Dynamics of the polar mesopause region inferred by the 12 years of OH airglow observation at Syowa Station, Antarctic

OH (8-4) airglow (emitting at an altitude of about 86 km) spectral observations with an OH spectrometer were conducted at Syowa Station, Antarctic, during the winter season from February 2008 until October 2019. We developed a method to judge the night sky condition automatically and objectively from all sky images to select data taken under clear sky condition. All-sky images taken by a color digital camera for aurora monitoring are used to judge the sky condition. We derived the rotational line intensity of OH airglow (OH airglow intensity) and the rotational temperature for 12 years (from 2008 to 2019) from the valid data. We detected distinct intensity variations with various time scales; (1) a decadal scale that may be caused by the solar cycle, (2) seasonal intensity variations with a maximum around April and a minimum around the winter solstice, (3) relatively long-timescale events that lasted for several days and (4) relatively short-timescales of several tens of minutes to several hours.

The intensity of OH airglow is thought to fluctuate due to changes in atmospheric composition in the upper polar mesosphere associated with the energetic particle precipitation (EPP) and the vertical transport of air masses with rich [O] from altitudes higher than the OH airglow layer. Therefore, we compared the peak altitude of the airglow layer, temperature distribution, and timing of the vertical transport enhancement with satellite data from the TIMED/SABER and AURA/MLS. The vertical profiles of OH airglow (1.6 μm and 2.0 μm) and temperature observed by the SABER and the vertical profiles of CO and temperature measured by the MLS were used in the analysis. The results showed that the (1) ~ (3) variations can be generally understood by the supply of oxygen atoms associated with vertical transport to the OH layer. For the variations (2), analysis of wind data obtained by the MF radar at Syowa Station and the [CO] data obtained by the MLS showed that the beginning of annual downward transport coincided with a timing of a change in the dominant wind direction in the mesopause region from westward to eastward. Regarding the variations (4), we focused on the relationship with auroral particles, which had been reported only once by Suzuki et al. (2009). We extracted EPP events from the cosmic noise absorption data from the riometer observations at Syowa Station and conducted a superposed epoch analysis of OH airglow intensity for three hours before and after the EPP events. As a result, the OH airglow intensity was observed to decay for about an hour after the EPP events.

In this presentation, we will show the OH intensity variations detected over Syowa Station at various time scales and discuss the specific dynamics occurring in the upper polar mesosphere region.