Ion dynamics in single and multiple electrostatic waves: Simulation and e-POP data comparison

Heavy ion (O⁺) energization through wave-particle interaction processes near the topside ionosphere have been a long-standing puzzle in space physics. Previous observations suggest that electrostatic broadband extremely low-frequency (BBELF) waves are responsible for O+ heating perpendicular to the magnetic field at various altitudes [Kintner et al. 1996; Knudsen et al. 1998; Shen et al. 2018]. Usually there is no distinct feature in the power spectral density (PSD) near ion cyclotron frequencies in BBELF waves. To understand whether and how BBELF waves account for intense ion heating (e.g. 4.3 eV in 2 km spatial region in Shen et al. 2018) is critical to our knowledge of ion energizations at the initial stage of heavy ion outflows. We perform test particle simulations to investigate ion heating by both single and multiple electrostatic waves in a background magnetic field. Previous studies show that for a single coherent electrostatic wave, ion heating is controlled by inherently complex structures of wave potential in cylindrical coordinates. When wave potential is large, higher-order potential structures overlap and ions are stochastically heated by waves with a wide range of frequencies. Chaotic behavior initiates when the polarization drift over one wave period becomes comparable to the perpendicular wavelength. Saturation occurs as these ions are trapped into large potentials determined by the zeros of Bessel functions. We explore how the observed ion heating ( 4.3 eV in 0.2 s) from the e-POP satellite can be modelled by test particle simulations. Both on-resonance and off-resonance cases are studied for the stochastic ion heating. Addition of multiple electrostatic waves with different frequencies and wavenumber spectra will be investigated.