Variations of OH rotational temperature over Syowa Station in the austral winter of 2008
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2010
Received: 14 February 2010
Accepted: 15 July 2010
Published: 28 October 2010
A grating spectrometer for hydroxyl (OH) airglow installed at Syowa Station (69°S, 39.6°E) by the 49th Japanese Antarctic Research Expedition (JARE49) has been in operation since late February, 2008. A dataset of 153 nights was acquired at this location in the austral winter season of 2008. This dataset shows variations in the rotational temperature over a range of temporal scales. The rotational temperature around the polar mesopause region is high in winter and decreases toward summer, which is a pattern similar to that observed at Davis Station, located at almost the same latitude as Syowa Station. A large temperature variation with a period of several days was observed in early May, 2008. Based on a comparison with a simultaneous dataset obtained by the SABER instrument onboard the TIMED satellite, it is inferred that this rotational temperature variation was due to the modulation of vertical motions around the mesopause.
Hydroxyl (OH) airglow is the brightest emission in the nightglow and was first reported by Meinel (1950). The spectral lines of vibration-rotation bands called the Meinel bands (v′ < 9) appear in the visible to infrared wavelengths. The mean altitude and thickness of the OH airglow layer have been measured to be 87 and 8 km, respectively, by rocket-borne observations (Baker and Stair, 1988). A primary source of excited OH molecules at these altitudes is the reaction of atomic hydrogen with ozone (Bates and Nicolet, 1950).
OH airglow spectroscopy is a very reliable and useful method for determining the temperature of the upper mesosphere, and many optical observations have been made, particularly in the middle- and low-latitude regions (e.g., Krassovsky, 1972; Sivjee et al., 1972; Takahashi et al., 1974). In the northern polar region, Oznovich et al. (1995) used a Michelson interferometer to observe the OH (3-0) band and a scanning photometer to observed other airglow emissions (OI 557.7 nm; sodium 589.3 nm). Their aim was to investigate the behavior of atmospheric waves passing through these layers in Eureka, Canada (80°N). Nielsen et al. (2002) reported a long-term variation in the OH rotational temperature using a 20-year dataset obtained with an Ebert-Fastie scanning spectrometer at Longyearbyen, Norway (78°N). Although OH airglow measurements have been made at only a limited number of Antarctic locations, but the data obtained in a number of these studies provide important information. The first long-term observation of the OH rotational temperature that showed seasonal and interannual variations was a 7-year study using a Czerny-Turner scanning spectrometer at Davis Station (68.6°S) (Burns et al., 2002). French and Burns (2004) reported a 7-year trend in OH rotational temperature, identifying a long-term correlation between temperature and the solar cycle and effects of planetary wave penetration into the upper mesosphere during the winter season. Using the same OH rotational temperature dataset at Davis Station, French et al. (2005) reported an unusually high temperature for the austral winter of 2002 and found a 14-day oscillation with large amplitude (~ 15–20 K) in early spring related to a planetary wave penetration into the mesopause region. Espy et al. (2003) also found a rapid and large-scale variation in OH rotational temperature which occurred simultaneously at Rothera Station (68°S, 68°W) and Halley Station (76°S, 27°W). Since the meridional wind variation observed by a medium frequency (MF) radar at Rothera Station showed a very good correlation with the temperature variation during this period, these researchers concluded that the temperature variation was caused by an enhancement of the inter-hemispheric meridional flow from summer (north) to winter hemisphere (south). At a higher latitude, Azeem and Sivjee (2009) reported tidal features of OH rotational temperature at the South Pole using dataset of OH (3-1) band observations over the period between 1994 and 2007.
Recent global observations by satellites have revealed detailed characteristics of OH airglow. These have resulted in the construction of an exact empirical model of the height profile of OH emission based on 6 years of measurements by the Wind Imaging Interferometer (WINDII) onboard the UARS satellite (Liu and Shepherd, 2006). WINDII measures volume emission rate profiles of the OH (8-3) band P1(3) line airglow (734 nm). Liu and Shepherd (2006) used 50,000 profiles of volume emission rate obtained within latitudes of ±40° over the period from November 1991 to August 1997 and found universality of the inverse correlation between peak altitude and the integrated emission rate.
