Standing waves at low Mach number laminar bow shocks. [earth-solar wind interaction

Explorer 43 data have been used to study 34 bow shock crossings observed from 5 to 16 RE upstream from the average bow shock location. These shocks have magnetosonic Mach numbers between 1.2 and 2.0, and they display particularly simple laminar structures. In 17 of these cases waves are detected adjacent to and upstream of the shock with amplitudes ranging up to ΔB/B ≃ 1 and period ranging from 6 to 130 s. A variance analysis of the magnetic field data reveals that the vectors of these waves are oriented along the shock normals. The wave polarization relative to the average field direction is right-handed when the crossing is from the interplanetary medium to the magnetosheath but is left-handed when the crossing is made in the opposite direction. On the basis of these observations the waves are identified as the standing whistler waves that have been predicted by the theory of low Mach number oblique shocks. The observed amplitude and the occasional lack of waves are in general agreement with theory, which predicts that decreasing amplitudes should correspond to larger field-shock normal angles and lower plasma betas. Variations in the observed frequencies can be explained by changes in the wavelength of the standing wave as upstream parameters change. Shock velocities calculated from theoretical wavelengths typically range from 10 to 30 km/s, but in one instance the velocity was 150 km/s.

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