太阳风动量涨落激发磁层亚暴的机制

本文将太阳风涨落传输能量产生磁层亚暴的机制推广到无碰撞等离子体过程。太阳风的涨落在磁层顶激发压缩阿尔文波,并在磁尾的无碰撞等离子体中传播。尾瓣中满足条件β<<1,而等离子体片中β≥1,其中β为等离子体压力与磁压之比。这样,快磁声波在尾瓣中几乎不衰减,而在等离子体片中很快衰减,将波动能量耗散在等离子体片中使等离子体加热或者粒子加速。这种机制还表明,磁尾等离子体片中的高能粒子可以由太阳风涨落动能耗散而被加速,不一定是直接源于太阳。

On the magnetospheric structure of pulsars

The magnetospheric structure of a pulsar is discussed for a non-force-free magnetic field. The local solution to the axisymmetric equations of the pulsar is obtained by the method of expanding in the polar angle. Particular attention is given to the solutions near the polar axis and the equator. Near the pulsar surface, the magnetic field energy density is found to be larger than the other energy components; the gravitational potential and the kinetic energy are relatively larger far away from the pulsar surface. It is shown that these relations influence the mass distribution in the pulsar magnetosphere. The results also show that the plasma rotation may be nonrigid and, hence, that a corotational region with a closed magnetic field may not exist.

Multipole moments in general relativity and dynamical perturbations of black-hole magnetospheres

This thesis consists of two parts. In Part I, we develop a multipole moment formalism in general relativity and use it to analyze the motion and precession of compact bodies. More specifically, the generic, vacuum, dynamical gravitational field of the exterior universe in the vicinity of a freely moving body is expanded in positive powers of the distance r away from the body's spatial origin (i.e., in the distance r from its timelike-geodesic world line). The expansion coefficients, called "external multipole moments,'' are defined covariantly in terms of the Riemann curvature tensor and its spatial derivatives evaluated on the body's central world line. In a carefully chosen class of de Donder coordinates, the expansion of the external field involves only integral powers of r ; no logarithmic terms occur. The expansion is used to derive higher-order corrections to previously known laws of motion and precession for black holes and other bodies. The resulting laws of motion and precession are expressed in terms of couplings of the time derivatives of the body's quadrupole and octopole moments to the external moments, i.e., to the external curvature and its gradient.

In part II, we study the interaction of magnetohydrodynamic (MHD) waves in a black-hole magnetosphere with the "dragging of inertial frames" effect of the hole's rotation - i.e., with the hole's "gravitomagnetic field." More specifically: we first rewrite the laws of perfect general relativistic magnetohydrodynamics (GRMHD) in 3+1 language in a general spacetime, in terms of quantities (magnetic field, flow velocity, ...) that would be measured by the ''fiducial observers” whose world lines are orthogonal to (arbitrarily chosen) hypersurfaces of constant time. We then specialize to a stationary spacetime and MHD flow with one arbitrary spatial symmetry (e.g., the stationary magnetosphere of a Kerr black hole); and for this spacetime we reduce the GRMHD equations to a set of algebraic equations. The general features of the resulting stationary, symmetric GRMHD magnetospheric solutions are discussed, including the Blandford-Znajek effect in which the gravitomagnetic field interacts with the magnetosphere to produce an outflowing jet. Then in a specific model spacetime with two spatial symmetries, which captures the key features of the Kerr geometry, we derive the GRMHD equations which govern weak, linealized perturbations of a stationary magnetosphere with outflowing jet. These perturbation equations are then Fourier analyzed in time t and in the symmetry coordinate x, and subsequently solved numerically. The numerical solutions describe the interaction of MHD waves with the gravitomagnetic field. It is found that, among other features, when an oscillatory external force is applied to the region of the magnetosphere where plasma (e⁺e^-) is being created, the magnetosphere responds especially strongly at a particular, resonant, driving frequency. The resonant frequency is that for which the perturbations appear to be stationary (time independent) in the common rest frame of the freshly created plasma and the rotating magnetic field lines. The magnetosphere of a rotating black hole, when buffeted by nonaxisymmetric magnetic fields anchored in a surrounding accretion disk, might exhibit an analogous resonance. If so then the hole's outflowing jet might be modulated at resonant frequencies ω=(m/2) Ω_H where m is an integer and Ω_H is the hole's angular velocity.

