The aurorally associated absorption of cosmic noise at College, Alaska

A simultaneous study of radio wave absorption and luminous aurora at College, Alaska, using narrow beam antennas, 5577 A photometers, and all-sky cameras resulted in the conclusion that the nighttime radio wave absorption observed at College falls into the following two main categories. The absorption belonging to category 1 is observed at any time between 2000 and 0200h, correlates well with the intensity fluctuations of 5577 A, and is limited to luminous regions of the sky. Included in this category is the absorption associated with the quiet as well as bright and active phases of the display. The absorption belonging to category 2 is observed only in the postmidnight hours, does not correlate with the intensity fluctuations of 5577 A and, most probably, is not limited to luminous regions of the sky. With the absorption making a transition from category 1 to category 2, a 10- to 100-fold increase takes place in the ratio of absorption to 5577 A intensity. The close association between radio wave absorption and luminous aurora during absorption events of category 1 suggests that the primary particles are approximately in the energy range 10–20 kev. It is shown that the absorption associated with the quiet phase of the display is easily explained by a flux of 107–108 electrons cm−2 sec−1 in the range 10–20 kev. It is also shown that the transition from the quiet phase to the bright and active phase can be explained by a momentary 10- to 100-fold increase in the flux of low energy electrons. The lack of correlation between absorption and 5577 A intensity fluctuations and the pronounced increase in the ratio of absorption to 5577 A intensity observed during absorption events of category 2 are indicative of a hardening of primary particle energy spectrum, possibly because of injection of a large number of electrons in the energy range 30–100 kev. It is estimated that a flux of 106–107 electrons cm−2 sec−1 in the above energy range can adequately account for the observed absorption during events of category 2.