MAGION-4 observations of bow shock crossings

The bow shock is a highly dynamic boundary controlled by steady and transie nt variations in solar wind parameters. During the time interval from Augus t 1995 to August 1997, the MAGION-4 satellite (subsatellite of INTERBALL-1) observed more than 850 bow shock crossings. Due to the high inclination an d temporal evolution of its elliptical orbit, the MAGION-4 data set covers a broad range of latitudes and local times. The set of MAGION-4 bow shock c rossings was completed with solar wind and interplanetary magnetic field (I MF) observations to determine upstream conditions. These data were taken fr om the WIND observations. The time of propagation of the solar wind feature s from the WIND position to the bow shock was computed as a two-step approx imation based on the measured solar wind velocity. The values of the solar wind dynamic pressure and IMF used for the comparison with models were comp uted as five-minute averages centered around the time of the crossing, dete rmined as given above. As a measure of the uncertainty of the determination of the solar wind parameters relating to the particular crossing, we have computed the dispersion of each parameter. The solar wind dynamic pressure varied from 1 to 12nPa and the Alfvenic Mac h number from 4 to 50. These observations provide an excellent opportunity to test empirical models describing the bow shock position as a function of upstream parameters. We have calculated the predicted bow shock position f or various models and then determined the distance between it and the obser ved locations. We have tested influence of the solar wind and IMF fluctuati ons, solar wind aberration, Mach number (M-A), and magnetic field strength on the precision of the mentioned models. The results show the uncertainty of the prediction of the bow shock position cannot be explained by errors i n the determination of upstream parameters, and that either additional para meters or new interactions should be incorporated into the model.