STEREO and Wind observations of intense cyclotron harmonic waves at the Earth's bow shock and inside the magnetosheath

Abstract

[1] 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 a variety of shock geometries ranging from quasi-parallel to nearly perpendicular along with whistler mode waves, ion acoustic waves, and electrostatic solitary waves. Large amplitude cyclotron harmonic waveforms are also observed in the magnetosheath in association with magnetic field gradients convected past the bow shock. Amplitudes of the cyclotron harmonic waves range from a few tens to more than 500 mV/m peak-peak. A comparison between the short (15 m) and long (100 m) Wind spin plane antennas shows a similar response at low harmonics and a stronger response on the short antenna at higher harmonics. This indicates that wavelengths are not significantly larger than 100 m, consistent with the electron cyclotron radius. Waveforms are broadband and polarizations are distinctively comma-shaped with significant power both perpendicular and parallel to the magnetic field. Harmonics tend 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 by STEREO during bow shock crossings and magnetosheath traversals and our observations suggest that they are an important component of the high-frequency turbulent spectrum in these regions.

Quantified energy dissipation rates in the terrestrial bow shock: 2. Waves and dissipation

“…The long duration of the THEMIS waveform captures compared to those observed by Wind and STEREO [e.g., Breneman et al, 2013] showed that these fluctuations could remain enhanced for >10 s. We also note that the observation of these high-frequency waves was not limited to quasi-perpendicular geometry (e.g., see Table I:1 in Paper I). Thus, we conclude that large-amplitude high-frequency electromagnetic fluctuations are ubiquitous in quasi-parallel and quasi-perpendicular collisionless shocks, consistent with previous results at interplanetary shocks [e.g., Wilson et al, 2007] and the bow shock [e.g., Breneman et al, 2013].…”

“…ECDI, IAWs, and trains of ESWs (similar to those shown in Figures II:5-II:7) were observed in nearly every bow shock crossing we examined with not only THEMIS, but Wind [e.g., Breneman et al, 2013] and STEREO [e.g., Breneman et al, 2013]. The amplitude of these fluctuations ranged from approximately tens of mV/m to >300 mV/m.…”

“…Large localized currents are important because they can act as a source of free energy for numerous plasma instabilities [e.g., Lemons and Gary, 1978]. Sagdeev, 1966;Gary, 1981;Treumann, 2009] and previous observations [e.g., Wilson et al, 2007;Breneman et al, 2013]. These results are primarily for high-frequency electrostatic waves.…”

“…Previous studies focusing on electric field observations of some of the higher-frequency modes observed amplitudes exceeding ∼200 mV/m [e.g., Breneman et al, 2013;Hull et al, 2006;Mozer and Sundkvist, 2013;Wilson et al, 2007. Recent Vlasov simulations using realistic mass ratios [e.g., Petkaki et al, 2006;Petkaki and Freeman, 2008;Lui, 2006, 2007] show that the effective collision frequencies, wave-particle interactions, previously estimated from quasi-linear theory are ∼2-3 orders of magnitude too small.…”

“…In addition, previous studies of collisionless shocks have found any combination of the following electromagnetic and electrostatic fluctuations: (1) magnetosonic whistler waves [e.g., Hull et al, 2012;Sundkvist et al, 2012;Wilson et al, 2009Wilson et al, , 2012, (2) high-frequency electromagnetic whistler mode waves [e.g., Hull et al, 2012;Wilson et al, 2013a], (3) trains of electrostatic solitary waves (ESWs) or electron phase space holes [e.g., Bale et al, 1998Bale et al, , 2002, (4) ion-acoustic waves (IAWs) [e.g., Fuselier and Gurnett, 1984;Balikhin et al, 2005;Wilson et al, 2007], and/or (5) the electron cyclotron drift instability (ECDI) [Breneman et al, 2013;. Note that the ECDI is an instability, not a wave.…”