中低纬度变化磁场的暴时特征研究

Magnetic storm is a kind of severe disturbances in the whole solar-earth electromagnetic space. It has significant effects on communication, electric power, oil transport pipe and human activities in space. Therefore, magnetic storms are worth for applications systems, not only being a favorable issue for scientists. In this paper, the spatial and temporal distributions of the magnetic fields produced by the magnetosphere-ionosphere current systems during storms are studied. Four parts are included in this paper decomposion of different disturbances with different origins, topological structure of the ring current, the asymmetric characteristics of the ring current, and the statistic peculiarities of the day-to-day variability (DTD) of Sq. 1 The decomposition of magnetic disturbances at mid-low latitudes and its evolutions during storms Transient variations in the geomagnetic field recorded at mid-low latitudes mainly include the storm-time variation (Dst), solar quiet daily variation (Sq) and disturbance daily variation (SD). With the data of the geomagnetic meridian chain observatories in China, 25 storms during the period of 1997 to 1999 have been analyzed. According to the features of different variations, a method of “three-steps decomposition” is developed by using the method of Natural Orthogonal Components (NOC), Correlation Analysis and Fourier Analysis to separate those three components in turn. The results show that, the first eigenmode by the MNOC clearly describing the special distribution and temporal evolution of storm-time variation, in addition, Correlation Analysis and Fourier Analysis offer a useful method to extract the Sq and SD variations. The latitudinal shift of the Sq current focus seems to be the principal reason of the day-to-day variaitons in the daily range of Sq. The magnitude of SD reaches a maximum during the main phase, and then gradually decreases. 2 The topology structure of the ring current during storms Both the mechanism of the ring current and the geomagnetic data suggest that the central plane of the ring current is declining to the geomagnetic equator plane with a tilt angle δ. Using the H and Z component data at two stations in a meridian chain, we deduce a new parameter describing the invariable peculiarity of different storms. Then the δ angle is calculated by using the data from a meridian chain and tested with the ERC model. Finally the deduced tilt angles are used to modify Dst index. 3 The asymmetric characteristics of the ring current during storms The variations of the geomagnetic field at mid-low latitudes show a significant dawn-dusk asymmetry, resulting from the superposition of the fields from the symmetric ring current and the partial ring current. On the basis of the data from the 20°E, 30°E meridian chains and 30°N latitudinal chain, the dawn-dusk asymmetry is investigated by using three methods, namely, statistic analysis, ring current model calculation and typical event analysis. This characteristic implies the asymmetry of the spatial distribution of the ring current. In addition, during the main phase after the sudden commencement (SC), H field increases and reaches maximum around noontime, implying the effect of the Chapman-Ferraro current. 4 The statistic characteristics of the day-to-day variability and its mechanism The day-to-day variability of the geomagnetic Sq field is studied by using the magnetic data from a meridian chain of magnetometers along 120° E longitude. The method of NOC is applied to separate the Sq variation from complicated disturbances. The first eigenmode with the largest eigenvalue represents fairly well the Sq variation with a conspicuous day-to-day variability in the daily range. For the stations on the same north- or south-side of the Sq current system focus, the day-to-day variations show a positive correlation. In contrast, for the stations on the different sides of the Sq focus, they show a negative correlation, suggesting an important role of latitudinal shift of the Sq current system focus to the day-to-day variability of the Sq daily range. The Sq daily range is correlated with the magnetic indices Ap and Dst in a peculiar way: on some severe disturbed days, noticeably enhancements of the Sq are observed, implying increases of the ionospheric conductivities and/or tidal wind velocities; on other severe disturbed days, however, dramatically reduced Sq variations occur, suggesting dominant effects of the ‘disturbance dynamo’ process.

