V. Angelopoulos

Bursty bulk flows in the inner central plasma sheet

High speed flows in the Earths Inner Central Plasma Sheet (ICPS) occur during enhanced flow intervals that have been termed Bursty Bulk Flow (BBF) events. The importance of different flow magnitude samples for Earthward transport in the ICPS are statistically evaluated and several representative BBFs and their relevance to Earthward transport are discussed. The selection of BBFs is automated in a database and they are shown to be responsible for most of the Earthward transport that occurs within the ICPS. The BBF related transport is compared to the transport measured within the entire plasma sheet during the 1985 AMPTE/IRM crossings of the magnetotail. The results show that BBFs last only a small fraction of the time in the plasma sheet but can account for several tens of percent of the Earthward particle and energy transfer and possibly all of the Earthward magnetic flux transfer in the plasma sheet.

Neutral line model of substorms: Past results and present view

The near-Earth neutral line (NENL) model of magnetospheric substorms is reviewed. The observed phenomenology of substorms is discussed including the role of coupling with the solar wind and interplanetary magnetic field, the growth phase sequence, the expansion phase (and onset), and the recovery phase. New observations and modeling results are put into the context of the prior model framework. Significant issues and concerns about the shortcomings of the NENL model are addressed. Such issues as ionosphere-tail coupling, large-scale mapping, onset trigger- ing, and observational timing are discussed. It is concluded that the NENL model is evolving and being improved so as to include new observations and theoretical insights. More work is clearly required in order to incorporate fully the complete set of ionospheric, near-tail, midtail, and deep- tail features of substorms. Nonetheless, the NENL model still seems to provide the best avail- able framework for ordering the complex, global manifestations of substorms.

THEMIS observations of an earthward‐propagating dipolarization front

[1] We report THEMIS observations of a dipolarization front, a sharp, large-amplitude increase in the Z-component of the magnetic field. The front was detected in the central plasma sheet sequentially at X = -20.1 R E (THEMIS P1 probe), at X = -16.7 R E (P2), and at X = -11.0 R E (P3/P4 pair), suggesting its earthward propagation as a coherent structure over a distance more than 10 R E at a velocity of 300 km/s. The front thickness was found to be as small as the ion inertial length. Comparison with simulations allows us to interpret the front as the leading edge of a plasma fast flow formed by a burst of magnetic reconnection in the midtail.

Tail Reconnection Triggering Substorm Onset

Magnetospheric substorms explosively release solar wind energy previously stored in Earths magnetotail, encompassing the entire magnetosphere and producing spectacular auroral displays. It has been unclear whether a substorm is triggered by a disruption of the electrical current flowing across the near-Earth magnetotail, at ∼10 RE (RE: Earth radius, or 6374 kilometers), or by the process of magnetic reconnection typically seen farther out in the magnetotail, at ∼20 to 30 RE. We report on simultaneous measurements in the magnetotail at multiple distances, at the time of substorm onset. Reconnection was observed at 20 RE, at least 1.5 minutes before auroral intensification, at least 2 minutes before substorm expansion, and about 3 minutes before near-Earth current disruption. These results demonstrate that substorms are likely initiated by tail reconnection.

Detection of localized, plasma‐depleted flux tubes or bubbles in the midtail plasma sheet

Recent studies have shown that most Earthward transport hi the midtail, high-beta plasma sheet takes place in the form of short-lived, high-speed plasma flow bursts. Bursty bulk flows are observed both when the plasma sheet is thin, such as during substorm expansion, and when it is thick, such as during substorm recovery. We present multi-instrument observations from the ISEE1 and ISEE 2 spacecraft to argue that when the plasma sheet becomes thick and close to its equilibrium state, the plasma and magnetic field signatures of high-speed flow events are consistent with the theoretically predicted signatures of plasma-depleted flux tubes or “bubbles” [Pontius and Wolf, 1990; Chen and Wolf, 1993]. These signatures consist of a decrease in the plasma pressure and an increase in the Bz-component of the magnetic field accompanying the high speed flow. We show that the Earthward moving bubbles are separated from the plasma ahead of them by a sharp tangential discontinuity. The layer ahead of the bubbles exhibits flow and magnetic field shear consistent with flow around an Earthward moving obstacle. The bubble is in approximate total pressure balance with the surrounding medium. We show that there is a systematic difference in the orientation of the discontinuity measured at ISEE 1 and 2, implying a small (about 1–3 RE) cross-tail size of the bubbles.

Global distribution of whistler-mode chorus waves observed on the THEMIS spacecraft

[1] Whistler mode chorus waves are receiving increased scientific attention due to their important roles in both acceleration and loss processes of radiation belt electrons. A new global survey of whistler-mode chorus waves is performed using magnetic field filter bank data from the THEMIS spacecraft with 5 probes in near-equatorial orbits. Our results confirm earlier analyses of the strong dependence of wave amplitudes on geomagnetic activity, confinement of nightside emissions to low magnetic latitudes, and extension of dayside emissions to high latitudes. An important new finding is the strong occurrence rate of chorus on the dayside at L > 7, where moderate dayside chorus is present >10% of the time and can persist even during periods of low geomagnetic activity. Citation: Li, W., R. M. Thorne, V. Angelopoulos, J. Bortnik, C. M. Cully, B. Ni, O. LeContel, A. Roux, U. Auster, and W. Magnes (2009), Global distribution of whistler-mode chorus waves observed on the THEMIS spacecraft, Geophys. Res. Lett., 36, L09104, doi:10.1029/2009GL037595.

