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Comparison of OH rotational temperatures measured by the spectral airglow temperature imager (SATI) and by a tilting-filter photometer
Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 891 – 897
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Comparison of OH rotational temperatures measured by the spectral airglow temperature imager (SATI) and by a tilting-.lter photometer K. Shiokawaa;∗ , Y. Otsukaa , T. Ogawaa , H. Takahashib , T. Nakamurac , T. Shimomaid a Solar-Terrestrial
Environment Laboratory, Nagoya University, 3-13, Honohara, Toyokawa, Aichi 442-8507, Japan b Instituto Nacional de Pesquisas Espaciais, S. J. dos Campos, SP, Brazil c Radio Science Center for Space and Atmosphere, Kyoto University, Uji, Japan d Interdisciplinary Faculty of Science and Engineering, Shimane University, Matsue, Japan Received 29 June 2001; received in revised form 4 June 2002; accepted 3 March 2004
Abstract Measurements of OH rotational temperatures have been carried out at Shigaraki, Japan (34:8◦ N, 136:1◦ E), for 8 nights in September–December 2000, using the Spectral Airglow Temperature Imager (SATI) and a tilting-.lter zenith photometer (MC4), simultaneously. Clear positive correlation was obtained between the two temperatures. However, the absolute temperatures obtained by SATI were unusually low (∼130–180 K) with di;erences of ∼60 K from the MC4 temperatures. We discuss possible causes of the temperature di;erences, particularly for SATI, which newly uses OH(6-2) Q-branch lines (MC4 uses OH(6-2) P-branch lines) and imaging optics for the temperature measurement. c 2004 Elsevier Ltd. All rights reserved. Keywords: Airglow; Rotational temperature; Mesopause region; Spectral measurement; SATI; Aeronomy
1. Introduction The Spectral Airglow Temperature Imager (SATI) has been developed by Wiens et al. (1997), as a revision of the Mesospheric Oxygen Rotational Temperature Imager (MORTI, Wiens et al., 1991). Both instruments employed a new optical system, which uses cooled-CCD detectors to image airglow lines as concentric circles (spectral scanning in the radial direction and sky azimuth in the azimuthal direction). The instruments use a narrow-band .lter, which works as a Fabry–Perot etalon. MORTI measures only the O2 lines, while changing the .lter allows the revised SATI to measure O2 and OH bands. SATI has been widely introduced in Spain, Japan, and Canada during the Planetary Scale Mesopause Observing System (PSMOS) period. ∗
Corresponding author. Fax: +81-533-89-1539. E-mail address: [email protected] (K. Shiokawa).
Although the great capability of spectral and azimuthal imaging of SATI, the OH temperatures obtained by SATI at mid-latitude Shigaraki, Japan (34:8◦ N, 136:1◦ E), are systematically low at ∼130–180 K. Typical temperature in the OH emission altitudes (∼86 km) at midlatitudes is 170–220 K (She et al., 2000). To investigate the ambiguity in SATI OH temperatures, we simultaneously measured OH rotational temperatures at Shigaraki using SATI and a zenith tilting-.lter photometer (MC4). Large di;erences (∼60 K) were found in the temperatures measured by the two di;erent techniques. In this paper, we discuss possible causes of the temperature ambiguities particularly for SATI.
2. Observations Using an automatic system controlled by a personal computer, SATI has measured O2 and OH temperatures at Shigaraki since December 1997 as part of the Optical
c 2004 Elsevier Ltd. All rights reserved. 1364-6826/$ - see front matter doi:10.1016/j.jastp.2004.03.003
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K. Shiokawa et al. / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 891 – 897 2500
SATI Shigaraki
Q1 (3) Dec.22, 2021UT Q2 (3) 20 T=150K 30
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30 Nov.23, 0944UT 20 T=148K 10 Nov.23, 1824UT T=150K 0 30 Sep.29, 1912UT 20 T=164K 10 Sep.29, 1028UT T=147K 0 30 Sep.27, 1036UT 20 T=149K 10 Sep.27, 1453UT T=161K 0 0 10 20 30 40 50 60 70 80 90 100 110 120 Radius (pixel)
Fig. 1. Examples of the observed (thick solid curves, averaged for all azimuth) and .tted synthetic (thin solid curves) spectra of OH(6-2) Q-branch by SATI at Shigaraki on September–December, 2000. The values of temperature T , which give the minimum di;erence between the observed and .tted spectra, are shown in the left. The .tted synthetic spectra for T + 20 K is also shown by the dotted curves for comparison. In the .tting procedure, 50 times higher signi.cance is weighted for the radius smaller than 76 pixel (indicated by the vertical dashed line).
