[P-EM13] Dynamics of the Inner Magnetospheric System

[PEM13-P23] Possible roles of magnetospheric density ducts in controlling the shape of pulsating aurora and the energy of precipitating electrons

*Yuri Ito¹, Keisuke Hosokawa¹, Yasunobu Ogawa^2,5, Yoshizumi Miyoshi³, Mizuki Fukizawa², Fuminori Tsuchiya⁴, Kiyoka Murase⁵, Shin-ichiro Oyama^3,2, Satoko Nakamura³, Yoshiya Kasahara⁶, Shoya Matsuda⁶, Satoshi Kasahara⁷, Tomoaki Hori³, Shoichiro Yokota⁸, Kunihiro Keika⁷, Iku Shinohara⁹ (1.The University of Electro-Communications, 2.National Institute of Polar Research, 3.Institute for Space-Earth Environmental Research, Nagoya University, 4.Tohoku University, 5.Graduate University for Advanced Studies, 6.Kanazawa University, 7.University of Tokyo, 8.Osaka University, 9.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency)

Keywords:Arase, EISCAT radar, Pulsating aurora, Chorus waves, Ducts

Auroras are classified into two broad categories: the discrete aurora, which has a distinct arc-like shape, and the diffuse aurora, which has an indistinct patchy shape. Most of the diffuse auroras are known to show a quasi-periodic luminosity modulation called pulsating aurora (PsA). Magnetospheric electrons causing PsA are generally scattered through wave-particle interactions with magnetospheric chorus waves and precipitated into the ionosphere, being referred to as “PsA electrons”. Previous studies clarified that sub-relativistic electrons originating from the radiation belt also precipitate down to lower altitudes together with PsA electrons, and strongly ionize the middle atmosphere (Miyoshi et al., 2015). Recent numerical simulations suggested that precipitations of PsA electrons having an energy of tens keV to sub-relativistic range require chorus waves to propagate to higher magnetic latitudes (MLAT) of ~20° and to resonate with trapped energetic electrons (Miyoshi et al., 2015, 2020). However, there are no actual cases of simultaneous observations of PsAs and chorus showing such propagation to high latitudes; thus, we still do not know under what conditions PsA electrons become harder and precipitate down to lower altitudes.
To reveal this issue, we have investigated a PsA event on January 12, 2021, during which simultaneous observations with the Arase satellite, ground-based all-sky imagers and the European Incoherent SCATter (EISCAT) radar were conducted. Through the analysis of the simultaneous measurements, we have tried to clarify the relationship between the morphology of PsA and the energy of PsA electrons, and then to understand what factors control the relationship. One of the main results is that, when the shape of PsA was patchy, the energy of the corresponding PsA electrons exceeded tens keV. In addition, during this interval of relatively harder precipitation, chorus wave was observed by Arase at MLAT higher than 20°. Furthermore, 1) the energy flux of scattered electrons, 2) the filling ratio of loss cone at the satellite location, and 3) the energy flux of PsA electrons estimated from EISCAT showed a reasonable correlation. On the basis of these observational results, we hypothesize that the spatial structure of PsA and the energy of PsA electrons are controlled by the existence of “density ducts,” which are tube-like regions where the electron density is lower or higher than the surrounding area. Those structures guide chorus waves along the magnetic field, allowing them to propagate to higher MLAT. In order to test this hypothesis, we compared the irregularity of the background electron density measured by Arase in the magnetosphere with the emission intensity of PsA patches at the footprint. The irregularity of ~2—18% in the electron density possibly due to the existence of ducts and the emission intensity of PsA patches show a good spatiotemporal correspondence, which supports the above-mentioned hypothesis. In the presentation, we show the observational results and discuss the factors controlling the morphology of PsA and the energy of PsA electrons.