ON THE ABSENCE OF PLASMA-WAVE EMISSIONS AND THE MAGNETIC-FIELD ORIENTATION IN THE DISTANT MAGNETOSHEATH

In early September, 1983 ISEE3 made a long traversal of the distant dawnside magnetosheath starting near x = -150 R(E) downstream. The distant magnetosheath often contains moderately intense plasma wave emissions at frequencies from several hundred Hz to 5 kHz. However, over time scales of many days, a clear correlation exists between the occurrence of the plasma waves and the cone angle (theta(xB)) between the magnetic field and the plasma flow velocity (x-direction). For theta(xB) large (small), the plasma wave amplitudes are near background (high). Sudden(< 1 minute) changes in the local magnetic field orientation produce correspondingly sudden changes in the wave amplitudes. Statistically, the wave amplitudes decrease continuously with increasing theta(xB).