Linear and nonlinear kinetic Alfvén waves at Venus

Abstract

Space observations show that Venus suffers significant atmospheric erosion caused by the solar wind forcing. Plasma acceleration is found to be one of the main mechanisms contributing to the global atmospheric loss at Venus through its magnetotail. Motivated by these observations, we propose that kinetic Alfvén waves (KAW) may be a possible candidate for charged particle energization at the upper atmosphere of Venus. To test this hypothesis, we explored the basic features of both linear and nonlinear KAW structures at Venus. We considered a low-but-finite β plasma consisting of ionospheric populations (consisting of hydrogen H+, oxygen O−, and isothermal ionospheric electrons) and solar wind populations (protons and isothermal electrons). In the linear regime, we obtain a linear dispersion relation that exhibits a dependence on the intrinsic plasma configuration at Venus. The linear analysis predicts wave structures with wavelengths of ~10–102 km and frequencies of up to ~5 Hz. In the nonlinear regime, small-but-finite-amplitude solitary excitations with their corresponding bipolar electric field are obtained through the reductive perturbation technique. We discuss the influence of the intrinsic plasma parameters (the ionic concentration, solar wind electron temperature, magnetic field strength, and obliqueness) on the nature of the structures of the solitary KAWs and their corresponding electric field. We find that the ambipolar field is amplified with increasing propagation angle, magnetic field strength, and relative temperature of electrons. Our theoretical analysis predicts the propagation of elliptically polarized ultra-low-frequency (ULF) solitary structures with a maximum magnitude of ~0.01–0.034 mV m−1 and a time duration of 20–30 s. The result of the fast Fourier transform (FFT) power spectra of the ambipolar parallel electric field is broadband electromagnetic noise in the frequency range of ~0.5–2 Hz.