SM23C-01
Plasma sheet ion pressure profiles during substorm
Over 200 substorm events were carefully selected from observations made by Polar UVI, VIS, and an ensemble of ground instruments. From these events, more than 140 of them have simultaneous DMSP observations. Four substorm phases were defined, namely, growth, expansion, early recovery, and late recovery. Plasma sheet 2-D ion pressure, density, and temperature profiles inferred from DMSP satellite observations were constructed for each of these four substorm phases. The growth phase profiles show that the ion pressure is higher in the inner edge of the plasma sheet. The premidnight pressure enhancement can be attributed to the temperature enhancement while the postmidnight pressure enhancement can be attributed to the density enhancement. The temperature enhancement at premidnight has been previously reported and attributed to the curvature and gradient drift of the ions. The postmidnight density enhancement may result from the enhanced convection. The profiles show that the ion pressure near the midnight meridian in the midtail region increases right after the substorm onset, during the expansion phase. The pressure subsequently declines during the early recovery phase and declines further during the late recovery phase, but the late recovery pressure is still higher than that of the growth phase. This near midnight pressure enhancement during the expansion and recovery phases can be attributed to both temperature and density enhancements. The 2-D pressure, temperature, and density profiles provide observational constraints to the competing substorm theories.
SM23C-02
Do Global Auroral Images and Simultaneous Magnetospheric Observations Support the NENL Model of Substorm Onset?
Correlations between global auroral images and in-situ measurements in the plasma sheet and magnetotail
have led to many new insights into how the magnetosphere and ionospheric auroral region are coupled. Several
studies have shown that intense auroral emission in the ionosphere is well correlated with plasma sheet
disturbances characterized by one or more of the following: large ion velocity moments, enhancements in the
energetic ion and electron fluxes, increases in plasma temperature, and high frequency fluctuations in the
magnetic field. In addition global images provide unambiguous timing of auroral brightenings and direction and
speed of propagation of auroral forms. The timing and propagation determined from images, in turn, can be
used to put the plasma sheet observations in context. We show several examples of auroral substorms and
other auroral activity observed by global auroral imagers and simultaneous, in-situ near-Earth plasma sheet
plasma and magnetic field observations. We conclude that when a spacecraft in the near-Earth plasma sheet (X
< 20 RE) detects plasma sheet activity, it is in a region magnetically connected to intense auroral emission in
the ionosphere. This implies a near Earth source (near 10 RE) and that plasma sheet disturbances propagate
tailward as regions of intense auroral emission migrate poleward. This result is mostly inconsistent with the
NENL model of substorm onset. Several case studies which were originally interpreted to support the NENL
model are re-interpreted to be inconsistent with this model. Future refinements to the NENL model must
address these inconsistencies.
http:sprg.ssl.berkeley.edu/matt/AGUS2007/NENL/
SM23C-03
Role of Reconnection and Ballooning Modes in the Near-Earth Neutral Line Model
Recent developments in the theory of collisionless reconnection and nonlinear ballooning instabilities in the Earth's magnetotail, tested by observations from Wind and Cluster, have brought new insights to bear on the classical Near-Earth Neutral Line model. These developments underscore the importance of collisionless reconnection in producing an X-line at mid-tail distances (~30 Re) and the growth of a thin and intense current sheet that mediates the development of very strong pressure gradients at near-Earth distances (~10 Re). In turn, these pressure gradients excite ballooning instabilities that are manifested as a westward traveling surge, which can be inferred by analyzing the time-delay between earthward and tailward flux enhancements of energetic ions. The eneregtic ion data also suggests a strong pre-onset pressure gradient that is reduced impulsively at onset. New developments in the theory and simulation of nonlinear ballooning modes appear to be consistent with these observations. Furthermore, the theory predicts the generation of finger-like structures and strongly sheared flows as a consequence of the instability, which pose new challenges for observations. However, despite the robust nonlinear growth of ballooning modes at near-Earth distances, it cannot be claimed that ballooning modes, by themselves, can account for all the important signatures of substorm onset. We will discuss open questions, and directions in which theoretical and modeling studies need to be extended in order to obtain a more complete theory of substorm onset.
SM23C-04
The relationship between substorm onset location and the polar cap boundary
A topological difference between the classical near earth neutral line (1) and current disruption (2) substorm models is the latitude position of the open closed field line boundary (OCFB) immediately after substorm onset. In model 1 the OCFB should be near the onset point which is the location of the reconnection between open and closed field lines while in model 2 the onset and subsequent poleward expansion should take place in a regions of closed field lines and where the expansion move towards the OCFB. Data sets were examined in which a NASA FAST satellite magnetic footprint occurred within 2000 km of a substorm onset point as determined from auroral brightening in the global images obtained by the NASA IMAGE spacecraft. The OCFB was determined from the FAST electron and proton precipitation poleward boundary. The latitude difference between the OCFB and the onset latitude was plotted against the time difference of the FAST observation and the substorm onset. From the resulting scatter plot it can be seen that nearest to onset there is a large difference between the latitude of the OCFB and the onset. This is consistent with prior observations of substorm onsets occurring deep in a closed field line region of the magnetosphere. However there is a trend that for FAST passes occurring soon (within a few minutes) after onset the OCFB is very close to the onset latitude just as if the closed field line region poleward of the onset had disappeared. For FAST passes later than 5 minutes after the OCFB is observed more and more poleward consistent with snapshots of poleward expanding surges located at the OCFB during the substorm expansion phase.