Characteristics of plasma flows at the inner edge of the plasma sheet

Abstract

[1] All known types of auroral zone magnetic activity are associated with closure of open magnetic flux in the magnetotail. As closure is caused by magnetic reconnection we expect to observe fast flows during geomagnetic activity. We have scanned the ion flow data during the first pass of the THEMIS D spacecraft through the tail (December 2007 to May 2008, identifying all flows with |V ?x | > 150 km/s. These flows generally occur in a sequence of several short bursts (bursty bulk flows). Earthward flows are much more common than tailward flows and are faster than tailward flows. Earthward flows have a longer duration; tailward flows are seen alone or after an earthward flow. Both directions of flow are associated with an increase in tail B z (dipolarization). Fast flows in either direction are rarely seen inside of 9 R E . Earthward flows are strongly localized in the local time sector 2100-0100 and have a probability distribution identical to that seen in auroral substorm expansions by the IMAGE spacecraft. Tailward flows are also localized but with a peak shifted to 2330 LT. Very close to midnight the flows are slowed and reflected. At other local times they appear to be deflected around the Earth. Fast flows often follow a reduction in E s (GSM VB s ) and occur close to the time of a sudden decrease in the AL index. Generally, the first flow burst in a sequence is most closely associated with the AL onset, and its peak follows the AL onset by about 2 min. The probability of observing a fast flow at THEMIS D during steady magnetospheric convection (SMC) events is quite low compared with the probability during an interval before the SMC. Since most of the fast flows carry magnetic flux earthward and are associated with substorm onset seen in the aurora by IMAGE and in the AL index, we interpret them as evidence that magnetic reconnection has occurred in the tail. Near 30 R E in the tail plasmoid ejection has also been associated with substorm onset, so we conclude that the fast flows are created by a new X line formed outside the 11.9 apogee of THEMIS D some time earlier than they are seen at THEMIS D.During SMC it appears that fast flows due to reconnection are deflected around the Earth outside the apogee of the satellite.

A Statistical Study of Near‐Earth Magnetotail Evolution During Pseudosubstorms and Substorms With THEMIS Data

“…At X = −10 and X = −11R E , where relatively fast earthward flows are observed as shown above, the occurrence rate of V x > 100 km/s clearly increases around t = 0 for both cases, but this increase is larger for substorms than for pseudosubstorms. We found that there are more fast earthward flows in the outermost regions for both cases, especially for substorms, consistent with McPherron et al (2011). Figure 7 shows spacecraft locations for pseudosubstorm and substorm events accompanied by fast earthward flows with a peak of more than 100 km/s during the period from t = −5 to 5 min.…”

“…It is generally very difficult, however, to determine the reliable onset location in the magnetotail from only one or a few spacecraft. Here V is not negligible, since the flow deflects in the azimuthal direction in the near-Earth region (McPherron et al, 2011). There is, however, some ambiguity of the corresponding magnetotail location determined from mapping with a magnetic field model particularly just before onset when magnetic field lines are highly stretched and hence possibly deviated from the model field lines.…”

M–I coupling across the auroral oval at dusk and midnight: repetitive substorm activity driven by interplanetary coronal mass ejections (CMEs)

“…8). Events of plasma intrusion from the midtail followed by diversion closer to Earth (as indicated in the figure; see also McPherron et al, 2011) and association with auroral arc brightening have been recently documented by Yau et al (2013). On the relationship between the Harang reversal boundary and the substorm injection boundary we refer to the early study by Brekke (1977).…”

Magnetotail origins of auroral Alfvénic power

“…[7] The distribution of BBF events in the plasma sheet cluster in the 2100-0100 MLT sector with a centroid near 2300 MLT [McPherron et al, 2011]. Given the statistical distribution of BBFs, the event study by Angelopoulos et al [2002] connecting an Alfvén wave burst at Polar satellite altitudes with BBFs in the plasma sheet and the propensity of braking BBFs to generate Alfvén wave and associated auroral activity, it seems plausible that the dawn-dusk asymmetry in the distribution of Alfvén wave power reported by Keiling et al [2003] is connected with the dawn-dusk asymmetry in the plasma sheet distribution of BBFs.…”