[P-EM14] Study of coupling processes in solar-terrestrial system

[PEM14-12] Response of the sodium atom density to auroral particle precipitation

*Satonori Nozawa¹, Takuo T. Tsuda², Norihito Saito³, Yasunobu Ogawa⁴, Takuya Kawahara⁵, Toru Takahashi⁶, Tetsuya Kawabata¹, Kairi Ishiguro¹, Satoshi Wada³ (1.Institute for Space-Earth Environmental Research, Nagoya University, 2.The University of Electro-Communications, 3.RIKEN Center for Advanced Photonics, RIKEN, 4.National nstitute of Polar Research, 5.Shinsyu University, 6.Electronic Navigation Research Institute, National Institute of Maritime, Port and Aviation Technology)

Keywords:sodum layer, polar MLT, electron density, auroral particle precipitation, sodium lidar, EISCAT VHF radar

This talk will present results of response of the sodium layer (i.e. change of sodium atom density) at ~80-110 km height to auroral particle precipitation using simultaneous observations of the sodium lidar and the EISCAT VHF radar at the EISCAT Tromsoe site (69.6 eg N, 19.2 deg E). It is still an open question how the sodium layer responds to auroral particle precipitation; in other words, the sodium density increases or decreases during auroral particle precipitation periods. For last ~4 decades, some scientists reported increase of the sodium layer and even formation of sporadic sodium layers (SSLs; enhanced sodium density layers) (e.g. von Zahn et al., 1987; Gu et al., 1995), while some scientists reported decrease (e.g. Nomura et al., 1987; Heinselman et al., 1998; Heinselman 2000). Tsuda et al. (2013) pointed out the response must be different due to the effect of the electric field (strength and direction).

Nomura et al. (1987) reported decrease of sodium column density during a cosmic noise absorption event as well as a large variation in geomagnetic H-component at Syowa Station in Antarctica (69 deg N, 39 deg W). Heinselman et al. (1998) reported decrease in sodium column density due to auroral particle precipitation derived from sodium lidar and incoherent scatter (IS) radar observations made at Sondrestrom, Greenland (66 deg N, 50 deg W). Furthermore, Heinselman (2000) conducted a modeling study and reported that neutral sodium atoms can be ionized via charge exchange with major molecule ions (O2+ and NO+) in the lower E-region.

The ionospheric electric field plays an important role in auroral effects. Ion motions driven by the electric fields (and winds) could induce vertical (along the local geomagnetic field) and horizontal transportation (convergence or divergence) of sodium ions (e.g. Kirkwood and von Zahn, 1991), and cause change of sodium atom density through ion-molecule chemistry. This effect is also considered in the probable hypothesis of SSL formation (e.g. Cox and Plane, 1998; Takahashi et al., 2015).

To facilitate understanding of this issue, we need simultaneous observational data of sodium atom density and electron density as function of height with a good time resolution. By using simultaneous observational data with the sodium lidar and the EISCAT VHF radar obtained at Tromsoe, Tsuda et al. (2013) presented the first investigation of the effect of particle precipitation showed decrease of the sodium density where the electron density was enhanced during the absence of an electric field effect period. We have extended the Tsuda’s study using more simultaneous observational data for different conditions; we gathered (so far) 11 nights of datasets when the simultaneous observations were conducted. We will show and discuss how the sodium density vary during auroral particle precipitation periods.

References
Cox, R. M., and J. M. C. Plane (1998), An ion-molecule mechanism for the formation of neutral sporadic Na layers, J. Geophys. Res., 103, 6349–6359.

Gu, Y. Y., J. Qian, G. C. Papen, G. R. Swenson, and P. J. Espy (1995), Concurrent observations of auroral activity and a large sporadic sodium layer event during ANLC-93, Geophys. Res. Lett., 22, 2805–2808.

Heinselman, C. J., J. P. Thayer, and B. J. Watkins (1998), A highlatitude observation of sporadic sodium and sporadic E-layer formation, Geophys. Res. Lett., 25, 3059–3062.

Heinselman, C. J. (2000), Auroral effects on the gas phase chemistry of meteoric sodium, J. Geophys. Res., 105, 12,181–12,192.

Kirkwood, S., and U. von Zahn (1991), On the role of auroral electric fields in the formation of low altitude sporadic-E and sudden sodium layers, J. Atmos. Terr. Phys., 53, 389–407.

Nomura, A et al., (1987), Lidar observations of the mesospheric sodium layer at Syowa Station, Antarctica, Geophys. Res. Lett., 14, 700–703.

Takahashi, T., S. Nozawa, T. T. Tsuda, Y. Ogawa, N. Saito, T. Hidemori, T. D. Kawahara, C. Hall, H. Fujiwara, N. Matuura, A. Brekke, M. Tsutsumi, S. Wada, T. Kawabata, S. Oyama, and R. Fujii (2025) A case study on generation mechanisms of a sporadic sodium layer above Tromsoe (69.6 deg N) during a night of high auroral activity, Ann. Geophys., 33, 941-953.

Tsuda, T., S. Nozawa, T. D. Kawahara, T. Kawabata, N. Saito, S. Wada, Y. Ogawa, S. Oyama, C. M. Hall, M. Tsutsumi, M. K. Ejiri, S. Suzuki, T. Takahashi, T. Nakamura (2013), Decrease in sodium density observed during auroral particle precipitation over Tromsoe, Norway, Geophys. Res. Lett., 40, 4486-4490, 10.1002/grl.50897.

von Zahn, U., P. von der Gathen, and G. Hansen (1987), Forced release of sodium from upper atmospheric dust particles, Geophys. Res. Lett., 14, 76–79.