Magnetospheric Dynamics Research Articles

Reply to Comment by Lamy et al. on “Locating the source field lines of Jovian decametric radio emissions”

Locating the source of decametric (DAM) radio emissions is a key step in the use of remote radio observations to understand the Jovian magnetospheric dynamics and their interaction with the planet’s moons. Wang YM et al. (2020) presented a method by which recorded arc-shaped DAM emissions in the radio dynamic spectra can be used to locate the source of a DAM. An Io-related DAM event on March 14, 2014 was used to demonstrate the method. A key parameter in the method is whether the DAM is emitted in the northern or the southern hemisphere; the hemisphere of origin can be determined definitively from the polarization of the emission. Unfortunately, polarization information for the emission on March 14, 2014 event was not recorded. Our analysis assumed the source to be in the northern hemisphere. Lamy et al. (2022) argue convincingly that the source was probably in the southern hemisphere. We appreciate the helpful contribution of Lamy et al. (2022) to this discussion and have updated our analysis, this time assuming that the DAM source was in the southern hemisphere. We also explore the sensitivity of our method to another parameter — the height at which the value of <italic>f</italic>_ce,max, which is the maximal electron cyclotron frequency reached along the active magnetic flux tube, is adopted. Finally, we introduce our recent statistical study of 68 DAM events, which lays a more solid basis for testing the reliability of our method, which we continue to suggest is a promising tool by which remote radio observations can be used to locate the emission source of Jovian DAMs.

Untangling magnetic massive star properties with linear polarization variability and the analytic dynamical magnetosphere model

ABSTRACT Light scattered off particles can become linearly polarized. Stars surrounded by oblique, co-rotating envelopes are therefore expected to manifest periodic linear polarimetric variations. The electron scattering magnetospheres of magnetic massive stars are expected to be suitable candidates to observe this effect. In this paper, we present the first semi-analytical model capable of synthesizing the continuum polarimetric signatures of magnetic O-type stars in an optically thin, single electron scattering limit. The purpose of this investigation is to improve our general understanding of magnetic hot stars by characterizing their polarimetric behaviour. Our linear polarization model is constructed by combining the analytical expressions for the polarimetric variations of an obliquely rotating envelope with the analytic dynamical magnetosphere model to represent a physical model for the envelope density structure. We compute grids of model Stokes Q and U curves and show that their shapes are unique to the choice of inclination and obliquity angles. We apply our model to HD 191612, a prototypical Of?p-type star, having both polarimetric and photometric observations. We find that the polarimetric modulations are best reproduced with $i=19^{+12}_{-3}$○, $\beta =71^{+3}_{-9}$○, and $\log \dot{M}_{B=0}=-6.11^{+0.12}_{-0.06}$ [M⊙ yr−1]. These results agree with previous investigations of this star. By combining both polarimetric and photometric synthesis tools, we simultaneously model the observations thus adding further refinement of the wind and magnetic properties of HD 191612.

Ganymede’s magnetic footprint brightness and location in respond to main emission

Jupiter’s aurora features have been observed by the Hubble space telescope (HST) for over two decades. One of the auroral features, Ganymede’s magnetic footprint, appears close to the main emission and is sometimes embedded in the main emission. The latter case causes difficulty in identifying Ganymede’s magnetic footprint from in the main emission. The FUV aurora images were taken by Advanced Camera for Surveys (ACS) onboard the HST. The fluctuations of Ganymede’s footprint brightness over time will be analyzed. Moreover, the correlation between the brightness and locations of the main emission and Ganymede’s magnetic footprint will be analyzed to characterize the connection between ionospheric phenomena and the magnetospheric dynamics. Since the main emission is very bright in comparison with the footprint, therefore, the variation of the main emission can affect the Ganymede’s magnetic footprint. Furthermore, the expansion of the main emission is consistent with the location shift of Ganymede’s magnetic footprint in equatorward direction. The brightness and location of the main emission can be influenced by the plasma variation in Jupiter’s magnetosphere which is affected partly by the volcanic eruption on Io and solar wind dynamic pressure. The variation of Ganymede magnetic footprint’s brightness and location in respond to the main emission could be an important indicator of the magnetospheric variation under the influences of internal and external factors.

