Nighttime thermospheric meridional winds at Cachoeira Paulista (23°S, 45°W): Evidence for effects of the equatorial midnight pressure bulge

Abstract

We have studied the local time and seasonal variations of thermospheric nighttime meridional winds at the low-latitude station, Cachoeira Paulista, Brazil, corresponding to quiet geomagnetic conditions of high solar activity (Fl0.7 > 150 units). The meridional winds are derived from F layer peak height (h max) data using a modified form of the “servo” model wherein plasma transport due to electric field is taken into account. It is found that during the solstices the meridional winds exhibit the transequatorial neutral airflow from the summer to the winter hemisphere, i.e., the nighttime winds are primarily equatorward (poleward) during the December (June) solstice. It is shown that this neutral wind flow pattern is effectively modulated by winds related to equatorial midnight temperature maximum (MTM) and the associated midnight pressure bulge, especially in the December solstice (local summer). The seasonal visibility of the effect of MTM on low/equatorial latitude meridional winds seems to be different in the east Brazil (45°W) and Indian (75°E) sectors.