Mei-Ching Fok

Ring current development during storm main phase

The development of the ring current ions in the inner magnetosphere during the main phase of a magnetic storm is studied. The temporal and spatial evolution of the ion phase space densities in a dipole field are calculated using a three dimensional ring current model, considering charge exchange and Coulomb losses along drift paths. The simulation starts with a quiet time distribution. The model is tested by comparing calculated ion fluxes with Active Magnetospheric Particle Tracer Explorers/CCE measurement during the storm main phase on May 2, 1986. Most of the calculated omnidirectional fluxes are in good agreement with the data except on the dayside inner edge (L < 2.5) of the ring current, where the ion fluxes are underestimated. The model also reproduces the measured pitch angle distributions of ions with energies below 10 keV. At higher energy, an additional diffusion in pitch angle is necessary in order to fit the data. The role of the induced electric field on the ring current dynamics is also examined by simulating a series of substorm activities represented by stretching and collapsing the magnetic field lines. In response to the impulsively changing fields, the calculated ion energy content fluctuates about a mean value that grows steadily with the enhanced quiescent field.

Radiation Belt Environment model: Application to space weather nowcasting

[1]xa0A data-driven physical model of the energetic electrons in the Earths radiation belts, called the Radiation Belt Environment (RBE) model, has been developed to understand Earths radiation belt dynamics and to predict the radiation conditions found there. This model calculates radiation belt electron fluxes from 10 keV to 6 MeV in the inner magnetosphere. It takes into account the realistic, time-varying magnetic field and considers effects of wave-particle interactions with whistler mode chorus waves. The storm on 23–27 October 2002 is simulated and the temporal evolutions of the radial and pitch angle distributions of energetic electrons are examined. The calculated electron fluxes agree very well with particle data from the low-orbit SAMPEX and LANL geosynchronous satellites, when the wave-particle interactions are taken into account during storm recovery. Flux increases begin near the plasmapause and then diffuse outward to higher L shells, consistent with previous findings from statistical studies. A simplified version of the RBE model is now running in real time to provide nowcasting of the radiation belt environment. With further improvements and refinements, this model will have important value in both scientific and space weather applications.

Three-dimensional ring current decay model

This work is an extension of a previous ring current decay model. In the previous work, a two-dimensional kinetic model was constructed to study the temporal variations of the equatorially mirroring ring current ions, considering charge exchange and Coulomb drag losses along drift paths in a magnetic dipole field. In this work, particles with arbitrary pitch angle are considered. By bounce averaging the kinetic equation of the phase space density, information along magnetic field lines can be inferred from the equator. The three-dimensional model is used to simulate the recovery phase of a model great magnetic storm, similar to that which occurred in early February 1986. The initial distribution of ring current ions (at the minimum Dst) is extrapolated to all local times from AMPTE/CCE spacecraft observations on the dawnside and duskside of the inner magnetosphere spanning the L value range L = 2.25 to 6.75. Observations by AMPTE/CCE of ring current distributions over subsequent orbits during the storm recovery phase are compared to model outputs. In general, the calculated ion fluxes are consistent with observations, except for H+ fluxes at tens of keV, which are always overestimated. A newly invented visualization idea, designated as a chromogram, is used to display the spatial and energy dependence of the ring current ion differential flux. Important features of storm time ring current, such as day-night asymmetry during injection and drift hole on the dayside at low energies (<10 keV), are manifested in the chromogram representation. The pitch angle distribution is well fit by the function, jo(1 + Ayn), where y is sine of the equatorial pitch angle. The evolution of the index n is a combined effect of charge exchange loss and particle drift. At low energies (<30 keV), both drift dispersion and charge exchange are important in determining n.

Impulsive enhancements of oxygen ions during substorms.