Mesosphere Thermosphere Imagers (OMTIs, Shiokawa et al., 1999). SATI measures these bands alternately with an exposure time of 2 min. The time resolution of the O2 and OH temperatures obtained by SATI is thus ∼4 min. SATI has an annular .eld of view in the sky, with a radius of 30◦ ± 3:5◦ zenith angle centered on the zenith. The temperature and intensity of each emission are obtained for 12 azimuthal sectors. In this paper, we use azimuthally averaged data for comparison with the zenith-looking photometer. The rotational temperature was calculated using rotational transition probabilities given by Gattinger (1984). Fig. 1 shows examples of the observed (thick solid curve) and .tted synthetic (thin solid curve) spectra of OH(6-2) Q-branch by SATI. Three Q-branch lines (Q1 (1): 834:5 nm, Q1 (2): 835:3 nm, and Q1 (3): 836:5 nm) are identi.ed. Minor contributions from the second Q-branch lines (Q2 (1): 834:4 nm, Q2 (2): 835:0 nm, and Q2 (3): 836:1 nm) should
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Photometer output counts/sec.
observed calculated calculated (T+20K)
1500 1000 500 0 836.0
838.0
840.0
842.0
844.0
846.0
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Fig. 2. An example of the OH(6,2) P branch spectrum obtained by MC4 at 1000 UT (1900 LT) on September 19, 2000.
be also included in the observed spectrum. The peak heights were mostly reproduced by the synthetic curves (which includes both the Q1 and Q2 lines), particularly for Q1; 2 (1) and Q1; 2 (3). Detailed discussion of the spectral .tting procedures will be cited in Section 3. The zenith-looking tilting-.lter photometer (MC4) has measured airglow emission intensities of OI (557:7 nm), OI (630:0 nm), NaD (589:3 nm), OH(6-2), and O2 b (0-1) bands at the same place in Shigaraki since September 2000. The .eld of view of MC4 is 1:2 × 5:7◦ , equivalent to a 2 × 9 km rectangular area at around 90 km altitude in the zenith direction. The spectral resolutions of the OH(6-2) P lines are 0:96 nm for P1 (2) at 839.9 and 0:85 nm for P1 (4) at 846:6 nm. The background light level was measured at 847:7 nm. An example of the observed OH(6-2) P branch spectrum is shown in Fig. 2. The background is subtracted in this plot. The P1 (2) (839:9 nm), P1 (3) (843:0 nm) and P1 (4) (846:5 nm) lines can be seen clearly. The P2 series (small secondary peaks) are also well separated from the P1 series. In the present study intensity ratio between P1 (4) and P1 (2) was used to calculate the rotational temperature. The P1 (3) line was not used because there are P1 (12) and P2 (12) lines of the OH(5,1) band near to the P1 (3) line at 843:0 nm. It is diIcult to estimate the contamination of these lines. In the temperature calculation of MC4, we used .tting of synthetic spectra, similarly to the procedure for SATI, considering the e;ect of these lines and contamination of minor Q-branch lines in this wavelength region. The MC4 photometer was calibrated using a laboratory light standard, Eppley ES 8315, and a MgO white di;user screen. Overall, sensitivity was around 20 counts/Rayleigh/s, which provides a statistical signal-to-noise ratio of better than 30. Estimated errors originating from .lter transmission functions and absolute sensitivity for temperature calculation are ±2 K, and there is a random error range of ±3 K. In the present study, the intensity ratio between P1 (4) and P1 (2) was used to calculate the rotational temperature. The temperatures depend on the rotational transition probability, A(j; j ), where j is rotational quantum number. At
K. Shiokawa et al. / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 891 – 897
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TOH(SATI, fitted to MC4)
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Fig. 3. Comparison of OH rotational temperatures measured by SATI (thin solid curves) and MC4 (thick solid curves) at Shigaraki on December 22 and September 29, 2000. The SATI temperatures .tted to the MC4 temperatures by Eq. (1) are shown by thick dashed curves. The thin dotted curves are the SATI temperatures modi.ed by using Q-branch parameters of French et al. (2000).
present three published tables are available for the OH(6-2) band (Mies, 1974; Langho; et al., 1986; Turnbull and Lowe, 1989). According to French et al. (2000), there are significant di;erences between them. We used the A(j; j ) values given by Mies (1974). In this case, the di;erence from Turnbull and Lowe (1989) was −7 K, and from Langho; et al. (1986), +6 K. Discussion of the OH rotational transition probability is still open. Therefore, we have an uncertainty of ±6 K in the absolute temperature. The time resolution of the observation sequence is 1:5 min; for the rotational temperature determination, we took a running average of 5 data points, which is suIcient to compare the two data sets. In this paper, we use data on OH rotational temperatures obtained simultaneously by the two instruments for the 8 clear-sky nights of September 26, 27, and 29, October 26, November 22–24, and December 22. Fig. 3 shows an example of the measured OH rotational temperatures on September 29 and December 22, 2000. On both nights, the temperatures obtained by SATI were signi.cantly lower than those obtained by MC4. Looking at the other data set, we noticed that the di;erence in temperature between the two instruments is not constant, varying from 50 to 80 K on average. Concerning the short time variations, however, the relative
Fig. 4. Correlation of OH rotational temperatures measured by SATI and MC4. Each data point is indicated by colors, which show OH band intensity measured by SATI. The line shows the linear least-squares .tting (Eq. (1) in the text). The number of data points, average and standard deviation (sigma) of the temperatures, and the correlation coeIcient are shown at the top of the .gure.
