Variations in Cosmic Noise Absorption in Association With Equatorward Development of the Pulsating Auroral Patch: A Case Study to Estimate the Energy Spectra of Auroral Precipitating Electrons

Abstract

AbstractThis study focused on a pulsating aurora event associated with aurora morphological changes in Fennoscandia in the early morning on March 7, 2017. A high‐speed sampling all‐sky camera captured equatorward development of the pulsating auroral patch in association with a substorm centered over the Greenland/North America region. Of particular interest in this event is the interconnection between the auroral intensity and the cosmic noise absorption (CNA) derived from three riometers aligned meridionally in Finland (from north to south: Ivalo, Sodankylä, and Rovaniemi). The analysis was made by dividing optical measurements into two oscillation components: longer and shorter than 40 s, that is, nonpulsating and pulsating auroral modulations. The interrelation between the auroral brightness and CNA showed a linear correlation. The inclination of the regression line changed with time depending on latitudes, which was interpreted as hardening or softening of the precipitating electron spectrum. Especially in the case of the low‐pass component, the inclination of the CNA‐vs‐intensity interrelation increased at the three riometer latitudes in the substorm recovery phase. On the other hand, for the high‐pass component, the inclination decreased at Rovaniemi (lower latitude) but remained uniform at Sodankylä (higher latitude). These features suggest that the precipitating electron spectrum has softened in the low‐pass or nonpulsating auroral component, but the spectrum has hardened in the high‐pass or pulsating auroral component on the lower latitude part of the auroral patch region. This study proposes a new application of riometer‐camera measurements to examine auroral particle precipitation.