Longitudinal Development of Cosmic Noise Absorption Based on Multipoint Observations at Subauroral Latitudes During Storm-Time Substorms on 25–28 August 2018

Enhancements in electron density in the D-region ionosphere attributed to the precipitation of high-energy electrons, have previously been inferred from increases in cosmic radio noise absorption (CNA) using ground-based riometers. However, there have been few studies of CNA observations at multi-point stations distributed in longitudes. Thus, the spatio-temporal development of the global distribution of CNA is not well understood. In this study, we investigated the longitudinal extent of CNA using simultaneous riometer observations at six stations at subauroral latitudes in Canada, Alaska, Russia, and Iceland. These stations are located encircling the earth at ∼60° north magnetic latitudes. We have conducted simultaneous observations of CNA at these stations since October 2017. Here we focus on seven substorms during a geomagnetic storm 25–28 August 2018 and study the spatio-temporal development of the global distribution of CNA during these substorms. For all seven substorms, some stations observed CNA enhancements after the substorm onsets. In five cases, the CNA enhancements started around midnight and expanded eastward. The other two cases show westward and anti-sunward development of CNA. The eastward expansion of CNA indicates the eastward drift of high-energy electrons, which is the source of the CNA, due to gradient and curvature drift in the geomagnetic field. The westward expansion of CNA may correspond to westward expansion of the substorm injection region due to dawn-to-dusk electric fields. These results indicate that spatio-temporal development of CNA at subauroral latitudes corresponds to high energy electron drift in the inner magnetosphere.