Periodic oscillations in the high-latitude ionosphere driven by ultralow frequency waves: simultaneous measurements using SuperDARN radars and GNSS-TEC technique
Authors: Shinbori Atsuki , Hosokawa Keisuke , Hori Tomoaki , Teramoto Mariko , Ponomarenko Pavlo V. , Obana Yuki , Nishitani Nozomu , Yukimatu Akira Sessai , Otsuka Yuichi , Nishioka Michi , Perwitasari Septi , Kumamoto Atsushi , Tsuchiya Fuminori , Matsuda Shoya , Kasahara Yoshiya , Matsuoka Ayako , Miyoshi Yoshizumi , Shinohara Iku 
Published: March 2024

Analyzing the propagation characteristics of ultralow frequency (ULF: ~1–100 mHz) magnetohydrodynamic waves through ground- and satellite-based magnetometer data offers insights into the plasma conditions within the magnetosphere, plasmasphere, and ionosphere. Although a network of ground magnetometers provides estimations of ULF waves' macroscopic properties, their ability to capture small-scale structures (< 100 km) is limited. This limitation arises from the spatial integration of ionospheric current effects, which effectively "smears out" these delicate features. Therefore, to elucidate the generation mechanism of ionospheric electron-density variations associated with Pc5 ultralow-frequency (ULF) waves, from subauroral to high latitudes, we analyzed the global navigation satellite system (GNSS)-total electron content (TEC), ionospheric plasma flow observed by the Super Dual Auroral Radar Network (SuperDARN), and electron density in the inner magnetosphere measured by the Arase satellite. On 23 November, 2022, the SuperDARN Prince George (PGR) radar in the dusk sector detected meridional plasma flow oscillations with periods and amplitudes of 5 min and 10–60 m/s, respectively. The plasma flow oscillations started at approximately 01:10 UT and persisted until 03:30 UT over a magnetic latitude range of 65–72°, with an increasing amplitude as the magnetic latitude increased. The electron density did not exhibit a sharp gradient during the inner magnetosphere pass, indicating that the plasmasphere extended beyond the apogee of the Arase satellite (6.1 Re) under quiet geomagnetic conditions. A detailed comparison between SuperDARN radar and GNSS-TEC data showed that meridional plasma flow oscillations appeared in the mid-latitude trough and auroral oval (increased TEC region). Additionally, the equatorward boundary of the auroral oval was located at a between magnetic latitudes of 72 and 74 °. The 15-min detrended TEC measured over the Fort Simpson radar, inside the field-of-view of the PGR radar, showed oscillations similar to the ionospheric plasma flow variations. Through a spectral analysis of the detrended TEC and meridional plasma flow oscillations, we identified a phase difference of ~ 135° (~ 1.9 min) between them. This result is consistent with a simple model calculation using an oscillating electric field with a period of 5 min and an amplitude of 30 m/s for the vertical \(\mathbf{E}\times \mathbf{B}\) drift. Based on these observational and model calculation results, the TEC oscillations can be explained by the upward and downward motion of the ionosphere owing to an external electric field caused by Alfvén waves propagating along the magnetic field lines from the dusk-side magnetosphere.