电离层行进式扰动的GPS台网监测研究

In this dissertation, we investigated two types of traveling ionospheric disturbances (TIDs)/gravity waves (GWs) triggered separately by auroral energy input during super geomagnetic storms and solar terminator (ST) under quiet geomagnetic conditions (kp<3+) using TEC measurements from the global network of GPS receivers. Research into the generation and propagation of TIDs/GWs during storms greatly enhance our understandings on the evolution processes of energy transportation from the high-latitude’s magnetosphere to the low-latitude ionosphere and the conjugated effect of TIDs propagation between the northern and southern hemispheres. Our results revealed that the conjugacy of propagation direction between the northern and southern hemispheres was subject to the influence of Coriolis force. We also figure out the evolution processes of ionospheric disturbances at the global scale. These are important topics that had not been well addressed previously. In addition, we also obtained thee wave structures of medium scale TIDs excited by the solar terminator (ST) moving over the northern America and physical mechanisms involved. Our observations confirm that the ST is a stable and repetitive source of ionospheric wave disturbances and the evidence of solar terminator generated disturbances has been demonstrated experimentally via the GPS TEC measurement. The main researches and results of this dissertation are as follows. First, the global traveling ionospheric disturbances (TIDs) during the drastic magnetic storms of October 29–31, 2003 were analyzed using the Global Position System (GPS) total electron content (TEC) data observed in the Asian-Australian, European and North American sectors. We collected the most comprehensive set of the TEC data from more than 900 GPS stations on the International GNSS Services (IGS) website and introduce here a strategy that combines polynomial fitting and multi-channel maximum entropy spectral analysis to obtain TID parameters. Moreover, in collaboration with my thesis advisor, I have developed an imaging technique of 2-dimensional map of TIDs structures to obtain spatial and temporal maps of large scale traveling ionospheric disturbances (LSTIDs). The clear structures of TEC perturbations map during the passage of TIDs were displayed. The results of our study are summarized as follows: (1) Large-scale TIDs (LSTIDs) and medium-scale TIDs (MSTIDs) were detected in all three sectors after the sudden commencement (SC) of the magnetic storm, and their features showed longitudinal and latitudinal dependences. The duration of TIDs was longer at higher latitudes than at middle latitudes, with a maximum of about 16 h. The TEC variation amplitude of LSTIDs was larger in the North American sector than in the two other sectors. At the lower latitudes, the ionospheric perturbations were more complicated, and their duration and amplitude were relatively longer and larger. (2) The periods and phase speeds of TIDs were different in these three sectors. In Europe, the TIDs propagated southward; in North America and Asia, the TIDs propagated southwestward; in the near-equator region, the disturbances propagated with the azimuth (the angle of the propagation direction of the LSTIDs measured clockwise from due north with 0°) of 210° showing the influence of Coriolis force; in the Southern Hemisphere, the LSTIDs propagated conjugatedly northwestward. Both the southwestward and northeastward propagating LSTIDs are found in the equatorial region. These results mean that the Coriolis effect cannot be ignored for the wave propagation of LSTIDs and that the propagation direction is correlated with the polar magnetic activity. (3) The day (day of year: 301) before the SC (sudden commencement) of magnetic storm, we observed a sudden TEC skip disturbances (±10 TECU). It should be a response for the high flux of proton during the solar flare event, but not the magnetic storms. Next, the most comprehensive and dense GPS network’s data from North-America region were used in this paper to analyze the medium scale traveling ionospheric disturbances (MSTIDs) which were generated by the moving solar terminator during the quiet days in 2005. We applied the multi-channel maximum entropy spectral analysis to calculated TID parameters, and found that the occurrence of ST-MSTIDs depends on the seasonal variations. The results of our study are summarized as follows: (1) MSTIDs stimulated by the moving ST (ST-MSTIDs) are detected at mid-latitudes after the passage of the solar terminator with the life time of 2～3 hours and the variation amplitude of 0.2～0.8 TECU. Spectral analysis indicated that the horizontal wavelength, average period, horizontal phase velocity of the MSTIDs are around 300±150 km，150±80 m/s and 25±15 min, respectively. In addition, ST-MSTIDs have wave fronts elongating the moving ST direction and almost parallel to ST. (2) The statistical results demonstrate that the dusk MSTIDs stimulated by ST is more obvious than the dawn MSTIDs in summer. On the contrary, the more-pronounced dawn MSTIDs occurs in winter. (3) Further analysis indicates that the seasonal variations of ST-MSTIDs occurrence frequency are most probably related to the seasonal differences of the variations of EUV flux in the ionosphere region and recombination process during sunrise and sunset period at mid-latitudes. Statistical study of occurrence characteristics of TIDs using the GPS network in North-American and European during solar maximum, In conclusion, statistical studies of the propagation characteristics of TIDs, which excited by the two common origins including geomagnetic storms and moving solar terminator, were involved with global GPS TEC databasein this thesis. We employed the multichannel maximum entropy spectral analysis method to diagnose the characteristics of propagation and evolvement of ionospheric disturbances, also, the characteristics of their regional distribution and climatological variations were revealed by the statistic analysis. The results of these studies can improve our knowledge about the energy transfer in the solar-terrestrial system and the coupling process between upper and lower atmosphere (thermosphere-ionosphere-mesosphere). On the other hand, our results of the investigation on TIDs generated by particular linear origin such as ST are important for developing ionospheric irregularity physics and modeling the transionosphere radio wave propagation. Besides, the GPS TEC representation of the ST-generated ionospheric structure suggests a better possibility for investigating this phenomenon. Subsequently, there are scientific meaning of the result of this dissertation to deeply discuss the energy transfer and coupling in the ionosphere, as well as realistic value to space weather forecast in the ionosphere region.

