On the formation of pre‐onset azimuthal pressure gradient in the near‐Earth plasma sheet

Xing et al. (2011) demonstrated by case study that the plasma sheet pressure gradient at ∼11 R E near the substorm onset meridian undergoes a substantial duskward enhancement shortly before substorm onset as identified from the auroral poleward expansion. It was suggested that the increased upward FAC driven by this pressure gradient enhancement lead to the thin onset arc intensification from which the poleward expansion initiates. In the present study, we employ the multiTHEMIS spacecraft in azimuthal conjunction ‐at ∼−11 R E and examine the ion flux and distributions during the period of pressure gradient enhancement, and identify two categories of events. For events with pressure gradient change ≤2 min prior to onset, strong earthward ion flux enhancements covering the energy range from several keV to above 25 keV were observed by the spacecraft identifying the higher pressure increase, while at the same time the ion distributions show a substantial earthward shift in velocity space. These resemble the ion acceleration ahead of earthward moving dipolarization fronts. On the other hand, the spacecraft observing the lower pressure increase found weaker or no ion flux enhancements and had nearly isotropic distributions, indicating that longitudinally localized dipolarization fronts are responsible for the pressure gradient development. These pressure gradient enhancements were in the duskward (dawnward) direction for measurements to the east (west) of the onset region. For pressure gradient enhancement >2 min prior to onset, stronger plasma sheet thinning was observed by the spacecraft identifying the larger pressure increase and the ion distributions appear nearly isotropic, which suggests plasma redistribution in the undisturbed plasma sheet. These two categories suggest that both dipolarization fronts and growth phase plasma sheet localized thinning can drive azimuthal pressure gradient enhancement near the onset meridian and cause pre‐onset thin auroral arc intensification, though the dipolarization front events appear to be substantially more common based on our 14 events. Key Points Dipolarization front precursor may lead to short pre‐onset pressure gradient Dipolarization is the dominant trigger in growth phase plasma redistribution Pre‐onset flow channel may be a crucial trigger for substorm onset