[P-EM13] Coupling Processes in the Atmosphere-Ionosphere System

[PEM13-30] Determine the origin of tidal oscillations in the ionospheric transition region with EISCAT radar and global model data

*Florian Ludwig Guenzkofer¹, Dimitry Pokhotelov¹, Gunter Stober², Huixin Liu³, Hanli Liu⁴, Anders Tjulin⁵, Claudia Borries¹ (1.Institute for Solar-Terrestrial Physics, German Aerospace Center (DLR), Neustrelitz, Germany, 2.Institute of Applied Physics & Oeschger Center for Climate Change Research, Microwave Physics, University of Bern, Bern, Switzerland, 3.Department of Earth and Planetary Science, Kyushu University, Fukuoka, Japan, 4.High Altitude Observatory, National Center for Atmospheric Research, Boulder, CO, USA, 5.EISCAT Scientific Association, Kiruna, Sweden)

Dynamics in the ionospheric transition region are strongly impacted by both lower atmosphere waves and geomagnetic forcing. This is due to the transition from a collision dominated plasma to a nearly collisionless plasma that takes place at ∼ 90 − 150 km altitude. Tidal-like oscillations of the neutral wind velocity can be used to identify impacts from above and below. Determining their origin could help understanding the complex solar-terrestrial coupling processes. Neutral wind velocities can be calculated from Incoherent Scatter Radar (ISR) measurements applying the steady state ion mobility equation, following the method described e.g. in (Nozawa et al., 2010, doi:10.1029/2009JA015237). Three dimensional ion velocity vectors can be retrieved from line of sight measurements with the EISCAT ISRs operated in the beam swinging mode. The ion-neutral collision frequency as central coupling parameter is obtained from a model neutral atmosphere and an empirical collision model. According to the classical picture of tides, semidiurnal oscillations in the ionosphere are caused by upward propagating atmospheric tides. Diurnal modulations in the high latitude ionosphere are mostly forced at high altitudes by the plasma convection pattern over the polar cap according to this. The transition of predominant oscillation period takes place at roughly ∼ 120 km altitude, though 12h oscillations can be detected at higher altitudes (∼ 250 km) as well (Q. Wu et al., 2017, doi:10.1002/2016JA023560). The onset of this transition was shown by a 22 day long EISCAT UHF campaign conducted in September 2005 (Nozawa et al., 2010). For the here presented research, data from the same measurement campaign is evaluated, extending the investigation up to an altitude of ∼ 142 km. Simultaneous EISCAT and meteor radar measurements are combined to validate the ISR neutral wind determination method and to extend the total range of measurements. Measurement results are compared to global circulation models (GAIA, WACCM-X(SD) and TIE-GCM). A transition of predominant oscillation period at ∼ 120 km is detected, confirming the results in (Nozawa et al., 2010). Additionally, an upper band of strong semidiurnal oscillations above ∼ 130 km is detected in experimental and model data. Amplitudes and phases of tidal oscillations have been determined with the Adaptive Spectral Filtering (ASF) technique (G. Stober et al., 2020, doi:10.5194/acp-20-11979-2020). The ASF technique is a robust frequency analysis method for irregularly spaced data, which is the case above 120 km for the used dataset. The retrieved amplitudes also remain reliable at lower altitudes where the uncertainty of ISR measurements is increased due to the lower electron density. Whether a tidal-like mode is propagating or in situ generated can be distinguished by phase progression analysis. The upper 12h oscillation band is in situ generated according to global model data. The tidal mode of this oscillation band is a sun-synchronous semidiurnal westward propagating wavenumber-2 (SW2) mode. It undergoes an autumn transition around the time of the September equinox which is found in both radar measurement and model results.