Configuration of high-latitude and high-altitude boundary layers

We report comprehensive statistical results for 4 years of Hawkeye observat ions totaling 1757 boundary crossings, Our objective is to use the full set of Hawkeye plasma, magnetic field, and plasma wave data to identify every cusp-associated data interval (entry layer, cusp, plasma mantle), to spatia lly map these regions, and to isolate the primary variables affecting their occurrence frequency and location. We find that SM coordinates best order the angular position of cusp region data intervals and that GSM coordinates are better for ordering in radial distance, especially for the plasma mant le. Dipole tilt and external pressure are the primary variables affecting h igh-latitude and high-altitude boundary configuration. Compared to these, t he effects of IMF parameters are minor although discernable when adequate c orrections are made for dipole tilt and pressure. New results on cusp flari ng and indentation of the high-latitude boundary are obtained by limiting t his data set in pressure, varying dipole tilt ranges, acid examining change s in boundary configuration in both SM and GSM coordinates. We find that as the dipole tilts more toward the oncoming magnetosheath plasma flow, inden tation becomes enlarged and the cusp outflow region, the plasma mantle, bec omes more flared out relative to the Earth-Sun line. In contrast, as the di pole lilts away from the Sun direction, cusp indentation is reduced but the cusp outflow region remains flared out compared to boundary shapes inferre d from low-latitude observations. A semiempirical magnetopause model by Boa rdsen et nl. [this issue] compares very well with the high-latitude boundar y layer observations reported here including a test for hemispheric symmetr y, which is assumed by the model.