A direct measurement of the winter helium bulge

Abstract

In late December 1975, the orbital configuration of the Atmosphere Explorer‐D satellite made possible the measurement of the winter helium bulge within a single perigee pass. Shortly after the winter solstice, the perigee of the polar orbiting AE‐D satellite crossed the equator, at which time descending and ascending portions of the orbit stretched symmetrically over the southern and northern hemispheres. The open source neutral mass spectrometer (OSS) on board AE‐D measured helium densities between the perigee (about 150 km) and altitudes of 650 km. During the time the perigee was at the equator, altitudes above approximately 550 km were located north and south at latitudes greater than 50°, Helium showed, in the winter hemisphere, a factor of 20 higher densities than at corresponding altitudes and latitudes in the southern, summer hemisphere. Absolute densities of helium agree well with previous measurements.