Superposed Epoch Analyses of Electron‐Driven and Proton‐Driven Magnetic Dips

AbstractIn this study, we statistically investigate the features of magnetic dips by constructing superposed epoch analysis on Van Allen Probe data. Based on the values of electron and proton plasma betas, we categorize dips into two types: electron‐dominant and proton‐dominant. The global distributions of dips are obtained. Superposed epoch analysis on two types reveals a correlation between the magnetic fluctuations and plasma betas. Moreover, the occurrences of butterfly pitch angle distributions of relativistic electrons driven by the magnetic dips are confirmed on a statistical basis. Our results reveal the statistical characteristics of magnetic dips and establish the relationship among the magnetic fluctuations and background plasma parameters, indicating the potentially important role of magnetic dips in the inner magnetosphere dynamics.

Bifurcated Current Sheets in Mercury's Magnetotail: Observations and Implications

AbstractCross‐tail current sheets play an important role in magnetospheric dynamics and plasma transport. We present a comprehensive study of the structure and behavior of Mercury's cross‐tail current sheet, as well as constraints on the plasma environment imposed by this structure, using magnetic field measurements from the MESSENGER spacecraft. We find that the current sheet dominantly exhibits a bifurcated, or double‐peaked, current density structure throughout the tail region, but that a single‐peaked sheet is sometimes observed. The time‐averaged current distribution in the nightside magnetosphere is as follows: close to the planet on the midnight plane at an altitude of 0.15 RM (where RM is Mercury radius = 2,440 km), current density peaks are 0.2 RM north and south of the magnetic equator. The peak‐to‐peak separation decreases with down‐tail distance, until the current density profile resembles that of a single‐peaked sheet. We find no correlation between the sheet geometry and long‐period variations in solar wind and magnetospheric conditions. We attribute the observed bifurcated structure in the current sheet to non‐adiabatic behavior (Speiser orbits) of the major plasma constituents in Mercury's magnetosphere (H+, Na+ and, to a lesser extent e−). Finally, we infer the spatial distribution of the current carriers by comparing a simple model of their resultant current sheets to observations. We find that, close to the planet, a combination of planetary sodium ions and solar wind protons with energies of a few keV are the main current carriers, but in the far‐tail region, the current is dominantly carried by solar wind protons.

Modeling of Ultraviolet Aurora Intensity Associated With Interplanetary and Geomagnetic Parameters Based on Neural Networks

AbstractThe spatial distribution of aurora intensity is an important manifestation of solar wind‐magnetosphere‐ionosphere energy coupling process, and it oscillates with the change of space environment parameters and geomagnetic index. It is of great significance to establish an appropriate aurora intensity model for the prediction of space weather and the study of magnetosphere dynamics. Based on Ultraviolet Imager (UVI) data of Polar satellite, we constructed two auroral models by using two different neural networks, that is, the generalized regression neural network (GRNN), and the conditional generation adversarial network (CGAN). Input parameters of the models include interplanetary magnetic field, solar wind velocity and density, and the geomagnetic AE index. Output result is the spatial distribution of auroral intensity in altitude adjusted corrected geomagnetic (AACGM) coordinates. The structural similarity index (SSIM) of image quality is used as an evaluation standard of detail similarity between the prediction results of auroral intensity model and corresponding UVI images (complete similarity is 1, dissimilarity is 0, SSIM is generally considered to have good similarity if it is greater than 0.5). Based on the respective training datasets of GRNN and CGAN models, the evaluating results showed that the mean values (standard deviation) of SSIM were 0.5409 (0.0912) and 0.5876 (0.0712), respectively, so the prediction results from both models can restore the auroral intensity distribution of the original images of UVI. In addition, the value of SSIM can increase with the increase of the number of training data. Therefore, more training data will help improve the effectiveness of these models.

