Steepened magnetosonic waves at comet Giacobini‐Zinner

Abstract

We examine intense hydromagnetic waves at comet Giacobini‐Zinner to investigate the mode and direction of wave propagation and thereby provide important constraints on potential mechanisms for wave origin in the vicinity of the comet. The character of the wave polarization changes from basically elliptical far from the comet to almost linear followed by either a rapid partial rotation (less than 360°) or multiple rotations (high‐frequency wave packets) in the vicinity of the bow wave. Simultaneous high‐resolution measurements of electron density fluctuations demonstrate that the long‐period (∼100 s) waves are propagating in the magnetosonic (fast MHD) mode. Principal axis analyses of the long‐period waves and accompanying partial rotations show that the sum of the wave phase rotations is 360°. This indicates that both are parts of the same wave. The change in polarization characteristics near the comet is simply a consequence of wave steepening. From the time sequence of the steepened waveforms observed by ICE, we demonstrate that the waves must propagate in the general direction along the magnetic field toward the Sun with phase velocities less than the solar wind speed. They are consequently blown back across the spacecraft and observed with a left‐hand sense of rotation. All available observations are therefore consistent with wave generation by the right‐hand resonant ion ring instability which predicts waves propagating in the ion beam (solar) direction. Based on the Wu‐Davidson linear growth rate expression, the total convective gain is shown to exhibit a weak (logarithmic) dependence on distance from the comet reaching a maximum of 30 e‐folding in the vicinity of the bow wave. Conditions necessary for the origin of multiple rotations currently are not well understood. Arguments for their being standing whistler waves consisting of a partial rotation plus multiples of 360° rotations are presented. Models of such emissions should be able to explain them as integral parts of the steepened magnetosonic waves. The large amplitudes, ΔB/|B| ∼ O(1), and small‐scale sizes (rotational discontinuities) of the cometary waves suggest that rapid pitch angle scattering and energy transfer with energetic ions should occur. Since the waves are highly compressive, Δ|B|/|B| ≃ O(0.5), one can also anticipate possible first‐order Fermi and gradient B drift acceleration.