Electrostatic and electromagnetic turbulence associated with the Earth's bow shock

Authors

  • Paul Rodriguez,

  • Donald A. Gurnett


Abstract

The electric and magnetic field spectral densities of plasma waves in the earth's bow shock have been measured in the frequency range 20 Hz to 200 kHz by using two 16-channel spectrum analyzers on the Imp 6 spacecraft. The electric field spectrum in the bow shock consists of two distinct components: one component has a broad peak typically centered between 200 and 800 Hz with an average (5.12-s time constant) spectral density at the peak of about 10−9 V² m−2 Hz−1, and the other component increases monotonically with decreasing frequency approximately as f−(2.0±0.5) and has an average spectral density of about 3.0 × 10−9 V² m−2 Hz−1 at 36.0 Hz. The magnetic field spectrum in the shock has only one component that increases monotonically with decreasing frequency approximately as f−(4.0±0.5) and has an upper cutoff frequency near the local electron gyrofrequency. This magnetic field spectrum appears to be associated with the monotonic component of the electric field spectrum. The electric to magnetic energy density ratio ϵEB of this noise is about 10−3 to 10−4, which is consistent with the energy density ratio expected for electromagnetic whistler mode waves in the bow shock. The broad peak in the electric field spectrum between 200 and 800 Hz has a large electric to magnetic energy density ratio, ∼10² to 10³, indicating that this component consists of almost purely electrostatic waves. Electrostatic noise with a spectrum similar to the turbulence in the shock but with lower intensities is observed throughout the magnetosheath region downstream of the shock. This magnetosheath electric field turbulence often includes many bursts with a distinct ‘parabolic’ frequency-time variation on a time scale of a few seconds. Spin modulation measurements of the electric field direction show that the electric field vectors in both the shock transition region and the magnetosheath region are preferentially oriented parallel to the static magnetic field direction. The electric field of upstream electron plasma oscillations also is oriented parallel to the static magnetic field.

Ancillary