M.-C. Fok

Comprehensive computational model of Earth's ring current

A comprehensive ring current model (CRCM) has been developed that couples the Rice Convection Model (RCM) and the kinetic model of Fok and coworkers. The coupled model is able to simulate, for the first time using a self-consistently calculated electric field, the evolution of an inner magnetosphere plasma distribution that conserves the first two adiabatic invariants. The traditional RCM calculates the ionospheric electric fields and currents consistent with a magnetospheric ion distribution that is assumed to be isotropic in pitch angle. The Fok model calculates the plasma distribution by solving the Boltzmann equation with specified electric and magnetic fields. To combine the RCM and the Fok model, the RCM Birkeland current algorithm has been generalized to arbitrary pitch angle distributions. Given a specification of height-integrated ionospheric conductance, the RCM component of the CRCM computes the ionospheric electric field and currents. The Fok model then advances the ring current plasma distribution using the electric field computed by the RCM and at the same time calculates losses along particle drift paths. We present the logic of CRCM and the first validation results following the H+ distribution during the previously studied magnetic storm of May 2, 1986. The H+ fluxes calculated by the coupled model agree very well with observations by AMPTE/CCE. In particular, the coupled model is able to reproduce the high H+ flux seen on the dayside at L ∼ 2.3 that the previous simulation, which employed a Stern-Volland convection model with shielding factor 2, failed to produce. Though the Stern-Volland and CRCM electric fields differ in several respects, the most notable difference is that the CRCM predicts strong electric fields near Earth in the storm main phase, particularly in the dusk-midnight quadrant. Thus the CRCM injects particles more deeply and more quickly.

Decay of equatorial ring current ions and associated aeronomical consequences

The decay of the major ion species which constitute the ring current is studied by solving the time evolution of their distribution functions during the recovery phase of a moderate geomagnetic storm. In this work, only equatorially mirroring particles are considered. Particles are assumed to move subject to E×B and gradient drifts. They also experience losses along their drift paths. Two loss mechanisms are considered: charge exchange with neutral hydrogen atoms and Coulomb collisions with thermal plasma in the plasmasphere. Thermal plasma densities are calculated with a plasmaspheric model employing a time-dependent convection electric field model. The drift-loss model successfully reproduces a number of important and observable features in the distribution function. Charge exchange is found to be the major loss mechanism for the ring current ions; however the important effects of Coulomb collisions on both the ring current and thermal populations are also presented. The model predicts the formation of a low-energy (< 500 eV) ion population as a result of energy degradation caused by Coulomb collisions of the ring current ions with the plasmaspheric electrons; this population may be one source of the low-energy ions observed during active and quiet periods in the inner magnetosphere. The energy transferred to plasmaspheric electrons through Coulomb collisions with ring current ions is believed to be the energy source for the electron temperature enhancement and the associated 6300 A (stable auroral red [SAR] arc) emission in the subauroral region. The calculated energy-deposition rate is sufficient to produce a subauroral electron temperature enhancement and SAR arc emissions that are consistent with observations of these quantities during moderate magnetic activity levels.

Observations of neutral atoms from the solar wind

We report observations of neutral atoms from the solar wind in the Earths vicinity with the low-energy neutral atom (LENA) imager on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft. This instrument was designed to be capable of looking at and in the direction of the Sun. Enhancements in the hydrogen count rate in the solar direction are not correlated with either solar ultraviolet emission or suprathermal ions and are deduced to be due to neutral particles from the solar wind. LENA observes these particles from the direction closest to that of the Sun even when the Sun is not directly in LENAs 90° field of view. Simulations show that these neutrals are the result of solar wind ions charge exchanging with exospheric neutral hydrogen atoms in the postshock flow of the solar wind in the magnetosheath. Their energy is inferred to exceed 300 eV, consistent with solar wind energies, based on simulation results and on the observation of oxygen ions, sputtered from the conversion surface in the time-of-flight spectra. In addition, the sputtered oxygen abundance tracks the solar wind speed, even when IMAGE is deep inside the magnetosphere. These results show that low-energy neutral atom imaging provides the capability to directly monitor the solar wind flow in the magnetosheath from inside the magnetosphere because there is a continuous and significant flux of neutral atoms originating from the solar wind that permeates the magnetosphere.

