On the local time dependence of the bow shock wave structure

In the first 6 months after its launch, Ogo 3 crossed the earth's bow shock over 500 times. From this group a set of 494 shock crossings were chosen for analysis. These crossings, as they were recorded by the UCLA/JPL search coil magnetometer, were scanned and classified according to the nature of the plasma waves detected near the shock. More than 85% of the shocks detected fell into a single category showing the predominance of two independent wave trains near the shock, the higher frequency appearing upstream and the lower downstream. The other 15%, which constitute an upper limit, appear to be composed of shocks dominated by a single wave pattern and of chaotic shocks showing no orderly progression of wave frequencies as the shock was penetrated. This division of wave pattern was found to occur at all local times, that is, in all regions where the satellite penetrated the shock. Further, the center frequency and power spectra of the waves present in the dominant category were found to vary as widely from crossing to crossing on one orbit as they did across the face of the shock surface as the satellite moved from dawn to dusk. To the extent that the waves present near the front indicate the shock dissipation process, it appears that there is little local time dependence of the shock structure in comparison with those changes in structure that occur on time scales of tens of minutes.