Printed in Great Britain. GLOBAL DISTRIBUTION OF HELIUM IN THE UPPER ATMOSPHERE DURING SOLAR MINIMUM

Abstract-Helium in the Earth’s thermosphere traces the dynamical systems that redistribute energy and mass. Measurements of the global helium distribution in the thermosphere, using Atmosphere Explorer satellite C. (A&C), show a gradual seasonal change in the number density of helium for all latitudes. The enhancement in helium over the winter pole (the helium bulge) changes in magnitude slowly as seasons progress. The bulge builds and recedes following the progression of winter North to South and back again. This progression of the winter helium enhancement is presented in this paper using latitudinal profiles of helium number density for each month during the year. The absolute magnitude of the winter helium enhancement in the aurora1 regions is affected by aurora1 heating at low altitudes. The reduction in the winter helium bul~eatlowaItitudesshownin~~-Cdata~an betraced tothisio~ali~edbeating.Thegradua~~ari~ltion in helium concentration measured at many latitudes for all seasons of the year implies that global thermosph~ric wind systems change gradually with the seasons