Earth Planets Space, Vol. 63 (No. 4), pp. 371-375, 2011
Huixin Liu1,2 and Mamoru Yamamoto3
1Earth and Planetary Science Division, Kyushu University, Japan
2Space Environment Research Center, Kyushu University, Japan
3Research Institute for Sustainable Humanosphere, Kyoto University, Japan
(Received July 16, 2010; Revised October 27, 2010; Accepted November 21, 2010; Online Online published June 14, 2011)
This brief report presents geomagnetic storm effects on the formation and characteristics of the midlatitude summer nighttime anomaly (MSNA). This anomaly is a phenomenon where the diurnal variation of the plasma density maximizes at night instead of day. Under disturbed geomagnetic conditions, the MSNA is found to have smaller spatial coverage, lower magnitude of the reversed diurnal cycle, and shorter duration of the nighttime enhancement. All these features demonstrate a weakening of the MSNA. In addition, the nighttime maximum tends to occur at earlier local time. These effects can be reasonably understood in the frame of storm-induced equatorward wind and the molecular-rich air it carries along with. For instance, the shrink of the spatial coverage is essentially a dominant effect of the molecular-rich air, which tends to deplete the plasma significantly on the poleward edge of the MSNA region. On the other hand, the smaller magnitude and the shorter duration seem to be mainly caused by the storm-induced equatorward wind. Storm effects presented here add further evidence to the pivot role of effective neutral wind in the formation of MSNA.
Key words: Middle-latitude ionosphere, summer night anomaly, neutral wind, magnetic storms.