STEREO and wind observations of intense electron cyclotron harmonic waves at the earths bow shock and inside the magnetosheath

Abstract

We present the first observations of electron cyclotron harmonic waves at the Earth's bow shock from STEREO and Wind burst waveform captures. These waves are observed at magnetic field gradients at shocks ranging from quasi-parallel to nearly perpendicular along with whistler mode waves, ion acoustic waves, and electrostatic solitary waves. We also observe similar waveforms in the magnetosheath in association with magnetic field gradients convected past the bow shock. Amplitudes of the electron cyclotron harmonic waves are typically a few 10s of mV/m but are seen up to more than 500 mV/m peak-peak. Wavelengths are estimated to be not significantly larger than 100 m, consistent with the electron cyclotron radius. Waveforms are broadband and polarizations are distinctively comma-shaped due to significant wave power both perpendicular and parallel to the magnetic field. Harmonics are observed to be more prominent in the perpendicular directions. These observations indicate that the waves consist of a combination of perpendicular Bernstein waves and field-aligned waves without harmonics. A likely source is the electron cyclotron drift instability, which is a coupling between Bernstein and ion acoustic waves. These waves are the most common type of high frequency wave seen in the STEREO dataset during bow shock crossings and magnetosheath traversals. Our observations suggest that they are an important component of the high frequency turbulent spectrum in these regions.