Dissertations / Theses on the topic 'Magnetospheric magnetic field'
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Schwarte, Judith. "Modelling the earth's magnetic field of magnetospheric origin from CHAMP data." [S.l. : s.n.], 2004. http://deposit.ddb.de/cgi-bin/dokserv?idn=971057001.
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Topliss, Stephen Mark. "Particle features at the equatorward edge of the cusp." Thesis, University College London (University of London), 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.342233.
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Patra, Swadesh. "The Contribution of Magnetospheric Currents to Ground Magnetic Perturbation during Geomagnetic Storms." DigitalCommons@USU, 2013. https://digitalcommons.usu.edu/etd/1719.
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A geomagnetic storm is triggered in response to a disturbance in the solar wind. The earth's ring current gets energized during a geomagnetic storm, which leads to a decrease in the horizontal component of the geomagnetic field on the earth's surface. The Disturbance Storm Time (Dst) index, which is a measure of the intensity of the ring current, is calculated by taking the average of this decrease in the horizontal intensity across four low latitude magnetometer stations and removing the quiet time secular variations. The rate of decrease of the Dst index is an indicator of the deenergization of the ring current particles. But there are several issues with the Dst measurement as a proxy of the ring current energy. In particular, the percentage contribution of the tail current effect to the Dst index is still debated. In this work, an effort has been made to separate and quantify the possible contribution of the tail current to the Dst index. The relative contribution for a selected set of storms for which the interplanetary magnetic field turned northward abruptly after the peak in Dst was observed is estimated. The WINDMI model of the nightside magnetosphere is used to investigate the contributions of ring current, magnetotail current, and magnetopause current on the observed two-phase decay of the Dst index. The role of different solar wind magnetosphere coupling functions on the Dst index calculated by the WINDMI model is also investigated. The performance of four other coupling functions in addition to the rectified vBs is evaluated. These coupling functions emphasize different physical mechanisms to explain the energy transfer into the magnetosphere due to solar wind velocity, dynamic pressure, magnetic field, and Mach number. One coupling function is due to Siscoe, another by Borovsky, and two by Newell. The results indicate that for a majority of cases, at most only vx, By, and Bz are needed to sufficiently account for the supply of energy to the ring current and geotail current components that contribute to the Dst index. The capabilities of the WINDMI model to reliably determine the state of the global magnetosphere are improved by employing the the Magnetotail (MT) index as a measurement constraint during large geomagnetic storms. The MT index is used as a proxy for the strength of the magnetotail current in the magnetosphere. The inclusion of the MT index as an optimization constraint in turn increases our confidence that the ring current contribution to the Dst index calculated by the WINDMI model is correct during large geomagnetic storms. To improve the models prediction of AL index, we also modify the ionospheric conductivity and fit to two substorms. The rate of reduction of convection in the magnetotail for some of these storms is numerically simulated by using inner magnetospheric models like the Fok Ring Current (FRC) and the Rice Convection Model along with the global BATSRUS model at the community coordinated modeling center. Model results are compared against magnetometer data by creating movie maps from several low-latitude magnetometer stations. The results indicate the contribution from the tail current to the Dst is important. In addition, the reduction of the cross-tail current during substorm dipolarization is predicted by the measured isotropic boundary locations. Several well known phenomena are identified in the magnetometer movie maps.
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Schwarte, Judith [Verfasser]. "Modelling the earth's magnetic field of magnetospheric origin from CHAMP data / Geoforschungszentrum Potsdam. Von Judith Schwarte." Potsdam : Geoforschungszentrum, 2004. http://d-nb.info/971057001/34.
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Winslow, Reka Moldovan. "Investigation of Mercury's magnetospheric and surface magnetic fields." Thesis, University of British Columbia, 2014. http://hdl.handle.net/2429/50100.
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This thesis is devoted to the study of Mercury’s magnetic field environment, to reveal the nature of the interaction between a weak planetary magnetic field and the interplanetary medium. Due to the lack of orbital spacecraft observations at Mercury prior to the MErcury Surface, Space Environment, GEochemistry, and Ranging (MESSENGER) mission, work in this thesis presents some of the first analysis and interpretation of observations in this unique and dynamic environment.
The bow shock and magnetopause define the boundary regions of the planet’s magnetosphere, thereby representing the initial interaction of the planetary field with the solar wind. We established the time-averaged shapes and locations of these boundaries, and investigated their response to the solar wind and interplanetary magnetic field (IMF). We found that the solar wind parameters exert the dominant influence on the boundaries; we thus derived parameterized model shapes for the magnetopause and bow shock with solar wind ram pressure and Alfven Mach number, respectively.
The cusp region is where solar wind plasma can gain access to the magnetosphere, and in Mercury’s unique case, the surface. As such, this area is expected to experience higher than average space weathering and be a source for the exosphere. Using magnetic field observations, we mapped the northern cusp’s latitudinal and longitudinal extent, average plasma pressure and observed its variation with the solar wind and IMF. From the derived plasma pressure estimates we calculated the flux of plasma to the surface.
Mercury’s internal dipole field is not centered on the planet’s geographic equator but has a significant northward offset. We developed the technique of proton-reflection magnetometry to acquire the first measurements of Mercury’s surface field strength. Proton loss cones are evident in both the northern and southern hemispheres, providing confirmation of persistent proton precipitation to the surface in these regions. We used the size of the loss cones to estimate the surface magnetic field strength, which confirm the offset dipole structure of the planetary field. With additional proton-reflection magnetometry observations, we generated a global proton flux map to Mercury’s surface and searched for regional-scale surface magnetic fields in the northern hemisphere.Science, Faculty of
Earth, Ocean and Atmospheric Sciences, Department of
Graduate
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Eriksson, Stefan. "Global Magnetospheric Plasma Convection." Doctoral thesis, Stockholm : Tekniska högsk, 2001. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3230.