空间电流体系地磁反演及理论研究

The space currents definitely take effects on electromagnetic environment and also are scientific highlight in the space research. Space currents as a momentum and energy provider to Geospace Storm, disturb the varied part of geomagnetic field, distort magnetospheric configuration and furthermore take control of the coupling between magnetosphere and ionosphere. Due to both academic and commercial objectives above, we carry on geomagnetic inverse and theoretical studies about the space currents by using geomagnetic data from INTERMAGNET. At first, we apply a method of Natural Orthogonal Components (NOC) to decomposition the solar daily variation, especially for (solar quiet variation). NOC is just one of eign mode analysis, the most advantage of this method is that the basic functions (BFs) were not previously designated, but naturally came from the original data so that there are several BFs usually corresponding to the process really happened and have more physical meaning than the traditional spectrum analysis with the fixed BFs like Fourier trigonometric functions. The first two eign modes are corresponding to the and daily variation and their amplitudes both have the seasonal and day-to-day trend, that will be useful for evaluating geomagnetic activity indices. Because of the too strict constraints of orthogonality, we try to extend orthogonal contraints to the non-orthogonal ones in order to give more suitable and appropriate decomposition of the real processes when the most components did not satisfy orthogonality. We introduce a mapping matrix which can transform the real physical space to a new mathematical space, after that process, the modified components which associated with the physical processes have satisfied the orthogonality in the new mathematical space, furthermore, we can continue to use the NOC decomposition in the new mathematical space, and then all the components inversely transform back to original physical space, so that we would have finished the non-orthogonal decomposition which more generally in the real world. Secondly, geomagnetic inverse of the ring current’s topology is conducted. Configurational changes of the ring current in the magnetosphere lead to different patterns of disturbed ground field, so that the global configuration of ring current can be inferred from its geomagnetic perturbations. We took advantages of worldwide geomagnetic observatories network to investigate the disturbed geomagnetic field which produced by ring current. It was found that the ring current was not always centered at geomagnetic equator, and significantly deviated off the equator during several intense magnetic storms. The deviation owing to the tilting and latitudinal shifting of the ring current with respect to the earth’s dipole can be estimated from global geomagnetic survey. Furthermore those two configurational factors which gave a quantitative description of the ring current configuration, will be helpful to improve the Dst calibration and understand the dependence of ring current’s configuration on the plasma sheet location relative to the equator when magnetotail field warped. Thirdly, the energization and physical acceleration process of ring current during magnetic storm has been proposed. When IMF Bz component increase, the enhanced convection electric field drive the plasma injection into the inner magnetosphere. During the transport process, a dynamic heating is happened which make the particles more ‘hot’ when the injection is more deeply inward. The energy gradient along the injection path is equivalent to a kind of force, which resist the plasma more earthward injection, as a diamagnetic effect of the magnetosphere anti and repellent action to the exotically injected plasma. The acceleration efficiency has a power law form. We use analytical way to quantitatively describe the dynamical process by introducing a physical parameter: energization index, which will be useful to understand how the particle is heated. At the end, we give a scheme of how to get the from storm time geomagnetic data. During intense magnetic storms, the lognormal trend of geomagnetic Dst decreases depend on the heating dynamic of magnetosphere controlling ring current. The descending pattern of main phase is governed by the magnetospheric configuration, which can be describled by the energization index. The amplitude of Dst correlated with convection electric field or south component of the solar wind. Finally, the Dst index is predicted by upstream solar wind parameter. As we known space weather have posed many chanllenges and impacts on techinal system, the geomagnetic index for evaluating the activity space weather. We review the most popular Dst prediction method and repeat the Dst forecasting model works. A concise and convnient Key Points model of the polar region is also introduced to space weather. In summary, this paper contains some new quantitative and physical description of the space currents with special focus on the ring current. Whatever we do is just to gain a better understanding of the natural world, particularly the space environment around Earth through analytical deduction, algorithm designing and physical analysis, to quantitative interpretation. Applications of theoretical physics in conjunction with data analysis help us to understand the basic physical process govering the universe.