Prediction of Soft Proton Intensities in the Near-Earth Space Using Machine Learning

Abstract The spatial distribution of energetic protons contributes to the understanding of magnetospheric dynamics. Based upon 17 yr of the Cluster/RAPID observations, we have derived machine-learning-based models to predict the proton intensities at energies from 28 to 962 keV in the 3D terrestrial magnetosphere at radial distances between 6 and 22 RE. We used the satellite location and indices for solar, solar wind, and geomagnetic activity as predictors. The results demonstrate that the neural network (multi-layer perceptron regressor) outperforms baseline models based on the k-nearest neighbors and historical binning on average by ∼80% and ∼33%, respectively. The average correlation between the observed and predicted data is about 56%, which is reasonable in light of the complex dynamics of fast-moving energetic protons in the magnetosphere. In addition to a quantitative analysis of the prediction results, we also investigate parameter importance in our model. The most decisive parameters for predicting proton intensities are related to the location—Z geocentric solar ecliptic direction—and the radial distance. Among the activity indices, the solar wind dynamic pressure is the most important. The results have a direct practical application, for instance, for assessing the contamination particle background in the X-ray telescopes for X-ray astronomy orbiting above the radiation belts. To foster reproducible research and to enable the community to build upon our work we publish our complete code, the data, and the weights of trained models. Further description can be found in the GitHub project at https://github.com/Tanveer81/deep_horizon.

Magnetopause ripples going against the flow form azimuthally stationary surface waves

Surface waves process the turbulent disturbances which drive dynamics in many space, astrophysical and laboratory plasma systems, with the outer boundary of Earth’s magnetosphere, the magnetopause, providing an accessible environment to study them. Like waves on water, magnetopause surface waves are thought to travel in the direction of the driving solar wind, hence a paradigm in global magnetospheric dynamics of tailward propagation has been well-established. Here we show through multi-spacecraft observations, global simulations, and analytic theory that the lowest-frequency impulsively-excited magnetopause surface waves, with standing structure along the terrestrial magnetic field, propagate against the flow outside the boundary. Across a wide local time range (09–15h) the waves’ Poynting flux exactly balances the flow’s advective effect, leading to no net energy flux and thus stationary structure across the field also. Further down the equatorial flanks, however, advection dominates hence the waves travel downtail, seeding fluctuations at the resonant frequency which subsequently grow in amplitude via the Kelvin-Helmholtz instability and couple to magnetospheric body waves. This global response, contrary to the accepted paradigm, has implications on radiation belt, ionospheric, and auroral dynamics and potential applications to other dynamical systems.

Drift Resonance Between Particles and Compressional Toroidal ULF Waves in Dipole Magnetic Field

AbstractWe examine the drift‐resonant particle dynamics for toroidal ultralow frequency (ULF) waves in a pure dipole background geomagnetic field. We confirm that the resonant condition originally believed to apply only for poloidal ULF waves, , also applies for compressional toroidal waves. The predicted phase relationships have been confirmed from Van Allen Probes observations. Their good agreement provides the first observational evidence for the drift resonance condition controlled by the compressional toroidal ULF wave. Moreover, we extend the drift resonance theory into a nonlinear regime. The resulting particle motion can be described by a modified pendulum equation with solutions depending on the wave number . For high‐ toroidal waves, the resonant islands become asymmetric to perturb the particle trajectories within each potential well and consequently increase the trapping widths in both energy and L‐shell. We further carry out test‐particle simulations to show the evolution of electron distribution functions when they interact with either toroidal or poloidal waves. These findings demonstrate that toroidal ULF waves, like their poloidal counterparts, play an important role in magnetospheric particle dynamics.

Diffusivity in force-free simulations of global magnetospheres

ABSTRACT Assuming that the numerical diffusivity triggered by violations of the force-free electrodynamics constraints is a proxy for the physical resistivity, we examine its impact on the overall dynamics of force-free aligned pulsar magnetospheres endowed with an equatorial current sheet (ECS). We assess the constraint violations as a diffusivity source. The effects of modifications on electric fields used to restore force-free conditions are not confined to the ECS, but modify the magnetospheric dynamics on time-scales shorter than the pulsar rotational period. These corrections propagate especially via a channel that was unexplored, namely, changes induced to the electric charge density, ρ. We quantify the global consequences of diffusivity by comparing different techniques to model ρ. By default, we combine a conservative ρ-evolution with hyperbolic/parabolic cleaning of inaccuracies in the Maxwell equations. As an alternative, we enforce a constrained evolution, where ρ is directly computed as the electric field divergence. The conservative approach reduces the Poynting flux dissipated in the ECS by an order of magnitude, along with an increase of the pulsar luminosity driven by a shift of the Y-point location. The luminosity changes according to LY ∝ α0.11, where α is the ratio of diffusion to advection time-scales, controlling the amount of (numerical) diffusivity. Our models suggest interpreting the luminosity dependence on the Y-point location as differences in resistivities encountered at the ECS. Alternatively, they could be interpreted in terms of the pair formation multiplicity, κ, smaller diffusion being consistent with κ ≫ 1.