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Stetler, Fredrik. "Isolated magnetic field structures in the Saturn magnetosphere." Thesis, KTH, Rymd- och plasmafysik, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-214821.
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This report’s primary focus is to use the data gathered by the Cassini satellite and analyzeits magnetic field data around Saturn. By looking for isolated changes in magneticfield values locations of potential plasmoids can be determined and examined. Theseso called plasmoids are pockets of higher density plasma ,associated with an increaseor decrease of the magnetic field strength, inside the magnetosheath, which may be importantfor the interaction between the solar wind plasma and the magnetosphere. Thestudy has been made over 7 years, from the beginning of 2010 to the end of 2016. Duringthis period a number of magnetic field structures have been found and documentedin this report, along with analyzing some of their properties such as their width andmagnetic field strength.Denna rapports primära fokus är att använda data insamlad av Cassini satelliten ochanalysera dess magnetiska fältdata runt Saturnus. Genom att titta efter isolerade förändringari magnetiska fältvärdena går det att lokalisera och examinera potentiella plasmoider.Dessa så kallade plasmoider är fickor med högre densitet av plasma, associerademed en ökning eller minskning av magnetisk fältdata, inne i magnetoskiktet, vilket kanvara viktigt för interaktionen mellan solvindens plasma och magnetosfären. Studien hargjorts över 7 års tid, från början av 2010 till slutet av 2016. Under denna period harett antal magnetiska fältstrukturer hittats och dokumenterats i denna rapport, genom attanalysera några av deras egenskaper så som deras bredd och magnetisk fältstyrka.
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Yuen, Rai. "Pulsar Magnetosphere Revisited: Emission Geometry and the Synthesis of the Vacuum-Dipole and the Rotating-Magnetosphere Models." Thesis, The University of Sydney, 2013. http://hdl.handle.net/2123/10011.
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We reconsider the vacuum-dipole model (VDM) and the corotating-magnetosphere model (CMM) for pulsar electrodynamics. Both the VDM and the CMM are fatally flawed as stand-alone models. The former model is used for deriving certain pulsar parameters, such as the surface magnetic field strength and characteristic age, but it lacks the plasma required to emit the observed radiation. The latter model introduces important concepts, such as the Goldreich-Julian charge density and corotation electric field, which form the basis for more detailed models, but it neglects the inductive electric field. When this field is included, the model is unstable to growth of large-amplitude electric oscillations when subject to a temporal perturbation. Furthermore, the predicted highly-relativistic magnetospheric plasma given by the two models is inconsistent with results obtained from observations with the Double Pulsar system. We therefore propose a way of synthesizing the VDM and the CMM for obliquely rotating pulsars. We first modify the VDM to a "minimal" model by assuming that the parallel component of the inductive electric field is screened by charges. We define a class of synthesized models as a linear combination of a fraction y times the minimal model and 1 - y times the CMM. We suggest that the synthesized model provides a basis for understanding the abrupt changes in the magnetospheres of some pulsars, which can alter their slowing down rates. The synthesized model also implies that the velocity of the magnetospheric plasma depends on y and the position of the emission point, which is determined numerically based on the obliquity and viewing angles for emission heights close to stellar surface in dipolar magnetic field structure. We also explore the field structure by including higher order terms in the ratio of the radius to the light-cylinder radius in the magnetic field and explore the implications of these additional terms.
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Bunting, Robert J. "Development and use of a current wedge modelling method for analysis of multiple onset substorms." Thesis, University of York, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.338555.
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Dimmock, Andrew. "The study of magnetic and electric field structures at planetary magnetospheres." Thesis, University of Sheffield, 2012. http://etheses.whiterose.ac.uk/2679/.
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Gralla, Samuel E., Alexandru Lupsasca, and Alexander Philippov. "PULSAR MAGNETOSPHERES: BEYOND THE FLAT SPACETIME DIPOLE." IOP PUBLISHING LTD, 2016. http://hdl.handle.net/10150/622675.
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Most studies of the pulsar magnetosphere have assumed a pure magnetic dipole in flat spacetime. However, recent work suggests that the effects of general relativity are in fact of vital importance and that realistic pulsar magnetic fields will have a significant nondipolar component. We introduce a general analytical method for studying the axisymmetric force-free magnetosphere of a slowly rotating star of arbitrary magnetic field, mass, radius, and moment of inertia, including all the effects of general relativity. We confirm that spacelike current is generically present in the polar caps (suggesting a pair production region), irrespective of the stellar magnetic field. We show that general relativity introduces a similar to 60% correction to the formula for the dipolar component of the surface magnetic field inferred from spindown. Finally, we show that the location and shape of the polar caps can be modified dramatically by even modestly strong higher moments. This can affect emission processes occurring near the star and may help explain the modified beam characteristics of millisecond pulsars.
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Sazykin, Stanislav. "Theoretical Studies of Penetration of Magnetospheric Electric Fields to the Ionosphere." DigitalCommons@USU, 2000. https://digitalcommons.usu.edu/etd/7152.
