Dissertations / Theses on the topic 'Inner-Heliosphere'

Impulsive solar electron beams have an attractive diagnostic potential for poorly understood particle acceleration processes in solar flares. Solar flare accelerated electron beams propagating away from the Sun can interact with the turbulent interplanetary media, producing Langmuir waves and type III radio emission. In this thesis, we simulate electron beam propagation from the Sun to the Earth in the weak turbulent regime taking into account the self-consistent generation of Langmuir waves. We show that an injected single power-law spectrum will be detected at 1 AU as a broken power-law due to wave-particle interaction in the inhomogeneous plasma. We further extend these results by investigating the Langmuir wave interaction with background electron density fluctuations from low frequency MHD turbulence. We find a direct correlation between the spectra of the double power-law below the break energy and the turbulent intensity of the background plasma. Solar flares are believed to accelerate both upward and downward propagating electron beams which can radiate emission at radio and X-ray wavelengths correspondingly. The correlation between X-ray and radio emissions in a well observed solar flare allowed us detailed study of the electron acceleration region properties. We used the Nancay Radioheliograph, Phoenix-2 and RHESSI to infer the type III position, type III starting frequency and spectral index of the HXR emission respectively. Using these datasets and numerical simulations of the electron beam transport in the corona plasma, we were able to infer not only the location (the height in the corona), but to estimate the spatial length of the electron acceleration site.

The heliosphere, the volume inflated by the solar wind, is impenetrable to inter-stellar plasma, except for high-energy galactic cosmic rays. Neutral components of the local interstellar medium (LISM), however, do enter at the speed of Sun's relative motion to LISM. On their journey through the heliosphere, interstellar neutrals are subject to ionization by solar wind protons and electrons, solar photons, and also the gravitational pull of the Sun. Once ionized, the newly created ions, called "pickup ions" (PUIs), are swept out by the solar wind toward the termination shock (TS). Helium atoms, having a higher ionization potential, penetrate deeper into the solar system, with their trajectories gravitational focused to form a cone of high concentration of LISM He in the wake of the Sun's motion through LISM. This He cone provides a rich source of He⁺ PUIs. Recent theories suggest that PUIs are the seed particles for the detected anomalous cosmic rays (ACRs)--singly-charged ions highly enriched in He, N, O, and Ne, and with energies < 50 MeV/nuc after solar modulation. To reach such high energies, it would be necessary that the PUIs be pre-accelerated before reaching the TS, where the ions are finally accelerated to become ACRs. This thesis investigates two aspects of the PUIs life cycle by using the data gathered by Solar Wind Composition Spectrometer (SWICS) on the Advance Composition Explorer (ACE). The first is a study of pre-acceleration of He⁺ PUIs up to 100 KeV, by shocks generated by coronal mass ejecta. Our result implies that quasi-parallel shocks are more efficient at accelerating He⁺ PUIs in this energy interval than quasi-perpendicular shocks. The second is a study of time-variability of the gravitationally focused He cone. Our results suggest that the most likely cause of observed variability is due to the changing ionization rate of He over the solar cycle.

This thesis investigates the implications of relaxing assumptions inherent in techniques that analyse solar wind transients observed by NASA's Solar TErrestrial RElations Observatory (STEREO). In the first research chapter, I relaxed the assumption that the STEREO spacecraft are stationary while observing a transient. For much of the parameter space investigated, this effect was minimal, however in some cases it resulted in differences in derived radial speeds of hundreds of km s-1, leading to large errors. Using real data examples, the difference this effect makes was shown. The second research chapter applies the previous analysis to Corotating Interactions Region (CIR) observations. CIR events were identifed in STEREO HI J-maps, analysed, and their predicted arrival times calculated at each of the STEREO and Advanced Composition Explorer (ACE) spacecraft. A superposed epoch analysis was conducted using the predicted arrival times as the zero epoch time. It was found that when the fixed STEREO spacecraft assumption was relaxed, the CIR related transients that I observed had their estimated propagation speed increase such that they were propagating at (or close to) the slow solar wind speed, a physically realistic change. Changes in the structure of a stream interface over 1-2 days were seen, calling into question some of the underlying assumptions, which assume constant propagation characteristics over longer time-scales. Finally, I consider acceleration of solar wind transients close to the Sun. I use the analysis from previous chapters to perform fits to transient trajectories close to the Sun and infers the size of the acceleration region required to achieve convincing fits at low elongation values. It was found that the behaviour of the transients is consistent with an acceleration region within which the transient accelerates and then adopts a constant propagation speed. The acceleration region does not appear to occur at a fixed radial distance, but rather is different for each event analysed.

