Ground-based measurements of exospheric hydrogen density

A new observation technique, suggested by recent advance in exospheric theory, appears useful for timely measurement of exospheric atomic hydrogen density, [H]. Fabry‐Perot interferometer measurements of the geocoronal 6563 Å (Hα) emission in the plane of the solar azimuth are used to approximately isolate the singly scattered resonant fluorescent intensity of Hα (ISS) thereby simplifying the derivation of [H]. This initial use of the technique produces profiles of [H] from 400 to 10,000 km. The theoretical bulge of [H] in the early morning hours, previously escaping detection based on simple comparison of relative intensities, is now revealed with a magnitude exceeding the theoretical value. The measured ratio of [H] at 0430 local time to [H] at 2030 local time is 3.0−0.36+0.71 at 700 km, compared with a theoretical value of 1.72. At 2000 km we find no local time variation, in agreement with theory. Unlike nearly all previous studies, we find no arbitrary increase in the solar Lyman beta (Ly‐β) flux is necessary to reconcile derived ISS with current models of exospheric [H]. Deriving absolute [H] remains subject to large systematic uncertainties (on the order of 50%), but the new method substantially improves upon linear extrapolation of relative density variations from Hα intensity changes.