Hybrid Modeling of the Formation of Thin Current Sheets in Magnetotail Configurations

Abstract. The present study investigates the tail current intensification prior to dipolarization, focusing on the dynamics of energetic ions. The event selected for this study, which was initially reported by Lui et al. [ 1988], occurred on June 1, 1985. The uniqueness of this event is that the AMPTE/CCE spacecraft remained close to the neutral sheet until the local magnetic field was dipolarized. Energetic (30-200 keV) ion fluxes were larger when the telescope was looking dawnward than when it was looking in the opposite direction. Until several tens of seconds before the (local) onset of dipolarization, this dawn-dusk flux anisotropy was more manifest for higher-energy ions, suggesting that the anisotropy was caused by the radial gradient of the energetic particle density. That is, the tail current was driven by radial pressure gradient. However, just prior to the onset, the flux level and the dawn-dusk anisotropy of the lowest-energy (31-43 keV) channel increased significantly, causing a further intensification of the tail current. The feature was less obvious in higherenergy channels. This energy dependence is better explained in terms of ion motion across the magnetic field. The velocity of this motion is a function of the ion energy and is estimated at 4% of the velocity corresponding to a given energy. It is likely that the associated enhancement of the tail current density was comparable to the enhancement during the entire growth phase. A positive feedback process between the unmagnetization of ions and the thinning of the current sheet is proposed for explaining this explosive intensification of the tail current. It is also found that electron pressure anisotropy did not contribute to the tail current intensification in the present event. The present result strongly suggests that the ion kinetics is important in the tail current intensification prior to the local onset of tail current disruption.

Section: Mitchell Et Almentioning

confidence: 99%

Ion dynamics and tail current intensification prior to dipolarization: The June 1, 1985, event

Ohtani¹,

Lui²,

Takahashi³

et al. 2000

“…Several interesting magnetotail models based on a Vlasov or hybrid description of plasma have been developed [Hesse et al, 1996;Pritchett et al, 1996;Biichner et al, 1998]. These models realistically describe the system's particle dynamics and the plasma instability; their internal evolution is almost perfect for studies of local processes like reconnection but is significantly less valuable when applied to a global magnetotail configuration.…”

Section: 808 Peroomian Et Al: Self-consistent Magnetotail Configumentioning

confidence: 99%

Dynamical properties of self‐consistent magnetotail configurations

Peroomian¹,

Ashour‐Abdalla²,

Zelenyǐ³

2000

Abstract. This study of the magnetotail employs a large-scale kinetic ion model specifically developed to consider the region's approach to equilibrium and its dynamics under the influence of an imposed electric field. Results from the self-consistent, two-dimensional model indicate that the magnetotail achieves equilibrium by maintaining a delicate balance between the influx of plasma from a mantle source and the accelerated loss of these particles resulting from nonadiabatic particle dynamics. The quasi-steady oscillations that result from instantaneous imbalances between sources and losses occur independent of solar wind variations. This variability occurs even for steady solar wind conditions and is caused by the rapid nonadiabatic energization of ions incident on the current sheet and their loss through the flanks of the magnetotail faster than they can be replenished. To determine the influence of external parameters on the dynamical state of the magnetotail, we performed a parameter search in which we varied the magnitude of the convection electric field and the influx of plasma mantle ions. This parameter search showed that the magnetotail configuration can adjust itself to compensate for the various combinations of solar wind parameters. As a rule, this self-adjustment occurs in the form of quasi-steady magnetotail states, where only the average characteristics of the configuration are steady but pronounced cyclic variations occur in the magnetotail topology. For cases in which the magnetotail is underpopulated, the current sheet is depleted via the flanks. In overpopulation cases a flaring of magnetotail field lines caused by the additional current in the neural sheet limits access of additional mantle particles to the region. The magnetotail system is flexible enough to self-consistently regulate its own population. However, the resulting configuration becomes dynamic and only a quasi-steady state is achieved.

“…The corresponding electron channel would be even thinner than the ion channel, and for consistency we get the so-called thin current sheet, characterized by /k = A = Pio [Cowley, 1971[Cowley, , 1973Ashour-Abdalla et al, 1994]. However, recent studies of the distant tail magnetic structure clearly indicate that in general the current sheet is relatively thick [Hoshino et al, 1996a], the thin current sheet being more appropriate for the growth phase of substorm events [e.g., Hesse et al, 1996]. In particular, the average current sheet thickness has been estimated from the observations to be of the order of -• 2.5Rz [Pulkkinen et al, 1993[Pulkkinen et al, , 1996, while the ion thermal Larmor radius pio is only • 200 km.…”

Section: Introductionmentioning

confidence: 99%

Effect of magnetic turbulence on the ion dynamics in the distant magnetotail

Veltri¹,

Zimbardo²,

Taktakishvili³

et al. 1998

Abstract. The ion dynamics in the distant Earth's magnetotail is studied in the case that a cross tail electric field E0 and reconnection-driven magnetic turbulence are present in the neutral sheet. The magnetic turbulence observed by the Geotail spacecraft is modeled numerically by a power law magnetic fluctuation spectrum. The magnetic fluctuations have the tearing mode parity with respect to the neutral sheet and are superimposed on a modified Harris sheet. A test particle simulation is performed for the ions, and the particle density, current density, bulk velocity, temperature, pressure, and heat flux are obtained for every point in the distant tail and as a function of the magnetic fluctuation level, 5B/Bo. It appears that the magnetic turbulence is very effective in maintaining the stationary structure of the current sheet and in changing the ion acceleration due to the electric field to thermal motion. Also, magnetic turbulence can inflate the current carrying region up to a thick current sheet, in contrast with the often assumed thin current sheet.