Modes of energy transfer from the solar wind to the inner magnetosphere

Abstract

Energy transport from the interplanetary plasma to Earth’s inner magnetosphere occurs in a range of time scales and efficiencies. It is often hypothesized that this range is smoothly varying with radial geocentric distance, indicating the transport involves many processes, whose ranges overlap. Here we report evidence from observations, and time series analysis, and other data-based modeling which indicates that the coupling of magnetospheric relativistic electron fluxes to solar wind variables occurs in specific ranges of radial distance (L shell). These findings probably have important consequences for the understanding of physical mechanisms responsible for the acceleration in each region. We identify three distinct regions: P0 at approximately 3<L<4 RE, P1 at 4<L<7 RE, and P2 at L>7 RE. Each one responds to a different combination of solar wind variables, and couples to the main driver variable, the solar wind speed VSW, in a different way. Mode P1 is the prototypical response of the inner magnetosphere. The electron flux responds more slowly than the other two regions to VSW (2–3 days): high-speed streams are the most geoeffective structures for that region. Mode P0 responds significantly faster (<1 day) and seems to be more affected by the negative Bz component of the interplanetary field (probably through magnetic reconnection) and the magnitude of the field, rather than by variations in solar wind plasma variables. Region P2 contains much lower fluxes of trapped particles than the other two, and responds rapidly (∼1 day) to positive Bz and to lower solar wind speed. The interpretation is that these regions are representative of different modes of energy transfer from the interplanetary medium to the inner magnetosphere with implications for very different particle acceleration mechanisms.