Substorm triggering by new plasma intrusion: THEMIS all‐sky imager observations

[1] A critical, long‐standing problem in substorm research is identification of the sequence of events leading to substorm auroral onset. Based on event and statistical analysis of THEMIS all‐sky imager data, we show that there is a distinct and repeatable sequence of events leading to onset, the sequence having similarities to and important differences from previous ideas. The sequence is initiated by a poleward boundary intensification (PBI) and followed by a north‐south (N‐S) arc moving equatorward toward the onset latitude. Because of the linkage of fast magnetotail flows to PBIs and to N‐S auroras, the results indicate that onset is preceded by enhanced earthward plasma flows associated with enhanced reconnection near the pre‐existing open‐closed field line boundary. The flows carry new plasma from the open field line region to the plasma sheet. The auroral observations indicate that Earthward‐transport of the new plasma leads to a near‐Earth instability and auroral breakup ∼5.5 min after PBI formation. Our observations also indicate the importance of region 2 magnetosphere‐ionosphere electrodynamic coupling, which may play an important role in the motion of pre‐onset auroral forms and determining the local times of onsets. Furthermore, we find motion of the pre‐onset auroral forms around the Harang reversal and along the growth phase arc, reflecting a well‐developed region 2 current system within the duskside convection cell, and also a high probability of diffuse‐appearing aurora occurrence near the onset latitude, indicating high plasma pressure along these inner plasma sheet field lines, which would drive large region 2 currents.

Kinetic structure of the sharp injection/dipolarization front in the flow-braking region

[1] Observations of three closely-spaced THEMIS spacecraft at 9-11 Re near midnight and close to the neutral sheet are used to investigate a sharp injection/ dipolarization front (SDF) propagating inward in the flow-braking region. This SDF was a very thin current sheet along the North-South direction embedded within an Earthward-propagating flow burst. A short-lived depression of the total magnetic field (down to 1 nT), devoid of wave activity and intense particle fluxes, stays ahead of the SDF. Clear finite proton gyroradius effects, which help visualize the geometry and sub-gyroscale of the SDF, are seen centered at the thin current sheet. The SDF nearly coincides with the narrow interface between plasmas of different densities and temperatures. At that interface, we observed strong (40―60 mV/m peak) E-field bursts of the lower-hybrid time scale that are confined to a localized region of density depletions. This sharp dipolarization/injection front propagating in the flow-braking region appears to be a complicated kinetic-scale plasma structure that combines a number of small-scale elements (Bz drops, thin current sheets, LH cavities, injection fronts) previously discussed as separate objects.

Magnetotail flow bursts: Association to global magnetospheric circulation, relationship to ionospheric activity and direct evidence for localization

A series of bursty bulk flow events (BBFs) were observed by GEOTAIL and WIND in the geomagnetotail. IMP8 at the solar wind showed significant energy coupling into the magnetosphere, while the UVI instrument on POLAR evidenced significant energy transfer to the ionosphere during two substorms. There was good correlation between BBFs and ionospheric activity observed by UVI even when ground magnetic signatures were absent, suggesting that low ionospheric conductivity at the active sector may be responsible for this observation. During the second substorm no significant flux transport was evidenced past WIND in stark contrast to GEOTAIL and despite the small intersatellite separation ((3.54, 2.88, −0.06) RE). Throughout the intervals studied there were significant differences in the individual flow bursts at the two satellites, even during longitudinally extended ionospheric activations. We conclude that the half-scale-size of transport-bearing flow bursts is less than 3 RE.

Identifying the Driver of Pulsating Aurora

Auroral Chorus Energetic particles that arrive from near-Earth space produce photon emissions—the aurora—as they bombard the atmosphere in the polar regions. The pulsating aurora, which is characterized by temporal intensity variations, is thought to be caused by modulations in electron precipitation possibly produced by resonance with electromagnetic waves in Earths magnetosphere. Nishimura et al. (p. 81) present a detailed study of an event that showed a good correlation between the temporal changes in auroral luminosity and chorus emission—a type of electromagnetic wave occurring in Earths magnetosphere. The results points to chorus waves as the driver of the pulsating aurora. Correlations are found between aurora light intensity and a type of electromagnetic wave in Earth’s magnetosphere. Pulsating aurora, a spectacular emission that appears as blinking of the upper atmosphere in the polar regions, is known to be excited by modulated, downward-streaming electrons. Despite its distinctive feature, identifying the driver of the electron precipitation has been a long-standing problem. Using coordinated satellite and ground-based all-sky imager observations from the THEMIS mission, we provide direct evidence that a naturally occurring electromagnetic wave, lower-band chorus, can drive pulsating aurora. Because the waves at a given equatorial location in space correlate with a single pulsating auroral patch in the upper atmosphere, our findings can also be used to constrain magnetic field models with much higher accuracy than has previously been possible.