Electrostatic noise at the plasma frequency beyond the Earth's bow shock

TLDR

The authors attribute the observed electrostatic noise to a two‑stream instability driven by a finite‑time‑of‑flight induced double‑humped electron distribution, and they characterize the resulting spectra and amplitudes in the downstream run‑out region. Electrostatic noise at the solar‑wind plasma frequency is detected only when interplanetary magnetic field lines connect to the bow shock, with maximum amplitude along tangent lines, and the observed wave amplitudes—sufficient to generate first‑ and second‑harmonic electromagnetic waves—confirm a two‑stream instability that fills the shock flanks with electrostatic waves.

Abstract

Electrostatic noise at or near the ambient solar wind plasma frequency, detected by the University of Minnesota plasma waves experiment aboard the Imp 6 satellite beyond the earth's bow shock, is correlated with times when the interplanetary magnetic field probably connects to the shock. The results of this analysis are (1) the noise is present when the field lines connect to the shock, and no noise at the plasma frequency is observed when the field lines do not connect and (2) the noise amplitude is largest along field lines that are tangent to the shock. The measurements are consistent with a two‐stream instability mechanism. The necessary double‐humped distribution function is probably produced by a finite time of flight mechanism; that is, of the energetic electrons generated in the shock, the lower‐energy electrons are swept downstream by the solar wind convection field, leaving a higher‐energy bump on the distribution function. A theoretical discussion shows that the amplitude of the instability is probably limited by the oscillating two‐stream mechanism in the sunward part of the unstable region and by quasilinear diffusion in the downstream part. The amplitude is sufficiently large to generate electromagnetic waves at both the first and the second harmonic of the plasma frequency, and these waves are observed. The instability then leaves a large region, on the flanks of the bow shock, which is filled with electrostatic waves. Spectra and amplitudes in this ‘runout region’ are given and discussed.