AuroralH-emission and interplanetary neutral hydrogen

Summary Auroral spectra taken from within the auroral zone reveal H-emission as not limited to the magnetic zenith but as extending over the sky. The surface brightness forH andH may tentatively be estimated to a few airglows. The assumption is made that some of this emission may extend to the very low gm latitudes and be practically universal and the possible mode of excitation of neutral hydrogen in space is speculated upon. At the earth's orbit the density number of the cosmic, infalling, hydrogen accreted from beyond the solar system towards the sun is about 10² neutral atoms cm^–3 and coronal plasma contributions cannot encrease this number by more than a factor of two. If an exceptionally violent solar disturbance ejects stream-electrons attaining velocities-of 2,200 km sec^–1 corresponding to the threshold energy forH excitation of 12.8 eV and if these electrons, at the earth's orbit, have a density number of 10³ cm^–3, then a shell about 100,000 km thick may contributeH emission having a surface brightness of about 0.1 airglow this being the higher probable value. Such anH-emission could be detectable from all latitudes during, or, most probably, just before the occurence of an auroral storm.