Spatiotemporal Development of Cosmic Noise Absorption at Subauroral Latitudes Using Multipoint Ground-Based Riometers

Electron density enhancements in the ionospheric D-region due to the precipitation of high-energy electrons (>30 keV) have been measured as increases in cosmic radio noise absorption (CNA) using ground-based riometers. CNA has been studied since the 1960s. However, there have been few studies of the spatiotemporal development of CNA at multi-point ground stations distributed in longitude at subauroral latitudes, where plasma particles with a wide energy range are intermingled. In this study, we analyzed the longitudinal development of CNA steep increases using simultaneous riometer observations at six stations at subauroral latitudes in Canada, Alaska, Russia, and Iceland over 3 years from 2017 to 2020. The results revealed that the occurrence rate of steep increases in CNA was highest at midnight at 22-08 magnetic local time (MLT), and lowest near dusk at 17-21 MLT. We also showed statistically that the CNA steep increases expanded eastward on the dawn side and westward on the dusk side. The CNA expansion velocity was slightly faster than the results of previous studies in the auroral zone. Correlation and superposed epoch analyses of CNA with solar wind and geomagnetic parameters revealed that CNA intensity was dependent on the Interplanetary Magnetic Field Bz, Interplanetary Electric Field Ey, SYM-H index, and SME index. These results indicate that the CNA at subauroral latitudes is closely related to solar wind and geomagnetic activities, and its propagation characteristics correspond to the dynamics of high energy electrons in the inner magnetosphere.