LOCAL TIME AND ALTITUDE VARIATION OF EQUATORIAL THERMOSPHERE MIDNIGHTDENSITY MAXIMUM (MDM) - SAN-MARCO DRAG BALANCE MEASUREMENTS

We present the first study of the local time and altitude variation of the MDM, the density component of the equatorial midnight pressure bu lge, an important feature in the nighttime motions of the ionosphere-t hermosphere system in the equatorial region. The neutral density data of the San Marco 3 (SM3) and San Marco 5 (SM5) satellites were average d to obtain 24 hr density variations from April to December in 1971 an d 1988. From these variations, the MDM amplitude as a function of alti tude and local time was obtained at altitudes from 220 to 400 km at th e geographic equator. We show the first evidence of downward phase pro pagation of the MDM together with a vertical structure not observed be fore, which may suggest an ionospheric interaction or perhaps a viscou s dissipation effect. In 1971, the MDM propagates rather gradually fro m about 350 to 220 km in about two hours. in 1988, there appear two re gions in which the MDM time shows little change with height with a dis continuous jump in between the two regions. The net effect is stilt a downward displacement bf the MDM with local time. The results show sig nificant seasonal and solar activity effects. During low solar activit y periods (1971), the MDM occurs earlier in equinox than in solstice f or all altitudes, consistent with the seasonal variation of the midnig ht temperature maximum (MTM). However, with higher solar activity (198 8) and at altitudes below 340 km, the MDM occurs earlier in solstice t han in equinox, raising new questions on solar activity effects on the relative phases of the tidal oscillations of the neutral density and the neutral temperature.