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Ionospheric disturbance electric fields of magnetospheric origin play an important role in determining the global morphology and dynamics of the ionosphere of the Earth. In this work, we present a number of numerical simulations of the transient electric fields in the middle and inner magnetosphere and the ionosphere equatorward of the auroral zone caused by idealized changes in the magnetospheric driving parameters. For these studies, we u se the Rice Convection Model (RCM), a large computer code of the magnetosphere-ionosphere coupling which consistently computes the electric fields, currents, and plasma densities in the magnetosphere and the electric field and currents in the ionosphere in the quasi-static slow-flow approximation. We made substantial upgrades to the code, which include a module computing realistic solar EDY-produced ionospheric conductances and a new potential solver. Our upgraded version of the RCM also includes a time - varying magnetospheric magnetic field and a self-consistently estimated auroral zone. We first discuss numerical problems encountered in modeling electrodynamics of convection with a time-varying magnetic field, realistic ionospheric conductances, and a self-consistent auroral zone, and our solutions to those difficulties. We then present a number of "computer experiments" with the new version of the RCM with idealized changes in the magnetospheric parameters such as sudden changes in the cross polar cap potential drop, magnetic field reconfiguration corresponding to the overall changes in the high-latitude convection, as well as rotations of the electric field on the polar cap boundary. Prompt penetration ionospheric electric fields simulated with the upgraded RCM are shown to be consistent with the previous simulations. The new simulations and their results are discussed in the context of (1) possible contribution to the variability of the ionospheric electric fields, and (2) role of time-varying magnetic field on the characteristic lifetimes of prompt penetration electric fields at subauroral, middle, and low latitudes.
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Gralla, Samuel E., Alexandru Lupsasca, and Alexander Philippov. "Inclined Pulsar Magnetospheres in General Relativity: Polar Caps for the Dipole, Quadrudipole, and Beyond." IOP PUBLISHING LTD, 2017. http://hdl.handle.net/10150/626414.
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In the canonical model of a pulsar, rotational energy is transmitted through the surrounding plasma via two electrical circuits, each connecting to the star over a small region known as a "polar cap." For a dipole-magnetized star, the polar caps coincide with the magnetic poles (hence the name), but in general, they can occur at any place and take any shape. In light of their crucial importance to most models of pulsar emission (from radio to X-ray to wind), we develop a general technique for determining polar cap properties. We consider a perfectly conducting star surrounded by a force-free magnetosphere and include the effects of general relativity. Using a combined numerical-analytical technique that leverages the rotation rate as a small parameter, we derive a general analytic formula for the polar cap shape and charge-current distribution as a function of the stellar mass, radius, rotation rate, moment of inertia, and magnetic field. We present results for dipole and quadrudipole fields (superposed dipole and quadrupole) inclined relative to the axis of rotation. The inclined dipole polar cap results are the first to include general relativity, and they confirm its essential role in the pulsar problem. The quadrudipole pulsar illustrates the phenomenon of thin annular polar caps. More generally, our method lays a foundation for detailed modeling of pulsar emission with realistic magnetic fields.
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Shore, Robert Michael. "Improved description of Earth's external magnetic fields and their source regions using satellite data." Thesis, University of Edinburgh, 2013. http://hdl.handle.net/1842/8935.
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In near-Earth space, highly spatio-temporally variant magnetic fields result from solar-terrestrial magnetic interaction. These near-Earth external fields currently represent the largest source of error in efforts to model the magnetic field produced in the Earth’s interior. Starting in 1999, the Decade of Geopotential Field Research (Friis-Christensen et al., 2009) has greatly increased the amount of available low-Earth orbit (LEO) satellite magnetic data. These data have driven many advances in field modelling, yet have highlighted that LEO measurements are particularly susceptible to contamination from external fields. This thesis presents a series of studies attempting to describe the external fields in more detail, in order that they can be more effectively separated from the internal fields in magnetic modelling efforts. A range of analysis methods, different for each study, are applied to satellite and ground-based observatory data. Mandea and Olsen’s (2006) method of estimating the secular variation (SV) of the internal field from satellite data via ‘Virtual Observatories’ (VOs) is applied to synthetic data from the upcoming Swarm constellation satellite mission of the European Space Agency. Beggan (2009) found VOs constructed from CHAMP satellite data to be contaminated with external field signals which appeared to have a significant local time (LT) dependence. I find that utilising the increased coverage of LT sectors offered by the Swarm constellation geometry does not significantly decrease the contamination. Following this surprising result I tested a wide range of methods aimed at reducing the VO contamination from each parameterised external field source region. In anticipation of future studies using real data, I used the results of the tests to provide a more complete description of the external field variations affecting analyses of geographically-fixed magnetic phenomena when using satellite data and spherical harmonic analysis (SHA). Ionospheric electric currents flowing at LEO altitudes are known to violate the assumption of measurements taken in a source-free space, required in SHA-based models of the magnetic field. In order to better describe the electromagnetic environment at LEO altitudes, I use data from the Ørsted and CHAMP satellites to calculate the current density from Amp`ere’s integral. Vector magnetic data from discrete overflights of the two satellites (at different altitudes) are rotated into the along-track frame to define the integral loop and its ‘surface area’, permitting estimation of the predominantly zonal current density flowing in the region between the two orbital paths. I designed selection criteria to extract geometrically-stable overflights spanning the range of LTs twice in the 6 years of mutually available satellite vector data. From these overflights I resolve current densities in the range 0:1 μA=m2, with the distribution of current largely matching the LT progression of the Appleton anomaly. I applied detailed tests to check for biases intrinsic to the method, and present results free of systematic errors. The results are compared with the predictions of the CTIP (Coupled Thermosphere-Ionosphere-Plasmasphere) model of ionospheric composition and temperature, showing a typically good spatiotemporal agreement. I find persistent current intensifications between geomagnetic latitudes of 30 and 50 in the post-midnight, pre-dawn sector, a region which has been previously considered to be relatively free of currents. External fields induce currents in the Earth’s conducting mantle, the magnetic fields of which add to the field measured at and above the Earth’s surface. The morphology of the long-period inducing field is poorly resolved on timescales of months to years, reducing the accuracy of mantle induction studies (a key part of the Swarm mission). I improve the description of its morphology via the method of Empirical Orthogonal Functions (EOFs), which I apply to over a decade of ground-based observatory data. EOFs provide a decomposition of the spatiotemporal structures contained in the magnetic field data, with partitions arising from the data themselves, overcoming the relatively simplistic assumptions made about the inducing field morphology in LT. The results of vector data EOF analyses are presented, but I rely primarily on scalar analyses which are more fitting for this study. I overcome the limitations of the irregular observatory distribution with a novel spatial weighting matrix, combining the output from multiple EOF analyses to greatly improve the data coverage in LT. I find that the seasonal variation of the inducing field is more important than the variation of the symmetric ring current on annual periods, and that dawn-dusk asymmetry should be accounted for to increase the accuracy of mantle conductivity estimates based on data covering the decadal timescales of the solar cycle.
