1. Relating the Equatorward Boundary of the Diffuse Redline Aurora to its Magnetospheric Counterpart Grant, Jeff 1 ; Donovan, Eric 1 ; Spanswick, Emma 1 ; Jackel,Brian 1 1. Institute for Space Imaging Science, Space Physics University of Calgary, Calgary AB, CAN ABSTRACT THE FAST MISSION REFERENCES CONCLUSIONS Auroral boundaries are the ionospheric footprint of sharp gradients in magnetospheric properties. Examples include the poleward border of the diffuse “redline” (630nm) aurora and equatorward boundary of the proton aurora which have been shown to be excellent proxies for the ionospheric projections of the open-closed and ion isotropy boundaries, respectively. Recent ground-based ASI and MSP observations have shown that there is often an arc collocated with the equatorward boundary of the diffuse redline aurora (onset arcs for example are frequently located at this boundary). Even a cursory consideration of the location of the equatorward boundary of the redline aurora relative to that of the proton aurora leads us to the conclusion that the redline equatorward boundary is not the ionospheric projection of the earthward edge of the electron plasma sheet. In this study, we use ground-based optical, together with in situ topside ionospheric and magnetospheric plasma observations to identify the magnetospheric feature that most likely corresponds to the equatorward boundary of the redline aurora. Launched in 1996, the Fast Auroral Snapshot (FAST) crosses the northern auroral oval twice for every orbit and collects high resolution data, “snapshots”, in these regions. Its instruments include electrostatic analyzers (ESA), electric field sensors and magnetometers. The path of the satellite is primarily meridional, thus crossing the auroral oval transversely and making it ideal to study auroral boundaries. In this study, we use ESA ion and electron data from FAST available from CDAweb. The data set we use consists of FAST orbits between 1998 and 2001 that have primarily meridional orbit footprints and occur at a pre-midnight magnetic local time. Further criteria included orbits with a well defined isotropy boundary and a clear inner edge of the electron plasma sheet, lowering the number of orbits to approximately 300. The above figure shows the location of each orbit in magnetic latitude and magnetic local time. IDENTIFYING BOUNDARY LOCATIONS The equatorward ion, b2i and polar cap boundaries are easy to identify using ion energy data (left above). The inner edge of the electron plasma sheet is more difficult to determine as it is energy dependent. We consider five energy intervals (2, 5, 10, 20 and 30 keV) of electrons and look for the location where the flux drops exponentially to approximately 1/e of its flux in the plasma sheet region [1]. BOUNDARY DISTRIBUTIONS [1] Frank, L. A. (1971), Relationship of the Plasma Sheet, Ring Current, Trapping Boundary, and Plasmapause near the Magnetic Equator and Local Midnight, J. Geophys. Res., 76(10), 2265– 2275. FUTURE WORK Extending this process to all FAST data is essential for creating a complete data set. Ideally, we would like to use this data to determine the dependence of these boundaries on the state of the solar-terrestrial system. The variations in these boundaries will be compared to the variations in the magnetosphere boundaries to determine any correlation. The end goal is to establish the magnetospheric counterparts to these ionospheric boundaries. BOUNDARY DISTRIBUTIONS