A case study of electron precipitation in the late substorm growth phase on and nearby of a preonset arc by A. Olsson and P. Janhunen Submitted to Annales Geophysicae, 1997 ABSTRACT: We follow the electron precipitation characteristics of a preonset arc using the high resolution Freja TESP instrument. Our data coverage extends from about 10 minutes before onset up to one minute before onset. The arc is the most equatorward one (around mlat $62^{\circ}$) of a system of growth phase arcs, and it was close to the radiation belt precipitation. Within the preonset arc, the spectra are dominated by warm ($0.7-1.3$ keV) electrons accelerated by a field-aligned potential drop of about $2-3$ kV. On the poleward side of the preonset arc, there is a cold ($0.1-0.3$ keV) electron population superposed to a warm population. 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 field lines and colder electrons on open field lines is so small that mixing e.g. due to finite electron Larmor radius effects can take place. This view is also supported by a comparison with a global MHD simulation model. 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 five, and the Lyons-Evans-Lundin $K$ parameter is larger by a factor of two. The pressure ratio between the warm and cold electron populations is about ten. Thus there is a significant electron pressure gradient in the preonset region.