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Yapici, Tolga. "Influences Of Interplanetary Magnetic Field On The Variability Of Aerospace Media." Master's thesis, METU, 2007. http://etd.lib.metu.edu.tr/upload/3/12608784/index.pdf.
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The Interplanetary Magnetic Field (IMF) has a controlling effect on the Magnetosphere and Ionosphere. The objective in this work is to investigate the probable effects of IMF on Ionospheric and Geomagnetic response. To fulfill the objective the concept of an event has been created based on the polarity reversals and rate of change of the interplanetary magnetic field components, Bz and By. Superposed Epoch Method (SPE) was employed with the three event definitions, which are based on IMF Bz southward turnings ranging from 6 to 11 nT in order to quantify the effects of IMF By and Bz. For the first event only IMF Bz turnings were taken into account while for the remaining, positive and negative polarity for IMF By were added. Results showed that the increase in the magnitude of IMF Bz turnings increased the drop of F layer critical frequency, f0F2. The drop was almost linear with the increase in magnitude of polarity reversals. Reversals with a positive IMF By has resulted in the continuation of geomagnetic activity more than 4 days, that is to say, the energy, that has penetrated as a consequence of reversal with a positive By polarity, was stored in outer Magnetosphere,whereas, with a negative IMF By the energy was consumed in a small time scale.
At the second step of the work, although conclusions about geomagnetic activity could be done, as a consequence of data gaps for f0F2 in addition to having low numbers of events, characterization of f0F2 due to constant IMF By polarity could not be accomplished. Thus, a modeling attempt for the characterization of the response due to polarity reversals of IMF components with the Genetic Programming was carried out. Four models were constructed for different polarity reversal cases and they were used as the components of one general unique model. The model is designed in such a way that given 3 consecutive value of f0F2, IMF By and IMF Bz, the model can forecast one hour ahead value of f0F2. The overall model, GETY-IYON was successful at a normalized error of 7.3%.
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Hatch, Spencer Mark. "Stormtime and Interplanetary Magnetic Field Drivers of Wave and Particle Acceleration Processes in the Magnetosphere-Ionosphere Transition Region." Thesis, Dartmouth College, 2017. http://pqdtopen.proquest.com/#viewpdf?dispub=10603779.
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The magnetosphere-ionosphere (M-I) transition region is the several thousand--kilometer stretch between the cold, dense and variably resistive region of ionized atmospheric gases beginning tens of kilometers above the terrestrial surface, and the hot, tenuous, and conductive plasmas that interface with the solar wind at higher altitudes. The M-I transition region is therefore the site through which magnetospheric conditions, which are strongly susceptible to solar wind dynamics, are communicated to ionospheric plasmas, and vice versa. We systematically study the influence of geomagnetic storms on energy input, electron precipitation, and ion outflow in the M-I transition region, emphasizing the role of inertial Alfven waves both as a preferred mechanism for dynamic (instead of static) energy transfer and particle acceleration, and as a low-altitude manifestation of high-altitude interaction between the solar wind and the magnetosphere, as observed by the FAST satellite. Via superposed epoch analysis and high-latitude distributions derived as a function of storm phase, we show that storm main and recovery phase correspond to strong modulations of measures of Alfvenic activity in the vicinity of the cusp as well as premidnight. We demonstrate that storm main and recovery phases occur during ~30% of the four-year period studied, but together account for more than 65% of global Alfvenic energy deposition and electron precipitation, and more than 70% of the coincident ion outflow. We compare observed interplanetary magnetic field (IMF) control of inertial Alfven wave activity with Lyon-Fedder-Mobarry global MHD simulations predicting that southward IMF conditions lead to generation of Alfvenic power in the magnetotail, and that duskward IMF conditions lead to enhanced prenoon Alfvenic power in the Northern Hemisphere. Observed and predicted prenoon Alfvenic power enhancements contrast with direct-entry precipitation, which is instead enhanced postnoon. This situation reverses under dawnward IMF. Despite clear observational and simulated signatures of dayside Alfvenic power, the generation mechanism remains unclear. Last, we present premidnight FAST observations of accelerated precipitation that is best described by a kappa distribution, signaling a nonthermal source population. We examine the implications for the commonly used Knight Relation.
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Kullen, Anita. "Polar auroral arcs." Doctoral thesis, KTH, Alfvén Laboratory, 2003. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-3562.
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Lotz, Stefanus Ignatius. "Empirical modelling of the solar wind influence on Pc3 pulsation activity." Thesis, Rhodes University, 2012. http://hdl.handle.net/10962/d1005249.