Nonlinear impact of plasma sheet density on the storm‐time ring current

We investigated the nonlinear impact of the plasma sheet density on the total energy of the storm-time ring current by means of a numerical simulation that self-consistently solves the kinetic equation of ring current protons and the closure of the electric current between the magnetosphere and ionosphere. Results of the simulation indicate that when the convection electric field is self-consistently coupled with the ring current, the total energy of the ring current ions trapped by the Earths magnetic field is roughly proportional to ∼N 1/2 ps , where N ps is the plasma sheet density. This nonlinear response results from the strengthened shielding electric field with increasing N ps . The total energy is almost proportional to N ps when using an empirical convection electric field, which is independent of the condition of the simulated ring current. An empirical relationship between N ps and the solar wind density was used to estimate time-dependent N ps . The result shows that the calculated Dst* tends to overshoot the observed one when the non-self-consistent electric field is employed. A better agreement was obtained with the self-consistent electric field. We suggest that the nonlinear response of the ring current to N ps is one of the mechanisms that impedes the growth of the storm-time ring current. Another mechanism is probably the saturation of the polar cap potential drop for high solar wind electric field.

The Comprehensive Inner Magnetosphere-Ionosphere Model

Simulation studies of the Earths radiation belts and ring current are very useful in understanding the acceleration, transport, and loss of energetic particles. Recently, the Comprehensive Ring Current Model (CRCM) and the Radiation Belt Environment (RBE) model were merged to form a Comprehensive Inner Magnetosphere-Ionosphere (CIMI) model. CIMI solves for many essential quantities in the inner magnetosphere, including ion and electron distributions in the ring current and radiation belts, plasmaspheric density, Region 2 currents, convection potential, and precipitation in the ionosphere. It incorporates whistler mode chorus and hiss wave diffusion of energetic electrons in energy, pitch angle, and cross terms. CIMI thus represents a comprehensive model that considers the effects of the ring current and plasmasphere on the radiation belts. We have performed a CIMI simulation for the storm on 5–9 April 2010 and then compared our results with data from the Two Wide-angle Imaging Neutral-atom Spectrometers and Akebono satellites. We identify the dominant energization and loss processes for the ring current and radiation belts. We find that the interactions with the whistler mode chorus waves are the main cause of the flux increase of MeV electrons during the recovery phase of this particular storm. When a self-consistent electric field from the CRCM is used, the enhancement of MeV electrons is higher than when an empirical convection model is applied. We also demonstrate how CIMI can be a powerful tool for analyzing and interpreting data from the new Van Allen Probes mission.

Plasma sheet and (nonstorm) ring current formation from solar and polar wind sources

We consider the formation of the plasma sheet and geosynchronous region (nonstorm) ring current in the framework of collisionless test particle motions in three-dimensional magnetospheric fields obtained from self-consistent MHD simulations. Simulation results are compared with observations of the near-Earth plasma sheet from the Polar spacecraft during 2001 and 2002. Many particles were initiated in two regions representative of the solar wind source upstream of the bow shock and the polar wind source outside the plasmasphere, both of which are dominated by protons (H+). Proton trajectories are run until they precipitate into the atmosphere, escape from the simulation space, or become stably trapped. These calculations produce a database of proton characteristics in each 1 RE3 volume element of the magnetosphere and yield velocity distributions as well as bulk plasma properties. We report results reflecting steady growth phase conditions after 45 min of southward interplanetary field, BZ = −5 nT (BY = 0), and for conditions resulting after 2 hours of northward BZ = +5 nT. The results for simulated velocity distributions are consistent with the Polar soundings of the current sheet from lobe to lobe and with AMPTE/CCE observations of (nonstorm) ring current region protons. The simulations help us identify the differentiation between solar and polar wind H+ ions in observations. The weak NBZ ring current-like pressure is primarily polar wind protons, while the moderately active SBZ ring current-like pressure is primarily solar wind protons. The solar and polar wind contributions to the SBZ ring current are comparable in density, but the solar protons have a higher average energy. For SBZ, solar wind protons enter the nonstorm ring current region primarily via the dawn flank and to a lesser degree via the midnight plasma sheet. For NBZ, solar wind protons enter the ring current-like region via the cusp and flanks. Polar wind protons enter the nonstorm ring current through the midnight plasma sheet in both cases. Solar and ionospheric plasmas thus take different transport paths to the geosynchronous (nonstorm) ring current region and may thus be expected to respond differently to substorm dynamics of the magnetotail.

