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. Plain Language Summary: The inner magnetosphere contains plasma particles with a wide range of energies. High‐energy electrons (>30 keV) in the inner magnetosphere are accelerated and lost while rotating longitudinally around the earth. They cause satellite failures and astronaut exposure. It is important to study high‐energy electrons and understand the dynamics of electrons in the inner magnetosphere for safe space utilization and space weather forecasting. The electron density in the ionosphere, which is enhanced by the precipitation of these high‐energy electrons, has been inferred using sensitive ground‐based radio receivers called riometers as an increase in cosmic radio noise absorption (CNA). However, there have been few studies of cosmic radio noise absorption (CNA) observations at multi‐point stations distributed in longitude to show how this takes place on a global scale. 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, focusing on the 25–28 August 2018, geomagnetic storm. Our results show that CNA enhancements started around midnight and expanded both eastward and westward around the Earth. Electrons are known to systematically drift in the inner magnetosphere and we discuss the propagation of CNA by considering that. Key Points: This is the first report of longitudinal development of cosmic radio noise absorption (CNA) for storm‐time substorms using six ground riometers at subauroral latitudesCNA enhancements expanded eastward (5 cases), westward (1 cases), and anti‐sunward (1 case)Spatio‐temporal development of CNA at subauroral latitudes corresponds to high energy electron drift in the inner magnetosphere [ABSTRACT FROM AUTHOR]