Michael W. Liemohn

Dominant role of the asymmetric ring current in producing the stormtime Dst

Three storms are examined to determine the contribution to the Dst* index from the symmetric and asymmetric (partial) components of the ring current. The storms (September 24–25, 1998, October 18–19, 1998, and May 14–15, 1997) all have a similar solar wind trigger (an initial shock followed by a coronal mass ejection with southward interplanetary magnetic field) and placement in the solar cycle (rising phase). The near-Earth ion distribution function is simulated for each storm using a kinetic transport model. The use of a McIlwain magnetospheric electric field description improves the simulation results over the Volland-Stern field used previously. It is found that most of the main phase magnetic field depression is due to the asymmetric component of the ring current (≥80% at the Dst* minimum for the three storms). Note that this is a minimum asymmetric ring current contribution, because the closed-trajectory ions may also be spatially asymmetric. Ions in the partial ring current make one pass through the inner magnetosphere on open drift paths that intersect the dayside magnetopause. Changes in the density of the inner plasma sheet are transmitted directly along these open drift paths. For a steady convection field, an increase in the source population produces a decrease (more intense perturbation) in Dst*, while a decrease produces a Dst* recovery. As the storm recovery proceeds, a decrease in the electric field results in a conversion of open to closed drift paths, forming a trapped, symmetric ring current that dominates Dst*. The mostly H+ composition of the ring current for all three storms rules out the possibility of differential charge exchange being the cause of the fast and slow decay timescales, confirming that outflow is the main loss of ring current-generated Dst* during the early phase decay. The slow decay timescale in the late recovery, however, is dominated by charge exchange with the hydrogen geocorona. The symmetric-asymmetric ring current is also placed in the context of the solar wind and plasma sheet drivers.

Geomagnetic storms driven by ICME- and CIR-dominated solar wind

The interaction of the solar wind and the Earths magnetosphere is complex and the phenomenology of the interaction is very different for solar wind dominated by interplanetary coronal mass ejections (ICMEs) compared to solar wind dominated by corotating interaction regions (CIRs). We perform a superposed epoch study of the effects of ICME- and CIR-dominated solar wind upon the storm-time plasma at geosynchronous orbit using data from the magnetospheric plasma analyzer (MPA) instruments on board seven Los Alamos National Laboratory (LANL) satellites. Using 78 ICME events and 32 CIR events, we examine the electron and ion plasma sheets that are formed during each type of solar wind driver, at energy-per-charge between ∼0.1 and 45 keV/q. The results demonstrate that CIR events produce a more significant modulation in the plasma sheet temperature than ICME events, whilst ICME events produce a more significant modulation in the plasma sheet density than CIR events. We attribute these differences to the average speed in the solar wind and a combination of the density of the solar wind and the ionospheric component of the plasma sheet, respectively. We also show that for CIR events, the magnitude of the spacecraft potential is, on average, significantly greater than during ICME-events, with consequent effects upon the performance of instrumentation within this environment.

Analysis of early phase ring current recovery mechanisms during geomagnetic storms

A time-dependent kinetic model is used to investigate the relative importance of various mechanisms in the early phase decay rate of the ring current. It is found that, for both the solar maximum storm of June 4–7, 1991 and especially the solar minimum storm of September 24–27, 1998, convective drift loss out the dayside magnetopause is the dominant process in removing ring current particles during the initial recovery. During the 1998 storm, dayside outflow losses outpaced charge exchange losses by a factor of ten.

