Magnetopause characteristics at 0840–1040 hours local time

We have analyzed three‐dimensional energetic (≳24 keV) particle distributions for 14 consecutive inbound ISEE orbits during times of magnetopause crossings and close approaches. The data are from the November 10 through December 11, 1977, time period and cover local times of ∼0840–1040 hours. We find that in this period the magnetopause as defined by energetic particles can be represented as a sharp (very much less than the ion gyroradius) and very well defined boundary for magnetospherically trapped particles for a wide range of magnetospheric activity and magnetosheath field conditions. Using the technique described by Williams (1979) we have determined the magnetopause position, orientation, and velocity for all identified magnetopause crossings and close approaches. We find that the magnetopause is nearly always in motion with velocities ranging from near zero to ≳±25 km/s. Large‐amplitude (>1000 km) radial motions often are present and at times are quasi‐periodic. Superimposed on these large‐scale motions is the nearly continuous presence of orientation angle variations consistent with boundary waves having periods of ∼100–450 s and amplitudes of a few hundred kilometers. During a rare interval when the boundary orientation was steady, large magnetopause radial motion was observed. Conversely, during an interval of near‐zero magnetopause velocity, orientation angle variations were observed. Correlations with published plasma, magnetic field, and plasma wave magnetopause identifications show the energetic (≳24 keV) particle results to be accurate and an important factor in determining magnetopause characteristics and behavior. Comparison of our results with published plasma data for the November 10, 1977, inbound magnetopause encounters yields a scale length ( e ‐folding distance) for magnetosheath plasma penetration into the magnetosphere of ∼60–90 km for electrons and ∼90–110 km for protons.