Increasingly Parallel Pressure Anisotropic Ballooning: Substorm Growth Phase Drift Orbits Resulting in a Locally Ballooning Unstable Geomagnetic Tail

Recent work by Oberhagemann and Mann (2020, https://doi.org/10.1029/2019GL085271) has shown that ballooning instabilities in the transition region between dipolar and taillike fields associated with substorm onset may be triggered by an anisotropic pressure distribution that moves from an initially mild perpendicular anisotropy toward more parallel anisotropy. Here we examine the mechanism which can result in this transition toward more parallel pressure during growth phase tail stretching. We trace particles through a two‐dimensional model magnetic field to examine the evolution of particle distributions as they periodically drift through a tail which changes from a less stretched, quiet time to a more stretched late growth phase configuration. Our findings show that these particle pitch angle distributions become more parallel anisotropic through a combination of drift shell splitting and preferential de‐energization in the perpendicular direction. Drift shell splitting causes low pitch angle particles from a high‐pressure region to mix with high pitch angle particles from a low‐pressure region in a radially localized area in the tail transition region where high earthward pressure gradients exist. Preferential de‐energization occurs due to a transfer of particles from less stretched to more stretched field lines that reverses the usual balance between Fermi and betatron acceleration and causes particle pitch angle distributions to become more field aligned. Our model predicts onset characteristics and location in good agreement with observations, with the most unstable region consistent with the location of the auroral beads, which accompany the onset of the expansion phase of many substorms. We examine the substorm onset paradigm of increasingly parallel pressure anisotropic ballooning using a growth phase model magnetotailDuring tail stretching, drifting energetic particle distributions develop more parallel anisotropy, increasing ballooning instabilityThe results can explain why onset occurs when it does, an observationally testable hypothesis, with forecasting potential