Seasonal and solar cycle variations of the zonal mean circulation in the thermosphere

TLDR

The heat and momentum sources that drive the thermospheric circulation are solar EUV and UV heating, high‑latitude auroral heating, and a momentum source from the correlation of diurnal wind variations with ion drag. The authors use a zonally symmetric thermosphere model to calculate seasonal and solar cycle variations of zonal mean circulation and latitudinal temperature, requiring a high‑latitude energy input of 2 × 10¹⁸ erg s⁻¹ at solstice during solar maximum and 4.5 × 10¹⁷ erg s⁻¹ during solar minimum to match observations. The calculations show that solar electromagnetic energy absorption mainly drives the thermospheric circulation, which is modulated by auroral heating linked to geomagnetic activity; the circulation exhibits a persistent solstice‑type pattern that abruptly shifts near equinox depending on geomagnetic activity, and the summer‑pole to winter‑pole temperature difference and circulation strength are weaker during solar minimum than during solar maximum.

Abstract

The seasonal and solar cycle variations of the zonal mean circulation (longitudinal or, equivalently, diurnal average) and the latitudinal variation of temperature in the earth's thermosphere are calculated by using a numerical model of the zonally symmetric thermosphere. The heat and momentum sources that drive the thermospheric circulation are solar EUV and UV heating, high-latitude heating primarily due to auroral processes, and a momentum source that results from the correlation of diurnal variations of wind and ion drag. The calculations show that the thermospheric circulation is mainly driven by heating due to the absorption of solar electromagnetic energy. However, it is modulated by a high-latitude heat source associated with auroral processes and related to geomagnetic activity. The seasonal variation of the thermosphere is characterized by a persistent solstice type of circulation that undergoes an abrupt transition within a week or two of equinox depending upon geomagnetic activity. The temperature difference from summer pole to winter pole is smaller and the circulation weaker during solar minimum than during solar maximum. To obtain agreement between the calculated and observed latitudinal variation of temperature and circulation, it is necessary to assume a high-latitude energy input during solstice of 2 × 1018 ergs s−1 for average solar maximum conditions and 4.5 × 1017 ergs s−1 for average solar minimum conditions.