The Signature of Geomagnetic Field External Drivers in Virtual Observatory 30‐day Means Derived From Swarm Data

AbstractThe separation of contributions from different sources in the magnetic field signal measured at satellite altitude is an open challenge. An approach to this problem, using Principal Component Analysis, is here applied to geomagnetic external field series at Virtual Observatories (VOs). These series are computed from an enlarged data set of Swarm data covering all local times and all geomagnetic activity levels between January 2014 and December 2019. For each 30‐day time window, the Equivalent Source Dipole technique is used to reduce all measurements inside a cylinder to one single “observation” at its axis and 500 km altitude. Our results reveal a first principal mode with dipolar geometry and time variation following very closely the RC‐index of geomagnetic activity. They display a resolved second principal mode with annual periodicity and of approximately zonal quadrupolar radial pattern, reminiscent of results in a previous study using VO series from a filtered satellite data set and with lower time resolution. We resort to the recent comprehensive model CM6 to identify a possible source for this second mode. We propose that the dipolar mode is the expression of the magnetospheric ring current dynamics, at 30‐day time resolution, and the quadrupolar mode is the expression of the annual asymmetry between local summer and winter Sq current vortices. Two fainter modes could be related to the equinoctial amplification of Sq vortices and the ionospheric dynamo modulation by nonmigrating tides. We show that a more uniform local time sampling could contribute to better resolve ionospheric structures.

A Preliminary Study of Magnetosphere-Ionosphere-Thermosphere Coupling at Jupiter: Juno Multi-Instrument Measurements and Modeling Tools.

The dynamics of the Jovian magnetosphere are controlled by the interplay of the planet's fast rotation, its main iogenic plasma source and its interaction with the solar wind. Magnetosphere‐Ionosphere‐Thermosphere (MIT) coupling processes controlling this interplay are significantly different from their Earth and Saturn counterparts. At the ionospheric level, they can be characterized by a set of key parameters: ionospheric conductances, electric currents and fields, exchanges of particles along field lines, Joule heating and particle energy deposition. From these parameters, one can determine (a) how magnetospheric currents close into the ionosphere, and (b) the net deposition/extraction of energy into/out of the upper atmosphere associated to MIT coupling. We present a new method combining Juno multi‐instrument data (MAG, JADE, JEDI, UVS, JIRAM and Waves) and modeling tools to estimate these key parameters along Juno's trajectories. We first apply this method to two southern hemisphere main auroral oval crossings to illustrate how the coupling parameters are derived. We then present a preliminary statistical analysis of the morphology and amplitudes of these key parameters for eight among the first nine southern perijoves. We aim to extend our method to more Juno orbits to progressively build a comprehensive view of Jovian MIT coupling at the level of the main auroral oval.

Solar Wind Control of Magnetosheath Jet Formation and Propagation to the Magnetopause

AbstractMagnetosheath jets are localized high‐dynamic pressure pulses originating at Earth's bow shock and propagating earthward through the magnetosheath. Jets can influence magnetospheric dynamics upon impacting the magnetopause; however, many jets dissipate before reaching it. In this study we present a database of 13,096 jets observed by the Time History of Events and Macroscale Interactions during Substorms spacecraft from 2008 to 2018, spanning a solar cycle. Each jet is associated with upstream solar wind conditions from OMNI. We statistically examine how solar wind conditions control the likelihood of jets forming at the shock, and the conditions favorable for jets to propagate through the magnetosheath and reach the magnetopause. We see that, for each solar wind quantity, these two effects are separate, but when combined, we find that jets are over 17 times more likely to reach and potentially impact the magnetopause when the interplanetary magnetic field (IMF) orientation is at a low cone angle, and approximately 8 times more likely during high speed solar wind. Low IMF magnitude, high Alfvén Mach number, and low density approximately double the number of jets at the magnetopause, while and dynamic pressure display no net effect. Due to the strong dependence on wind speed, we infer that jet impact rates may be solar cycle dependent as well as vary during solar wind transients. This is an important step towards forecasting the magnetospheric effects of magnetosheath jets, as it allows for predictions of jet impact rates based on measurements of the upstream solar wind.

