Low‐frequency oscillations and transport processes induced by multiscale transverse structures in the polar wind outflow: A three‐dimensional simulation

Abstract

Multiscale transverse structures in the magnetic‐field‐aligned flows have been frequently observed in the auroral region by FAST and Freja satellites. A number of multiscale processes, such as broadband low‐frequency oscillations and various cross‐field transport effects are well correlated with these structures. To study these effects, we have used our three‐dimensional multifluid model with multiscale transverse inhomogeneities in the initial velocity profile. Self‐consistent generation of the low‐frequency mode driven by local transverse gradients in the field‐aligned ion flow and associated transport processes were simulated. Effects of particle interaction with the self‐consistent time‐dependent three‐dimensional wave potential have been modeled using a distribution of test particles. For typical polar wind conditions it has been found that even large‐scale (∼ 50 – 100 km) transverse inhomogeneities in the flow can generate low‐frequency oscillations that lead to significant flow modifications, cross‐field particle diffusion, and other transport effects. It has also been shown that even small‐amplitude (∼ 10 – 20%) short‐scale (∼ 10 km) modulations of the original large‐scale flow profile significantly increases low‐frequency mode generation and associated cross‐field transport, not only at the local spatial scales imposed by the modulations but also on global scales. Note that this wave‐induced cross‐field transport is not included in any of the global numerical models of the ionosphere, ionosphere‐thermosphere, or ionosphere‐polar wind. The simulation results indicate that the wave‐induced cross‐field transport not only affects the ion outflow rates but also leads to a significant broadening of particle phase‐space distribution and tranverse particle diffusion.