The causes of mid-latitude F layer behavior

Abstract

Ten years of measurements of the ionospheric F-layer peak height h max and peak density nmax and of the horizontal neutral wind at these heights with the Japanese MU Radar are analyzed to detail the causes of the temporal variations of the peak parameters. In the absence of winds hmax rides the height of a constant product of the atomic and molecular densities of the thermosphere. Since the mean wind does not change with solar activity, the hmax change with solar activity is driven almost solely by a thermal expansion of the thermosphere. The seasonal variation of hmax, on the other hand, is driven almost solely by the seasonal change in winds. The diurnal variation of hmax is driven most importantly by winds, secondarily by thermal expansion. nmax is proportional to the ratio of the atomic and molecular densities of the thermosphere at altitude hmax but that ratio is insensitive to thermal expansion. The solar-activity change in nmax is largely due to the change in solar EUV intensity, secondarily due to changes in neutral composition at the base of the thermosphere. The seasonal change in n max is semiannual in response to the semiannual change in neutral O density. The wind itself shows features of ion-drag control in almost every facet of its behavior except that its diurnal amplitude does not change with season, a result consistent with the explanation that one day is insufficient time to set up a nondivergent circulation pattern in the upper atmosphere. Our numerical results are valid only for the location of the MU Radar, but the understandings involved are broadly applicable to the midlatitude ionosphere. Copyright 2008 by the American Geophysical Union.