Alfvénic Thermospheric Upwelling in a Global Geospace Model

Motivated by low-altitude cusp observations of small-scale (~1 km) field-aligned currents (SSFACs) interpreted as ionospheric Alfvén resonator modes, we have investigated the effects of Alfvén wave energy deposition on thermospheric upwelling and the formation of air density enhancements in and near the cusp. Such density enhancements were commonly observed near 400 km altitude by the CHAMP satellite. They are not predicted by empirical thermosphere models, and they are well correlated with the observed SSFACs. A parameterized model for the altitude dependence of the Alfvén wave electric field, constrained by CHAMP data, has been developed and embedded in the Joule heating module of the National Center for Atmospheric Research (NCAR) Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model. The CMIT model was then used to simulate the geospace response to an interplanetary stream interaction region (SIR) that swept past Earth on 26–27 March 2003. CMIT diagnostics for the thermospheric mass density at 400 km altitude show (1) CMIT without Alfvénic Joule heating usually underestimates CHAMP's orbit-average density; inclusion of Alfvénic heating modestly improves CMIT's orbit-average prediction of the density (by a few %), especially during the more active periods of the SIR event. (2) The improvement in CMIT's instantaneous density prediction with Alfvénic heating included is more significant (up to 15%) in the vicinity of the cusp heating region, a feature that the MSIS empirical thermosphere model misses for this event. Thermospheric density changes of 20–30% caused by the cusp-region Alfvénic heating sporadically populate the polar region through the action of corotation and neutral winds.