[P-EM13] Study of coupling processes in solar-terrestrial system

[PEM13-16] Statistical study of the semidiurnal tide in the polar lower Thermosphere using EISCAT radar data over 33 years

*Hirotaka Koyama¹, Satonori Nozawa¹, Yasunobu Ogawa², Asgeir Brekke³ (1.Institute for Space-Earth Environmental Research, Nagoya University, 2.National Institute of Polar Research, 3.UiT The Arctic University of Norway)

Keywords:EISCAT radar, Semidiurnal tide, Lower thermosphere

The semidiurnal tide plays a crucial role in the atmospheric dynamics in the polar lower thermosphere (Nozawa and Brekke, 1999a; Wu et al., 2003, etc.). Although the polar lower thermosphere is a key region for understanding of coupling processes between the Magnetosphere and the atmosphere as well as between the atmospheric regions, our understanding of the polar lower thermosphere, in particular knowledge of the semidiurnal tide, is still insufficient. In our previous studies in terms of neutral wind variations using the EISCAT UHF radar (Brekke et al., 1994; Nozawa and Brekke, 1995, 1999a, b), dependence of season, geomagnetic disturbance, and solar activity of mean wind and diurnal and semidiurnal tidal waves have been investigated. Due to limitation of the number of data (56 days), however, the three effects could not be investigated separately. The purpose of this study is to reveal individually variations of the semidiurnal tide with season, geomagnetic disturbance, and solar activity.

In this study, 328 days of wind data at altitudes between 93 and 119 km derived from the EISCAT UHF radar data obtained at Tromsoe (69.6°N, 19.2°E) over 33 years (1986-2019) are used. Temporal and altitude resolutions of the data are 6 min and 3 km, respectively. Data are averaged with time and altitude, and then averaged wind data with 24 hour length for each category are made. Twelve hour components are extracted from the averaged wind data using the Lomb-Scargle method (Hocke, 1998). Classifications used for evaluation of the dependence of the amplitude of the semidiurnal tide on season, solar activity, and geomagnetic disturbances are as follows:
Season: Summer (between April 23 - August 2), and Winter (October 23 - February 22)
Geomagnetic disturbance: Kp≦3, 3<Kp<5, 5≦Kp
Solar activity: F10.7≦75, 75<F10.7<150, 150≦F10.7
The evaluation criterion in terms of seasonal dependence is that the dependence is significant when the amplitude difference of peak altitude from 102 km to 108 km between summer and winter is 10% and more. The evaluation criterion in terms of solar activity and geomagnetic disturbance dependence is that the dependence is significant when the amplitude difference of peak altitude from 102 km to 108 km is 10% and more for each condition.

The dependence of the semidiurnal tide for each factor is summarized as follows:
Seasonal dependence: The amplitude in summer is greater than that in winter over the height region under conditions of Kp<5 and 75<F10.7.
Geomagnetic dependence: In summer the amplitude has no geomagnetic dependence, while in winter the larger geomagnetic disturbance, the larger amplitude.
Solar activity dependence: The higher solar activity, the larger amplitude under any conditions of season and geomagnetic disturbance.

We will present these results and discuss possible causes of the variations.



References
Brekke, A., S. Nozawa, and T. Sparr, Studies of the E-region neutral wind in the quiet auroral ionosphere, J. Geophys. Res., 99, 8801-8825, 1994.

Nozawa, S., and A. Brekke, Studies of the E region neutral wind in the disturbed auroral ionosphere, J. Geophys. Res., 100, 14,717-14,734, 1995.

Nozawa, S., and A. Brekke, Studies of the auroral E region neutral wind through a solar cycle: Quiet days, J. Geophys. Res., 104, 45 -66, 1999a.

Nozawa, S., and A. Brekke, Seasonal variation of the auroral E region neutral wind for different solar activities, J. Atmos. Sol. Terr. Phys., 61, 585 – 605, 1999b.

Wu, Q.,Killeen,T.L.,Nozawa,S.,McEwen,D.,Guo,W.,Solomon,S.C., Observations of mesospheric neutral wind 12-hour wave in the Northern Polar Cap. J. Atmos. Sol. Terr. Phys., 65,971–978, 2003