Auroral hydrogen emission

Abstract

A photometer using a tilting interference filter to measure auroral hydrogen emission is described, and the method of analysis of records from such a photometer is discussed. Average Hbeta intensities measured at the Australian National Antarctic Research Expeditions station, Mawson, were 25-50 R in a period of low solar activity. Doppler shift of the peak of the magnetic zenith Hbeta profile was 6 ± 1 A over a wide range of conditions of intensity and auroral and magnetic activity. No "narrow profile" auroral hydrogen emission was observed and the auroral hydrogen emission showed no polarization. The intensity of hydrogen emission was not dependent on the intensity of lambda 4709 or lamba 6300 A emissions from visual auroras, nor on the type or intensity of visual auroral forms. However, when visual auroras were not present, a correlation was sometimes observed between changes of intensity in Hbeta and lambda 4709 A emissions. The hydrogen emission was associated closely with r type Es ionization. There appeared to be no correlation with magnetic activity or with v.l.f. radiation. The region of maximum hydrogen emission always appeared equatorwards of visual forms. An equatorwards movement of the zone of emission was observed before midnight; the zone spread back polewards after auroral breakup and then returned equatorwards; a return of the zone polewards was sometimes observed in the morning hours. An attempt to measure the height of hydrogen emission by the van Rhijn method was unsuccessful but led to a means of selecting the most appropriate of the several published magnetic zenith hydrogen line profiles. Reported asymmetry of magnetic horizon hydrogen line profiles is explained in terms of a wide height distribution of the emission and possible small deviations of the direction of pointing from the true magnetic horizon direction.