In this dissertation, I explore the relationship between solar energetic particles (SEPs) and the interplanetary magnetic field, and I use observations of SEPs to probe the region of space between the Sun and the Earth. After an introduction of major concepts in heliospheric physics, describing some of the history of energetic particles and defining the data sets used in the work, the rest of this dissertation is organized around three major concepts related to energetic particle transport: magnetic field-line length, interplanetary turbulence, and particle scattering and diffusion. In Chapter 2, I discuss how energetic particles can be used to measure the lengths of field lines and how particle scattering complicates the interpretation of these measurements. I then propose applying these measurements to a particular open problem: the origin and properties of heliospheric current sheets. In the next chapter, I move from the large to small scale and apply energetic particle measurements to important problems in interplanetary turbulence. I introduce two energetic-particle features, one of which I discovered in the course of this work, which have size scales roughly that of the correlation scale of the turbulence (the largest scale over which observations are expected to be similar). I discuss how multi-spacecraft measurements of these energetic particle features can provide a measure of the correlation scale independent of the magnetic field measurements. Finally, I consider interplanetary scattering and diffusion in detail. I describe new observations of particle diffusion in the direction perpendicular to the average magnetic field, showing that particles only scatter a few times between their injection at the Sun and observation at the Earth. I also provide numerical simulation results of diffusion parallel to the field which can be used to correct for the effects of transport on the particles. These corrections allow inferences to be made about the particle energies at injection from observations of the event-integrated fluences at 1 AU. By carefully including scattering, cooling, field line meandering and turbulence effects, solar-energetic particles become a powerful tool for studying the inner heliosphere.

A two-dimensional (2-D) time-dependent cosmic ray modulation model is used to calculate the modulation of cosmic-ray protons and electrons for 11-and 22-year modulation cycles using a compound approach to describe solar cycle related changes in the transport parameters. The compound approach was developed by Ferreira and Potgieter (2004) and incorporates the concept of propagation diffusion barriers, global changes in the magnetic field, time-dependent gradient, curvature and current-sheet drifts, and other basic modulation mechanisms. By comparing model results with 2.5 GV Ulysses observations, for both protons and electrons, it is shown that the compound approach results in computed intensities on a global scale compatible to observations. The model also computes the expected latitudinal dependence, as measured by the Ulysses spacecraft, for both protons and electrons. This is especially highlighted when computed intensities are compared to observations for the different fast latitude scan (FLS) periods. For cosmic ray protons a significant latitude dependence was observed for the first FLS period which corresponded to solar minimum conditions. For the second, which corresponded to solar maximum, no latitude dependence was observed as was the case for the third FLS period, which again corresponded to moderate to minimum solar activity. For the electrons the opposite occurred with only an observable latitude dependence in intensities for the third FLS period. It is shown that the model results in compatible intensities when compared to observations for these periods. Due to the success of the compound approach, it is also possible to compute charge-sign dependent modulation for 2.5 GV protons and electrons. The electron to proton ratio is presented at Earth and along the Ulysses trajectory. Lastly, it is also shown how the modulation amplitude between solar minimum and maximum depends on rigidity. This is investigated by computing cosmic ray intensities for both protons and electrons, not only at 2:5 GV, but also up to 7:5 GV. A refinement for the compound approach at higher rigidities is proposed.
Thesis (M.Sc. (Space Physics))--North-West University, Potchefstroom Campus, 2011.

