S. Y. Fu

Ballooning instability in the presence of a plasma flow: A synthesis of tail reconnection and current disruption models for the initiation of substorms

The drift ballooning mode (DBM) instability near the inner edge of the plasma sheet (IEPS) is studied further by including a nonstationary earthward flow and flow shear in the analysis. Both equatorial and off-equatorial regions are considered. It is found that the presence of a decelerated earthward flow destabilizes both the M− and M+ branches of the DBM in a large portion of the current sheet near the IEPS and substantially increases the growth rate of the instability. The flow shear in the premidnight sector causes the conventional ballooning mode to weakly subside, while it slightly enhances the growth rate for the Alfvenic ballooning mode. The combination of the earthward flow and flow shear makes both the Alfvenic ballooning mode and conventional ballooning mode grow much faster than they would without the flow, giving rise to coupled Alfvenic slow magnetosonic waves, field-aligned currents, and the formation of a current wedge. A synthesis of tail reconnection and cross-tail current disruption scenarios is proposed for the substorm global initiation process: When the fast flow produced by magnetic reconnection in the midtail abruptly decelerates at the IEPS, it compresses the plasma populations earthward of the front, transports momentum to them, and pushes them farther earthward. This creates the configuration instability in a large portion of the inner tail magnetic field lines on both the tailward side and earthward side of the braking point. As soon as the ionospheric conductance increases over a threshold level, the auroral electrojet is greatly intensified, which leads to the formation of the substorm current wedge and dipolarization of the magnetic field. This substorm paradigm combines the near-Earth neutral line and near-Earth current disruption scenarios for the initiation of substorms and may also synthesize dynamical processes in the rnagnetosphere-ionosphere coupling and field line resonance during the substorm onset. We intend to use this global model to explain substorm expansion onsets occurring under the southward interplanetary magnetic field condition.

Ring current oxygen ions escaping into the magnetosheath

Storm-related magnetospheric oxygen bursts were observed in the dayside magnetosheath during the coronal mass ejection on January 10, 1997. These singly charged oxygen ion events exhibited a clear antisunward flow. The oxygen ions are associated with a strong negative interplanetary magnetosheath field (IMF). The average Bz was almost − 50 nT, and the field projection in the x – y plane (GSE) was nearly constant in the Sun/dawn sector forming an angle of 45° relative to the Earth-Sun axis. The magnetopause was identified as a rotational discontinuity by using the principal axis analysis (PAA) method. The three-dimensional polar versus azimuthal angle distribution of the oxygen ions showed that the oxygen flow has a north to south velocity component. The observations suggest that the dayside reconnection process is generally steady. The energy dispersion can be explained with the time-of-flight (TOF) effect assuming oxygen ions are escaping from the magnetosphere along the reconnected field lines. The lack of hydrogen and helium ions during the observed oxygen bursts can be explained, as only oxygen ions are resupplied by the gradient drift in the inner magnetosphere because of their larger bounce periods with respect to hydrogen and helium ions. Therefore only oxygen ions are observed continuously in the magnetosheath. The estimated oxygen escape rate amounts to 0.61 × 1023 ions/s, about 33% of the input rate of the ring current. The observations imply that the stormtime ring current is asymmetric. A large amount of ring current oxygen ions escape from the magnetosphere into the magnetosheath.

Temporal and Spatial Variation of the Ion Composition in the Ring Current

A global view of the ring current ions is presented using data acquired by the instrument MICS onboard the CRRES satellite during solar maximum. The variations of differential intensities, energy spectra, radial profile of the energetic particles and the origin of the magnetic local time (MLT) asymmetry of the ring current have been investigated in detail. O+ ions are an important contributor to the storm time ring current. Its abundance in terms of number density increases with increasing geomagnetic activity as well as its energy density. However, a saturation value for the energy density of O+ ions has been found. The low-energy H+ ions show a dramatic intensification and a rapid decay. However, its density ratio during the storm maximum is almost constant. On the other hand, high-energy H+ ions first exhibit a flux decrease followed by a delayed increase. Its density ratio shows an anti-correlation with the storm intensity. Both the positions of the maximum flux of O+ and He+ depend on storm activity: they move to lower altitudes in the early stage of a storm and move back to higher L-values during the recovery phase. Whereas the position of H+ and He++ show almost no dependence on the Dst index. The energy density distributions in radial distance and magnetic local time show drastic differences for different ion species. It demonstrates that the ring current asymmetry mainly comes from oxygen and helium ions, but not from protons. The outward motion of O+ around local noon may have some implications for oxygen bursts in the magnetosheath during IMF Bz negative conditions as observed by GEOTAIL.

