Does the ballooning instability trigger substorms in the near-Earth magnetotail?

Details Does the ballooning instability trigger substorms in the near-Earth magnetotail? Does the ballooning instability trigger substorms in the near-Earth magnetotail?

Nov 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993jgr....9819369o&link_type=abstract

Journal of Geophysical Research (ISSN 0148-0227), vol. 98, no. A11, p. 19,369-19,379

Physics

53

Ballooning Modes, Curvature, Geomagnetic Tail, Geomagnetism, Magnetic Field Configurations, Magnetohydrodynamic Stability, Magnetohydrodynamics, Plasma Pressure, Polar Substorms, Earth Magnetosphere, Field Aligned Currents, Magnetohydrodynamic Waves, Plasma Drift, Ring Currents, Solar Terrestrial Interactions

Scientific paper

The stability of the near-Earth magnetotail against ballooning (or configurational) instability is examined in the framework of the MHD approximation. It is emphasized that a change in plasma pressure induced by a meriodional electric field drift delta un is an important factor that determines the stability. We have to consider two ways in which plasma pressure changes, that is, a convective change -delta un grad(P0), where P0 is background plasma pressure, and plasma expansion/compression -P0 dive (delta un). Since delta un is perpendicular to the magnetic field and its magnitude is inversely proportional to the magnetic field strength, delta un diverges/converges in usual tail magnetic field configurations. For the instability, the convective change must overwhelm the effects of the plasma expansion/compression. However, near the equator in the near-Earth tail, the latter may over-compensate for the former. We describe the ballooning instability in terms of a coupling between the Alfven and slow magnetosonic waves in an inhomogeneous plasma and derive instability conditions. The result shows that the excessive curvature stabilizes, rather than destabilizes, perturbations. It is also found that the field-aligned flow stabilizes perturbations, as well as the field-aligned current. We infer that under quiet conditions, the plasma pressure gradient in the near-Earth tail is not sharp enough to trigger the instability. The plasma sheet is expected to become more stable during the substorm growth phase because of an increase in the field line curvature associated with the plasma sheet thinning. In the region closer to the Earth, including the ring current, the plasma pressure gradient may be localized in a limited range of the radial distance during the growth phase. However, recently reported plasma and magnetic field parameters before substorm onsets do not provide very convincing evidence that the ballooning instability is the triggering mechanism of substorms.

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