[P-EM15] Study of coupling processes in solar-terrestrial system

[PEM15-10] Solar activity dependence of the semidiurnal tide and that modulation source in the polar lower thermosphere

*Hirotaka Koyama¹, Satonori Nozawa¹, Yasunobu Ogawa^2,3, Asgeir Brekke⁴ (1.Institute for Space-Earth Environmental Research, Nagoya University, 2.National Institute of Polar Research, 3.SOKENDAI, 4.UiT The Arctic University of Norway)

Keywords:EISCAT radar, Semidiurnal tide, Lower thermosphere

The focus of this study is to reveal individually seasonal, solar activity, and geomagnetic activity dependences of the semidiurnal tide at 92-118 km altitude using EISCAT UHF radar data for 328 days over 33 years. No study has ever statistically revealed each dependence individually in the polar lower thermosphere. We classified the seasons as summer and winter, the geomagnetic activity as Kp < 3 and Kp >= 3, and the solar activity as F10.7 < 120 and F10.7 >= 120 in this study. Under the conditions of high geomagnetic activity and summer, the amplitudes for high solar activity are greater than those for low solar activity at and above 101 km. At and below 98 km, the amplitudes for low solar activity are greater than those for high solar activity. Under the condition of low geomagnetic activity, the amplitudes in summer for high solar activity are greater than those for low solar activity at and above 105 km. At and below 101 km, the amplitudes for low solar activity are greater than those for high solar activity. The phase in summer for high geomagnetic activity (Kp >= 3) and high solar activity (F10.7 >= 120) above 105 km is shifted by about 1 hour toward earlier time. Under the conditions of high geomagnetic activity and winter, the amplitude variation tendency is the same as summer. Under the conditions of low geomagnetic activity and winter, the amplitude differences are smaller than those under others conditions at and above 108 km. In terms of the phase, for high solar activity at all altitudes it is shifted toward earlier time, while it is shifted toward later time for high geomagnetic activity (Kp >= 3). The phase variation with the solar activity may be due to a forcing that varies with the solar activity. The amplitude variations in the lower polar thermosphere at low altitudes are consistent with previous studies in the mesosphere (e.g., Andrioli et al. 2022), but it is not understood what is responsible for the amplitude variations at high altitudes.
We discussed the main modulation source of the solar activity dependence for the semidiurnal tide.

Reference
Andrioli, V. F., Xu, J., Batista, P. P., Resende, L. C. A., Da Silva, L. A., Marchezi, J. P., Li, H.,735 Wang, C., Liu, Z., Guharay, A. (2022). New findings relating tidal variability and solar736 activity in the low latitude MLT region. Journal of Geophysical Research: Space Physics, 737 127, e2021JA030239, https://doi.org/10.1029/2021JA030239