Characteristics of the helium component of the local interstellar medium

Abstract

Extreme ultraviolet instruments on the Solrad 11B satellite observed helium in the local interstellar medium (LISM) flowing through the solar system through resonantly scattered 584 Â solar radiation. Since this satellite orbited the Earth at 130,000 km altitude and, thus, was far removed from the geocoronal airglow, reliable measurements over many directions at a given time were made. As a result these data, together with " hot gas " density-resonance scatter computations and our previous results, permit accurate determination of quite narrow ranges of LISM velocity, V = 22-28 km s-1 , and temperature, T = 9,000-15,000 K. The derived LISM helium density of 1.1-2.7 x 10" 2 cm-3 , when compared with the derived density of the inflowing LISM hydrogen 4-6 x 10" 2 cm-3 , suggests that the local interstellar gas is substantially ionized.