Ballooning stability of the Earth's magnetosphere
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A substorm is a frequently occurring ordered sequence of global energetic events in the magnetosphere and ionosphere. The most obvious manifestation of a substorm is auroral brightening, due to an increase in the number of electrons precipitating into the ionosphere. During a substorm, quiet auroral arcs suddenly intensify. Electrojets are produced in the ionosphere, and magnetic disturbances are observed on the surface of the Earth with magnetometers. Plasma instabilities in the inner magnetosphere, in the region where the nightside auroral magnetic field lines cross the magnetic equator, are a candidate to explain the triggering mechanism of these substorms. This region is at a distance of about ten times the radius of the Earth in the midnight sector. In this region the plasma thermal pressure gradient reaches its highest value and the plasma is susceptible to unstable motions. We have investigated the linear stability of the inner magnetosphere against fast interchange-ballooning dynamics as a possible candidate for the magnetospheric substorm trigger, using different models of the quiet-time magnetospheric magnetic field. The region most likely unstable to these dynamics is found to map to the lower edge of stable auroral arcs. We then extend the ideal fast-MHD analysis to include local gyrokinetic effects such as waveparticle resonances, in order to explain the low-frequency oscillations that are observed prior to and during the onset of substorms. We generalize the local kinetic analysis to include non-local orbital effects due to the mirroring motion of the particles between the Earth’s magnetic poles. We developed a new numerical technique to solve the resulting non-linear integral eigenvalue equations. We also investigated a magnetic compressional trapped particle instability in detail and obtained the conditions for instability as well as the growth rates and mode structures. We invoke this low-frequency drift wave instability to explain compressional Pi2 oscillations observed throughout the substorm onset period and argue that they play an important role in substorm dynamics.