Hemispheric asymmetry of the annual and semiannual variation of thermospheric composition

We examine hemispheric asymmetry of the annual and semiannual variation of the ratio of O and N-2 concentrations (O/N-2) using observations by the Global Ultraviolet Imager (GUVI) instrument onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite and compare them with Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCM-X) model simulations. We found that in the equatorial region, the "equinox peaks" of the observed O/N-2 are near the end of March and October, and the two annual lows are near the beginning of July and January.

Compared to the equatorial region, in the northern hemisphere (NH) low latitudes, the first "equinox peak" clearly shifts toward the December solstice, whereas in the southern hemisphere (SH) low latitudes, the "equinox peaks" shift toward the June solstice (JS), forming the hemispheric asymmetry characteristics of the annual and semiannual variation. Seasonal variation of O/N-2 shows no apparent phase variation with altitude, and the annual and semiannual pattern is consistent from year to year.