Ion-Neutral Coupling at the Thermospheric Footprint of the Northern Geomgnetic Cusp, Observed During the C-REX Sounding Rocket Mission

Abstract:

For reasons that are not well understood, there are permanent enhancements of neutral mass density in Earth's thermosphere in the vicinity of the northern and southern geomagnetic cusps, and at altitudes of around 400 km. Such enhancements are expected to cause small but important and currently unpredictable perturbations to the orbits of spacecraft flying through them. Here we report on a NASA sounding rocket mission to study mechanisms responsible for establishing and maintaining these enhancements. On November 24, 2014, a Black-Brant 12 sounding rocket was launched from Andoya Space Center out over the Greenland Sea, and into the enhancement region associated with the ionospheric footprint of the northern geomagnetic cusp. It released ten rocket-propelled "grenades" that dispersed barium and strontium tracer clouds into the thermosphere throughout a 3D volume extending over many tens of km around the main trajectory, and spanning heights from 190 to 400 km. Subsequent motions of the ionized barium and neutral barium/strontium components of the clouds were determined by photographic triangulation, using cameras based at Longyearbyen, Ny-Alesund, and aboard a NASA aircraft flying just south of Svalbard. Results showed very dynamic ion convection that was producing substantial Joule heating per unit mass at around 250 km altitude. Surprisingly, the resulting vertical flows were only very weak and, if anything, directed downward. Nevertheless, a picture is emerging in which Joule heating associated turbulent convection in the cusp is a major contributor to the processes that enhance the neutral density.