Multiscale whistler waves within Earth's perpendicular bow shock

Abstract

We present observations of intense whistler waves made by Polar in the frequency range from a few Hz to 600 Hz within Earth's nearly perpendicular bow shock. The long duration burst data provided by Polar reveal the detailed properties of whistler waves in context with the macrostructure of the layer of this supercritical shock. We show that the pedestal and ramp have superposed quasiperiodic, large amplitude precursor substructure occurring at a cadence of ∼3 sec, which is near the ion cyclotron period. With increasing penetration into the shock front, the amplitude of this substructure increases and ultimately reaches downstream values. The nonlinear substructure is shown to be concentrated regions of intense whistler wave activity. Power spectra in the whistler range show strong enhancements in two distinct bands: a relatively broadband lower frequency component occurring near the lower hybrid frequency (a few tens of Hertz) and a higher frequency component at a few hundred Hertz. The lower frequency component is composed of right‐hand polarized whistler wave packets propagating quasiparallel to the magnetic coplanarity plane at oblique angles with respect to both the magnetic field and shock normal, with respective anglesθkb varying from 50°–70° and θkn ∼ 50°. These waves generally have relatively large amplitude (δB/B0∼ 0.1–0.4) magnetic fields ranging from a few nT to 15 nT. Given their preferential upstream propagation near the magnetic coplanarity plane, they are likely generated by a kinetic cross‐field streaming instability driven by the relative drift between the reflected ion beam and the electrons. The high‐frequency component appears to be the shock analog of whistler “lion roars” often observed in the magnetosheath. The lion roars occur within the foot and into the shock ramp in regions where sufficiently intense low‐frequency whistlers exist. These are right‐hand circularly polarized wave packets lasting up to ∼10 cycles, with amplitudes reaching 1 nT, and propagating nearly parallel or antiparallel with respect to the background magnetic field (θkb ⪅ 30° or θkb ⪆ 150°). These packets, which appear regularly with a cadence of a few tens of Hertz, occur within local magnetic field (and also electric field) minima of the lower frequency whistler waves. These waves are likely generated by the whistler anisotropy instability driven by preferential perpendicular anisotropies in the electron distribution function induced by the strong magnetic field of the ramp substructure. The growth rate for the instability is estimated at γ ∼ ωci, which is rather low. We suggest that the low‐frequency whistler waves play a key role in the growth and spatial distribution of the lion roar by forming a quasiperiodic series of magnetic field ducts (troughs) that confine their propagation nearly along the surface of the shock. This allows enough time for the lion roar to grow to observed amplitudes before they leave the region of growth. The low‐frequency waves may also be contributing to the growth of the lion roar by inducing enhancements in the electron anisotropy within magnetic troughs.