The Bow Shock File. This file gives the orbit-by-orbit times andlocations of the bow shock crossings, which are characterized by distinctchanges in Ne. Multiple shock crossing are listed if they are sufficientlyseparated to be resolved accurately. (Bow shock crossings will be evidentin the High Resolution Ne File when they occurred within 30 minutes ofperiapsis).The ionopause and bow shock crossing times and locations are easilyidentified in the high resolution Ne measurements (Theis et al., 1980).These files contain the UT, altitude, latitude, SZA and local time of eachcrossing.The bow shock is a much more discrete feature in the data than theionopause. Multiple shock crossings sometimes occur because the shockoften moves at higher velocities than the satellite. In these cases, only theouter most shock crossing is recorded, unless the separation between thecrossings is greater than a minute or two. The occurrence of multipleshocks in the Bow Shock File provides a record of the orbits in which thesolar wind itself was probably highly variable. Because of the geometry ofthe orbit, most shock crossings were in the range of 45 to 135 deg. SZA.However, the nose region of the shock was explored between 1985 and1987 when PVO periapsis was near the equator and was at altitudesbetween 2000 and 2300 km. During these years near solar minimum thenose of the shock often moved down into that altitude range (Russell et al,1988). During the subsolar passages of these years, the orbitapproximately paralleled the shock, sometimes inside, sometimes outside,thus providing interesting snapshots of its movements.

The bow shock is selected from 200 minute pass plots by marking the UTof the sharp change in the amplitude of Ne at the shock discontinuity. Theresolution of the shock crossing time is of the order of 1 minute on theseplots, but this could be improved to a few seconds if expanded plots wereused. There is no plan currently to provide the ultimate resolutionavailable in bow shock crossing time and location.