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Geomagnetic pulsations are ultra-low frequency (ULF) oscillations of the geomagnetic field that have been observed in the magnetosphere and on the Earth since the 1800’s. In the 1960’s in situ observations of the solar wind suggested that the source of pulsation activity must lie beyond the magnetosphere. In this work the influence of several solar wind plasma and interplanetary magnetic field (IMF) parameters on Pc3 pulsations are studied. Pc3 pulsations are a class of geomagnetic pulsations with frequency ranging between 22 and 100 mHz. A large dataset of solar wind and pulsation measurements is employed to develop two empirical models capable of predicting the Pc3 index (an indication of Pc3 intensity) at one hour and five minute time resolution, respectively. The models are based on artificial neural networks, due to their ability to model highly non-linear interactions between dependent and independent variables. A robust, iterative process is followed to find and rank the set of solar wind input parameters that optimally predict Pc3 activity. According to the parameter selection process the input parameters to the low resolution model (1 hour data) are, in order of importance, solar wind speed, a pair of time-based parameters, dynamic solar wind pressure, and the IMF orientation with respect to the Sun-Earth line (i.e. the cone angle). Input parameters to the high resolution model (5 minute data) are solar wind speed, cone angle, solar wind density and a pair of time-based parameters. Both models accurately predict Pc3 intensity from unseen solar wind data. It is observed that Pc3 activity ceases when the density in the solar wind is very low, even while other conditions are favourable for the generation and propagation of ULF waves. The influence that solar wind density has on Pc3 activity is studied by analysing six years of solar wind and Pc3 measurements at one minute resolution. It is suggested that the pause in Pc3 activity occurs due to two reasons: Firstly, the ULF waves that are generated in the region upstream of the bow shock does not grow efficiently if the solar wind density is very low; and secondly, waves that are generated cannot be convected into the magnetosphere because of the low Mach number of the solar wind plasma due to the decreased density.
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Caudal, Gérard. "Sur la facon dont le plasma piege a l'interieur des magnetospheres planetaires deforme le champ electrique et le champ magnetique qui y regnent." Paris 7, 1987. http://www.theses.fr/1987PA077101.
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Description du magnetodisque de jupiter dans laquelle les gradients de pression et les forces d'inertie agissent sur le plasma et engendrent des courants perpendiculaires au champ magnetique et le deforment. Les observations effectuees par les sondes spatiales sont en accord avec cette description. La ou les courants perpendiculaires sont divergents ils creent des courants alignes qui se referment dans l'ionosphere conductrice en deformant le champ electrique. On propose un mecanisme tenant compte de la perte d'electrons par diffusion en angle d'attaque, compatible avec les observations du radar eiscat de la distorsion du champ electrique
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Hill, Meirian Jane. "The microwave palaeointensity technique and its application to lava." Thesis, University of Liverpool, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.367068.
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Uwamahoro, Jean. "An analysis of sources and predictability of geomagnetic storms." Thesis, Rhodes University, 2011. http://hdl.handle.net/10962/d1005236.
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Solar transient eruptions are the main cause of interplanetary-magnetospheric disturbances leading to the phenomena known as geomagnetic storms. Eruptive solar events such as coronal mass ejections (CMEs) are currently considered the main cause of geomagnetic storms (GMS). GMS are strong perturbations of the Earth’s magnetic field that can affect space-borne and ground-based technological systems. The solar-terrestrial impact on modern technological systems is commonly known as Space Weather. Part of the research study described in this thesis was to investigate and establish a relationship between GMS (periods with Dst ≤ −50 nT) and their associated solar and interplanetary (IP) properties during solar cycle (SC) 23. Solar and IP geoeffective properties associated with or without CMEs were investigated and used to qualitatively characterise both intense and moderate storms. The results of this analysis specifically provide an estimate of the main sources of GMS during an average 11-year solar activity period. This study indicates that during SC 23, the majority of intense GMS (83%) were associated with CMEs, while the non-associated CME storms were dominant among moderate storms. GMS phenomena are the result of a complex and non-linear chaotic system involving the Sun, the IP medium, the magnetosphere and ionosphere, which make the prediction of these phenomena challenging. This thesis also explored the predictability of both the occurrence and strength of GMS. Due to their nonlinear driving mechanisms, the prediction of GMS was attempted by the use of neural network (NN) techniques, known for their non-linear modelling capabilities. To predict the occurrence of storms, a combination of solar and IP parameters were used as inputs in the NN model that proved to predict the occurrence of GMS with a probability of 87%. Using the solar wind (SW) and IP magnetic field (IMF) parameters, a separate NN-based model was developed to predict the storm-time strength as measured by the global Dst and ap geomagnetic indices, as well as by the locally measured K-index. The performance of the models was tested on data sets which were not part of the NN training process. The results obtained indicate that NN models provide a reliable alternative method for empirically predicting the occurrence and strength of GMS on the basis of solar and IP parameters. The demonstrated ability to predict the geoeffectiveness of solar and IP transient events is a key step in the goal towards improving space weather modelling and prediction.
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Leto, P., Courtney Trigilio, Lidia M. Oskinova, Richard Ignace, C. S. Buemi, G. Umana, A. Ingallinera, et al. "A Combined Multiwavelength VLA/ALMA/Chandra Study Unveils the Complex Magnetosphere of the B-Type Star HR5907." Digital Commons @ East Tennessee State University, 2018. https://dc.etsu.edu/etsu-works/2682.
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We present new radio/millimeter measurements of the hot magnetic star HR 5907 obtained with the VLA and ALMA interferometers. We find that HR 5907 is the most radio luminous early type star in the cm–mm band among those presently known. Its multi-wavelength radio light curves are strongly variable with an amplitude that increases with radio frequency. The radio emission can be explained by the populations of the non-thermal electrons accelerated in the current sheets on the outer border of the magnetosphere of this fast-rotating magnetic star. We classify HR 5907 as another member of the growing class of strongly magnetic fast-rotating hot stars where the gyro-synchrotron emission mechanism efficiently operates in their magnetospheres. The new radio observations of HR 5907 are combined with archival X-ray data to study the physical condition of its magnetosphere. The X-ray spectra of HR 5907 show tentative evidence for the presence of non-thermal spectral component. We suggest that non-thermal X-rays originate a stellar X-ray aurora due to streams of non-thermal electrons impacting on the stellar surface. Taking advantage of the relation between the spectral indices of the X-ray power-law spectrum and the non-thermal electron energy distributions, we perform 3-D modelling of the radio emission for HR 5907. The wavelength-dependent radio light curves probe magnetospheric layers at different heights above the stellar surface. A detailed comparison between simulated and observed radio light curves leads us to conclude that the stellar magnetic field of HR 5907 is likely non-dipolar, providing further indirect evidence of the complex magnetic field topology of HR 5907.