The inverse relationship between altitude and the intensity of OH airglow peak has also been verified by the Sounding of the Atmosphere by Broadband Emission Radiometry (SABER) instruments onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite in the polar region (see Russell et al., 1999). Winick et al. (2009) demonstrated a relationship between OH layer height and volume emission rate at the peak altitude at night in the latitude region between 60° S and 80°N during the period from February to March of 2004, 2005, and 2006. According to their results, the northern high-latitude region was under an unusual condition in the boreal winters of 2004 and 2006, namely, an extremely low OH peak altitude that sometimes descended down to 80 km. This is thought to be related to a major stratospheric warming event. The inverse relationship is still maintained even in such extreme cases. Most of these variations in OH rotational temperature and integrated intensity are consistent with vertical motions which cause modulations of the OH peak altitude (Ward, 1999).
The results of these earlier studies using a nearly global dataset provided by satellite measurements has enabled great advances to be made in our understanding of OH airglow features. It is now known that most variations in the intensity and rotational temperature of OH airglow are caused by modulation of the OH layer by a vertical motion induced by atmospheric waves or circulations. Even with such knowledge of OH airglow, however, ground-based observations are still considered to be an important method by which to monitor not only the mesopause temperature but also local phenomena in the upper mesosphere. Ground-based OH airglow observations are particularly useful in the Antarctic where few such observations have been performed in the past.
In order to monitor the polar mesopause temperature, a grating spectrometer for OH airglow was installed at Syowa Station (69°S, 39.6°E) by the 49th Japanese Antarctic Research Expedition (JARE49) in early 2008. Here we report the first results of the OH rotational temperature observation over Syowa Station in the winter of 2008.
Since this spectrometer was originally designed to observe the behavior of OH rotational temperature during an active aurora event, the OH (8-4) band with relatively less contamination from strong aurora emissions is selected. However, it should be noted that a weak auroral emission by N2 1PG overlaps the OH band. This auroral emission may cause contamination of the rotational temperature measurement. Consequently, two rotational temperatures were derived from two pairs of rotational lines, i.e. P1(2)/P1(5) and P1(3)/P1(5), to remove data affected by auroral contamination. If their difference is larger than 10 K, the temperatures are omitted as inaccurate values due to auroral contamination. The relationship between auroral activity and the behavior of OH airglow is reported in Suzuki et al. (2010).
In this study, we used a dataset from the SABER instrument onboard the TIMED satellite to examine the causes of the ground observed variations. SABER is a ten-channel radiometer providing near-global measurements of the emissions from several sources. Two of these channels are designed to observe the emission from OH airglow. These channels measure the 2.0- and 1.6-µm radiation from the pair of OH (9-7) and OH (8-6) and the pair of OH (5-3) and OH (4-0) bands, respectively (Russell et al., 1999). Vertical profiles of the volume emission rate from OH airglow can be derived from these datasets. The other channels obtain near infrared to infrared radiation from CO2, and these are used to retrieve atmospheric temperature and pressure up to an altitude of approximately 120 km. The orbit of the TIMED satellite has a 60-day yaw cycle, switching between the northern hemisphere viewing mode (83°N–52°S) and southern hemisphere viewing mode (52°N–83°S) every 60 days.
Observations were made between 29 February and 18 October 2008. This period corresponds to the period from early autumn to late spring, when the dark night occurs. The observation time of each night was determined by the solar zenith angle. In principle, the observations were conducted when the solar elevation angle was < 18° below the horizon (astronomical twilight). The exposure time was set to 1 min throughout the entire 2008 season. The observations were conducted over 158 nights, including cloudy nights, with the exception of nights when weather conditions were extremely severe, such as heavy snow or blizzards. Of the observed nights, 59 were perfectly clear, 57 were partly clear, and the others were completely cloudy. Although clouds degrade a signal-to-noise ratio due to extinction— and hence the precision of the temperature measurement may decrease—the observation itself is not impossible. However, moonlight scattered by clouds contaminates the observed spectrum and can cause a bias in the temperature measurement due to a complex absorption spectrum in the water in the clouds. Therefore, we did not use the datasets obtained only cloudy night under the moonlight in the data analysis.