ULF‐Modulation of Whistler‐Mode Waves in the Inner Magnetosphere During Solar Wind Compression

AbstractThe solar wind plays important roles in terrestrial magnetosphere dynamics, including the particle population and plasma wave generation. Here, we report an interesting event that ultralow frequency (ULF) waves are enhanced right after solar wind compression and the compressional mode ULF wave subsequently modulates both the intensity and energy flux direction of whistler‐mode waves. Quasi‐periodic whistler‐mode wave packets are observed around = 5.6 at noon sector by Van Allen Probes. Growth rate calculation demonstrates that the compressional mode ULF wave can modulate the whistler‐mode wave intensity by modulating the energetic electron anisotropy. Moreover, the direction of wave energy flux is observed to alternate between northward and southward at equator, which is probably because the intense ULF waves periodically alter the relative direction of the wave source region with respect to the spacecraft. The current results provide a chain of observational evidences to illustrate how the generation and propagation of whistler‐mode waves in the inner magnetosphere are affected by ULF waves during the solar wind dynamic pressure enhancement.

Alfvénicity-related Long Recovery Phases of Geomagnetic Storms: A Space Weather Perspective

Abstract This paper reports, for the first time on a statistical basis, on the key role played by the Alfvénic fluctuations in modulating the recovery phase of the geomagnetic storms, slowing down the restoration of the magnetosphere toward its pre-storm equilibrium state. Using interplanetary and geomagnetic measurements collected over more than one solar cycle, a high correlation between the durations of Alfvénic streams and concurrent recovery phases is found, pointing to a clear coupling between Alfvénic turbulence and magnetospheric ring current dynamics. By exploiting current solar wind models, these observations also provide space weather opportunities of predicting the total duration of any geomagnetic storm induced by any solar driver provided that it is followed by an Alfvénic stream, a crucial piece of information for ground technologies and infrastructures that are affected by time-integrated effects throughout the duration of the storm.

Ultraviolet line profiles of slowly rotating massive star winds using the ‘analytic dynamical magnetosphere’ formalism

ABSTRACT Recent large-scale spectropolarimetric surveys have established that a small but significant percentage of massive stars host stable, surface dipolar magnetic fields with strengths on the order of kG. These fields channel the dense, radiatively driven stellar wind into circumstellar magnetospheres, whose density and velocity structure can be probed using ultraviolet (UV) spectroscopy of wind-sensitive resonance lines. Coupled with appropriate magnetosphere models, UV spectroscopy provides a valuable way to investigate the wind–field interaction, and can yield quantitative estimates of the wind parameters of magnetic massive stars. We report a systematic investigation of the formation of UV resonance lines in slowly rotating magnetic massive stars with dynamical magnetospheres. We pair the analytic dynamical magnetosphere (ADM) formalism with a simplified radiative transfer technique to produce synthetic UV line profiles. Using a grid of models, we examine the effect of magnetosphere size, the line strength parameter, and the cooling parameter on the structure and modulation of the line profile. We find that magnetic massive stars uniquely exhibit redshifted absorption at most viewing angles and magnetosphere sizes, and that significant changes to the shape and variation of the line profile with varying line strengths can be explained by examining the individual wind components described in the ADM formalism. Finally, we show that the cooling parameter has a negligible effect on the line profiles.

Impacts of Ionospheric Ions on Magnetic Reconnection and Earth's Magnetosphere Dynamics

AbstractIonospheric ions (mainly H+, He+, and O+) escape from the ionosphere and populate the Earth's magnetosphere. Their thermal energies are usually low when they first escape the ionosphere, typically a few electron volt to tens of electron volt, but they are energized in their journey through the magnetosphere. The ionospheric population is variable, and it makes significant contributions to the magnetospheric mass density in key regions where magnetic reconnection is at work. Solar wind—magnetosphere coupling occurs primarily via magnetic reconnection, a key plasma process that enables transfer of mass and energy into the near‐Earth space environment. Reconnection leads to the triggering of magnetospheric storms, auroras, energetic particle precipitation and a host of other magnetospheric phenomena. Several works in the last decades have attempted to statistically quantify the amount of ionospheric plasma supplied to the magnetosphere, including the two key regions where magnetic reconnection occurs: the dayside magnetopause and the magnetotail. Recent in situ observations by the Magnetospheric Multiscale spacecraft and associated modeling have advanced our current understanding of how ionospheric ions alter the magnetic reconnection process, including its onset and efficiency. This article compiles the current understanding of the ionospheric plasma supply to the magnetosphere. It reviews both the quantification of these sources and their effects on the process of magnetic reconnection. It also provides a global description of how the ionospheric ion contribution modifies the way the solar wind couples to the Earth's magnetosphere and how these ions modify the global dynamics of the near‐Earth space environment.