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Loridan, Vivien. "Physical and numerical modeling of the dynamics of high-energy electrons trapped in the outer radiation belt of the Earth’s magnetosphere." Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLN043/document.
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Les satellites sont vulnérables aux particules de forte énergie piégées dans les ceintures de Van Allen. Afin d’en assurer la protection, il est nécessaire de prédire avec précision la dynamique des électrons au sein de la magnétosphère. Dans un premier temps nous proposons une méthode originale de résolution analytique de l’équation de Fokker-Planck réduite qui modélise le transport et les pertes des électrons de la magnétosphère interne. La résolution repose sur une technique de décomposition spectrale. Si la solution analytique s’avère utile pour mettre en exergue certaines propriétés physiques des ceintures de radiation, elle est également pertinente pour valider le code numérique de résolution de l’équation de Fokker-Planck réduite, développé durant la thèse. Ce dernier nous amène à généraliser l’étude précédente en illustrant l’évolution des flux d’électrons pour diverses énergies et positions. Nous prouvons notamment que la structure des ceintures de radiation ainsi que leur temps d’évolution ne dépendent que de quelques facteurs bien choisis. Dans la perspective de reproduire un événement particulier de retour au calme après un orage magnétique, mesuré par les satellites de la NASA dédiés aux ceintures de radiation, nous sommes en mesure de simuler la précipitation des électrons dans l’atmosphère terrestre causée par les interactions avec les ondes électromagnétiques de la magnétosphère. L’utilisation de conditions bâties sur des données empiriques et spécifiques à la période en question nous permet de corroborer les flux observés. Enfin, l’influence du champ magnétique terrestre sur la dynamique des ceintures de radiation est étudiée sous divers aspects. Nous nous concentrons sur la ceinture externe pour comprendre comment les asymétries du champ magnétique, considérablement façonnées par l’activité solaire, affectent notre manière de concilier théorie et observations. Nous explorons également l’importance de certains processus diffusifs nouveaux et cachés, qui émergent à cause de l’irrégularité naturelle du champ magnétique au plus proche voisinage de la TerreSatellites are vulnerable to high-energy particles trapped in the Van Allen belts. To ensure their protection, it is necessary to predict properly the electron dynamics in the magnetosphere. We first propose an original method to find the analytical solution of the reduced Fokker-Planck equation that models the transport and loss of electrons in the inner magnetosphere. The resolution relies on an eigenfunction expansion approach. If the analytical solution is proven to be useful at uncovering some of the physical properties of the radiation belts, it is also relevant to validate the numerical code that solves the reduced Fokker-Planck equation, which has been developed during the PhD. The latter code is used to generalize the previous study in illustrating the evolution of the electron fluxes for various energies and locations. We demonstrate that the structure of the radiation belts as well as their dynamical timescales only depend on a few well-chosen parameters. In the perspective of reproducing a specific storm-recovery event reported by the NASA Van Allen Probes, we are able to simulate the electron scattering in the Earth’s atmosphere due to the interaction with magnetospheric electromagnetic waves. The consideration of data-driven and event-specific conditions enables us to corroborate the observed fluxes. Finally, various influences of the Earth’s magnetic field on the dynamics of the radiation belts are investigated. We focus on the outer belt to see how the magnetic field asymmetries, which are strongly shaped by solar activity, affect the way of conciliating theory and observations. We also explore the importance of new hidden diffusive processes that emerge due to the natural irregularity of the magnetic field in the closest vicinity of the Earth
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Yalim, Mehmet S. "An artificial compressibility analogy approach for compressible ideal MHD: application to space weather simulation." Doctoral thesis, Universite Libre de Bruxelles, 2008. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/210427.
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Ideal magnetohydrodynamics (MHD) simulations are known to have problems in satisfying the solenoidal constraint (i.e. the divergence of magnetic field should be equal to zero, $ablacdotvec{B} = 0$). The simulations become unstable unless specific measures have been taken.
In this thesis, a solenoidal constraint satisfying technique that allows discrete satisfaction of the solenoidal constraint up to the machine accuracy is presented and validated with a variety of test cases. Due to its inspiration from Chorin's artificial compressibility method developed for incompressible CFD applications, the technique was named as \
Doctorat en Sciences de l'ingénieur
info:eu-repo/semantics/nonPublished
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André, Philippe. "Emission radio des etoiles pre-sequence principale du nuage rho ophiuchi : observations et interpretations." Paris 6, 1987. http://www.theses.fr/1987PA066240.
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L'observation radiointerferometrique du nuage moleculaire rho ophiuchi a permis de detecter une dizaine de sources stellaires emettant dans le domaine radio parmi la centaine d'etoiles jeunes de ce nuage. La nature de cette emission est magnetique et produite par effet gyrosynchrotron. Des eruptions magnetiques importantes provoquent la polarisation circulaire de l'emission de l'un des objets detectes et la variabilite de l'emission de deux autres etoiles. Plus generalement, ces observations semblent selectionner une population specifique d'etoiles tres jeunes, en presence d'un champ magnetique etendu et en l'absence d'environnement circumstellaire dense
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Hilmer, Robert Vincent. "A magnetospheric magnetic field model with flexible internal current systems." Thesis, 1989. http://hdl.handle.net/1911/16240.
