Two‐dimensional local modeling of thermospheric heating and neutral mass density enhancement driven by Alfvén waves

In the cusp region, a significantly enhanced thermospheric mass density is commonly observed around 400 km altitude. Despite a number of studies, the enhancement mechanism has not been fully characterized. In order to determine how the Joule heating and resultant mass density enhancements are generated in the F region of the ionosphere during a few hours after the Alfven resonator modes are set up, we have developed a new efficient method to calculate Alfven waves. In this method, the Fourier transform was used, and Alfven waves were solved as frequency-domain boundary value problems. We employed a two-dimensional local model and performed five modeling runs. The result from the modeling runs shows that the Alfven resonator modes generate significant neutral upwelling at similar to 300 km altitude, which creates a "cell" of the neutral mass density enhancement at altitudes centered between 350 and 400 km. This cell becomes evident roughly 1 hr after the Alfven resonator modes are set up, and this region continues to exist stably for 2 more hours. A fractional mass density enhancement at 400 km altitude 3 hr after the Alfven resonator modes having an Alfvenic field-aligned current of 20 mu Am-2 $\mu \mathrm{A}{\mathrm{m}}<^>{-2}$ at the top boundary are set up reaches similar to 30%, which is consistent with the result obtained from satellite observations. In terms of the Poynting flux, this corresponds to similar to 20 mWm(-2), which is also consistent with previous satellite observations.