Multi-Instrument Characterization of Magnetospheric Cold Plasma Dynamics in the June 22, 2015 Geomagnetic Storm.

We present a comparison of magnetospheric plasma mass/electron density observations during an 11‐day interval which includes the geomagnetic storm of June 22, 2015. For this study we used: Equatorial plasma mass density derived from geomagnetic field line resonances (FLRs) detected by Van Allen Probes and at the ground‐based magnetometer networks EMMA and CARISMA; in situ electron density inferred by the Neural‐network‐based Upper hybrid Resonance Determination algorithm applied to plasma wave Van Allen Probes measurements. The combined observations at L ∼ 4, MLT ∼ 16 of the two longitudinally separated magnetometer networks show a temporal pattern very similar to that of the in situ observations: A density decrease by an order of magnitude about 1 day after the Dst minimum, a partial recovery a few hours later, and a new strong decrease soon after. The observations are consistent with the position of the measurement points with respect to the plasmasphere boundary as derived by a plasmapause test particle simulation. A comparison between plasma mass densities derived from ground and in situ FLR observations during favorable conjunctions shows a good agreement. We find however, for L < ∼3, the spacecraft measurements to be higher than the corresponding ground observations with increasing deviation with decreasing L, which might be related to the rapid outbound spacecraft motion in that region. A statistical analysis of the average ion mass using simultaneous spacecraft measurements of mass and electron density indicates values close to 1 amu in plasmasphere and higher values (∼2–3 amu) in plasmatrough.

Synthetic gamma-ray light curves of Kerr black hole magnetospheric activity from particle-in-cell simulations

Context. The origin of ultra-rapid flares of very high-energy radiation from active galactic nuclei remains elusive. Magnetospheric processes, occurring in the close vicinity of the central black hole, could account for these flares. Aims. Our aim is to bridge the gap between simulations and observations by synthesizing gamma-ray light curves in order to characterize the activity of a black hole magnetosphere, using kinetic simulations. Methods. We performed global axisymmetric 2D general-relativistic particle-in-cell simulations of a Kerr black hole magnetosphere. We included a self-consistent treatment of radiative processes and plasma supply, as well as a realistic magnetic configuration, with a large-scale equatorial current sheet. We coupled our particle-in-cell code with a ray-tracing algorithm in order to produce synthetic light curves. Results. These simulations show a highly dynamic magnetosphere, as well as very efficient dissipation of the magnetic energy. An external supply of magnetic flux is found to maintain the magnetosphere in a dynamic state, otherwise the magnetosphere settles in a quasi-steady Wald-like configuration. The dissipated energy is mostly converted to gamma-ray photons. The light curves at low viewing angle (face-on) mainly trace the spark gap activity and exhibit high variability. On the other hand, no significant variability is found at high viewing angle (edge-on), where the main contribution comes from the reconnecting current sheet. Conclusions. We observe that black hole magnetospheres with a current sheet are characterized by a very high radiative efficiency. The typical amplitude of the flares in our simulations is lower than is detected in active galactic nuclei. These flares could result from the variation in parameters external to the black hole.

Global and local processes of thin current sheet formation during substorm growth phase

Formation of thin current sheets in Earth's magnetotail is a critical element of the magnetosphere dynamics. The configuration and stability of these thin current sheets control the onset of magnetic reconnection and particle acceleration in the magnetotail during substorms and other geomagnetic activities. In recent years, multipoint observations, kinetic and hybrid simulations have significantly improved our understanding of the structure and dynamics of thin current sheets, including their ion kinetic physics. Global hybrid simulations have also revealed many details of ion kinetics in thin current sheet physics. In this paper we review current understanding of current sheet physics and discuss our recent observational and simulation results on thin current sheet formation, emphasizing the kinetics of current sheet thinning, multi-component ion populations, the role of electron anisotropy and ion agyrotropy, and the role of the ionosphere.