Bulk plasma properties at geosynchronous orbit

We present a comprehensive study of plasma properties at geosynchronous orbit for electron and ion energies between ∼1 eV and ∼45 keV, between 1990 and 2001. The variations of temperature and density are analyzed as functions of local time, magnetospheric convection strength, and the strength of the ring current. Various parameters derived from temperature and density are calculated to elucidate the temporal and spatial location of delivery of plasma sheet material into the inner magnetosphere. We find that the electron and proton densities are greatest in the dawn region for the periods of highest convection and ring current strength. We perform a superposed epoch analysis of 283 geomagnetic storms which occurred between 1991 and 2001 and examine the temporal variation of the plasma at geosynchronous orbit as a function of storm phase. This analysis demonstrates the local time variability of delivery from the plasma sheet into the inner magnetosphere and the concurrent changes in temperature and pressure. We demonstrate that the density of electrons in the plasma sheet is strongly dependent upon the magnetospheric convection strength and, for the first time, upon solar activity. Electron density at geosynchronous orbit is strongly correlated with solar activity. The average plasma sheet electron density at solar maximum can be a factor of two or more higher than that at solar minimum. We also outline a method to estimate the composition of the plasma sheet from MPA measurements and calculate the O+ and H+ density variations with solar cycle as a function of Kp and local time. We show that the O+ and H+ plasma sheet densities increase with increasing solar activity, as does the O+/H+ density ratio. During times of high solar activity and strong convection, the O+ and H+ densities may be comparable.

Computational analysis of the near-Earth magnetospheric current system during two-phase decay storms

Several two-phase decay magnetic storms are examined using a kinetic transport model to find the spatial and temporal distribution of the perpendicular and field-aligned currents in the inner magnetosphere. The global morphology of these currents in the calculational domain (inside of geosynchronous orbit) is discussed as a function of storm epoch, obtaining good comparison between the numerically derived features and observed values of stormtime currents in this region. The model results are also consistent with quiet time plasma observations showing an increasing pressure in to L = 3 or 4, including a pressure maximum near midnight for the generation of region 2 Birkeland currents in the proper direction. A detailed analysis of the characteristic features of these currents is also presented and discussed. It is found that most of the ring current (>90%) during the main phase and early recovery phase is partial rather than symmetric, closing mostly (up to 90%) through field-aligned currents into the ionosphere. Conversely, the quiet time ring current is largely (>60%) symmetric, with most of the asymmetry produced by minor injections of near-Earth plasma sheet material. In general, the peak asymmetric current (which occurs during the main phase) is 2-3 times larger than the peak symmetric current (which occurs during the recovery phase) for any particular two-phase decay event. This is the case for all of the events studied, regardless of storm size, solar wind parameters, or solar cycle. The maximum azimuthal current (integrated over a local time slice) reaches 5 to 20 MA, compared with <2 MA of symmetric current at quiet times.

Pickup oxygen ion velocity space and spatial distribution around Mars

[1] We report a newly created highly parallelized global test particle model for resolving the pickup oxygen ion distribution around Mars. The background magnetic and convection electric fields are calculated using a three-dimensional multispecies magnetohydrodynamic model, which includes the effect of the Martian crustal magnetic field. In addition to photo-ionization, charge exchange collisions and solar wind electron impact ionization are included for the pickup ion generation. The most novel feature of our model is that more than one billion test particles are launched in the simulation domain in total. This corresponds to a profound enhancement by at least 3 orders of magnitude in the total number, compared to all existing test particle models. This substantial improvement enables an unprecedented examination of the pickup ion flux distribution in velocity space, which is not achievable in previous simulation studies due to the insufficient statistics arising from the limited number of test particles. Using the velocity space distribution of pickup O + ions as a tool, the Mars-solar wind interaction can be investigated in a unique way. It is shown that the velocity space distribution is highly non-Maxwellian, exhibiting non-gyrotropic and non-symmetric distributions, including many beam-like features. In the tail region, pickup ions have a prominent outflowing component in the whole energy range. The energy examination of particles traveling across the tail region shows that the acceleration highly depends on the source region where the particles originate. The strong convection electric field in the magnetosheath region is favorable to the pickup ion acceleration.

