The Energy Partition of Substorm Growth--Phase Arcs

During a substorm growth phase, auroral arcs observed on the ground are known to migrate southward. This motion is thought to be associated with a stretching of the nightside magnetotail as a result of an enhanced transfer of solar wind energy to the magnetosphere. Following slow growth, substorm onset is then observed as a brightening of the pre-existing arc. The is evidence for a region with enhanced coupling between the magnetosphere and ionosphere that exists before onset. Recent analysis of data from FAST and CANOPUS has shown that these arcs can persist as stationary arcs or, at times, can exhibit ULF fluctuations. In the few studies of energy fluxes associated with auroral arcs, results show that a distinct spatial boundary exists between Poynting fluxes and particle energy fluxes, implying that the energy flux at high altitudes consists of only Poynting flux and that conversion to particle energy flux occurs at some lower altitude. In this study, we investigate the partition of energy fluxes associated with growth--phase arcs by Fourier analyzing FAST data to determine the spatial dependence of the partitioning. We then use this information to estimate the altitude at which the conversion takes place and discuss implications for substorm onset.