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A New Substorm Onset Mechanism: Increasingly Parallel Pressure Anisotropic Ballooning

Recent observations demonstrating that auroral beads are observed prior to the majority of substorm onsets (Kalmoni et al., 2017, https://doi.org/10.1002/2016GL071826) have reinforced the potential importance of ballooning instabilities for near‐Earth magnetospheric substorm onset. Here we examine pressure anisotropic ballooning instabilities in stretched magnetotail geometries. Our results show that transition from an initially perpendicular anisotropy toward parallel anisotropy reduces the plasma β threshold for triggering a ballooning instability. Such increasingly parallel anisotropies can form as a direct consequence of tail stretching that occurs during the late substorm growth phase through the well‐known effects of drift shell splitting and the competition between betatron and Fermi processes in the tail. We propose such increasingly parallel ballooning triggers auroral substorm onset on field lines in the transition region between dipolar and tail‐like fields, consistent with observational constraints on the location of the onset arc with respect to the ion isotropy boundary in the magnetotail.