The small‐scale structure of electrostatic shocks

Small-scale regions of large electric fields have been observed above the auroral zone by the S3-3 satellite. The data from five such electrostatic shocks have been examined in as great a detail as is available. The three higher altitude shocks (all above 5700-km altitude) were associated with upward-going ion beams, indicating that the potential associated with the shock closed below the satellite to give rise to the parallel electric field required for the acceleration of the ion beam. Electrostatic ion cyclotron waves were found in all these cases adjacent to the shock and extending throughout the upward-going ion beam region. The lack of noticeable Doppler shift in the electrostatic ion cyclotron waves in association with large convective drift velocities indicates that the wavelength of the electrostatic ion cyclotron wave can be several kilometers and the potential difference within the wave can be the order of 100 V. Regions having anisotropic electron distributions with the signature of parallel electric field acceleration close to but above the satellite were found on the other side of the shock from the ion beam region in two of these examples. Both these regions were associated with V shaped VLF hiss. In contrast, lower altitude shocks were associated with VLF saucers and conical ion distributions. The association of the lower altitude shocks with VLF saucers indicates that the low-altitude shock region is a permanent feature on the scale of tens of seconds and that the low-altitude shock region extends along the magnetic field. Since they occur in different current regions, the low-altitude shocks described here are not merely the low altitude extension of the kind of higher altitude shocks associated with ion beams. Electrostatic shocks are always embedded in broader regions of low frequency turbulence. The shock itself is a complex low-frequency structure that, if interpreted as a Doppler shift effect, implies a complicated small scale spatial structure. The important theoretical question of whether the parallel electric field inside the electrostatic shock exists over a large region of space parallel to the magnetic field or is confined to a narrow double layer or similar structure is not answered. The data are consistent with both possibilities.

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