It has been observed that H+ is the dominant ion species in the plasma sheet and the ring current during quiet times. However, the O+/H+ density ratio increases with increasing geomagnetic storm and substorm activity. Energetic neutral atom (ENA) images from Imager for Magnetopause-to-Aurora Global Exploration/High Energy Neutral Atom (IMAGE/HENA) reveal the rapid increase of O+ ring current at substorm expansion. Finding the cause of this substorm-associated O+ enhancement is the main focus of this paper. Two possible sources are suggested: direct injection from the ionosphere and energization of the preexisting oxygen ions in the magnetosphere. We perform numerical simulations to examine these two mechanisms. Millions of O+ are released from the auroral region during a simulated substorm by the Lyon-Fedder-Mobarry MHD model. The subsequent trajectories of these outflowing ions are calculated by solving the full equation of particle motion. A few minutes into the substorm expansion phase, an enhancement in O+ pressure is found on the nightside at ∼12 RE. After careful analysis, we conclude that this pressure peak is coming from energization of the preexisting O+ in the plasma sheet. The direct injection mechanism will introduce a significant time lag between strong ionospheric outflow and magnetospheric enhancement, so that it cannot explain the observed O+ bursts. Using the temperature and density established by the test-particle calculations as boundary conditions to a ring current model, we calculate the O+ fluxes and the corresponding ENA emissions during the model substorm. We are able to reproduce observable features of oxygen ENA enhancements as seen by IMAGE/HENA.

Modeling of inner plasma sheet and ring current during substorms

The evolution of the inner plasma sheet and the ring current during substorm dipolarizations is simulated. A substorm cycle is treated by stretching and dipolarizing the magnetosphere according to the Tsyganenko 89 model. In order to clarify the relative influences of steady convection and induction electric field on ring current development, the inductive electric field is superposed on two baseline convective states: a nonstorm state using a weak electric field, and a storm-time state using a stronger electric field. Ion distributions on the nightside at 12 Earth radii (RE) during these two substorms are obtained using our single-particle code to trace particle trajectories backward in time to source regions assumed to have steady characteristics. The subsequent acceleration and transport of these boundary ions into the inner magnetosphere is modeled by our kinetic model of the ring current. The simulation generates many frequently observed features of substorm injections, including the sudden appearance of hot plasma tailward of a sharply defined injection boundary, the earthward motion of an injection front, the azimuthal and tailward expansion of this enhanced region, and the creation of characteristic ion dispersion patterns near geosynchronous orbit. Comparison of the nonstorm and storm cases suggests that substorms occurring without a convection enhancement produce mainly an enhancement of the cross-tail current but little change in the ring current. With strong convection, the role of substorms is to enable the convection enhancement to create robust ring current in the inner magnetosphere.

Global ENA IMAGE Simulations

The energetic neutral atom (ENA) images obtained by the ISEE and POLAR satellites pointed the way toward global imaging of the magnetospheric plasmas. The Imager for Magnetopause to Aurora Global Exploration (IMAGE) is the first mission to dedicate multiple neutral atom imagers: HENA, MENA and LENA, to monitor the ion distributions in high-, medium- and low-energy ranges, respectively. Since the start of science operation, HENA, MENA and LENA have been continuously sending down images of the ring current, ionospheric outflow, and magnetosheath enhancements from high pressure solar wind. To unfold multiple-dimensional (equal or greater than 3) plasma distributions from 2-dimensional images is not a trivial task. Comparison with simulated ENA images from a modeled ion distribution provides an important basis for interpretation of features in the observed images. Another approach is to develop image inversion methods to extract ion information from ENA images. Simulation studies have successfully reproduced and explained energetic ion drift dynamics, the transition from open to closed drift paths, and the magnetosheath response to extreme solar wind conditions. On the other hand, HENA has observed storm-time ion enhancement on the nightside toward dawn that differs from simple concepts but can be explained using more sophisticated models. LENA images from perigee passes reveal unexpected characteristics that now can be interpreted as evidence for a transient superthermal exospheric component that is gravitationally-influenced if not bound. In this paper, we will report ENA simulations performed during several IMAGE observed events. These simulations provide insight and explanations to the ENA features that were not readily understandable previously.

Rapid enhancement of radiation belt electron fluxes due to substorm dipolarization of the geomagnetic field

