[P-EM18] Study of coupling processes in solar-terrestrial system

[PEM18-06] Statistical study of Sporadic Sodium Layers (SSLs) above Tromsø (2)

*Satonori Nozawa¹, Takuo T. Tsuda², Norihito Saito³, Takuya Kawahara⁴, Satoshi Wada³, Yasunobu Ogawa⁵, Hitoshi Fujiwara⁶, Toru Takahashi⁷, Tetsuya Kawabata¹, Chris Hall⁸, Asgeir Brekke⁸ (1.Institute for Space-Earth Environmental Research, Nagoya University, 2.The University of Electro-Communications, 3.RIKEN Center for Advanced Photonics, 4.Faculty of Engineering, Shinshu University, 5.National Institute of Polar Research, 6.Faculty of Science and Technology, Seikei University, 7.Department Physics, University of Oslo, 8.UiT The Arctic University of Norway)

Keywords:SSL, Tromsoe, Coupling between plasma and neutral particles, Sodium LIDAR

This study is the first statistical study of SSLs differentiating in-situ generated SSLs from advected SSLs at high latitudes, and evaluates necessary conditions for generating a SSL. Based on about 3000 hours of sodium density data obtained with the Tromsø sodium LIDAR over 7 year seasons (October-March) between 2012 and 2018, we have identified 36 events of Sporadic Sodium Layer (SSL) in the polar mesosphere and lower thermosphere (MLT) region. A SSL is a thin sodium layer (about 1-2 km) with high sodium density (usually factor of 2 or more higher than that of a normal layer), and its life time is said to be about a few minutes to a few hours. Observational results of SSLs are summarized as follows: (1) SSLs appeared for shorter than 5 % of the overall observational time, confirming it is a rare event at high latitude. (2) The advent altitude distributed from 94 km to 106 km; no clear trend is found. (3) The advent time distributed from 18 LT to 1 UT: no events are found before 18 LT or later 1 LT. (4) No relationship is found between advent time and altitude. (5) The peak density tends to be lower as height increasing. (6) Lifetimes do not show a clear relationship with advent altitudes. We have investigated necessary conditions for a SSL to form in the polar MLT region. Auroral electron precipitation as well as appearance of a sporadic E layer would be one of necessary conditions to form a SSL.



By using an advantage of five directional measurements, we have derived movement velocities of SSLs using detection times (i.e., advent times) at five positions by assuming a SSL has a linear front perpendicular to the movement direction, and have compared wind velocity. The movement directions are dominantly from south-eastward to south westward except for 3 events: most SSLs moved southward in the meridional direction. Based on comparison of the velocities, we have found that 29 out of 36 events (81%) are likely classified to be advection events, while 7 events are left for candidates of in-situ generation events. We will discuss difference of “advection” and “in-situ” events in terms of height/horizontal structures of sodium densities.