A 2.5 dimensional MHD simulation of multiple‐plasmoid‐like structures in the course of a substorm

As well known, the magnetic cross-tail component By in the magnetotail is well correlated with the interplanetary magnetic field (IMF) By component, and its variation is significant during substorms. A 2.5-dimensional MHD simulation has been carried out on the basis of equilibrium solutions of the quiet magnetotail. Two types of distributions of the By component and the plasma density (Type I and Type II) are used as the initial states of the simulation studies. The results of all cases with different initial conditions illustrate that the formation of plasmoids occurs intermittently and repeatedly. Also, all the plasmoids are high-density and high-temperature regions in comparison with the ambient environment; that means the plasma with high-temperature is embedded in plasmoids formed repeatedly. These results are in line with features of multiple-plasmoid-like structures observed on January 15, 1994, with Geotail at 96 RE in the tail. Thus it can be concluded that a large amount of the energy stored in the magnetotail is gradually dissipated by ejecting multiple plasmoids in the course of substorms. In this simulation it is shown that the lasting inflow caused by the electric field E imposed on the boundary of the simulation box controls the recurrent formation of plasmoids. Further, as an additional evidence, the multiple-plasmoid-like structures have been detected by Geotail in conditions of high-speed solar wind streams and southward IMF. Therefore our simulation suggests that the solar wind and the IMF have close control over the magnetotail dynamic process. The features of a classic plasmoid (bipolar Bz) and a travelling compression region (TCR) in the (X, Z) plane can be seen in all cases. Taking the time evolution of the By component into account, two kinds of plasmoid-like structures with a flux rope core and with both By and Bz bipolar signatures can be reproduced for the Type I and Type II conditions, respectively. Therefore the occurrence of various magnetic structures in the magnetotail might be related to nonsteady driven reconnection with different initial distributions of the By component.

Vortex-induced magnetic reconnection and single X line reconnection at the magnetopause

Vortex-induced magnetic reconnection (VIMR) and bursty single X line reconnection (BSXR) at the magnetopause are studied using two-dimensional MHD simulation. The VIMR and BSXR occur when the flow-aligned Alfven Mach number is greater or less, respectively, than a critical value which typically ranges from 0.3 to 0.6 for different anomalous resistance conditions. In the former case, the streamlines and magnetic field lines are in alignment and construct a concentric vortical pattern, whereas in the latter case, magnetic field lines do not form closed loops, and vortices occur around the X line. The topological structure of the asymptotic quasi-steady state of 2-D reconnection depends strongly upon the topological invariants such as the cross helicity. The different sites where VIMR and BSXR work at the magnetopause to form flux transfer events are discussed.

Simulation study on stochastic reconnection at the magnetopause

A two-dimensional MHD simulation model is presented to study stochastic reconnection at the magnetopause. The simulation indicates that in an open system with strong velocity shear and large Reynolds and magnetic Reynolds number (R and R(sub m)), stochastic reconnection can take place in the magnetopause current sheet when the interplanetary magnetic field has a southward component, leading to the appearance of irregular mesoscale and small-scale structures (IMSSSs). The larger the R and R(sub m) are, the more IMSSSs are produced. The stronger the flow shear intensity is, the faster IMSSSs occur. Mesoscale and small-scale structures can coalesce to form large-scale ones if R and R(sub m) are midranged (10 to 200), while large and mesoscale structures can break into smaller ones if R and R(sub m) increase. It is expected that stochastic reconnection plays an important role in the solar wind-magnetosphere coupling.

Transient magnetic reconnection at the magnetopause in the presence of a velocity shear

Sheared plasma flows between the magnetosheath and magnetosphere exist in vast regions of the magnetopause and contain much more free energy than other energy sources. It is of importance to consider the presence of the velocity shear while studying reconnection at the magnetopause. This paper briefly reviews our current knowledge of vortex-induced magnetic reconnection obtained from two-dimensional MHD simulations with emphasis on the dynamic features of the vortex-induced reconnection process.

Energetic oxygen ions in the magnetosheath in the negative BZ phase of the CME on January 10, 1997

Abstract Energetic oxygen ion flux intensifications were observed by the HEP/LD instrument on board the GEOTAIL satellite thoughout the Bz negative phase of the CME event on January 10, 1997. At this time, the spacecraft was moving in the magnetosheath at 1500 LT on a magnetopause skimming segment of its orbit. The very steady southward magnetic field in the magnetosheath (negative Bz of the CME) was highly inclined forming an angle of 45° with respect to the north direction. The observed oxygen enhancements in the magnetosheath show anisotropic angular distributions which occupy a varying fraction of the unit sphere. These distributions became particularly narrow during the passage of a solar wind pressure pulse between between 1050 and 1113 UT. The details of the angular distributions in the magnetosheath favour a leakage model, although the reconnection model cannot be denied.

Particle dynamics in the magnetotail with time dependent electric field

The motion of charged particles in a reversal field of the magnetotail with a time-dependent electric field is studied numerically using a test particle approach. Variations in the solar wind magnetic field and/or velocity may induce a time-dependent electric field in the magnetotail. Interaction of the magnetotail particles with this electric field can give rise to a varying degree of stochastic motion. The essential of magnetotail particle stochastic motion is the random change between two status of motion — meandering and noncrossing orbits to the other. Energy transfering from the field to plasma is due to stochastic motion of the particles and is termed “stochastic heating” or “stochastic acceleration”. If the amplitude of perturbation electric field is larger than the threshold required to make a particle move in stochastic orbits, the particle will undergo stochastic acceleration. This mechanism can provide an explanation to the temperature difference existing between different particle species in the magnetotail plasma sheet T O + ≥ T He ++ ≥ T H+ ≥ T e .