Photoelectron effects on the self-consistent potential in the collisionless polar wind

The presence of unthermalized photoelectrons in the sunlit polar cap leads to an enhanced ambipolar potential drop and enhanced upward ion acceleration. Observations in the topside ionosphere have led to the conclusion that large-scale electrostatic potential drops exist above the spacecraft along polar magnetic field lines connected to regions of photoelectron production. A kinetic approach is used for the O+, H+, and photoelectron (p) distributions, while a fluid approach is used to describe the thermal electrons (e) and self-consistent electric field (E‖). Thermal electrons are allowed to carry a flux that compensates for photoelectron escape, a critical assumption. Collisional processes are excluded, leading to easier escape of polar wind particles and therefore to the formation of the largest potential drop consistent with this general approach. We compute the steady state electric field enhancement and net potential drop expected in the polar wind due to the presence of photoelectrons as a function of the fractional photoelectron content and the thermal plasma characteristics. For a set of low-altitude boundary conditions typical of the polar wind ionosphere, including 0.1% photoelectron content, we found a potential drop from 500 km to 5 RE of 6.5 V and a maximum thermal electron temperature of 8800 K. The reasonable agreement of our results with the observed polar wind suggests that the assumptions of this approach are valid.

First medium energy neutral atom (MENA) Images of Earth's magnetosphere during substorm and storm-time

InitialENA images obtained with the MENA imager on the IMAGE observatory show that ENAs ema- nating from Earths magnetosphere at least crudely track both Dst and Kp. Images obtained during the storm of August 12, 2000, clearly show strong ring current asymme- try during storm main phase and early recovery phase, and a high degree of symmetry during the late recovery phase. Thus, these images establish the existence of both partial and complete ring currents during the same storm. Further, they suggest that ring current loss through the day side mag- netopause dominates other loss processes during storm main phase and early recovery phase.

Understanding storm‐time ring current development through data‐model comparisons of a moderate storm

[1] With three components, global magnetosphere (GM), inner magnetosphere (IM), and ionospheric electrodynamics (IE), in the Space Weather Modeling Framework (SWMF), the moderate storm on 19 May 2002 is globally simulated over a 24-hour period that includes the sudden storm commencement (SSC), initial phase, and main phase of the storm. Simulation results are validated by comparison with in situ observations from Geotail, GOES 8, GOES 10, Polar, LANL MPA, and the Sym-H and Dst indices. It is shown that the SWMF is reaching a sophistication level for allowing quantitative comparison with the observations. Major storm characteristics at the SSC, in the initial phase, and in the main phase are successfully reproduced. The simulated plasma parameters exhibit obvious dawn-dusk asymmetries or symmetries in the ring current region: higher density near the dawn and higher temperature in the afternoon and premidnight sectors; the pressure is highest on the nightside and exhibits a near dawn-dusk symmetry. In addition, it is found in this global modeling that the upstream solar wind/ IMF conditions control the storm activity and an important plasma source of the ring current is in the solar wind. However, the ionospheric outflow can also affect the ring current development, especially in the main phase. Activity in the high-latitude ionosphere is also produced reasonably well. However, the modeled cross polar cap potential drop (CPCP) in the Southern Hemisphere is almost always significantly larger than that in the Northern Hemisphere during the May storm.

Nonsteady state ionosphere-plasmasphere coupling of superthermal electrons

Numerical solutions to the nonsteady state kinetic equation which describes the transport of superthermal electrons in the ionosphere and plasmasphere between the magnetically conjugate regions of the ionosphere are presented. The distribution function in time, distance along arbitrary geomagnetic field lines, energy, and pitch angle are among the parameters calculated by the model. This model represents a unified approach by self-consistently coupling the interaction of the two hemispheres and the trapping of superthermal electrons in the plasmasphere. Our calculations take into account the various ionization and excitation processes and the effect of an inhomogeneous magnetic field (i.e., magnetic mirroring of precipitating electrons and focusing of escaping electrons along magnetic field lines). Omnidirectional flux spectra and pitch angle distributions are shown for various L shells and situations, and the features are described in detail. Nonsteady state calculations predict that a depleted flux tube can take several hours to reach steady state levels again. Plasmaspheric transparencies are calculated for different conditions of illumination, scattering processes in the conjugate ionospheres, and field-aligned gradients of the thermal plasma density. Plasmaspheric transparency is found to be a complicated function of not only the plasmaspheric thermal plasma but also the ionospheric sources and scattering processes. A phenomenological model is used to describe the energy transmission, reflection, and deposition in the plasmasphere. By studying the ionosphere and plasmasphere as one system rather than two separate ones, substantial corrections are introduced in the values of key parameters describing photoelectron fluxes.