Neutral composition and density effects in the October-November 2003 magnetic storms

Abstract

The thermospheric effects of magnetic storm activity occurring in October and November of 2003 are studied using a combination of in situ and remote imaging instruments. The CHAMP (Challenging Minisatellite Payload) satellite near 400 km can detect the slight satellite drag force from which the in situ neutral density is then determined. Global images from the Far Ultraviolet Spectrographic Imager onboard the NASA-IMAGE satellite provide a remote measure of the disturbance in thermospheric composition at lower altitudes (130-200 km). Observed from high altitudes, variations in atomic oxygen emissions at 135.6 nm closely follow the ratio of the column integrated abundances of O and N2. In this work, the relationship between density and composition perturbations is investigated. Highlatitude Joule heating is known to produce global-scale reductions of O density relative to that of N2, so when observed in daytime these areas are expected to exhibit both enhanced temperatures/densities and reduced 135.6-nm brightness. In this work we find that, in addition to effects that can be attributed to direct Joule heating, some large variations in temperature/density at 400 km can more likely be attributed to adiabatic compression and expansion associated with (1) traveling atmospheric disturbances launched by impulsive auroral inputs and (2) convergence and downwelling in the global-scale storm-time thermospheric circulation driven by those same auroral inputs. These are discussed along with other possible explanations for the departure of the present findings from recent work describing a straightforward relationship between thermospheric temperature/ density and composition perturbation.