Depletion of mesospheric sodium during extended period of pulsating aurora

Abstract

We quantitatively evaluated the Na density depletion due to charge transfer reactions between Na atoms and molecular ions produced by high‐energy electron precipitation during a pulsating aurora (PsA). An extended period of PsA was captured by an all‐sky camera at the European Incoherent Scatter (EISCAT) radar Tromsø site (69.6°N, 19.2°E) during a 2 h interval from 00:00 to 02:00 UT on 25 January 2012. During this period, using the EISCAT very high frequency (VHF) radar, we detected three intervals of intense ionization below 100 km that were probably caused by precipitation of high‐energy electrons during the PsA. In these intervals, the sodium lidar at Tromsø observed characteristic depletion of Na density at altitudes between 97 and 100 km. These Na density depletions lasted for 8 min and represented 5–8% of the background Na layer. To examine the cause of this depletion, we modeled the depletion rate based on charge transfer reactions with NO+ and normalO2+ while changing the R value which is defined as the ratio of NO+ to normalO2+ densities, from 1 to 10. The correlation coefficients between observed and modeled Na density depletion calculated with typical value R = 3 for time intervals T1, T2, and T3 were 0.66, 0.80, and 0.67, respectively. The observed Na density depletion rates fall within the range of modeled depletion rate calculated with R from 1 to 10. This suggests that the charge transfer reactions triggered by the auroral impact ionization at low altitudes are the predominant process responsible for Na density depletion during PsA intervals.

Statistical investigation of Na layer response to geomagnetic activity using resonance scattering measurements by Odin/OSIRIS

“…The present result, i.e., the Na density decreased owing to auroral activity, is consistent with those by Nomura et al [], Heinselman et al [], Heinselman [], Tsuda et al [], and Takahashi et al []. The question remains which of the two auroral effects, i.e., particle precipitation or electric field effects, was mainly responsible for the decrease in the Na density.…”

“…The reason for the decrease in the Na density due to energetic particles is considered to be as follows [cf. Heinselman , ; Takahashi et al , , and references therein]. First, ionization by energetic particles can produce NO + and ions.…”

Impact of M‐solar flare‐induced solar proton event on mesospheric Na layer over Utah (41.8°N,112°W)

“…In long-term prospect (50 years), the atmospheric sodium also shows less significant trends in the centroid height and RMS widths of sodium concentration layer [Dawkins et al, 2016;Clemesha et al, 2004]. The sodium concentration is found susceptible to the auroral particle precipitation [Tsuda et al, 2013;Takahashi et al, 2017]. Tsuda et al [2013], using a simultaneous and common-volume observation by a European incoherent scatter (EISCAT) VHF radar and a sodium lidar, have reported a depletion in sodium concentration over Tromso, Norway (69.6 ∘ N, 19.2 ∘ E) during 24-25 January 2012.…”

Relationship between Na layer and CNA variations observed at Syowa, Antarctic

“…In most of them, event-based investigations (without statistical dataset) were performed, and those reported conflicting results and/or suggestions regarding the response in the Na density to auroral activity. In some cases, the Na density increased (von Zahn et al 1987;Gu et al 1995), and in others, it decreased (Nomura et al 1987;Heinselman et al 1998;Heinselman 2000;Tsuda et al 2013;Takahashi et al 2017).…”

“…There have been several previous investigations related with this issue (von Zahn et al 1987;Nomura et al 1987;Gu et al 1995;Heinselman et al 1998;Heinselman 2000;Tsuda et al 2013;Takahashi et al 2017;Tsuda et al 2017). In most of them, event-based investigations (without statistical dataset) were performed, and those reported conflicting results and/or suggestions regarding the response in the Na density to auroral activity.…”