Some recent studies have revealed that the post-storm thermospheric mass density sometime drops below its pre-storm value. This phenomenon has been attributed to the over-cooling effect caused by enhanced infrared emissions from nitric oxide.

(a) GOCE neutral density on dusk side at a fixed local time of 19:17, (b) duskside orbital averages of GOCE density in Figure 2a (black line), orbital (sunlit side) averaged GUVI NO column density (blue line), SABER orbital average of NO 5.3 μm radiance (altitude integrated volume emission rate, light blue line), and DMSP auroral hemispheric power (HP, red line), (c) solar EUV (26-34 nm) flux from SOHO/SEM on March 16-19, 2013. An over-cooling effect is found on March 19 (DOY 78) when the GOCE neutral density drops below its prestorm value on March 16 (DOY 75).

This paper presents relevant observations for selected over-cooling events during both major and moderate storms. Thermospheric neutral density variations observed by GOCE and CHAMP satellites are investigated with simultaneous measurements of thermospheric nitric oxide density and radiance from TIMED/GUVI and TIEMD/SABER, auroral hemispheric power from DMSP, and solar EUV flux from SOHO/SEM and TIMED/SEE during storm and non-storm times. The apparent storm-time “over cooling” effect is a result of both an increase in nitric oxide density (or cooling/energy loss rate) and a decrease in solar EUV flux as well as auroral hemispheric power. Whether or not an over cooling exists also depends on whether there are auroral activities prior to the pre-storm reference day and how the reference day is being chosen. The relative contribution of the different drivers (namely, nitric oxide cooling, variations in auroral hemispheric power and solar EUV, and auroral activities before the pre-storm day) remains to be determined. We note that the nitric oxide cooling alone may not be sufficient to cause the over-cooling effect.

Publication Name: Space Weather Journal

First HAO Author's Name: Gang Lu