A search for interplanetary He II, 304‐A emission

Abstract

A region of the sky around the antisolar direction in the earth's shadow was systematically surveyed for the first time at high sensitivity and angular resolution by the extreme ultraviolet telescope on the Apollo‐Soyuz mission. The signal from the 170‐620 A channel of the instrument, which is sensitive primarily to the He II, 304‐A and He I, 584‐A resonance lines, shows a sharp decrease in intensity as the line of sight sweeps into the shadow region. Once the line of sight is completely contained within the shadow, a small residual signal above instrumental background and independent of view direction and spacecraft position is observed. Its intensity is consistent with the expected signal from the interplanetary He I, 584‐A emission measured by the 500‐780 A channel of the extreme ultraviolet telescope. An upper limit at the 2σ confidence level to a possible He II, 304‐A component that can escape detection by our instrument corresponds to 0.02 R (rayleigh) (1.6 × 10³ photons cm−2 s−1 sr−1) or to a column density of 109 ions cm−2 scattering at line center. This flux conceivably could be generated by a number of emission sources whose characteristics we investigate in detail. We find that the two mechanisms most likely to approach the observed limit are multiple scattering of solar radiation by He+ in the plasmasphere and resonance scattering of sunlight by interplanetary He+ produced by photoionization of interstellar neutral He. For the latter source, our upper limit can be used to set an upper limit of 109 s to the e‐folding time for the decrease of the suprathermal He+ velocity toward the bulk solar wind speed, if line radiation dominates the solar continuum beyond a full line width. If the opposite is true, the upper limit becomes 107 s. These high values conflict with commonly believed theories of the underlying relaxation process, and thus a drastic revision would be required if a 304‐A intensity of 0.02 R is confirmed as due to this source.