Abstract
Abstract. Depleted flux tubes, or plasma bubbles, are one possible explanation of bursty bulk flows, which are transient high speed flows thought to be responsible for a large proportion of flux transport in the magnetotail. Here we report observations of one such plasma bubble, made by the four Cluster spacecraft and Double Star TC-2 around 14:00 UT on 21 September 2005, during a period of southward, but BY-dominated IMF. In particular the first direct observations of return flows around the edges of a plasma bubble, and the first observations of plasma bubble features within 8 RE of the Earth, consistent with MHD simulations (Birn et al., 2004) are presented. The implications of the presence of a strong BY in the IMF and magnetotail on the propagation of the plasma bubble and development of the associated current systems in the magnetotail and ionosphere are discussed. It is suggested that a strong BY can rotate the field aligned current systems at the edges of the plasma bubble away from its duskward and dawnward flanks.
Highlights
The exact mechanism by which magnetic flux and plasma are transported sunward through the Earth’s magnetotail plasma sheet has been an active topic of research since the Dungey Cycle of magnetospheric convection (Dungey, 1961) was first proposed
Plasma bubbles were first suggested by Pontius and Wolf (1990) as a possible solution to the so-called “pressure balance inconsistency”, whereby the adiabatic transport of magnetic flux and plasma earthward from the mid- to near-tail was shown to result in plasma pressures in the near-tail which are far too high to be confined by magnetic field intensities that are consistent with observed values (Erickson and Wolf, 1980)
In this paper we present observations from the four Cluster spacecraft and the Double Star TC-2 spacecraft of what we interpret to be a plasma bubble penetrating to within 8 RE of the Earth, provide direct evidence of the existence of the expected return flows around the flanks of the plasma bubble, report other features of the plasma bubble that are not present in simulations, and discuss the possible effects of a strong magnetotail BY on plasma bubbles
Summary
The exact mechanism by which magnetic flux and plasma are transported sunward through the Earth’s magnetotail plasma sheet has been an active topic of research since the Dungey Cycle of magnetospheric convection (Dungey, 1961) was first proposed. Because of its lower entropy content, the depleted flux tube cannot support as much gradient-curvature-drift current as neighbouring flux tubes and excess current is diverted into field aligned current systems at the boundaries of the bubble, flowing into the ionosphere on the dawnward boundary and out of the ionosphere on the duskward boundary, similar to the substorm current wedge concept. Further away from the equatorial plane, in lower density regions, the plasma pressure would remain low and pressure balance would be attained through an increase in the magnetic pressure This would set up a plasma pressure gradient within the flux tube, resulting in field aligned flows from the equatorial plane to the poles. In this paper we present observations from the four Cluster spacecraft and the Double Star TC-2 spacecraft of what we interpret to be a plasma bubble penetrating to within 8 RE of the Earth, provide direct evidence of the existence of the expected return flows around the flanks of the plasma bubble, report other features of the plasma bubble that are not present in simulations, and discuss the possible effects of a strong magnetotail BY on plasma bubbles
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