Driving mechanisms for the cusp region density anomaly

The Earth's magnetospheric cusp provides direct access of energetic particles to the thermosphere causing an ionization anomaly. The energy from these particles along with Joule heating, and ion drag forcing play a direct role in determining the neutral density structure in the cusp region. Recent measurements at different altitudes by the CHAMP (390-460 km altitudes) and Streak (325-123 km altitudes) spacecraft have provided new information on the mesoscale structure in the cusp region. They have shown, respectively, a region of strong enhanced density and a region of depleted density relative to the surrounding area. This has led to conflicting explanations of the mechanism or mechanisms driving the density structure. For CHAMP, the observations (strong enhanced density) were attributed to upwelling of the neutral gas due to Joule heating associated with observed field-aligned currents. For Streak, the observations (relative depleted density) were attributed to soft particle precipitation not being adequate to cause upwelling at the lower altitudes sampled by Streak and relatively harder precipitation in adjacent areas. We use a high-resolution numerical model of the thermosphere to simulate the atmospheric response to the relevant forcing by realistically specifying the particle heating, Joule heating, ion drag, and other forcing. We compare the model results to the Streak and CHAMP observations and assess the relative contributions of these mechanisms in explaining the distinctive features of the observations. Acknowledgements: This research was supported by The Aerospace Corporation's Technical Investment program