  Motivated by low-altitude cusp observations of small-scale (~ 1 km)
field-aligned currents (SSFACs) interpreted as ionospheric Alfv\'en resonator
modes, we investigated the effects of Alfv\'en 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 Alfv\'en 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
Alfv\'enic Joule heating usually underestimates CHAMP's orbit-average density;
inclusion of Alfv\'enic 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 Alfv\'enic 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 Alfv\'enic heating sporadically populate the
polar region through the action of corotation and neutral winds.
