Interaction Between Oblique Whistlers and Reflected Ions in a Supercritical Quasiperpendicular Shock: Evidences from Themis

Abstract:

Whistler waves are characteristic of collisionless fast mode shocks. Recent Polar observations at Earth’s bow shock have revealed copious whistlers in the lower-hybrid frequency range [Hull et al., JGR, 2012]. The waves have wavelengths comparable to the ion inertia length and propagate obliquely to both the magnetic field B₀ and the shock normal. Earth’s bow shock being supercritical, reflected ions play a fundamental role and, in particular, determine its macrostructure which consists of a foot, a ramp, and an overshoot region. Here, we investigate the possible interaction between the oblique whistlers and the reflected ions in the shock’s foot. The problem considered is characterized by a double anisotropy: one defined by the direction of B₀, the other by that of the reflected beam. Let Ψ be the plane defined by these two directions, presumably the magnetic coplanarity plane. The Cerenkov resonance condition reads ω = k U_re sin θ_bk cos ψ_vk , where U_re denotes the drift of the beam of reflected ions seen in the solar wind frame, θ_bk is the angle between the wavevector and B₀, and ψ_vk is the angle between the wavevector and the plane Ψ.

We exploit Themis burst mode and its three-axis field measurements in order to simultaneously characterize the whistlers (wavevector direction and module) and the ion beam (velocity direction and module). The resonance condition above is then examined in this light. Modelling the ion beam, we use our electromagnetic dispersion code [see companion paper in this session] to compute the linear characteristics of the oblique whistlers that can be excited and compare them with the observations. Moreover, waveforms and hodograms obtained from an electromagnetic PIC simulation are also compared with Themis measurements.