JapaneseGEMSIS-Ionosphere
Substorm Processes
Auroral Breakup and Magnetotail Reconnection During Substorms
The longitudinal locations of reconnection in the near-Earth magnetotail at the time of isolated auroral breakup were studied. The near-Earth reconnection is identified by tailward plasma flows with a southward magnetic field. We first identified 66 breakups in the Polar satellite ultraviolet imager observations of the nightside polar ionosphere. We then studied tailward flows during breakups using Geotail satellite in situ observations of the plasma sheet between 25 and 31 Re down the tail. It was found that most tailward flows were observed near the breakup longitudes. Auroral breakup is therefore inferred to be always accompanied by near-Earth reconnection near breakup MLT.
Reference: Ieda et al., JGR, 2008
Preonset Time Sequence of Auroral Substorms
Based on event and statistical analysis of THEMIS all-sky imager data, we have discovered a distinct and repeatable sequence of events that is initiated by a poleward boundary intensification (PBI) followed by a north-south (N-S) arc moving equatorward toward the onset latitude leading to substorm auroral onset. We also found that each N-S arc leads to a small intensification of the growth phase arc. Furthermore, when the onset-related N-S arc reaches the equatorward portion of the auroral oval, the preexisting growth phase arc is much brighter than at times of nononset-related N-S arcs. The onset arc is typically a thin arc near the poleward boundary of a diffuse growth-phase arc. Assuming that the growth-phase arc is related to the enhanced pressure gradient, this difference indicates that the near-Earth plasma pressure distribution at the time of plasma sheet fast flows is crucial in substorm triggering. These observations suggest that substorm onset instability is possible only when the preexisting inner plasma sheet pressure and pressure gradient is sufficiently large.
Reference: Nishimura et al., JGR, 2011
Auroral Substorm Current System
- The spatial structures of the ionospheric electric field and the currents inside the auroral bulge during a substorm were studied based on simultaneous observations made by multiple satellites and ground magnetometers. The multi-instrument analysis showed that the eastward electrojet current, which flows from dusk to midnight, weakened substantially as it passed through the local time sector of ~2 hours. This suggests that the eastward electrojet could decay significantly over similar small spatial scales due to the net upward field-aligned current flowing out of the ionosphere to the magnetosphere.
- The spatial distribution of field-aligned currents (FACs) was studied for an intense substorm. We first estimated the height-integrated ionospheric Hall and Pedersen conductances from ultraviolet images taken by the Polar satellite. To derive FACs, we then employed Ohm’s law for the ionosphere, applying a magnetic inversion method to the input conductances and ground magnetic field data. As a result, we found that the Hall FACs connected to the ionospheric Hall currents were anticorrelated with the Pedersen FACs connected to the ionospheric Pedersen currents. Such anticorrelations were observed near the poleward and the equatorward edges of the auroral westward currents. This result indicates that the auroral current system, including the Cowling channel (that is, the auroral westward currents), were enhanced by the polarization electric field in this particular event.
Two Satellite Observations of Precipitating Electrons Associated with Auroral Breakup
We compared auroral electrons several minutes before and after an auroral breakup. FAST passed the breakup location 6 min before the breakup and observed diffuse 10-keV electrons. The diffuse electrons were accompanied by broadband electrons below 1 keV, which were attributed to Alfvén waves. Seven minutes after the breakup, a Defense Meteorological Satellite Program (DMSP) satellite crossed the onset arc. The DMSP satellite observed inverted-V type electrons at the surge horn, which were 15° west of the initial breakup location. These results suggest that the evolution of diffuse electrons to inverted-V electrons is associated with waves.
Auroral Observations during a THEMIS Major Conjunction on March 1, 2008
We took white-light all-sky videos (sampling rate: 30 Hz) of auroras at Fort Smith (67 MLAT, 23 MLT at 0720 UT), Canada when the THEMIS satellites were aligned on the premidnight plane on March 1, 2008. A variety of auroras were observed on these nights. Below, we concentrate on a pseudobreakup at 0647 UT on March 1. On March 1, 2008, an auroral arc, moving equatorward, disappeared around 0640 UT in the south. Six minutes later, faint patches were first recognized around 0646:28 UT near the latitude where the previous arc dimmed in the south sky. Colocated with the patches, a weak arc showed up and gradually intensified, and a pseudobreakup occurred there at 0647:27 UT, presumably inside the field of view of the camera. The pseudobreakup dimmed around 0648 UT. At the same time, other patches showed up and were followed by some breakups. Corresponding THEMIS satellite observations included a southward turning in the magnetic field with a slow but spiky tailward flow (~100 km/s) observed by THEMIS-B (GSM X, Y, Z = -20, 4, -2 Re) at 0645 UT, 2 minutes before the pseudobreakup. Following were slow earthward flows at 0646 UT in THEMIS-E (200km/s at -11, 4, -2 Re) and in THEMIS-D (50 km/s at -11, 3, -2 Re). A sharp dipolarization of 10 nT was also observed at 0646 UT in THEMIS-A (-7, 5, -2 Re). THEMIS-C (-16, 4, -3 Re) did not observe flows, but observed a peak in total pressure at 0646 UT. A comparison of these timings suggests that this particular pseudobreakup was caused by a reconnection inside 20 Re ~2 minutes before the pseudobreakup.