W. W. Liu

Intensification of preexisting auroral arc at substorm expansion phase onset: Wave-like disruption during the first tens of seconds

[1]xa0With the deployment of the all-sky imager array of the THEMIS mission, we were able to construct a preliminary database of auroral substorm expansion phase onsets, from which we have established a number of common features characterizing the first tens of seconds of the substorm auroral intensification. We find that the intensification occurs within ∼10 sec over an arc segment extending approximately 1 h MLT and featuring wave-like formations distributed in longitude. The longitudinal wave number ranges between 100 and 300 such that the wavelength is comparable to the ion gyroradius in the central plasma sheet. The scale the intensification is about 10–30 sec. This study casts important observational constraints on substorm onset theories.

Simultaneous THEMIS in situ and auroral observations of a small substorm

[1]xa0We present ground-based and in situ observations from March 13, 2007. The THEMIS satellites were in the evening sector conjugate to THEMIS ground-based imagers. At ∼0507 UT there was an optical onset on inner CPS field lines. This involved near-simultaneous brightening of 1 MLT hour longitudinal segment of the onset arc. The part of the arc that brightened was that closest to the equatorward boundary of the diffuse (proton) aurora. Within one minute, a dipolarization front moved across four THEMIS satellites. Based on their locations, the order in which they detected the dipolarization front, and the auroral evolution, we assert that the expansion phase began earthward of the four satellites and evolved radially outwards. We conclude that this onset occurred in an azimuthally localized region of highly stretched field lines.

Longitudinally propagating arc wave in the pre‐onset optical aurora

[1]xa0We present the first systematic observational evidence for a traveling periodic structure in the pre-onset optical aurora – the longitudinally propagating arc wave (LPAW) – associated with flapping oscillations in the magnetotail. The LPAW is characterized by azimuthally moving intensity enhancements inside auroral arcs as seen by THEMIS ground-based all-sky imagers. It travels westward in the pre-midnight auroral sector during the 10–20 minutes preceding auroral breakup with a velocity of 2–10 km/s, time period 40–110 s, and wavelength 250–420 km. Magnetically conjugate measurements by THEMIS satellites show low frequency plasma oscillations consistent with the parameters of the arc wave in the course of current sheet thinning. When mapped into tail, wavelength (4800–9400 km) and velocity (70–190 km/s) of the LPAW are compatible with observations and theoretical predictions for current sheet flapping motions. Our results strongly suggest that LPAW is an auroral footprint of the drift wave mode (kink, sausage, ballooning, etc.) in a stretched magnetotail.

Azimuthal structures of substorm electron injection and their signatures in riometer observations

[1]xa0We propose a theoretical model to investigate the effects of the curvature/gradient (c/g) drift and the finite azimuthal extent of the dipolarization region on the electron injection process associated with the substorm dipolarization. We study the azimuthal structure of high-energy (>30 keV) electron precipitation flux and compare the result with riometer observations. We are able to reproduce three basic archetypes of riometer responses to substorms, namely, the spike, dispersionless injection, and dispersed injection events catalogued in previous observations. The electron injection near the duskward edge of the dipolarization region is most subject to azimuthal c/g drift loss, appearing in riometer observations as the “spike” feature. The “dispersionless injection” response is seen inside the dipolarization region but some distance away from its western border: or, alternatively, when the substorm has a rapid westward expansion, so that the gain and loss of electrons from the duskside and dawnside of a dipolarizing flux tube roughly balance. The “dispersed injection” feature is seen east of the dipolarization region. Our theory successfully explains the statistical differences in terms of magnetic local time location and peak intensity between spikes and injection events. Through the substorm event on 23 May 1998 we demonstrate that our theoretical predictions of riometer responses are very consistent with the observations. We highlight the potential of riometers in resolving the azimuthal extent and evolution of the dipolarization region, which provides a new ground-based technique of remote sensing the substorm process.

A transient narrow poleward extrusion from the diffuse aurora and the concurrent magnetotail activity

[1] We report observation of a transient narrow auroral feature extruding from the poleward boundary of the diffuse aurora on March 19, 2009. It moved westward and poleward initially to form part of a vortex pattern, followed by its equatorward-dawnward retreat later. During this auroral activity, THEMIS satellites, projected near the same magnetic local time of the auroral feature, detected appreciable plasma flows, increase in the ratio of the ion energy over the electron energy, and some enhancements of electrostatic waves. The plasma flows were initially duskward-earthward and changed to duskward-tailward later. The overall development of the observed plasma flow pattern was detected during the equatorward-dawnward retreat of the auroral feature when the Alfven transit time between the magnetotail and the ionosphere is taken into account. This suggests that THEMIS satellites remotely sensed a counter-clockwise flow vortex (viewed from above the equatorial plane) in the magnetotail with decreasing strength. We suggest that the process generating the auroral feature is related to the flow vortex in association with the depletion of the electron energy relative to the ion energy and wave-particle interaction. An estimate of the possible associated current density is made. We provide reasoning for this auroral feature to be an auroral streamer and not a failed transpolar arc.