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A three dimensional B-field model of the Earth's magnetosphere satisfying the condition $\nabla$ $\cdot$ B = 0 is described. Highly flexible ring and cross-tail current systems are combined with the vacuum B-field model of Voigt (1981), a fully shielded dipole within a fixed magnetopause geometry. The ring current consists of nested eastward and westward flowing current distributions which tilt with and remain axially symmetric about the magnetic dipole axis. To include realistic flexing of the current sheet with dipole tilt, the intensity and position of the westward flowing cross-tail current in the midnight meridian can be represented by arbitrary functions of the distance along the magnetotail.
Model configurations are completely specified by four initial physical input parameters: the dipole tilt angle, the magnetopause stand-off distance, the geomagnetic index D$\sp{\rm st}$, and the midnight equatorward boundary of the diffuse aurora. These parameters determine the relative position and strength of both the ring and cross-tail currents and provide for a diverse array of configurations including many degrees of magnetotail field stretching. The resulting equatorial flux levels, $\triangle$B profiles, and the dipole tilt-dependent shape and position of the neutral sheet compare well with observations. With additional input parameters, the reconfiguration of the geomagnetic tail during magnetospheric substorms is modeled and incorporated into a magnetic field simulation of an observed substorm event. The ring and cross-tail currents, as prescribed by the set of initial input parameters, follow a physically reasonable sequence of development and magnetic flux densities are in general agreement with geosynchronous observations of the event.
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Naehr, Stephen M. "Quantitative modeling of time-dependent phenomena in the magnetospheric magnetic field." Thesis, 2002. http://hdl.handle.net/1911/18118.
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A series of improvements to the Rice Field Model (RFM) are described, which both increase the accuracy and extend the capabilities of the model. A new ring current parameterization improves the determination of storm-time fields in the inner magnetosphere. Replacement of the tail current module with a more flexible representation also contributes to improved accuracy in the inner magnetosphere, and enables realistic variations in current strength and orientation over the entire magnetotail length. Revision of the tail shielding/interconnection field eliminates inconsistencies in the model magnetotail, and permits variation in the normal component distribution over the tail portion of the magnetopause. The enhanced flexibilities of the interconnection field and cross-tail current module make possible the modeling of variations in the interplanetary magnetic field (IMF) as it propagates downstream, thereby advancing the steady-state RFM an important step toward time-dependent modeling.
The modified RFM is used to explore a number of time-dependent magnetospheric phenomena. In simulations of the March 1998 magnetic storm, the new model displays an improved representation of the inner magnetosphere, accurately predicting both storm-induced variations and day-night asymmetry in the field at geosynchronous orbit. The effects of time-dependent interplanetary fields on magnetospheric convection are examined, using a new method to compute ionospheric flow and electric fields in non-steady configurations. This method is applied to simulations of the growth and contraction of the polar cap in Southward and Northward turnings of the IMF. Model convection patterns for Southward turnings are shown to be consistent with theoretical expectations. The RFM is also used to simulate polar cap convection in the particular IMF conditions believed to trigger formation of the theta aurora. The results of the simulation prove to be consistent with several observed properties of the theta aurora, and shed light on the plasma sheet and magnetotail configurations associated with this phenomenon.
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Waters, Colin L. "Low latitude geomagnetic field line resonance." Thesis, 1992. http://hdl.handle.net/1959.13/1416267.
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Research Doctorate - Doctor of Philosophy (PhD)In the past many attempts have been made to use geomagnetic pulsations for diagnostic studies of the magnetospheric plasma. These techniques have often involved the observation of Alfvén wave magnetic field line resonances which can be detected as geomagnetic pulsations in space and on the ground. Previous ground based measurements of these resonant wave structures have required large magnetometer arrays and detailed analysis of wave polarisation. The spectral difference in wave amplitude and phase between two closely spaced magnetometer sites is shown to be capable of identifying this resonant structure. A technique based on the cross spectral phase is described and shown to be particularly suitable for monitoring temporal variations in field line and plasma characteristics. Geomagnetic pulsation data at L=1.8 and L=2.8 show that resonance structure is present on most days and occurs during all daytime hours. The structure at L= 1.8 shows a single resonant frequency while at L=2.8 up to 4 harmonics are seen, depending on geomagnetic conditions. Data recorded for 4 months between July and October, 1989 at L=1.8 showed a day-to-day variation in the resonant frequency between 38-56 mHz with the most common frequency between 44-45 mHz occuring on 30 of the 138 samples. A distinguishing feature at L=1.8 is a local morning decrease in the resonant frequency commencing at sunrise and lasting approximately 3 hours. The diurnal variation in geomagnetic field line eigenfrequency at both L = 1.8 and L=2.8 on two consecutive days that exhibited different temporal variation has been modelled by solving the hydromagnetic wave equation for the Alfvén wave mode using a dipole magnetic field and the IRI-90 ionospheric and diffusive equilibrium (DE) plasma density models. These calculations failed to predict the observed harmonic spacing at L=2.8 with the eigenfrequencies found to be lower by more than 50% compared to the experimental values at both L=1.8 and 2.8. However, the importance of mass loading due to 0⁺ at ionospheric altitudes in determining the harmonic spacing is demonstrated. Helium was found to be unimportant in this regard. A comparison of eigenfrequencies calculated for L= 1.7 and 2.4 using the plasma density model of Bailey (1983) and those for the IRI-DE model show the model of Bailey (1983) used by Poulter et al. (1984a, b, 1988) is more realistic for these latitudes. A time domain phase analysis technique, similar in principle to the operation of a phase-locked loop, has been developed to study rapid phase changes in geomagnetic pulsation wave trains. The resulting phase-time representation shows the demodulated input signal and provides wave group structure information. A study of the pulsation group structure using a magnetometer array in eastern Australia spanning 300520 geomagnetic latitude (L=1.4-2.8) and 2.2 hours in longitude is presented. It is shown that the observation of phase skips in pulsation records do not necessarily imply an impulsive source. The phase-time characteristics of signals at three frequencies are compared across the array. Two waves (40 and 43 mHz) show similar characteristics across the latitudinal extent of the array while the other (34 mHz) shows evidence of geomagnetic field line resonance around L∼2. An examination of the pulsation phase-time representation with latitude across this array shows a low latitude region below L∼2 where the spectra at all stations exhibit a similar shape with decreasing power as the latitude decreases. The phase-time characteristics do not show evidence of a broadband wave energy source as the excitation mechanism of low latitude field line resonances. An analysis of phase skip propagation speeds also shows that the assumption of plane wave type propagation is an over simplification. Consequently geomagnetic pulsation activity was modelled by frequency modulating the driving frequency of a forced, lightly damped simple harmonic oscillator. Best agreement with the observed phase-time characteristics was obtained with the driving frequency modulated at 8 mHz and centred on a resonant frequency at 50 mHz.
