ABSTRACT

The nature of particle precipitation on closed and open magnetic field lines is quite different, but the transition that the precipitation undergoes when a magnetic field line is first converted from a closed field line to an open one is not well understood. There are several important stages as particle precipitation changes character across the open/closed magnetic field boundary (OCB) which need to be documented, investigated, and understood, because the specifics of these changes have important geophysical consequences. This project will develop a detailed chronology of the transition of particle characteristics across the OCB, based on phenomena such as the low-energy ion cutoff, and basic calculations of electron and ion bounce times. The result of all this will be to provide a toolkit for placing particle observations near the OCB in context.

Among the detailed aspects of the transition that will be studied is the drop-out in high-energy magnetospheric ions. Loss at the magnetopause with partial reflection would lead to the highest energy ions disappearing first, while resupply from closed field lines would lead to the highest energy ions lasting longest. The relative importance of these two processes is not yet known. Other issues involve the high-energy magnetospheric electrons, which may more closely follow the convection reversal boundary rather than the OCB. Whether the energetic electrons are energy dispersed across the OCB, and whether the energetic electrons can co-exist with magnetosheath electrons for a time inconsistent with flux tube emptying is not yet known. Other little discussed effects involve the appearance of a "shadow" electron precipitation equatorward of the main cusp (before ions arrive). This population resembles the polar rain or "strahl" electrons. The size of this shadow region varies from virtually non-existent to more than 1 degree, for reasons unknown. All these changes in particle characteristics across the OCB seem to have systematic norms and variants, which may provide important clues for quantitative modeling. The results will also be of use in investigating the extent to which the low latitude boundary layer is open or closed.

This project will also continue to maintain a Defense Meteorological Satellite Program (DMSP) resource, distributing particle data through the World Wide Web. The web site (http://sd-www.jhuapl.edu/Aurora) provides researchers around the world with a valuable tool. Because of the ease of access and aids provided at the web site, graduate students and postdoctoral fellows have been heavy users of the system.

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