For work on my thesis dissertation, we have been studying some energetic processes in solar flares. On our work on hard X-ray (HXR) emission from flares, we have shown that non-thermal recombination emission can compare with the bremsstrahlung HXR flux for certain flare conditions. In this thesis, we show spectral features characteristic of non-thermal recombination HXR emission and suggest how it plays a signicant role in the flare HXR continuum, something that has been ignored in the past. It is important to note that these results could demand a reconsideration of the numbers of accelerated electrons since recombination can be much more efficient in producing HXR photons than bremsstrahlung. We go on to show that although nonthermal recombination is not likely to dominate the total HXR flux unless we consider extreme parameter regimes, it can still form a signicant proportion of the HXR flux for typical flare conditions, thereby remaining important for both spectral inversion and low energy electron cut-off diagnostic capabilities. In related work on diagnosing particle acceleration in flares, we also have an interest in studying solar neutrons. To this end, this thesis presents our work done with new-age neutron detectors developed by our colleagues at the University of New Hampshire. Using laboratory and simulated data from the detector to produce its response matrix, we then employ regularisation and deconvolution techniques to produce encouraging results for data inversion. As a corollary, we have been reconsidering the role of inverse Compton scattering (ICS) of photospheric photons. Gamma-ray observations clearly show the presence of 100 MeV electrons and positrons in the solar corona, by-products of GeV energy ions. We present results of ICS of solar flare photons taking proper account of radiation field geometry near the solar surface. If observed, such radiation would let us determine the number of secondary positrons produced in large flares, contributing to a full picture of ion acceleration and to predicting neutron fluxes to be encountered by future inner heliosphere space missions.

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.

As duas sondas Helios viajaram na heliosfera interna do final do ano de 1974 até o início do ano de 1986, variando sua posição em longitude e distância radial (de 0.3 a 1 AU). Elas coletaram dados de plasma e campo magnético de alta resolução durante um ciclo solar completo. Mais de 390 ondas de choque interplanetárias guiadas por Ejeções Coronais de Massa Interplanetárias (ICMEs) foram identificadas pelas duas sondas, H1 e H2. Associando-se os dados de plasma e campo magnético de ambas as sondas, fazemos um estudo estatístico da extensão espacial de frentes de choque no meio interplanetário. Determinamos a dependência da probabilidade de choques serem vistos por ambas as sondas como uma função da separação longitudinal das espaçonaves. Como resultado, encontramos que, para um ângulo de separação entre as sondas de aproximadamente 90°, um choque tem 50% de chance de ser visto por ambas as sondas. A inclusão de dados dos satélites ISEE-3 e IMP-8 orbitando nas proximidades da Terra melhorou nossa estatística consideravelmente. Dentre os choques estudados, encontramos alguns casos em que as sondas estavam em direções praticamente opostas ao Sol e ainda assim observaram frentes de choque dentro de um contexto temporal aceitável. No entanto, devido à falta de observações simultâneas da coroa, não pudemos decidir univocamente se tais choques têm a mesma origem solar. Dentre o grande grupo de ondas de choque identificadas por H1 e H2, muitas delas foram guiadas por Nuvens Magnéticas (MCs). Algumas destas MCs foram observadas por múltiplas espa¸conaves, enquanto que a maioria delas constituiu o grupo das MCs observadas por uma única espaçonave. Por outro lado, encontramos que a extensão longitudinal das MCs pode ser tão grande quanto 90°. Dentre as nuvens estudadas, encontramos um evento em que as sondas estavam separadas por apenas 15° e somente uma delas observou a MC e o choque guiado pela mesma. Usamos a técnica de Análise da Mínima Variância (MVA) para determinar a direção de rotação do campo magnético dentro das MCs e a orientação do eixo das mesmas. Nuvens magnéticas que são altamente inclinadas em relação ao plano da eclíptica têm menos chance de ser observadas por duas espaçonaves mesmo se elas estiverem próximas uma da outra. Em geral, como indicado pelas observações através de múltiplas espaçonaves, MCs comportam-se como estruturas bem organizadas na heliosfera interna.
The two Helios probes traveled at variable longitudinal and radial separations (from 0.3 to 1 AU distance from the Sun) through the inner heliosphere from the end of 1974 until the beginning of 1986. In this way, they collected high resolution plasma and magnetic field data for an entire solar cycle. More than 390 shock waves driven by Interplanetary Coronal Mass Ejections (ICMEs) could be detected. Combining the data from both probes, we made a statistical study of the spatial extent of shock fronts in the interplanetary medium. We determine the dependence of the probability for shocks to be observed by both probes as a function of the spacecraft separation. We found that for a longitudinal separation of about 90° a shock has 50% chance to be observed by both probes. Including plasma and magnetic field data from the near-Earth ISEE-3 and IMP-8 spacecraft improved our statistical evaluation substantially. Thus, we found a few cases where the observations were located on almost opposite sides of the Sun and yet observed shock fronts within reasonable timely context. However, due to the absence of simultaneous coronal observations we can no longer uniquely decide whether these shocks originated at one and the same solar event. Among the large set of shocks identified by H1 and H2, many of them were driven by Magnetic Clouds (MCs). Some of these MCs were observed by multi-spacecraft, while most part of them consituted the group of singlespacecraft observation of MCs. On the other hand, we found that the longitudinal extent of MCs can be as large as 90°. We found one event where the two probes were separated by about 15°, and only one of the probes observed the MC and the shock wave driven by the cloud. We used the local Minimum Variance Analysis (MVA) to determine the direction of rotation of the magnetic field inside the MCs and the orientation of the MC axis. Highly-inclined MCs are less likely to be observed by two space probes even if they are very close to each other. In general, as observation from multi-spacecraft, MCs behave as well-organized structures in the inner heliosphere.

