The spatial and temporal variations in high latitude cosmic noise absorption and their relation to luminous aurora

Abstract

The spatial and temporal variations in cosmic radio noise absorption were investigated at College, Alaska, during 1962-1963 by means of riometers using one narrow beam antenna and two relatively broad beam antennas which were pointed at different direction along the magnetic meridian. The narrow beam antenna had a 12° beamwidth and was periodically swung in the magnetic meridian from 12° north of zenith to 12° south of zenith. Each of the broad beam antennas had a 26° beamwidth and was directed to 40° from zenith, one to the south and the other to the north. In order to explore the relation of the spatial variations in absorption with the differences in auroral luminosity existing in different directions at a given time, two λ 5577A photometers were operated in the two switching directions of the narrow bean antenna i.e. 12°N and 12°S. The information about the auroral coverage of the various antenna beams was obtained from all-sky photographs. A simultaneous study of radio-wave absorption in relation to luminous aurora resulted in the conclusion that the nighttime radio-wave absorption observed at College, Alaska falls into the following two main categories. The absorption belonging to Category I is observed at any time between 2000-0200 hrs., correlates well with the intensity fluctuations of λ 5577A, and is limited to luminous regions of the sky only. Included in the above category is the absorption associated with the quiet as well as bright and active phases of the display. The absorption belonging to Category II is observed only in the post-midnight hours, does not correlate with the intensity fluctuations of λ 5577A and, most probably, is not limited to luminous regions of sky only. With the absorption making a transition from Category I to Category II, a 10-100 fold increase takes place in the ration of absorption to λ 5577A intensity. In order to explore the contribution of bremsstrahlung X-rays to the observed absorption of both categories, the X-ray intensity is calculated on top of the D-region assuming reasonable flux values of the primary electrons. Using the results of the generalized magnetoionic theory, it is shown that the contribution of X-rays to the observed absorption is at the most 2¹/₂% and therefore may be safely neglected. The close association between radio-wave absorption and luminous aurora during absorption events of Category I suggests that the primary particles responsible for the absorption are approximately in the energy range 10-20 kev. It is shown that the absorption associated with the quiet phase of the auroral display is easily explained by a flux of 107-108 electrons cm⁻² sec⁻¹ in the above energy range. It is also shown that the transition from the quiet phase to the bright and active phase is the result of a momentary 10-100 fold increase in the flux of low energy electrons. The lack of correlation between absorption and λ 5577A intensity fluctuations and the pronounced increase in the ratio of absorption to λ 5577A intensity observed during absorption events of Category II are indicative of a hardening of the primary particle energy spectrum, possibly due to the injection of a large number of electrons in the energy range 30-100 kev. It is estimated that a flux of 106-107 electron cm⁻² sec⁻¹ in the above energy range can adequately account for the observed absorption. In the light of the above observations, the apparent discrepancy between the results of two rocket flights at Fort Churchill, one by McIlwain and the other by McDiarmid et al, is easily resolved. This work was supported by National Science Foundation, Grants 14133 and 947.