SM51C-01 INVITED
Time History of Substorm Disturbances From the THEMIS Program: Preliminary Results
During the first tail season of THEMIS observations, data from a number of substorms were collected, providing an unprecedented view and timing of tail and ground processes. The orbits, designed for closest proximity to the neutral sheet near pre-midnight, achieved a systematic view of the mid-tail and near-Earth substorm signatures. Several substorm studies have been made to determine the substorm onset location based on the timing approach. These reports show that substorm disturbances could be detected first in the near-Earth tail (inside the downstream distance of ~15 RE) as well as first in the mid-tail (beyond the downstream distance of ~20 RE). We present the salient features of these observations, offer suggestions regarding the discrepant interpretations, and discuss how future mission plans will enable a consistent picture on timing to emerge.
SM51C-02 INVITED
Perspectives of Substorm Theories
More than forty years of efforts in the theoretical description of magnetospheric substorms have resulted in an impressive and multi-facet picture of the phenomenon. Its elements are clustered around two major competing models of the magnetotail reconnection and current disruption. However, there are still important elements that are missing in the substorm puzzle and that should be targeted in the future studies. One of them is the mechanism of the spontaneous reconnection onset in the magnetotail. The presently dominating view requires the onset to be triggered by the formation of an X-line in the original tail-like magnetic field geometry. This triggering either by the inside-out propagation of the disturbance from the current disruption region or directly by the solar wind appears to be at variance with Geotail statistics and recent Themis observations. At the same time, modern theories and particle simulations with open boundaries suggest that the reconnection instability in the tail may start before the formation of the X-line and may be drastically different from its simplified picture based on reconnection of anti-parallel magnetic fields. Further studies in this direction require 3D particle simulations to understand the role of the ballooning-interchange instability and the possible formation of entropy-depleted flux tubes. According to the current disruption model, the latter instability along with others, driven by the strong cross-tail current, may be an independent cause of substorms. However, in spite of many specific models proposed to date, the dominating current disruption mechanism and whether it is spontaneous or externally driven remains unclear. Probably even more efforts are required to obtain the detailed description of the global consequences of the current disruption, including the properties of the rarefaction wave, the main agent of the inside-out signal propagation.
SM51C-03
Tail Reconnection Triggering Substorm Onset
We report on simultaneous magnetotail measurements by THEMIS at multiple distances, at the time of substorm onset, which occured on Feb 26, 2008. Evidence for reconnection including bipolar magnetic field, inflow towards the neutral sheet and counterstreaming electrons was observed at P1 and P2, suggesting that the reconnection site was at 20 Re. Evidence for current disruption, including dipolarization, turbulence and plasma heating were observed at P3 and P4 at ~11Re. Remote sensing of the current disruption region onset was not possible but particle distributions suggest that the plasma had started moving Earthward ahead of the dipolarization, i.e., point towards a source tailward of the P3, P4 satellites. Auroral brightening and poleward expansion was observed after reconnection onset and before dipolarization onset, suggesting that this event was triggered by reconnection around 20Re. Observations from a number of similar events show that a consistent picture of onset is starting to emerge from the first tail season. We discuss implications for substorm models and for auroral particle acceleration.
SM51C-04
Longitudinal association between magnetotail reconnection and auroral breakup based on Geotail and Polar observations
The dawn-dusk locations of reconnection in the near-earth magnetotail at the time of isolated auroral breakup are studied to clarify whether breakup is always accompanied by reconnection. The near-earth reconnection is identified by tailward plasma flows faster than 200 km/s with southward magnetic field. We first identified 66 breakups in the Polar ultraviolet imager observations of the nightside polar ionosphere. We then studied tailward flows during breakups using Geotail in situ observations of the plasma sheet between 25 and 31 RE down the tail. It was found that the dawn-usk (Y) locations of relatively fast (≥ 400 km/s) tailward flows were associated with breakup magnetic local time (MLT) by a regression line of YAGSM = -5.7 × (MLT + 0.6) RE with a correlation coefficient of 0.8. Most tailward flows were observed within 5 RE of the modeled Y locations, where tailward flows occurred in 88% of the 26 cases of breakups between 22 and 0 MLT. It is thus inferred that in most cases, breakup is accompanied by tailward flow near the breakup MLT with its dawn-dusk dimension ~ 10 RE. There were only two events without tailward flows in the region where flows have been expected. These two events were an earthward flow event and a traveling compression region event, which are not inconsistent with the initiation of the near-earth reconnection. Auroral breakup is thus likely to always be accompanied by near-earth reconnection near breakup MLT. It is also inferred that reconnection and breakup occur simultaneously within a few minutes, assuming a time delay between reconnection onset and the arrival of tailward flows at satellite locations.
