS/CdTe bulk crystal cells have been Investigated together with the problems of doping and contacting to p-type CdTe. These studies have shown that the best results are obtained with devices that have been prepared on pad polished, phosphorus doped substrates using carbon contacts (efficiency = 7.2%). The influence of deposition conditions on the electrical and structural properties of thin CdS and CdTe layers, and their effect on CdS/CdTe device efficiency were also studied, and optimum growth conditions established. In the third group of CU(_2)Te/CdTe solar cells a number of structural and electrical aspects such as the phase of Cu(_2)Te, and the Influence of dopants, substrate resistivity and preparation and ageing on cell efficiency have been examined. As secondary objectives, an investigation Into the epitaxial growth of CdS on CdTe, and the characterisation of as-grown and doped CdTe have been carried out. It has been shown that epitaxy is possible on the {111} and {221} faces of CdTe. The characterisation of CdTe has revealed the presence of dominant levels at energies above the valence band of 0.50 eV in the as-grown crystals; 0.53, 0.71 and 0.84 eV in Te-annealed single crystals; and 0.35 eV in Cu doped CdTe thin films.