磁层-电离层耦合现象的分析研究

The magnetosphere-ionosphere coupling is mainly manifested by the trans- porting processes of energy into the ionosphere , the energy is carried by solar wind and firstly accumulate at the magnetosphere, and the coupling processes also significantly include the interaction between the magnetosphere and ionosphere for mass and energy. At the quiet condition, energy is delivered by the large-scale convection of the geomagnetic field; the huge energy from solar wind bulk will be injected into and consumed at the near magnetosphere and ionosphere by the geomagnetic storm and substorm activities. Aurorae and FACs (Field-aligned currents) are the important phenomena in the coupling processes. In the present work, firstly, we analyze the activity characteristics of auroral precipitating particle, secondly, we study the distribution characters of large-scale field aligned currents (LS FACs) at storm-time using the observations from different satellites at different altitudes. Finally, we investigate the evolution of the geomagnetic field configuration at the nightside sector on the onset of the expansion phase in a substorm event, the substorm event happened at 0430UT to 0630UT on 8th Nov. 2004. The main results as follows： At the first, the data of the estimated power input (EPI) of auroral particles from NOAA/POES (Polar orbiting environmental satellite) for some 30 years have been analyzed. The variation tendencies of the EPI generally coincide with aa, AE and Dst indices. The annual variation of EPI shows equinox peaks and an asymmetric-activity with a higher peak in the winter-hemisphere than in the summer-hemisphere. The diurnal UT variations are different from north and south hemisphere: for north hemisphere, the peak appears at 1200UT, and the relative deviation is 22% to the daily average of the north hemisphere. For south hemisphere, the maximal deviation is 22% at 2000UT. So the diurnal variation of EPI is more dominant than the annual variation which maximal deviation is 3% to 12% for different seasons. Studies on correlations of the hourly average of EPI, Pa, with AE and Dst indices show a correlation coefficient r=0.74 of Pa and AE, and r=-0.55 of Pa and Dst. The hourly EPIs for north and south polar regions, NPa and SPa, show a north-south asymmetry with a higher correlation of SPa and AE (or Dst). Time delays of EPI with respect to magnetic indices are examined, the maximum correlation coefficient of Pa with AE (r=0.78) occurs when the time delay =0, suggesting a synchronous activity of auroral electrojet and auroral precipitating particles, while =1-2h, the correlation coefficient of Pa with Dst is maximum (r=0.57), suggesting that the activity of auroral particle precipitating may influence the ring current on some extent. Sencondly, we use the high-resolution magnetic field vector data of the CHAMP satellite to investigate the distribution of large-scale FACs during the great magnetic storm on 7th to 8th Nov. 2004. The results show that, whether in the northern or southern hemisphere, the number and density of large-scale FACs during the main-phase are more and bigger than these during the recover-phase, and the number of large-scale FACs in morning sector obviously is more than that in afternoon sector. In terms of the magnetic indices, we find that large-scale FACs in morning sector significantly affected by the substorm activities, while in afternoon sector the large-scale FACs mainly indicate the fluctuations of the ring-current in storm time. Accordingly to the former studies, similarly, we find that in the morning sector, the scale of the large-scale FACs move to the high-latitude region, and in the afternoon sector, large-scale FACs distinctly expand to the low-latitude region. During the time periods that the NOAA/POES auroral precipitating particle power data temporally correspond to the large-scale FACs, the more the power of auroral particle is, the more and bigger the number and density of FACs are. At the same time, we use the magnetic field vector data of POLAR obtain a good form of region 1, region 2, and three pieces of cusp FACs during a single transit at 1930UT-2006UT on 07th. And the characteristics of simultaneous electric field and energy particles observations on Polar are coincide with the five FACs pieces. Finally, by means of the observation of Cluster 4 and Goes 10、 Goes 12, we analyze the evolution process of the change of the magnetic field configuration at night sector at the expansion phase of a substorm event which happened during 0430UT to 0630UT on 8th Nov. 2004, we find that the times of the beginning of the polarizations of magnetic field are observed from Goes 10 to Goes 12 then to Cluster 4. So, at the synchronous orbit ( 6.6 RE) to 10RE distance scale of the neutral sheet, the current disruption spread tailward. Simultaneously, the strengthen of the FACs deduced from these satellites’ magnetic field observations are almost consistent with the times of polarizations, as well as the high energy particles injection and the electric field dominant variation. The onset times determined by the magnetic field polarizations from these satellites are all ahead of the onset time that confirmed from the auroral electrojet indices. So, these characters of different observations can be used as the criterions to determine the onset time for the substorms of such type as we studied.

