A case study of electron precipitation in the late substorm growth phase on and nearby a preonset arc

We follow the electron precipitation characteristics on and nearby a preonset arc using the high resolution Freja TESP instrument. Our data coverage extends from about 10 min before onset up to 1 min before onset. The arc is the most equatorward one (around MLAT 62 ) of a system of growth phase arcs, and it was close to the radiation belt precipitation. Within the preonset arc, inverted-V type precipitation dominates. Poleward of the arc we also ®nd some precipitation regions, and here there is systematically a cold electron population superposed with a warm population. Using single and double Maxwellian ®ts to the measured electron spectra we ®nd the ionospheremagnetosphere coupling parameters (®eld-aligned conductance K and the parallel potential drop V ) as well as the e€ective source plasma properties (density and temperature) during the event. Compared to typical expansion phase features, the preonset parallel potential drop is smaller by a factor of ten, the electron temperature is smaller by a factor of at least ®ve, and the ®eld-aligned conductance is about the same or larger. The fact that there are two isotropic superposed electron populations on the poleward side of the preonset arc suggests that the distance between warm trapped electrons on dipolar ®eld lines and colder electrons on open ®eld lines has become so small near the onset that mixing e.g. due to ®nite electron Larmor radius e€ects can take place.