Self-Consistent Magnetosphere-Ionosphere Coupling: Theoretical Studies

[1] A theoretical examination of the electrodynamical interaction between the ionosphere and the inner magnetosphere is presented. A self-consistent ring current (RC) model has been developed that couples the electron and ion magnetospheric dynamics with the calculation of the electric field. Two new features were taken into account in order to close the self-consistent magnetosphere-ionosphere coupling loop. First, in addition to the RC ions, we have solved an electron kinetic equation in our model. Second, using the relation of Galand and Richmond [2001], we have calculated the height integrated ionospheric conductances as a function of the precipitated high energy magnetospheric electrons and ions that are produced by our model. To validate the results of our model we simulate the magnetic storm of May 2, 1986, a storm that has been comprehensively studied by Fok et al. [2001], and have compared our results with different theoretical approaches. The self-consistent inclusion of the hot electrons and their effect on the conductance results in deeper penetration of the magnetospheric electric field. In addition, a slight westward rotation of the potential pattern (compared to previous self-consistent results) is evident in the inner magnetosphere. These effects change the hot plasma distribution, especially by allowing increased access of plasma sheet ions and electrons to low L shells. These results are consistent with recent observations from the IMAGE satellite. INDEX TERMS: 2736 Magnetospheric Physics: Magnetosphere/ionosphere interactions; 7807 Space Plasma Physics: Charged particle motion and acceleration; 2753 Magnetospheric Physics: Numerical modeling; 2730 Magnetospheric Physics: Magnetosphere—inner; KEYWORDS: magnetic storms, ring current, electric fields, numerical simulations

Modeling global O+ substorm injection using analytic magnetic field model

[1] During storm-time substorms the IMAGE/High-Energy Neutral Atom (HENA) global magnetospheric plasma imager observes dramatic rise and fall of the oxygen energetic neutral atom (ENA) fluxes in the 29-180 keV range that are well correlated with substorm onsets. The corresponding hydrogen ENA flux (10-198 keV) shows a much more gradual rise and fall. We use a three-dimensional test particle code in time varying electric and magnetic fields to study mechanisms causing the dramatic oxygen variation. Consistent with our previous work, we find that nonadiabatic heating occurs during the dipolarization of the geomagnetic field. The simulation model reproduces key features including the high-altitude increase of the ENA flux and enhancement timescale.

Tailward flow of energetic neutral atoms observed at Venus

[1] The ASPERA-3 experiment on Mars Express provides the first measurements of energetic neutral atoms (ENAs) from Mars. These measurements are used to study the global structure of the interaction of the solar wind with the Martian atmosphere. In this study we describe the tailward ENA flow observed at the nightside of Mars. After characterizing energy spectra of hydrogen ENA signals, we present composite images of the ENA intensities and compare them to theoretical predictions (empirical and MHD models). We find that the tailward flow of hydrogen ENAs is mainly generated by shocked solar wind protons. Despite intensive search, no oxygen ENAs above the instrument threshold are detected. The results challenge existing plasma models and constrain the hydrogen exospheric densities and atmospheric hydrogen and oxygen loss rates at low solar activity.

Response of neutral atom emissions in the low-latitude and high-latitude magnetosheath direction to the magnetopause motion under extreme solar wind conditions

[1] On 11 April 2001 the high velocity and density of the solar wind and the strong southward interplanetary magnetic field moved the dayside magnetopause inside of geosynchronous orbit. The Low Energy Neutral Atom (LENA) imager on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft in the magnetosphere observed significant emission in the magnetosheath direction. The total neutral atom flux from the dayside region, ignoring the neutral solar wind flux directly from the Sun, shows a threefold enhancement, and each of the three increases is coincident with the occurrence of the magnetopause inside 6.6 RE. Observations by LENA also show that emission in the direction of the low-latitude and high-latitude magnetosheath is modulated in such a manner that the sources shift earthward/sunward and equatorward/ poleward in the low-latitude and high-latitude sheath, respectively. A model based on the distributions of the sheath flux and of the number density of the hydrogen exosphere explains these characteristics as a result of the motion of the magnetopause having an indentation at the cusp, suggesting a means for monitoring the cusp motion using IMAGE/LENA. INDEX TERMS: 2724 Magnetospheric Physics: Magnetopause, cusp, and boundary layers; 2728 Magnetospheric Physics: Magnetosheath; 2784 Magnetospheric Physics: Solar wind/ magnetosphere interactions; 2740 Magnetospheric Physics: Magnetospheric configuration and dynamics;