H E Spence, N U Crooker, and H E Petschek
Boston University Center for Space Physics, Boston, MA, 02215
617-353-7412
spence@bu.edu

We investigate properties of magnetic field merging between the interplanetary magnetic field (IMF) and the geomagnetic field in regions poleward of the magnetospheric cusp. Recent analysis of POLAR energetic particle measurements reveals a hot population of ions seen frequently at high magnetic latitudes when the spacecraft is near apogee ($\sim 8 R_E$ altitude). The polar energetic particle (PEP) events occur at locations traditionally thought to be connected magnetically with the open fields in the polar cap. We propose that PEPS may be a reliable signature of high-latitude merging. The occurrence of PEPs is controlled strongly by the orientation of the vector IMF. Preliminary analysis indicates that PEPs occur principally when the IMF has either a strongly northward or radially outward field. The occurrence probability maximizes when both conditions are present. We compare the dependence of PEP occurrence on IMF orientation (measured concurrently by the WIND, IMP8, and/or GEOTAIL spacecraft) to theoretical models of merging poleward of the cusp. For example, we investigate the dependence of hemispheric asymmetry in lobe merging probability on not only the sign of the field components but also their relative magnitudes, i.e. angle in the x-z plane. Finally, we discuss aspects of PEP spatial and spectral structure that may be diagnostic of the merging processes occuring remotely on these high latitude flux tubes.