Winds and ion drifts measured in the thermospheric footprint of Earth’s northern magnetic cusp during the C-REX sounding rocket mission

We report here on neutral wind and ion drift measurements recorded during the November 24, 2014, "C-REX" sounding rocket mission into the thermosphere beneath Earth's northern geomagnetic cusp. The rocket released ten tracer clouds, each comprised of a mixture of barium and strontium, at altitudes between 190 and 400 km. The clouds were created by launching rocket-propelled "grenades" at high velocity out from the parent payload, and were dispersed across a 3D volume extending over many tens of km around the main trajectory. Cameras located at Longyearbyen, Ny-Alesund, and aboard an aircraft stationed north of Bear Island were used to image the tracer clouds and to triangulate on their position and 3D motion. Sunlight striking the clouds ionized the barium within a few tens of seconds, whereas the strontium remained neutral. We were thus able to independently measure the flow velocity of both neutrals and ions at the release locations. Here we will present high-resolution maps of the tracer cloud motion, along with the resulting estimates of neutral and ion flow velocities. These results show very substantial ion-neutral velocity differences: the ions' drift direction was roughly perpendicular to that of the neutrals, while the magnitude of their velocity difference was of order 500 meters per second. Combining these data with ground-based measurements of temperature and electron density allows us to estimate that the specific power density for Joule heating at heights above 200 km was very substantial during the time of this experiment. If such Joule heating is typical, it is very likely to play a major role in establishing the (currently poorly understood) permanent enhancements in the neutral mass density of Earth's thermosphere in the geomagnetic cusp regions at altitudes of around 400 km.