On the absence of plasma wave emissions and the magnetic field orientation in the distant magnetosheath

In early September, 1983 ISEE‐3 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 (θ xB ) between the magnetic field and the plasma flow velocity (x‐direction). For θ 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 θ xB .