Dependence of neutral temperatures in the lower thermosphere on geomagnetic activity

Abstract

The neutral temperature (Tn) in the lower thermosphere has been measured using high-resolution (0.13 nm) N2 Lyman-Birge-Hopfield (LBH) band emissions, taken from the High-Resolution Ionospheric and Thermospheric Spectrograph (HITS) instrument aboard the Advanced Research and Global Observation Satellite (ARGOS). Tn in the lower thermosphere (below ∼200 km) has been examined for 12 days in 2000 and 2001. During geomagnetically quiet conditions, the observed HITS temperatures (T HITS) as a function of latitude and calculations using the NRLMSISE-00 model are in good agreement. During periods with geomagnetic disturbances, the model temperatures are unable to match the spatial and temporal variations exhibited by the measurements. An analysis of HITS temperatures and measurements of the auroral electrojet index AL indicates that THITS changes typically follow variations in AL obtained between 13 and 18.5 hours earlier. On average, the time delay is 16 hours and 25 min earlier (correlation coefficient of 0.55), yielding the following empirical formula between Tn and AL changes: ΔTn[K] = 2.1 × ΔAL[nT]. Also, the agreement between measured (HITS) and modeled (NRLMSISE-00) Tn values has been investigated for 8 days with similar geomagnetic conditions, yielding a correlation coefficient of 0.75 between the THITS-TMSIS agreement and the previously measured AL. Copyright 2007 by the American Geophysical Union.