Magnetospheric location of the equatorward prebreakup arc

Abstract

We address the long-standing problem of the location and origin of the equatorwardmost Pre-Breakup auroral Arc (PBA) by combining energetic particle observations from NOAA Polar Operational Environmental Satellites (POES) overpasses of prebreakup arcs with auroral imaging and magnetospheric observations from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission. The prebreakup arc was observed within a few minutes of auroral breakup and ∼1-2 hours in MLT from the breakup meridian. For three ideal conjunctions out of 16 PBA crossings, we also construct a dynamically-adapted magnetospheric model after adding concurrent magnetic observations by the GOES spacecraft. Model-predicted isotropy boundaries of energetic particles are compared with observations, informing us about model uncertainties. Direct mapping with adapted models as well as particle flux comparisons between the ionosphere and the magnetosphere confirm that the PBA source lies within the region of the steep equatorial magnetic field gradient, where the equatorial field is also small (5-20 nT). That equatorial location, at roughly 8-10 Re, is likely the earthwardmost edge of the thin cross-tail current sheet. From observations we find that the prebreakup arc nearly coincides in latitude with the energy-dispersed, 30-300 keV electron isotropy boundary. Here the non-adiabatic electron precipitation from the high flux region of the outer radiation belt near its outer edge produces a narrow, intense energetic electron precipitation region, called the energetic electron arc (EEA). Two fortuitous conjunctions with DMSP also confirm that energetic (>20 keV) EEA electrons and lower energy, inverted-V electrons associated with the PBA are collocated. We suggest that EEA formation is an inherent part of the PBA formation process. By creating an enhanced conductance strip, the EEA (a seed arc) produces ionospheric polarization that leads to field-aligned current generation and associated field-aligned electron precipitation. We also discuss implications of our findings for the substorm onset mechanism.