For comparison, the OH equivalent temperature, which is the atmospheric temperature weighted by the vertical profile of OH volume emission rate obtained by the SABER observations, is plotted in Fig. 3 (diamonds). Each value of OH equivalent temperature is given as a mean of typically three or four tangential points within a 500-km distance of Syowa Station. Since SABER passes closely over Syowa Station twice a day in its ascending and descending orbit, the universal time (UT) of these data points is also derived by averaging. The typical sampling times above Syowa Station in this period are 13–14 UT and 22–23 UT for ascending and descending orbits, respectively. The mean distance of these data points from Syowa Station is about 200 km. The numbers located near the diamond symbols specify the number of SABER profiles used to calculate the daily average value and UT. The process used to derive OH equivalent temperature and the altitude profiles of atmospheric temperature and OH volume emission rates is as follows. First, the averaged profiles of temperature and OH volume emission rates are calculated by an equal weighting of three to four soundings. Then, two profiles of OH volume emission rate of two SABER channels are normalized with integrated emission rates through the OH layer. The two SABER OH VER channels are at 1.6 µm, which mostly includes the OH (4-2) and OH (5-3) bands, and 2.0 µm, which mostly includes the OH (8-6) and OH (9-7) bands. These two altitude profiles are combined and averaged to obtain the weighting function. Finally, these weighting functions are combined with the altitude profiles of temperature to obtain OH equivalent temperatures. Good agreement between the OH rotational and equivalent temperatures is seen except on DOY 128. A possible reason for this difference is the effect of unknown local heating or a horizontal inhomogeneity of temperature because the field of view of SABER is not necessarily the same as that of the ground-based observation.
If the change in the peak position of the OH airglow layer without downward motion is responsible for the large temperature change during DOY 124–129, the vertical gradient of the background temperature should be —7 K/km. However, no such vertical gradients of temperature are seen in the SABER temperature profiles shown in the lower panel of Fig. 4. Furthermore, most of these profiles show positive vertical temperature gradients during this period.
The OH equivalent temperature obtained by the SABER observation was found to be in good agreement with the OH rotational temperature obtained by the ground-based observation, indicating that the large increase in OH rotational temperature occurred simultaneously with the descent of the OH layer. An enhanced downward motion in the OH layer causes adiabatic heating and moves the OH layer downward. Therefore, the large temperature variation observed in early May 2008 over Syowa Station is thought to be due to the enhancement of the downward motion and the resulting adiabatic heating.
5. Summary and Conclusions
A grating spectrometer for OH airglow was installed at Syowa Station (69°S, 39.6°E) by JARE49 in early 2008 to monitor the polar mesopause temperature. The observations were performed during 158 nights. Of the observed nights, 59 were perfectly clear in the austral winter of 2008. The dataset shows a range of variations over both short and long time scales.
The rotational temperature increases from the autumn to the winter, and then decreases toward the spring. This feature is consistent with those observed previously in the polar mesopause region (e.g. Burns et al., 2002) and obtained from the MSISE-90 model.
In addition to the seasonal variation, large day-to-day variability in rotational temperature was observed. The rotational temperature showed a 20-K increase during 2 days in early May 2008. Based on a comparison of OH volume emission rate and temperature profiles observed by the SABER instruments onboard the TIMED satellite, it is inferred that the temperature increase was induced by the enhancement of the downward motion followed by the lowering of the OH airglow layer over Syowa Station. A simple quantitative discussion to estimate the enhanced vertical wind during the temperature increase demonstrated that a downward motion twofold larger than the typical vertical wind can cause the observed increase in temperature at the height of the OH layer. However, the cause of the enhanced downward motion can not be clarified by the single MF radar observation.
Although it was not referred to in this paper, there is a significant reduction in rotational temperature during early August, as shown in Fig. 2. In this period, the upwelling of the OH layer height is also found in the SABER dataset (not shown). However, no remarkable enhancement of the equatorward wind is seen in the MF dataset for this period. A similar depletion has also been reported in other winters at Davis Station, which is located at a distance +39° in longitude from Syowa Station at nearly the same latitude (French et al., 2005). The observations of these researchers show nearly regular reductions in the OH temperature during middle August in each winter. We suggest that this is an interesting phenomenon and consider that the cause of this “August depletion” may not be the same as that described herein. This depletion will be analyzed and discussed in future work.
Additional interesting scientific issues, such as the nature of the long-term trend in upper mesospheric temperatures and auroral effects on the polar mesopause region, will be investigated in the future using the long-term data set produced by this instrument.
The authors are grateful to the SABER team of the NASA/TIMED mission for providing the temperature and OH emission dataset (Ver. 1.07).
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