SM13C-1460
Geotail observations of low-frequency MHD waves prior to dipolarization onsets in the near- Earth magnetotail
Spacecraft observations indicate that low-frequency (0.006-0.025 Hz) fluctuations of the magnetic field appear a few minutes prior to substorm-associated dipolarization onsets. On the basis of Geotail observations, we investigate such fluctuations in the vicinity of the magnetic equator in particular in the near-Earth magnetotail at X= -8 to -12 RE. In order to characterize the observed fluctuations both physically and quantitatively, we propose a new method for estimating characteristics of wave parameters, such as the mode, the phase velocity in the plasma frame, the wave number, and the Doppler shift. In this method, the inhomogeneity of the ambient magnetic field is taken into account. Band-passed data of magnetic field and ion velocity fluctuations are analyzed on the basis of the linear MHD equations. It is found that the fluctuations in the low-frequency range consist of various modes of MHD waves. Particularly, the fast magnetosonic wave was identified and was propagating tailward a few minutes prior to a dipolarization onset, though there are also dipolarization events in which no fast wave was identified. A more prominent finding in the present study is the presence of a mode that has almost zero frequency in the plasma frame. Such a zero-frequency mode can convect with the ambient plasma flow and hence can be measured with a finite frequency by a spacecraft. This mode may correspond to a linear stage of the ballooning instability. These characteristics of the MHD waves may provide crucial constraints on the conventional substorm initiation models.
SM13C-1461
Evolution of the near-Earth magnetotail associated with substorm expansion onsets
We have statistically studied substorm-associated evolution of the magnetotail and the inner magnetosphere to understand the triggering mechanism of the substorm expansion onset. In the present study we used ion and magnetic field data from Geotail and magnetic field data from Polar and GOES to cover not only the near-Earth magnetotail but also the inner magnetosphere. A total of 3950 substorm events were determined from auroral breakups observed by the Polar UVI and IMAGE FUV auroral imagers. A few min before auroral onset, the total pressure starts to decrease first in the premidnight tail at X ~ -18 RE, where electric and magnetic field fluctuations also seem to occur. This location corresponds to the tailward edge of a region of strong magnetic field line stretching that extends from X ~ -8 RE to X ~ -18 RE. The plasmoid starts to substantially evolve at X < -20 RE, simultaneous with the beginning of the dipolarization at X ~ -8 to -9 RE. The dipolarization region then expands tailward as well as in the dawn-dusk directions and earthward. These observations suggest that, on average, the magnetic reconnection occurs first in the premidnight tail at X ~ -18 RE just before expansion onset to trigger onset processes. It is still unclear how the magnetic reconnection and the dipolarization are connected.
SM13C-1462
Magnetotail dynamics in Hall MHD simulation
A Hall magnetohydrodynamic (MHD) simulation is conducted to study magnetotail dynamic processes starting with realistic magnetotail geometry. With different imposing boundary conditions, the magnetic reconnection can be initiated in the near Earth region inside15 or the middle tail outside 20 . For the case with the reconnection in the near Earth region, the results indicate that sudden disruption of cross-tail current is triggered by fast magnetic reconnection with the reconnection rate over 0.1. We observe all substorm phases: early slow growth phase, late impulsive growth phase, substorm onset, and expansion phase with time scales that well agree with the satellite observations. Substantial increases of parallel electric field, field-aligned currents, and Earthward bulk flow in this simulation are observed immediately after the substorm onset. For the case with the reconnection first initiated in the middle tail, the results indicate that the magnetotail dynamics behaves rather different. The possible applications to the substorm are also discussed.
SM13C-1463
Relationship of density in the tail lobe to the solar wind
Geotail observes electron density in the magnetotail lobes. We investigate relationship between the lobe density and the solar wind parameters. Lobe plasma density is higher, when the interplanetary magnetic field (IMF) is stronger, solar wind speed is lower, and solar wind density is higher. In the north tail lobe, field-aligned bidirectional fluxes are observed when the IMF has a –Bx component (away sector). In the south lobe, they are observed when the IMF has a +Bx component (toward sector). When these bidirectional fluxes are observed, Kp is larger. These IMF-controlled asymmetries are consistent with earlier observations.