SM51C-05
Substorm Dynamics: THEMIS Data and OpenGGCM Simulations
THEMIS has now completed its first tail season and thus has collected data on a substantial number of substorm events. In addition, THEMIS also observed substorms during the commissioning pahase in February and March of 2007. Outstanding among these events were the March 23, 2007 "first light" substorm, and the February 26, 2008 event that was recently analyzed in a Science article. In this presentation we augment the data analysis with OpenGGCM global magnetosphere simulations. We show that the simulations reproduce the salient features of these substorms such as the westward traveling surge and dipolarization of the tail magnetic field. We conclude that the simulations are generally consistent with the Near-Earth Neutral Line substorm model; however, OpenGGCM simulations suggest that tail reconnection is much more fragmented and does not occur at a single x-line.
SM51C-06
Relative Timing of Substorm Processes as Derived from Multi-Fluid/Multi-Scale Simulations : Reconnection Versus Current Disruption
The role of tail reconnection in initiating substorm onset remains highly controversial. Multi-fluid/multi-scale simulations with resolution of 400 km in the tail and which incorporate ion skin depth and ion cyclotron processes are used to examine the link made between auroral current intensifications, dipolarization and tail reconnection. It is shown that consistent the near-Earth neutral-line model, a thin current sheet form during the growth phase with the eventual formation of an X-line and the ejection of a plasmoid. However, these processes do not produce substorm onset, but are precursors to onset. The modeling indicates that in the wake of the plasmoid, a Y-line forms within which there is intermittent generation flux ropes that can propagate either earthward or tailward, with equal probability. Depending on the size of the earthward moving flux rope, pseudo-breakup or substorm onset occurs when the energy within the earthward moving flux rope is dissipated against the inner edge of the plasma sheet. This interaction leads to rapid (1 min) dipolarization, magnetospheric injection of energetic particles, and intensification of the nightside auroral currents that expand in local time as well as to higher latitudes. Thus current disruption at the inner edge of the plasma sheet is the direct cause of substorm onset. Ionospheric outflows and their energization with the thin current sheet play important roles in the build up of energy in the plasma sheet that eventually produces substorm onset and drives breakup.
SM51C-07
Evidence for Co-existence of Outside-in and Inside-out Scenarios in Substorm on January 9, 2008
In the previous and recent studies (Pu et al., 1999, 2001; Cao et al., 2008; Zhang et al., 2007) we have suggested the co-existence of outside-in and inside-out scenarios in fully developed substorms. In the present work we show further evidence for this picture from THEMIS, Polar and ground-based measurements of an isolated substorm on January 9, 2008. During the growth phase of the substorm, THEMIS P2 (located at (-23.3, -5.0, -8.4) Re in GSM) observes an earthward flow at ~0640UT, indicating the occurrence of reconnection in the mid-tail. At 0654:30UT, an earthward flow arrives at P4 ((-9.7, -4.2, -3.7) Re) and causes magnetic flux pileup. Meantime, at 0654:54 and 0656:21UT PGEO station and Polar UVI imager observe pre- midnight auroral activity around MLAT 62oand MLAT 65o, respectively. Later at 0657 and 0702UT P4 sees depolarization twice, during which Pi2 reaches maximum, Pi1 greatly enhances, plasma temperature rapidly increases and the ideal MHD condition is broken. Besides, P5 ((-7.2, 0.0, -2.4) Re), GOES12 ((-5.3, -3.6, - 1.8) Re) and GOES11 ((-5.5, 2.8, -2.6) Re) observe depolarization, respectively, at 0706, 0711 and 0716UT. Of particular interest are substorm activations after 0722UT when IMF at 1 AU starts northward turning. Rightafter 0722UT Polar UVI shows a major onset of aurora breakup; at 0722UT P3 and P4 observe depolarization/current disruption. Then at 0728 and 0732UT P5 and P2 also see this feature at ~7Re and 17 Re, respectively. Later substorm expansion arrives at GOES11 and P1 at 0738 and 0750UT, successively. Poleward expansion of auroral bulges and tailward progression of substorm expansion are shown to be closely related. This event displays the development of a fully developed substorm£ºMagnetic reconnection first occurs in the mid-tail and earthward flows cause flux pileup/dipolarization in the near-Earth tail; the substorm expansion then progresses tailward, earthward and westward/eastward, leading to the full development of substorm expansion phase. Underlying dynamical processes are briefly discussed.