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Ding, Cheng. "Mapping magnetic flux tubes and field aligned currents using two representative magnetospheric models." Thesis, 1993. http://hdl.handle.net/1911/13714.
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Among available quantitative magnetospheric models, the Tsyganenko models represent empirical modeling, while the Toffoletto and Hill model is theoretical analysis. By mapping flux tubes between the ionosphere and the magnetosphere, those two models are analyzed and compared. Both the implied and actual field-aligned currents (FAC) are calculated in both models. The implied FAC, required to maintain the model field in magnetostatic equilibrium and given by the Vasyliunas equation, has the same order of magnitude as observational data, even in non-equilibrium models. The actual FAC, given by Ampere's law, is much smaller than the implied FAC, which means no large extraneous FAC is included in those models. Several new approaches imply that none of these models are in magnetostatic equilibrium, while the discrepancies are considerably larger for the Tsyganenko models. The magnetic effects of a preliminary FAC model have been evaluated in the Toffoletto and Hill theoretical model.
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Schwarte, Judith [Verfasser]. "Modelling the earth's magnetic field of magnetospheric origin from CHAMP data / von Judith Schwarte." 2004. http://nbn-resolving.de/urn:nbn:de:gbv:084-5650.
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"Modelling the Geometric Structure of the Magnetic Field in the Nightside Magnetosphere." Thesis, 2013. http://hdl.handle.net/10388/ETD-2013-03-1448.
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In this thesis, a simple model of the stretched magnetic field lines in the nightside magnetotail was created. The nightside magnetosphere model contains four main regions: plasmasphere, plasma sheet, magnetic lobes, and low latitude boundary layers. The plasma sheet is split into three regions based on the shape of the closed field lines present: dipole plasma sheet, transition plasma sheet, and stretched plasma sheet (SPS). The SPS, the focus of this thesis, is split into two regions: disruption zones (DZs) and a central neutral sheet (NS). The shape of the stretched field lines contain four inflection points. The convex curvature regions form the DZs and the central concave curvature region forms the NS. The NS is split into two regions: outer neutral sheet (ONS) and inner neutral sheet (INS). Due to the reversal of the x-component of the magnetic field at the center line of the NS, the protons are magnetized in the ONS and "unmagnetized" in the INS.
There are two main current systems in the SPS. The first is a double vortex current system consisting of eastward current in the DZs that closes westward in the NS. The second system is the NS field-aligned current (FAC) system. It is generated in the INS mainly by the earthward convective drift of the electrons while the "unmagnetized" protons have little convective drift and remain tailward of the electrons. This FAC system produces the pre-onset electron auroral arc during the growth phase of the substorm.
A simple model of the stretched magnetic field lines was created in order to calculate the current systems present in the SPS. The simple model was based entirely upon the shape of the stretched field lines. It passed two physical tests, divergence of the magnetic field and limits at infinity, so it was used to calculate currents. The total current using Ampere's law and the curvature current was found. Both results agreed with the double vortex current system.
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"Magnetic field line connection between the ionosphere and the magnetosphere -Auroral activity and relevant magnetic field variations at geosynchronous orbit-." Thesis, 1992. http://hdl.handle.net/2237/6624.
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Nishitani, Nozomu, and 望. 西谷. "Magnetic field line connection between the ionosphere and the magnetosphere -Auroral activity and relevant magnetic field variations at geosynchronous orbit-." Thesis, 1992. http://hdl.handle.net/2237/6624.
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Fu, Wei-Tsung, and 傅瑋宗. "Interplanetary Magnetic Field (IMF) Control over the Global Structures of Laboratory Magnetosphere." Thesis, 2019. http://ndltd.ncl.edu.tw/handle/94brun.
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博士國立中央大學
太空科學研究所
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Both east - west and north - south view images of magnetosphere for various interplanetary magnetic field (IMF) parameters have been investigated simultaneously in a laboratory experiment of the solar wind - magnetosphere coupling. An aurora is observed for both case of southward and northward IMF but the luminosity becomes stronger for southward IMF. Also, the reconnections are investigated in the front and tail areas. Inserting the 20 - degree terrella tilt, the study of modeling analysis is also processed for comparison with photograph of the experimental results in the laboratory simulation. Applied the models on CCMC overview of the global structure, the comparisons of model and laboratory simulation show the curvatures of the magnetic lines are qualitatively matched for the un-tilted and tilted terrella.
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HILMER, ROBERT V. "A MAGNETO-HYDROSTATIC MODEL OF MAGNETIC FIELD ROTATION THROUGH THE GEOMAGNETIC TAIL (MAGNETOSPHERE, IMF)." Thesis, 1986. http://hdl.handle.net/1911/13166.
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