The classical pure radial diffusion mechanism appears not to fully explain the frequently observed rapid enhancement in the timescales of minutes to hours in the radiation belt electron fluxes in the Earths magnetosphere. We here consider other physical mechanisms, such as energization mechanisms associated with substorm processes, to account for these sudden increases. A three-dimensional electron kinetic model is used to simulate the dynamics of the geomagnetically trapped population of radiation belt electrons during a substorm injection event. In the past this model has been extensively used to study dynamics of energetic ions in the ring current. This work, for the first time, constitutes the development of a combined convection and diffusion model to radiation belt electrons in the 0.04–4 MeV kinetic energy range. The Tsyganenko 89 geomagnetic field model is used to simulate the time-varying terrestrial magnetosphere during the growth phase elongation and the expansion phase contraction. We find that inductive electric field associated with the magnetic reconfiguration process is needed in order to transport substorm electrons into the trapped particle region of the magnetosphere. The maximum enhancement in energetic electron fluxes is found to be located around the geosynchronous orbit location (L = 6.6), with up to 2 orders of magnitude enhancement in the total fluxes (0.04–4 MeV). Although this enhancement in the inner magnetosphere is very sensitive to the temperature and, to a less extent, density of the source population in the plasma sheet, we suggest that the substorm-associated energization in the magnetotail and the subsequent adiabatic acceleration in the earthward region account for the enhanced MeV electrons (killer electrons) seen at the geosynchronous orbit during storms and substorms.

Magnetic coupling of the ring current and the radiation belt

[1] The magnetic influence of the storm time ring current on high-energy particles is demonstrated by using a simulation of the ring current incorporating self-consistent magnetic and electric fields. Observations by the Polar satellite show that the magnetic field is occasionally depressed by 50% or more near the equatorial plane at <6 RE. We call them equatorially magnetic depression events (EMDEs) and focus on the most intense EMDE observed during an intense storm on 22 October 1999. The simulation predicts that under a strong convection electric field, the magnetic field strength is highly depressed around L = 5 by newly injected ions of energy 80 keV or less. The depressed magnetic field causes a significant adiabatic decrease in the high-energy ion flux at pitch angles near 90 to conserve the first adiabatic invariant. A more tail-like (shortened) magnetic field line causes an enhancement of the flux at pitch angles near 0 and 180 to conserve the second adiabatic invariant. Consequently, a butterfly-like pitch angle distribution (PAD) is formed, which agrees with the Polar observation. We propose that the adiabatic process could have acted not only on the high-energy component of the protons but also on relativistic electrons in the outer radiation belt. This notion is supported by simultaneous Polar observation of relativistic electron fluxes that show a decrease at pitch angles near 90 and a slight increase at pitch angles near 0 and 180. PADs of protons and electrons can be used to distinguish nonadiabatic processes acting selectively on electrons from adiabatic ones.

On ionospheric trough conductance and subauroral polarization streams: Simulation results

[1]xa0Subauroral polarization streams (SAPS) are usually associated with geomagnetically disturbed times and considered a manifestation of magnetosphere and ionosphere coupling. Previous research results using radar and satellite measurements have revealed many features of the SAPS events. In this paper we focus on the effects of subauroral trough conductance on the attributes of SAPS and the evolution of the coupled magnetosphere and ionosphere system through the comprehensive ring current model, which includes the coupled electrodynamics of the inner magnetosphere and ionosphere with a self-consistent description of the electric field. Our numerical analysis indicates that low conductance at the subauroral latitudes (due to midlatitude trough) is critical to the large amplitude of SAPS. The model results are generally in good agreement with common characteristics of SAPS and are consistent with their existing generation mechanism.

Dynamical property of storm time subauroral rapid flows as a manifestation of complex structures of the plasma pressure in the inner magnetosphere

[1] During the intense magnetic storm of 15 December 2006, the midlatitude Super Dual Auroral Radar Network (SuperDARN) Hokkaido radar observed a dynamical character of rapid, westward flows at 50-56 magnetic latitude. The simulation that couples the inner magnetosphere and the subauroral ionosphere was performed using a realistic boundary condition of the hot ion distribution determined from four Los Alamos National Laboratory satellites at 6.6 R E . The following results are obtained using the simulation: (1) In general, morphology of the azimuthal component of the simulated ionospheric plasma flow is consistent with that known as the subauroral polarization stream (SAPS), (2) an increase in the hot ion density in the plasma sheet results in the temporal reduction and subsequent intensification of the rapid flow at certain subauroral latitudes with a delay of ~40 min, and (3) influence of the plasma sheet temperature on the rapid flow is not evident. The simulated line-of-sight velocity is compared with that obtained by the SuperDARN Hokkaido radar. Agreement between them is found in terms of the temporal and spatial variations of the rapid flows as well as the flow velocity. It is suggested that the dynamical character of the subauroral plasma flow is a direct manifestation of the plasma pressure distribution in the inner magnetosphere (the ring current) especially during the magnetic storm.