EISCAT-Cluster observations of quiet-time near-Earth magnetotail fast flows and their signatures in the ionosphere

Abstract

Abstract. We report observations of a sequence of quiettime Earthward bursty bulk flows (BBFs) measured by the Cluster spacecraft in the near-tail plasma sheet (XGSM ∼ −12 to −14 R E ) in the evening sector, and by simultaneous highresolution measurements in the northern conjugate ionosphere by the EISCAT radars, a MIRACLE all-sky camera and magnetometers, as well as a meridian-scanning photometer (MSP) in the Scandinavian sector on 17 October 2005.The BBFs at Cluster show signatures that are consistent with the plasma "bubble" model Wolf, 1993, 1999), e.g. deflection and compression of the ambient plasma in front of the Earthward moving bubble, magnetic signatures of a flow shear region, and the proper flows inside the bubble. In addition, clear signatures of tailward return flows around the edges of the bubble can be identified. The duskside return flows are associated with significant decrease in plasma density, giving support to the recent suggestion by Walsh et al. (2009) of formation of a depleted wake. However, the same feature is not seen for the dawnside return flows, but rather an increase in density.In the ionosphere, EISCAT and optical measurements show that each of the studied BBFs is associated with an auroral streamer that starts from the vicinity of the polar cap boundary, intrudes equatorward, brakes at 68-70 • aacgm MLAT and drifts westward along the proton oval. Within the streamer itself and poleward of it, the ionospheric plasma flow has an equatorward component, which is the ionospheric manifestation of the Earthward BBF channel. A sharp velocity shear appears at the equatorward edge of a streamer. We suggest that each BBF creates a local velocity shear in the ionosphere, in which the plasma flow poleward of and inside the streamer is in the direction of the Correspondence to: T. Pitkänen (timo.pitkanen@oulu.fi) streamer and southeastward. A northwestward return flow is located on the equatorward side. The return flow is associated with decreased plasma densities both in the ionosphere and in the magnetosphere as measured by EISCAT and Cluster, respectively. In summary, we present the first simultaneous high-resolution observations of BBF return flows both in the plasma sheet and in the ionosphere, and those are in accordance with the bubble model. The results apply for the duskside return flows, but the manifestation of dawnside return flows in the ionosphere requires further studies.Finally, EISCAT measurements indicate increased nightside reconnection rate during the ∼35-min period of BBFs. We suggest that the observed temporal event of IMF rotation to a more southward direction produces enhanced open flux transport to the nightside magnetotail, and consequently, the nightside reconnection rate is increased.

Cluster observations of the substructure of a flux transfer event: analysis of high-time-resolution particle data

“…Paschmann et al, 1982;LaBelle et al, 1987; Published by Copernicus Publications on behalf of the European Geosciences Union. 1988, 2011Owen et al, 2008). Other studies have shown that these signatures are consistent with transient reconnection at the magnetopause.…”

Observations of an auroral streamer in a double oval configuration

“…6 around 20:16 UT clearly represent the ionospheric equivalent current signature of an auroral streamer in the Northern Hemisphere, caused by the magnetospheric BBF. While from the experience obtained from earlier studies as mentioned in the introduction, already this signature alone would be a sufficient evidence for the existence of an auroral streamer in the Northern Hemisphere, in addition also the EISCAT electron density observations clearly confirm this (compare, e.g., with the study by Pitkänen et al, 2011). A comparison with the Southern Hemisphere optical data shown in Fig.…”

“…Their study, as well as a number of other studies based on magnetospheric observations of BBF (e.g., Sergeev et al, 1996Sergeev et al, , 2000Nakamura et al, 2005;Snekvik et al, 2007;Takada et al, 2008;Pitkänen et al, 2011) are consistent with the plasma bubble concept of Wolf (1993, 1999). Hence this concept has established itself today as the major guide for the interpretation of BBF/auroral streamer observations.…”

Energy conversion regions as observed by Cluster in the plasma sheet

“…Instead, the main focus is on summarizing the observations of the ECRs and hint to their possible relation to BBFs. However, it should be noted that we have observed ECRs during varying geomagnetic and/or substorm activities, while many investigations concerning BBFs are related to disturbed times, even though there are exceptions [e.g., Pitkänen et al, 2011]. Figure 1 shows a schematic overview of some important energy conversion processes relevant for the Earth's plasma sheet.…”

“…As shown by Birn et al [2004], flow vortices appear at the flanks of the bubble, twisting the magnetic field, and causing a downward (upward) field-aligned current at the dawnside (duskside) flank, and forming a local wedge like current system [e.g., Wolf, 1993, 1999;Sergeev et al, 1996a;Birn and Hesse, 1996;Birn et al, 1999Birn et al, , 2004Snekvik et al, 2007;Zhang et al, 2009]. A return plasma flow may occur at the flanks of the bubble, and the corresponding shear against the plasma flow in the bubble main channel may also be involved in the generation of the field-aligned currents [e.g., Chen and Wolf, 1999;Kauristie et al, 2000;Birn et al, 2004;Keiling et al, 2009;Ohtani et al, 2009;Walsh et al, 2009;Panov et al, 2010aPanov et al, , 2010bBirn et al, 2011;Ge et al, 2011;Pitkänen et al, 2011].…”