Research Article

Mesospheric ionization during substorm growth phase

¹ Department of Polar Science, The Graduate University for Advanced Studies, SOKENDAI, Tachikawa, 190-8518 Tokyo, Japan
² National Institute of Polar Research, Tachikawa, 190-8518 Tokyo, Japan
³ Polar Environment Data Science Center, Research Organization of Information and Systems, Tachikawa, 105-0001 Tokyo, Japan
⁴ Department of Earth and Planetary Science, The University of Tokyo, Bunkyo-ku, 113-0032 Tokyo, Japan
⁵ Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, Sagamihara, 252-5210 Kanagawa, Japan
⁶ Graduate School of Science, Osaka University, Toyonaka, 560-0043 Osaka, Japan
⁷ Institute for Space–Earth Environmental Research, Nagoya University, Nagoya, 464-8601 Aichi, Japan
⁸ Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, 920-1192 Ishikawa, Japan
⁹ Data Analysis Center for Geomagnetism and Space Magnetism, Kyoto University, Sakyo-ku, 606-8502 Kyoto, Japan
¹⁰ Research Institute for Sustainable Humanosphere, Kyoto University, Uji, 611-0011 Kyoto, Japan
¹¹ International Center for Space Weather Science and Education, Kyushu University, Fukuoka, 819-0395 Fukuoka, Japan
¹² Japan Atomic Energy Agency, Tokai-mura, 319-1184 Ibaraki, Japan

^* Corresponding author: murase.kiyoka@nipr.ac.jp

Received: 1 October 2021
Accepted: 20 April 2022

Abstract

Many studies have been conducted about the impact of energetic charged particles on the atmosphere during geomagnetically active times, while quiet time effects are poorly understood. We identified two energetic electron precipitation (EEP) events during the growth phase of moderate substorms and estimated the mesospheric ionization rate for an EEP event for which the most comprehensive dataset from ground-based and space-born instruments was available. The mesospheric ionization signature reached below 70 km altitude and continued for ~15 min until the substorm onset, as observed by the PANSY radar and imaging riometer at Syowa Station in the Antarctic region. We also used energetic electron flux observed by the Arase and POES 15 satellites as the input for the air-shower simulation code PHITS to quantitatively estimate the mesospheric ionization rate. The calculated ionization level due to the precipitating electrons is consistent with the observed value of cosmic noise absorption. The possible spatial extent of EEP is estimated to be ~8 h MLT in longitude and ~1.5° in latitude from a global magnetohydrodynamic simulation REPPU and the precipitating electron observations by the POES satellite, respectively. Such a significant duration and spatial extent of EEP events suggest a non-negligible contribution of the growth phase EEP to the mesospheric ionization. Combining the cutting-edge observations and simulations, we shed new light on the space weather impact of the EEP events during geomagnetically quiet times, which is important to understand the possible link between the space environment and climate.