Un des enjeux majeurs dans l’étude du vent solaire est la compréhension de l’évolution de la turbulence entre échelles spatiales, mais aussi, radialement, lorsqu’on s’éloigne du Soleil. Nous abordons ces questions en nous servant des observations de champ magnétique effectuées par les missions WIND et HELIOS. D’abord, à partir de longues séries temporelles de WIND, nous identifions l’initiation de la cascade non linéaire dans le vent solaire lent. De la quasi-invariance du rapport entre le temps non linéaire et le temps d’Alfvén, nous déduisons que l’évolution comparable de la turbulence observée dans les vents lents et rapides peut s’expliquer par la constance du rapport (B) / δB. Ensuite, nous montrons comment une seule expression paramétrique permet de décrire la densité spectrale de puissance aux échelles cinétiques en tout lieu de l’héliosphère interne (entre 0.3 et 1 UA). Nous révélons également la présence dans l’héliosphère interne d’ondes de sifflement quasi-monochromatiques et montrons comment le vent lent offre les conditions nécessaires pour la présence préférentielle de ces ondes via le mécanisme d’anisotropie du halo. Enfin, nous montrons comment la non-stationnarité est inhérente à la turbulence du vent solaire dans le régime inertiel. Ceci remet en question l’utilisation fréquente de la fonction d’autocorrélation comme outil pour estimer les échelles caractéristiques
One of the key issues in solar wind studies is the understanding of the evolution of turbulence, both across spatial scales, and radially, when moving away from the Sun. We use magnetic field observations from the WIND and HELIOS missions to address this issue. First, using long records from WIND we identify the initiation of the non-linear turbulent cascade in the slow solar wind. From the quasi-invariance of the ratio between non-linear time and Alfvén time we conclude that the similar evolution of turbulence in fast and slow winds is primarily governed by the constant (B) / δB ratio. Second, we show how one single parametric expression can describe the power spectral density at kinetic scales at all positions in the inner heliosphere (between 0.3 and 1 AU). We also reveal the presence of narrow-band whistler waves in the inner heliosphere and using the halo anisotropy values show how the slow wind provides the proper conditions for the prevalence of whistlers. Finally, we reveal how non-stationarity is inherent to solar wind turbulence in the inertial range, which questions the use of autocorrelation functions to estimate characteristic scales

This thesis examines the two state structure of the solar wind and its influence upon proton temperatures. To examine this question, data from the Helios spacecraft are examined. The results from the data are interpreted in light of several theoretical models of the solar wind. In particular, it is found that the interplanetary heating of the protons observed by Helios is consistent with models that rely on extended deposition of energy and momentum in the form of Alfvenic waves. Analysis shows that between 4-10% of the time the data are consistent with two fluid models which do not include extended deposition of energy and momentum. For the rest of the data, the magnetic fluctuations are analyzed and it is found that there is dissipation of wave energy. Calculations show that the heating required by the protons can be accounted for by the apparent dissipation of Alfvenic wave energy. The relationships of temperature to velocity, to number density, and to momentum flux are also examined and are found to be consistent with a bifurcation of the solar wind based upon Alfven waves. A qualitative scenario for the generation of the two state solar wind wherein all the energy for the solar wind comes from convection in the sun is discussed.