Dynamic fluid-kinetic (DyFK) modeling of auroral plasma outflow driven by soft electron precipitation and transverse ion heating

摘要

We apply a recently developed dynamic fluid-kinetic (DyFK) model to simulate and investigate the effects of soft auroral electron precipitation and perpendicular ion heating by waves on the plasma outflow along auroral field lines. The DyFK model is constructed by coupling a fluid ionospheric model for the region from 120 to 800 km to a semikinetic treatment for topside through several R_E altitude region. This approach, which is described in detail here, allows a partially self-consistent description of the plasma transport along highlatitude flux tubes where both low-altitude ionospheric heating and ionization production and loss as well as high-altitude energization and kinetic effects are incorporated and stressed. In the present work, we investigate the combined effects of the F region plasma production and electron heating by soft auroral electron precipitation and ion perpendicular wave heating at high altitudes, which produces ion conics. The auroral event simulated here involves 1.5 hours of moderate soft electron precipitation and relatively weak ion cyclotron waves along the magnetic field lines. The simulations reveal the F region electron heating and ionization by the soft electron precipitation, driving a topside O~+ upflow of up to 10~9 cm~(-2)s~(-1) below 1000 km within 30 min after the electron precipitation is turned on. The enhanced O~+ upflow plumes would be still gravitationally bound in the absence of further energization at higher altitudes. However, the synergistic effects of the increased upwelling ion supply driven by the precipitation and the wave-driven ion heating at higher altitudes combine to enhance O~+ bulk outflow by and order of magnitude above the baseline polar wind level to a net outflow flux of 10~8 ions cm~(-2)s~(-1) with a density of 10 ions cm~(-3) and bulk velocity of 12 km s~(-1) at 3 R_E altitude. Various O~+ conic velocity distributions develop within 10 min after transverse heating is initiated, and their characteristic energies saturate at approximately 10 eV for the peak waveinduced heating rates of 10~(-14) ergs s~(-1) at 2 R_E here. H~+ is also affected by the increases of O~+ due to H~+-O~+ collisional drag in the 1000-4000 km altitude transition region. H~+ flow is much less affected by the wave heating because of the faster transit times through the high-altitude wave heating zone and the lower H~+ bulk flow consists of a flux of 10~8 ions cm~(-2)s~(-1), a density of 4 ions cm~(-3), and a velocity of 30 km s~(-1) at 3 R_E altitude.