SM13C-1464
Partial Dropouts of the Near-Earth Plasma Sheet at Substorm Onsets
This study utilizes particle and field data observed at the Geotail satellite in the nightside plasma sheet at -9 RE > XGSE > -15 RE to describe the partial dropout of the plasma sheet at times of substorm onsets. At distances -15 RE > XGSE > -24 RE, dropout of the plasma sheet and entry of an observing satellite into the tail lobe is the dominant signature of substorm onset. The plasma sheet expands back over the satellite near the beginning of substorm recovery [ Hones et al., J. Geophys. Res., 75, 7060, 1970]. For XGSE < -24 RE the dominant signature is the plasmoid [e.g., Baumjohann et al., J. Geophys. Res., 104, 24,995, 1999]. These signatures are understood within the context of the near-Earth X-line model. At locations XGSE > -15 RE dipolarization and earthward plasma injection are the usual features at onset. Near the center of the plasma sheet (|Bx/Bz| < 2.5), dipolarization/injection is nearly always coincident with onset. However, in a significant number of onsets, when |Bx/Bz| > 2.5, injection is delayed, and instead, a partial dropout of the plasma sheet commences at onset. The partial dropout as described here is reminiscent of a plasma-sheet dropout, but the satellite does not enter the tail lobes. One type of partial dropout that we will describe here has the following consistent set of characteristics: (1) it begins coincident with the first signs of ground Pi2 onset; (2) the density, temperature and plasma beta decrease toward lobe values; (3) Bz increases approximately linearly while Bx remains approximately constant; (4) |Vz| increases consistent with motion of plasma and magnetic flux toward the center of the plasma sheet; and (5) at the satellite's location, plasma-sheet recovery is coincident with plasma injection and dipolarization. While the partial dropout is not an unusual signature at onset, neither is it unusual for this signature to be absent. Our interpretation is that the satellite is located in a region of transition between the dipolar, inner plasma sheet and the thin, expansion-phase, middle plasma sheet. We suspect that the satellite's location maps tailward along the magnetic field to the equatorial region of the dropped-out, thin plasma sheet.
SM13C-1465
Modeling particle distributions with a global magnetospheric model with embedded magnetotail thin current sheet
The thin current sheet in the magnetotail is a site for particle energization during magnetic reconnection, and affects the particle distributions at other parts of the magnetosphere. The self-consistent models of the thin current sheet are combined with a global magnetic field (Tsyganenko et al., 2002) to obtain a model of the magnetospheric field suitable for the study of particle distributions at different locations. In order to identify the signatures of a reconnecting magnetic field at different regions, models of the magnetospheric field are obtained by combining the Tsyganenko model with a thin current sheet equilibrium and a current sheet with a magnetic island. The particle distributions in these cases are then studied using particle-in-cell techniques. Although the PIC simulations may not yield the self-consistent evolution of the magnetosphere due to the limitations in the number of particles per cell, it yields a very good sampling of the particle distributions under the specified conditions at different regions of the magnetotail. These simulations will be used to identify the signatures of processes such as plasma flows (tailward vs. Earthward), multiple resonances effects, etc. The particle distributions obtained from the combined magnetospheric field can be compared with multi-point measurements and thus provide a framework for the understanding of data from multispacecraft missions.
SM13C-1466
Exploring the Inner Edge of the Plasma Sheet with ENA Observations
It has been shown that ENA measurements are sensitive enough to detect plasma sheet flux increases coincident with increased convection. By examining continuous large time intervals in the near earth tail, correlations between ENA fluxes and solar wind parameters (geomagnetic activity levels) will be established for most of the time between the years 2000 and 2006. These observations are a natural gateway to the study of the evolution and dynamics of the inner edge of the plasma sheet by providing two dimensional maps inside L of 10. These maps serve as the means to characterize the plasma sheet, a necessary first step in understanding of plasma sheet response to magnetospheric activity.