Kinetic ballooning/interchange instability in a bent plasma sheet

Abstract

[1] We use Time History of Events and Macroscale Interactions during Substorms (THEMIS) and GOES observations to investigate the plasma sheet evolution on 28 February 2008 between 6:50 and 7:50 UT, when there developed strong magnetic field oscillations with periods of 100 s. Using multispacecraft analysis of the plasma sheet observations and an empirical plasma sheet model, we determine both the large-scale evolution of the plasma sheet and the properties of the oscillations. We found that the oscillations exhibited signatures of kinetic ballooning/interchange instability fingers that developed in a bent current sheet. The interchange oscillations had a sausage structure, propagated duskward at a velocity of about 100 km/s, and were associated with fast radial electron flows. We suggest that the observed negative gradient of the Z GSM magnetic field component (∂B Z /∂X) was a free energy source for the kinetic ballooning/interchange instability. Tens of minutes later a fast elongation of ballooning/interchange fingers was detected between 6 and 16 R E downtail with the length-to-width ratio exceeding 20. The finger elongation ended with signatures of reconnection in an embedded current sheet near the bending point. These observations suggest a complex interplay between the midtail and near-Earth plasma sheet dynamics, involving localized fluctuations in both cross-tail and radial directions before current sheet reconnection.

Further evidence for the role of magnetotail current shape in substorm initiation

“…A statistical study by Nowada et al (2009) showed that auroral electrojet indices reached higher values in the periods with smaller dipole tilt angles and thus, supported the idea by Kivelson and Hughes (1990) that larger number of smaller substorms occur in the periods of maximal dipole tilt. Additional evidence to revise the relationship between the substorm onset and tail configuration was provided by a number of case studies (e.g., Kubyshkina et al 2011;Sergeev et al 2011;Panov et al 2012, Ganushkina et al 2013, in which substorms were shown to be initiated in a curved magnetotail with excessively (due to additional external tilt) increased plasma sheet bending.…”

Current sheet flapping in the near-Earth magnetotail: peculiarities of propagation and parallel currents

“…Thus, the Cluster tetrahedron resolved the local (along the normal direction to the CS plane) gradients of magnetic field in flapping CS well but could not estimate variations in CS characteristics along the magnetotail. These variations, being weak in the mid-tail, should be stronger in the near-Earth region (see case studies in Nakamura et al, 2009;Panov et al, 2012a). The strong gradients of CS parameters along the x axis in this region are considered as potential drivers for CS instabilities resulting in flapping motion Pritchett and Coroniti, 2011;Panov et al, 2012b).…”

Profiles of electron temperature and <I>B</I><sub>z</sub> along Earth's magnetotail

“…So far, estimates of the gradient ∂/∂X in the magnetotail have been obtained by using three independent approaches: (1) small-scale gradients of the near-Earth dipolarized CS can be estimated using direct measurements of Cluster mission in 2007(Nakamura et al, 2009 or THEMIS mission in the case of specific spacecraft configuration (Saito et al, 2010;Panov et al, 2012); (2) statistical investigation can give average profiles of the main CS parameters (B z component, plasma pressure and plasma density) along the tail (see statistics collected by AMPTE/IRM, Geotail and THEMIS spacecraft in Kan and Baumjohann, 1990;Wang et al, 2012); and (3) thermal electrons could be used as Published by Copernicus Publications on behalf of the European Geosciences Union. tracers of the magnetotail structure (Artemyev et al, 2011c).…”