Opportunities for X-ray remote sensing at Mercury

Grande, Manuel; Dunkin, S. K.; Kellett, B., 'Opportunities for X-ray remote sensing at Mercury', Planetary And Space Science (2001) 49(14-15) pp.1553-1559 RAE2008

Dissociative charge exchange and ionization of O-2 by fast H+ and O+ ions: Energetic ion interactions in Europa's oxygen atmosphere and neutral torus

Measurements of electron capture and ionization of O-2 molecules in collisions with H+ and O+ ions have been made over an energy range 10 - 100 keV. Cross sections for dissociative and nondissociative interactions have been separately determined using coincidence techniques. Nondissociative channels leading to O-2(+) product formation are shown to be dominant for both the H+ and the O+ projectiles in the capture collisions and only for the H+ projectiles in the ionization collisions. Dissociative channels are dominant for ionizing collisions involving O+ projectiles. The energy distributions of the O+ fragment products from collisions involving H+ and O+ have also been measured for the first time using time-of-flight methods, and the results are compared with those from other related studies. These measurements have been used to describe the interaction of the energetic ions trapped in Jupiter's magnetosphere with the very thin oxygen atmosphere of the icy satellite Europa. It is shown that the ionization of oxygen molecules is dominated by charge exchange plus ion impact ionization processes rather than photoionization. In addition, dissociation is predominately induced through excitation of electrons into high-lying repulsive energy states ( electronically) rather than arising from momentum transfer from knock-on collisions between colliding nuclei, which are the only processes included in current models. Future modeling will need to include both these processes.

Ion-acoustic waves in a two-electron-temperatute plasma: oblique modulation and envelope excitations

Theoretical and numerical studies are carried out for the nonlinear amplitude modulation of ion-acoustic waves propagating in an unmagnetized, collisionless, three-component plasma composed of inertial positive ions moving in a background of two thermalized electron populations. Perturbations oblique to the carrier wave propagation direction have been considered. The stability analysis, based on a nonlinear Schrodinger-type equation, shows that the wave may become unstable; the stability criteria depend on the angle theta between the modulation and propagation directions. Different types of localized excitations (envelope solitary waves) are shown to exist in qualitative agreement with satellite observations in the magnetosphere.

Nonlinear Lagrangian theory of envelope electrostatic plasma waves in a two-electron-temperature plasma

An analytical model based on Lagrangian variables is presented for the description of ion-acoustic waves propagating in an unmagnetized, collisionless, three-component plasma composed of inertial positive ions and two thermalized electron populations, characterized by different temperatures. The wave's amplitude is shown to be modulationally unstable. Different types of localized envelope electrostatic excitations are shown to exist, and their forms are analytically and numerically investigated in terms of the plasma dispersion and nonlinearity laws. These results are in qualitative agreement with satellite observations in the magnetosphere. (C) 2004 American Institute of Physics.

