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Abstract 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. 

Abstract Strong Thermal Emission Velocity Enhancement (STEVE) is a latitudinally narrow, purple‐band emission observed at subauroral latitudes. Stable Auroral Red (SAR) arcs characterized by major red emission, and red/green arcs with both red and green emissions also occur at subauroral latitudes. Characteristics of magnetospheric source plasma and electromagnetic fields of these three types of arcs have not been fully understood because of the limited conjugate observations between magnetosphere and the ground. In this study, we report 11 conjugate observations (2 STEVEs, 7 SAR arcs, and 2 red/green arcs), using all‐sky images obtained at seven ground stations over more than four years from January 2017 to April 2021 and magnetospheric satellites (Arase and Van Allen Probes). We found that, in the inner magnetosphere, the source region of STEVEs and red/green arcs were located outside the plasmasphere, and that of the SAR arc was in the region of spatial overlap between the plasmasphere and ring current region. Electromagnetic waves at frequencies below 1 Hz were observed for STEVEs and red/green arcs. SuperDARN radar data showed a strong westward plasma flow in the ionosphere, especially during STEVE events, whereas the plasma flows associated with SAR arcs and red/green arcs were generally weaker and variable. The STEVE and SAR arc can appear simultaneously at slightly different latitudes and STEVEs and red/green arcs can transform into SAR arcs. These first comprehensive ground‐satellite measurements of three types of subauroral‐latitude auroras increase our understanding on similarlity, differences, and coupling of these auroras in the ionosphere and the magnetosphere.