Universal time variations of the auroral hemispheric power and their interhemispheric asymmetry from TIMED/GUVI observations

This paper quantitatively analyzes the auroral hemispheric power (HP) and its interhemispheric asymmetry as a function of universal time (UT) for geomagnetically quiet conditions (Kp 1-3) from Thermosphere Ionosphere Mesosphere Energetics and Dynamics/Global Ultraviolet Imager (TIMED/GUVI) imaging observations. The HP variation with UT can be approximately characterized as two cases: One is for similar HP variations in the equinoxes in the Northern Hemisphere and for the June solstices of both hemispheres, and the other is for similar HP patterns in the equinoxes in the Southern Hemisphere and for the December solstices of both hemispheres. In the equinoxes, the HP variations are interhemispherically asymmetric due to different occurrence time of the HP peak. In the solstices, the HP is generally interhemispherically symmetric in its diurnal variations, but there is interhemispheric asymmetry in the magnitudes of the maximum HP. For geomagnetically quiet conditions (Kp = 2), in the equinoxes relative interhemispheric differences are typically between 0 and 20%, with respect to the averaged HP from the two hemispheres, while during the solstices, the maximum relative interhemispheric asymmetry can be as large as 30% in December, but it is only similar to 15% in June. These two cases are mainly associated with variations of auroral precipitation power in the night side sector (21:00-03:00 magnetic local time/MLT), which are primarily controlled by solar illumination conditions in both hemispheres and are also attributed to the difference in the geographical area of the auroral oval in the two hemispheres. Furthermore, the general interhemispheric symmetry of the HP variations in solstices suggests that auroral acceleration is not only controlled locally by solar illumination conditions, which has been well known previously, but also might be affected by processes in the precipitation source region.