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

摘要

We report observations of a sequence of quiet-time Earthward bursty bulkflows (BBFs) measured by the Cluster spacecraft in the near-tail plasma sheet(XGSM ~ ?12 to ?14 R_E) in the eveningsector, and by simultaneous high-resolution measurements in the northernconjugate ionosphere by the EISCAT radars, a MIRACLE all-sky camera andmagnetometers, as well as a meridian-scanning photometer (MSP) in theScandinavian sector on 17 October 2005.The BBFs at Cluster show signatures that are consistent with the plasma"bubble" model (Chen and Wolf, 1993,1999), e.g. deflection and compression of theambient plasma in front of the Earthward moving bubble, magnetic signaturesof 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 canbe identified. The duskside return flows are associated with significantdecrease in plasma density, giving support to the recent suggestion byWalsh et al.(2009) of formation of a depleted wake. However, the same feature isnot seen for the dawnside return flows, but rather an increase in density.In the ionosphere, EISCAT and optical measurements show that each of thestudied BBFs is associated with an auroral streamer that starts from thevicinity of the polar cap boundary, intrudes equatorward, brakes at68–70° aacgm MLAT and drifts westward along the proton oval. Withinthe streamer itself and poleward of it, the ionospheric plasma flow has anequatorward component, which is the ionospheric manifestation of theEarthward BBF channel. A sharp velocity shear appears at the equatorward edgeof a streamer. We suggest that each BBF creates a local velocity shear in theionosphere, in which the plasma flow poleward of and inside the streamer isin the direction of the streamer and southeastward. A northwestward returnflow is located on the equatorward side. The return flow is associated withdecreased plasma densities both in the ionosphere and in the magnetosphere asmeasured by EISCAT and Cluster, respectively. In summary, we present thefirst simultaneous high-resolution observations of BBF return flows both inthe plasma sheet and in the ionosphere, and those are in accordance with thebubble model. The results apply for the duskside return flows, but themanifestation of dawnside return flows in the ionosphere requires furtherstudies.Finally, EISCAT measurements indicate increased nightside reconnection rateduring the ~35-min period of BBFs. We suggest that the observedtemporal event of IMF rotation to a more southward direction producesenhanced open flux transport to the nightside magnetotail, and consequently,the nightside reconnection rate is increased.