SPRG Seminars
November 15, 2011:
"Survey results for electron scale magnetic depletions in Earth's magnetotail following dipolarizations"
Mark Wilber, Space Science Laboratory, University of California, Berkeley
Recently, Ge et al. [2011] described observations of magnetic field depletions recorded by the THEMIS spacecraft in the magnetotail near midnight, following the passage of dipolarization fronts. These bore similarities to well-known mirror structures seen throughout the heliosphere, but exhibited key differences. The ambient plasmas in which these were observed were generally stable to the mirror mode, and their estimated sizes were only a fraction of a thermal (10 keV) plasma sheet ion gyroradius. The ambient plasma had a parallel electron temperature excess, but was nonetheless stable against the firehose instability as well. Inside the structures the electron distributions were nearly isotropic, with small perpendicular temperature excesses possible.
We present a first statistical picture of these structures, and case events from Cluster that confirm the small scales estimated previously. We have found that the ion distributions are insensitive to their presence, as might be expected for structures spanning only a fraction of an ion gyroradius. The range of temperature anisotropies observed suggests that the ambient plasma distributions alone do not drive instabilty. Important questions include: how are the driving conditions created within an ambient medium typically stable to the mirror and firehose instabilities? Are these structures formed further down tail under more favorable conditions and then convected to where we see them? There is evidence for electron distributions within some structures that differ from those in the ambient plasma by the presence of an extra ring component. This suggests that additional perpendicular electrons may have been injected into these flux tubes, creating a source of free energy. It is plausible that betatron heating of electrons during dipolarization provides a source for this extra component, which may subsequently gradient/curvature drift into the flux tubes where they are observed, although the localization of such particles is a mystery.