Generation of parallel and quasi‐perpendicular EMIC waves and mirror waves by fast magnetosonic shocks in the solar wind

Abstract

AbstractThis paper presents a simulation study on the generation of electromagnetic ion cyclotron (EMIC) waves and mirror waves associated with fast magnetosonic shocks in the solar wind or the bow shock ahead of the magnetosphere. Strong fast magnetosonic shocks are often observed in these regions. In the shock downstream region, hydrogen ions (H+) and helium ions (He++) are strongly heated to high ion beta and large temperature anisotropy. The helium ions have a bunch distribution in the perpendicular velocity phase space in the immediate downstream region. These conditions can lead to generation of strong EMIC waves and mirror waves, which are often identified in satellite observations. In the simulations with a background magnetic field being in the simulation plane, the dominant EMIC waves are left‐handed polarized and propagate parallel to the background field. The nearly perpendicular (~85°) EMIC waves of right‐handed polarization excited at large wave numbers (kVA1/ΩH1 ≈ 3) show weak but nonnegligible wave energy. The effect of minor helium ions on wave excitation is found to be small. Mirror waves grow to large amplitudes when both the downstream ion beta and temperature anisotropy are high. Various wave modes are identified by ω − k dispersion analysis. For the cases with a background magnetic field making a large angle with the simulation plane, the mirror waves dominate, and nonlinear coalescence of mirror waves is clearly observed downstream.