[P-EM13] Dynamics of the Inner Magnetospheric System

[PEM13-P15] Study of magnetospheric structures connected to the equatorward boundary of the oval using Arase and ground imagers

*Masaki Gomi¹, Kazuo Shiokawa¹, Yoshizumi Miyoshi¹, Yuichi Otsuka¹, Shin-ichiro Oyama¹, Atsuki Shinbori¹, Tomoaki Hori¹, Chae-Woo Jun¹, Kazuhiro Yamamoto¹, Iku Shinohara², Kazushi Asamura², Satoshi Kasahara³, Kunihiro Keika³, Shoichiro Yokota⁴, Fuminori Tsuchiya⁵, Atsushi Kumamoto⁵, Yoshiya Kasahara⁶, Yoichi Kazama⁷, Wang Shiang-Yu⁷, Tam Sunny W.Y.⁸, Ayako Matsuoka⁹, Connors Martin¹⁰ (1.Institute for Space-Earth Environmental Research, Nagoya University, 2.Japan Aerospace Exploration Agency, 3.Tokyo University, 4.Osaka University, 5.Tohoku University, 6.Kanazawa University, 7.Academia Sinica Institute of Astronomy and Astrophisics, 8.National Cheng Kung University, 9.Kyoto University, 10.Athabasca University)

Keywords:auroral oval, equatorward boundary of oval, plasma sheet, Arase satellite, high-sensitive all-sky imager, sub-auroral region

A typical region for auroral appearance is a zonal belt at latitude of 65-75 degrees in the geomagnetic coordinates, surrounding both the north and south magnetic poles, which is known as the aurora oval. Gussenhoven et al. [1981,1983] and Hardy et al. [1981] have shown the dependence of the equatorward boundary of the oval separately on the Kp index and solar wind parameters. In these studies, a latitudinal slope with precipitating electron flux enhancement above 45 degrees geomagnetic latitude is defined as the equatorward boundary of the oval based on datasets obtained by the DMSP/F2 and DMSP/F4 satellites. Because the satellites measured the ionosphere at altitude of ~840 km, they were unable to directly investigate the plasma structure in the inner magnetosphere corresponding to the equatorward boundary of the auroral oval, which would be the source region of the auroral electrons. In addition, the estimated boundary has not been confirmed with ground-based optical imagers, which may provide more direct information on the boundary. To improve our understanding on the source region in the inner magnetosphere and the optical boundary of the auroral oval, this study utilized the Arase satellite, which directly observes plasma structures and the magnetic and electric fields in the inner magnetosphere, and high-sensitive all-sky imagers installed at sub-auroral latitudes in the northern hemisphere under the PWING project. First, we defined the equatorward boundary of the auroral oval from the all-sky imager observations and searched simultaneous magnetically conjugated observations made by the Arase satellite passing over the same magnetic field lines above the oval. We applied the TS05 model to estimate the magnetic field line connecting the ionosphere and magnetosphere. We analyzed six cases that the satellite footprint passed the equatorward boundary of the oval taken between March 2017 and April 2021 at 557.7 nm and 630.0 nm wavelengths with all-sky images located at Athabasca (magnetic latitude: 62.5N, geographic latitude and longitude: 54.6N, 246.4E), Gakona (magnetic latitude: 61.5N, geographic latitude and longitude: 62.4N, 214.8E), and Kapuskasing (magnetic latitude: 59.0N, Geographic latitude and longitude: 49.4N, 277.8E). We found that in four cases based on 557.7 nm images and in five cases based on 630.0 nm images, the flux boundaries of the 20-1000 eV electrons collocated with the boundaries of the auroral oval. In five of the six cases, there was also a distribution of energetic electrons on the low-latitude side of the auroral oval boundary. In this presentation, we discuss the characteristics of magnetospheric ions, electric and magnetic fields, magnetic local times when crossing the low-latitude boundary of the oval occurred, as well as AL index, and auroral features for these six cases. We also note some issues on identifying the equatorward boundary of the oval using high-sensitive all-sky imagers at different wavelengths.