Magnetospheric Structure and Atmospheric Joule Heating of Habitable Planets Orbiting M-dwarf Stars

We study the magnetospheric structure and the ionospheric Joule Heating of planets orbiting M-dwarf stars in the habitable zone using a set of magnetohydrodynamic models. The stellar wind solution is used to drive a model for the planetary magnetosphere, which is coupled with a model for the planetary ionosphere. Our simulations reveal that the space environment around close-in habitable planets is extreme, and the stellar wind plasma conditions change from sub- to super-Alfvénic along the planetary orbit. As a result, the magnetospheric structure changes dramatically with a bow shock forming in the super-Alfvénic sectors, while no bow shock forms in the sub-Alfvénic sectors. The planets reside most of the time in the sub-Alfvénic sectors with poor atmospheric protection. A significant amount of Joule Heating is provided at the top of the atmosphere as a result of the intense stellar wind. For the steady-state solution, the heating is about 0.1%-3% of the total incoming stellar irradiation, and it is enhanced by 50% for the time-dependent case. The significant Joule Heating obtained here should be considered in models for the atmospheres of habitable planets in terms of the thickness of the atmosphere, the top-side temperature and density, the boundary conditions for the atmospheric pressure, and particle radiation and transport. Here we assume constant ionospheric Pedersen conductance similar to that of the Earth. The conductance could be greater due to the intense EUV radiation leading to smaller heating rates. We plan to quantify the ionospheric conductance in future study.

Un modèle à criticalité auto-régulée de la magnétosphère terrestre

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

An overview of Earth's global electric circuit and atmospheric conductivity

The Earth’s global atmospheric electric circuit depends on the upper and lower atmospheric boundaries formed by the ionosphere and the planetary surface. Thunderstorms and electrified rain clouds drive a DC current (∼1 kA) around the circuit, with the current carried by molecular cluster ions; lightning phenomena drive the AC global circuit. The Earth’s near-surface conductivity ranges from 10−7 S m−1 (for poorly conducting rocks) to 10−2 S m−1 (for clay or wet limestone), with a mean value of 3.2 S m−1 for the ocean. Air conductivity inside a thundercloud, and in fair weather regions, depends on location (especially geomagnetic latitude), aerosol pollution and height, and varies from ∼10−14 S m−1 just above the surface to 10−7 S m−1 in the ionosphere at ∼80 km altitude. Ionospheric conductivity is a tensor quantity due to the geomagnetic field, and is determined by parameters such as electron density and electron–neutral particle collision frequency. In the current source regions, point discharge (coronal) currents play an important role below electrified clouds; the solar wind-magnetosphere dynamo and the unipolar dynamo due to the terrestrial rotating dipole moment also apply atmospheric potential differences. Detailed measurements made near the Earth’s surface show that Ohm’s law relates the vertical electric field and current density to air conductivity. Stratospheric balloon measurements launched from Antarctica confirm that the downward current density is ∼1 pA m−2 under fair weather conditions. Fortuitously, a Solar Energetic Particle (SEP) event arrived at Earth during one such balloon flight, changing the observed atmospheric conductivity and electric fields markedly. Recent modelling considers lightning discharge effects on the ionosphere’s electric potential (∼+250 kV with respect to the Earth’s surface) and hence on the fair weather potential gradient (typically ∼130 V m−1 close to the Earth’s surface. We conclude that cloud-to-ground (CG) lightning discharges make only a small contribution to the ionospheric potential, and that sprites (namely, upward lightning above energetic thunderstorms) only affect the global circuit in a miniscule way. We also investigate the effects of mesoscale convective systems on the global circuit.

Probing the large-scale topology of the heliospheric magnetic field using Jovian electrons

Jupiter’s magnetosphere acts as a point source of near-relativistic electrons within the heliosphere. In this study, three solar cycles of Jovian electron data in near-Earth space are examined. Jovian electron intensity is found to peak for an ideal Parker spiral connection, but with considerable spread about this point. Assuming the peak in Jovian electron counts indicates the best magnetic connection to Jupiter, we find a clear trend for fast and slow solar wind to be over- and under-wound with respect to the ideal Parker spiral, respectively. This is shown to be well explained in terms of solar wind stream interactions. Thus, modulation of Jovian electrons by corotating interaction regions (CIRs) may primarily be the result of changing magnetic connection, rather than CIRs acting as barriers to cross-field diffusion. By using Jovian electrons to remote sensing magnetic connectivity with Jupiter’s magnetosphere, we suggest that they provide a means to validate solar wind models between 1 and 5 AU, even when suitable in situ solar wind observations are not available. Furthermore, using Jovian electron observations as probes of heliospheric magnetic topology could provide insight into heliospheric magnetic field braiding and turbulence, as well as any systematic under-winding of the heliospheric magnetic field relative to the Parker spiral from footpoint motion of the magnetic field.