Finite Ion Mobility Effects on Quasi-Longitudinal Whistlers Near the Resonance Cone

The study investigates the tendency of quasi-longitudinal (QL) whistlers, propagating along the resonance cone, to excite ion perturbations as a function of background plasma density. In the presence of finite ion mobility (i.e., departure from the m_i → ∞ limit), simulations were performed over a range of plasma densities corresponding to the ratio of plasma frequency to electron cyclotron frequency ω_pe/Ω_ce = 3, 4, 5, and 10. This allowed examination of how increasing plasma density influences the conditions under which QL whistlers operate. A key finding is that the initialized QL whistlers lead to the self-consistent excitation of strong ion density perturbations, which grow until their associated electric fields begin to couple with those of the QL whistlers, generating ion excitations at higher wave numbers. In addition to the formation of steep electron density structures also observed in the zero ion mobility limit the ion density structures in the present simulations are found to approach the lower-hybrid resonance frequency at higher densities (i.e., higher ω_pe/Ω_ce), without developing steep gradients at the QL whistler scale lengths. These ion structures also acquire an increasingly coherent form, suggesting a recurrent equilibrium with the upper-hybrid excitations.