[P-EM15] Dynamics of Magnetosphere and Ionosphere

[PEM15-08] Characteristics of oscillations in drifts of auroral patches observed in the morning sector

*Yoshimasa Tanaka^1,12,13, Yasunobu Ogawa^1,12,13, Mizuki Fukizawa¹, Yoshizumi Miyoshi², Keisuke Hosokawa³, Ayako Matsuoka⁴, Mariko Teramoto⁵, Kazuhiro Yamamoto², Satoshi Kasahara⁶, Shoichiro Yokota⁷, Kunihiro Keika⁶, Tomoaki Hori², Kazushi Asamura⁸, Shiang-Yu Wang⁹, Yoichi Kazama⁹, Chae-Woo Jun², Yoshiya Kasahara¹⁰, Shoya Matsuda¹¹, Iku Shinohara⁸ (1.National Institute of Polar Research, 2.Institute for Space-Earth Environmental Research, Nagoya University, 3.Department of Communication Engineering and Informatics, University of Electro-Communications, 4.Data Analysis Center for Geomagnetism and Space Magnetism, Graduate School of Science, Kyoto University, 5.Kyushu Institute of Technology, 6.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7.Graduate School of Science, Osaka University, 8.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 9.Academia Sinica Institute of Astronomy and Astrophysics, 10.Emerging Media Initiative, Kanazawa University, 11. Institute of Science and Engineering, Kanazawa University, 12.Polar Environment Data Science Center, ROIS-DS, 13.Graduate Institute for Advanced Studies, SOKENDAI)

Keywords:auroral patches, Pc 5 pulsations, poleward moving auroral arcs, field line resonance, plasma waves

There have been several studies on oscillations in drifts of auroral patches so far. The oscillations of auroral patches coincide with Pc 5 geomagnetic pulsations observed on the ground or plasma waves in the magnetosphere. We have demonstrated that the auroral patches oscillate predominantly in the east-west direction in synchronization with poleward moving auroral arcs (PMAAs) as well as Pc 5 pulsations, and that these three phenomena can be interpreted to be caused by the field-line resonance (FLR). The oscillations of auroral patches are important because they are the visualization of field line oscillations that are normally invisible to the eye, and the spatial distribution of the ionospheric electric field can be derived from the motion of the auroral patches. However, the previous studies on this phenomenon have been limited to case studies.
In this study, we statistically study the oscillations of auroral patches, especially focusing on (1) the occurrence conditions of the auroral patch oscillations, (2) the relationship between the patch oscillation frequency and the FLR frequency, and (3) the relationship between the auroral patch oscillations and plasma waves in the magnetosphere. We visually searched for patch oscillation events from images taken by all-sky imagers with a monochromatic filter (557.7nm) at six auroral-latitude stations (Tromsø, Skibotn, Kilpisjärvi, Kiruna, Tjautjas, and Sodankylä) in 2017-2022. As a result, we found more than 30 patch oscillation events in the morning sector of 0530-0900 magnetic local time (MLT). Most of these events were found to occur several hours after substorm onset under conditions of high-speed solar wind and weak magnetic storms. The east-west oscillations of the auroral patches were accompanied by geomagnetic disturbances in the Pc 5 frequency band, often observed simultaneously with PMAAs. These three phenomena oscillated at approximately the same frequency, which was consistent with the FLR frequency (about 5 to 10 mHz) obtained by the IMAGE magnetometers. Furthermore, when the footprint of the Arase satellite was near the MLT of the ground stations, compressional waves with approximately the same frequency were observed in the magnetosphere. These results indicate that the above-mentioned three phenomena are caused by the FLRs and that their wave source is the magnetohydrodynamic (MHD) waves excited by the Kelvin-Helmholtz instability on the morning side of the magnetopause, especially during the high-speed solar wind stream.