Evolution of Energetic Proton Parallel Pressure Anisotropy at Geosynchronous Altitudes: Potential Role in Triggering Substorm Expansion Phase Onset

The sequence of events associated with the triggering of energy release during substorm expansion phase onset is still not well‐understood. Oberhagemann and Mann (2020b, https://doi.org/10.1029/2019gl085271) proposed a new substorm onset mechanism, where the transition toward parallel proton pressure anisotropy during tail stretching in the late growth phase could trigger a pressure anisotropic ballooning instability. Here we examine the evolution of energetic proton parallel pressure anisotropy at geosynchronous altitudes, seeking evidence in support of the proposed substorm onset mechanism. We use the Geostationary Operational Environment Satellite (GOES) proton flux and magnetometer data combined with substorm onset indicators derived from ground‐based magnetometers. Superposed epoch analysis of substorm onset times for 2014 using the isolated substorm list (Ohtani & Gjerloev, 2020, https://doi.org/10.1029/2020ja027902) clearly shows signatures of energetic proton parallel pressure anisotropy immediately before substorm onset, potentially supportive of the Oberhagemann and Mann theory. Plain Language Summary Substorms are disturbances in the nightside region of the geospace associated with the rapid release of stored magnetic energy. In the ionosphere, the signatures of this energy release are the spectacular dancing lights known as aurorae (northern and southern lights). The processes that lead to energy storage are well‐known. However, there are competing theories on what triggers the release of this significant amount of energy at substorm onset. According to a new substorm onset theory proposed by Oberhagemann and Mann, when the magnetic field stretches in the nightside during the energy storage, the pressure becomes more parallel to the magnetic field, leading to a ballooning instability at substorm onset. Here, we look for observational support for the association of such pressure profile at geosynchronous altitudes with substorm onset to examine the proposed model. Superposed epoch analysis of isolated substorms in 2014 shows increasing energetic proton parallel pressure anisotropy at the onset, providing evidence to support the Oberhagemann and Mann theory. Key Points Oberhagemann and Mann theory proposes that proton parallel temperature anisotropy triggers ballooning instability leading to substorm onset We use pitch angle resolved energetic proton fluxes at geosynchronous altitudes seeking observational evidence in support of the model Superpo