The relationship of thermospheric density anomaly with electron temperature, small-scale FAC, and ion up-flow in the cusp region, as observed by CHAMP and DMSP satellites

摘要

We present in a statistical study a comparison of thermospheric mass densityenhancements (ρ) with electron temperature (),small-scale field-aligned currents (SSFACs), and vertical ion velocity() at high latitudes around noon magnetic local time (MLT).Satellite data from CHAMP (CHAllenging Minisatellite Payload) and DMSP (Defense Meteorological Satellite Program) sampling the Northern Hemisphere duringthe years 2002–2005 are used. In a first step we investigate thedistribution of the measured quantities in a magnetic latitude (MLat) versusMLT frame. All considered variables exhibit prominent peak amplitudes in thecusp region. A superposed epoch analysis was performed to examine causalrelationship between the quantities. The occurrence of a thermosphericrelative mass density anomaly, ρ 1.2, in the cusp region isdefining an event. The location of the density peak is taken as a referencelatitude (Δ MLat = 0°). Interestingly, all the consideredquantities, SSFACs, , and are co-located with the densityanomaly. The amplitudes of the peaks exhibit different characters ofseasonal variation. The average relative density enhancement of the moreprominent density peaks considered in this study amounts to 1.33 during allseasons. As expected, SSFACs are largest in summer with average amplitudesequal to 2.56 μA m, decaying to 2.00 μA m in winter. The event relatedenhancements of and are both largest in winter (Δ =730 K, =136 m s) and smallest in summer (Δ = 377 K, = 57 m s. Based on the similarity of the seasonalbehaviour we suggest a close relationship between these two quantities. Acorrelation analysis supports a linear relation with a high coefficientgreater than or equal to 0.93, irrespective of season. Our preferredexplanation is that dayside reconnection fuels Joule heating of thethermosphere causing air upwelling and at the same time heating of theelectron gas that pulls up ions along affected flux tubes.