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. Plain Language Summary: The aurorae, the northern and southern lights, occur when charged particles are accelerated along the magnetic field and impact the atmosphere. Here we provide a new potential explanation for a process that can trigger the rapid release of stored energy in the nightside of near‐Earth space. Such rapid releases of energy lead to the development of very spectacular, bright, and dancing displays of the aurorae in a process known as a substorm. While the processes responsible for powering this are well understood, why the lights begin to dance exactly when then do, a moment known as the onset of the substorm, is not. It is generally agreed that this onset occurs after a period of stretching of the magnetic field on the nightside of Earth. Our new model suggests that as this stretching occurs, there is more pressure parallel to the magnetic field than across it in a confined region of the field. This leads to an instability, called a ballooning instability, that explosively releases energy in the magnetic field, leading to a substorm. The confined region of the field that our work predicts should be associated with onset agrees well with known locations where these bright substorm aurorae begin. Key Points: We propose a new paradigm for substorm onset, where a ballooning instability is triggered by pressure anisotropy becoming more parallelThis increasingly parallel pressure anisotropy occurs as a direct consequence of tail stretching during the substorm growth phaseOur paradigm predicts increasingly parallel temperature anisotropies pre‐onset, and onset locations in the transition region, as observed [ABSTRACT FROM AUTHOR]