Global observations of substorm injection region evolution: 27 August 2001

We present riometer and in situ observations of a substorm electron injection on 27 August 2001. The event is seen at more than 20 separate locations (including ground stations and 6 satellites: Cluster, Polar, Chandra, and 3 Los Alamos National Laboratory (LANL) spacecraft). The injection is observed to be dispersionless at 12 of these locations. Combining these observations with information from the GOES-8 geosynchronous satellite we argue that the injection initiated near geosynchronous orbit and expanded poleward (tailward) and equatorward (earthward) afterward. Further, the injection began several minutes after the reconnection identified in the Cluster data, thus providing concrete evidence that, in at least some events, near-Earth reconnection has little if any ionospheric signature.

Interaction between kinetic ballooning perturbation and thin current sheet: Quasi‐electrostatic field, local onset, and global characteristics

[1]xa0We present observation of plasma waves in the current sheet across multiple THEMIS satellites in a substorm event on March 5, 2008. It is shown that the arrival of a kinetic ballooning perturbation interacts with the local current sheet to generate a quasi-electrostatic wave a few minutes before local onset, consistent with the prediction that the current sheet thins after the passage of a rarefaction wave. The propagation speed of current disruption front is found to be ∼100 km/s, about a tenth of the fast mode speed. The observed pattern of interaction was constant across radial distances between 10 and 20 RE in the event reported. It is further proposed that the presence of the quasi-electrostatic field may change the local stability condition and induce local current disruption.

On the equatorward motion and fading of proton aurora during substorm growth phase

[1]xa0Using 50 high-quality events from the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) Gillam Meridian Scanning Photometer over 10 a (1989–1998), we show that proton aurora in the substorm growth phase exhibits a systematic tendency to fade. The average pattern of fading consists of a period of relatively stable proton aurora brightness and then a period of 15–20 min before onset during which the brightness decreases by an average of 15%. We interpret the observed proton aurora brightness variation in terms of the magnetic field stretching in the near-Earth magnetosphere; in particular, the fading is interpreted as a result of the central plasma sheet magnetic field lines having stretched to such a degree that the loss cone closing effect dominates precipitation due to nonadiabatic proton motions.

Energy avalanches in the central plasma sheet

[1]xa0The central plasma sheet (CPS) is simulated as a 1D cellular automaton. The system is driven deterministically and globally by a spatially non-uniform energy loading (convection). Each node (a flux tube) evolves until one of two local instability criteria is exceeded. The unstable node releases a small amount of energy to the ionosphere and another small amount is distributed to its neighboring nodes. The partition between the two modes of energy distribution is the only random factor in the model. The energy redistribution relaxes the node deterministically to a stable state. The simulation suggests that a central plasma sheet driven in the above manner is in a self-organized critical state, with energy avalanches obeying a scale-free distribution. The avalanches, however, co-exist with quasi-periodic intermittencies manifested in ring-current injection, which is correlated with strong CPS avalanches, and tailward energy ejection, which shows no apparent correlation in this aspect.

Electrostatic field and ion temperature drop in thin current sheets: A theory

[1]xa0The observational evidence presented by Liang et al. (2009) showed that a neutral sheet–pointing electrostatic field frequently arises in the late growth-phase current sheet in the magnetotail. In this paper, we elaborate on the suggestion that this electric field is associated with the thinning of the current sheet to the ion scale at which the electron and ion current sheets begin to separate. The attendant effect of a decreasing ion temperature, also interpreted in terms of a thinning current sheet, suggests that a cold plasma population is involved. We review existing theories of “charged” Harris sheet that can produce electrostatic fields and show that they cannot explain the observations for various reasons. A particular problem is the over shielding of the electrostatic field by the cold population embedding the current sheet. We argue that this problem stems from not treating the cold plasma as a separate population from the hot plasma forming the thin current sheet (TCS). We show that if the cold population is treated as external to the TCS and behaving in a largely MHD manner, the resultant solution yields an electrostatic field and ion temperature drop consistent with the observations.