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We have quantitatively evaluated generation mechanisms of a sporadic sodiumlayer (SSL) based on observational data obtained by multiple instruments at ahigh-latitude station: Ramfjordmoen, Tromsø, Norway (69.6° N,19.2° E). The sodium lidar observed an SSL at 21:18 UT on 22January 2012. The SSL was observed for 18 min, with a maximum sodium densityof about 1.9 × 10<sup>10</sup> m<sup>−3</sup> at 93 km with a 1.1 kmthickness. The European Incoherent Scatter (EISCAT) UHF radar observed a sporadic E layer (Es layer) above90 km from 20:00 to 23:00 UT. After 20:00 UT, the Es layer graduallydescended and reached 94 km at 21:18 UT when the SSL appeared at the samealtitude. In this event, considering the abundance of sodium ions (10 %or less), the Es layer could provide only about 37 % or less of thesodium atoms to the SSL. We have investigated a temporal development of thenormal sodium ion layer with a consideration of chemical reactions and theeffect of the (southwestward) electric field using observational values ofthe neutral temperature, electron density, horizontal neutral wind, andelectric field. This calculation has shown that those processes, includingcontributions of the Es layer, would provide about 88 % of sodium atoms ofthe SSL. The effects of meteor absorption and auroral particle sputtering appearto be less important. Therefore, we have concluded that the major source ofthe SSL was sodium ions in a normal sodium ion layer. Two processes – namelythe downward transportation of sodium ions from a normal sodium ion layer dueto the electric field and the additional supply of sodium ions from theEs layer under relatively high electron density conditions (i.e., in theEs layer) – played a major role in generating the SSL in this event.Furthermore, we have found that the SSL was located in a lower-temperatureregion and that the temperature inside the SSL did not show any remarkabletemperature enhancements

A case study on generation mechanisms of a sporadic sodium layer above Tromsø (69.6° N) during a night of high auroral activity