amplitudes of variation obtained by both instruments was mostly similar. Based on 8-years observation by a sodium lidar at Fort Collins (41◦ N, 105◦ W), She et al. (2000) determined average temperatures in the mesopause region at midlatitudes. According to their results, the average temperatures in September and December at the OH emission altitude (86 km) are ∼195 and ∼210 K, respectively. The temperatures of SATI in Fig. 3 are in the range of ∼135–185 K, which is far lower than the average temperatures in the mesopause region. Fig. 4 shows the correlation between the OH rotational temperatures obtained by SATI and MC4 for the 8 nights at Shigaraki. The 5-point running-averaged MC4 temperatures are interpolated to obtain values at the time when SATI temperatures were measured. Thus, the time resolution of the data plotted in Fig. 4 is ∼4 min. A clear positive correlation is seen in Fig. 4. The correlation coeIcient r is 0.87. By .tting a linear function to this plot, we obtained the correction equation for converting from the SATI temperature TOH (SATI ) (K) to the MC4 temperature TOH (MC4) (K), i.e., TOH (MC4) = 1:59 × TOH (SATI ) − 24:3:
(1)
The dashed curves in Fig. 3 show SATI temperatures corrected by Eq. (1). They .t the MC4 temperatures with a di;erence of mostly less than 20 K. The di;erence in the amplitude of short-period variation (less than 1 h) could be
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due to the di;erence in the .eld of view. The SATI data are an average of a 30◦ circular ring view, while the MC4 data are from a small area on the zenith, as mentioned above. For the data of December 22, there seems to be a systematic di;erence of the .tted SATI temperatures, which is higher from the MC temperatures in the early evening, and lower from midnight to morning, suggesting that the parameters in Eq. (1) slightly vary depending on nights. The color scales showing the data points in Fig. 4 indicate the OH band intensity obtained by SATI. Clearly, both TOH (SATI ) and TOH (MC4) increase with increasing OH airglow intensity. A similar positive correlation has been discussed by previous researchers (e.g., Takahashi et al., 1974; Clemesha et al., 1991). This positive correlation may be due to a temperature-dependent reaction rate. If so, it is interesting to note that both the P- and Q-branch temperatures show the clear positive correlation in Fig. 4. However, this positive correlation can also be attributed to correlated temperature and density changes in the upper atmosphere, as suggested by Clemesha et al. (1991) using nocturnal mean data.
3. Discussion As shown in the previous section, there are large discrepancies (50–80 K) of the observed temperatures between SATI and MC4. Although the MC4 temperatures (180– 260 K in Fig. 4) may be slightly higher than the typical midlatitude temperatures from September to December (∼195–210 K), the SATI temperatures are much lower (130 –180 K in Fig. 4). Because SATI employed a new imaging optics with a CCD detector and is widely used at several places in the world, we particularly discuss the causes of the low temperatures of SATI in this section. 3.1. Spectral =tting Examples of spectral .tting of SATI are shown in Fig. 1. The following procedures are employed to .t the instrumental synthetic spectra S(T; k) to the observed spectra Io (k) for SATI (k and T denote the radius of the fringe in pixel number and temperature, respectively). (1) determination of e;ective refractive index and center wavelength for each fringe image to convert the synthetic spectra as a function of wavelength to those as a function of radius in pixels, (2) least-squares .tting of S(T; k) to Io (k) to determine Em (band intensity) and Gd (background continuum intensity) that convert S(T; k) to the intensity Is (T;k)(=Em S(T; k) + Gd ), and (3) calculation of residuals R = (Io (k) − Is (T; k))2 for each temperature to .nd out the temperature that gives minimum R. Because the Q1; 2 (3) peak is much smaller than the other two peaks, 50 times higher signi.cance is weighted for the radius smaller than 76 (indicated by vertical dashed line in Fig. 1) in the .tting procedures of (2) and (3). Thus,
the temperature is determined primarily from the intensity ratio of Q1; 2 (1) and Q1; 2 (3). The procedure (1) sometimes gives slightly inaccurate conversion from wavelength to radius, so that the peak locations of the observed and synthetic spectra become slightly di;erent, as shown in the spectra at 1028 UT of September 29 in Fig. 1. However, the observed intensity ratio is well reproduced by the best-.t synthetic spectra at T = 147 K (indicated by the dotted curve) particularly for Q1; 2 (1) and Q1; 2 (3). The curves of Is (T; k) for 20 K higher temperatures are also shown in Fig. 1 as dotted curves, which is clearly separated from the observed curves. Thus, it seems that the ambiguity of the spectral .tting cannot explain the systematic low temperatures of SATI. The 50 times higher weighting at the Q1; 2 (3) peaks below 76 pixel may slightly change the result of the least-squares .tting. We compared the results with the case of no weighting. For the case of September 29, 2000, T = 164 K (with weighting) → 133 K (no weighting) at 1912 UT and T = 147 K → 131 K at 1028 UT. For the case of December 22, 2000, T = 150 K (with weighting) → 157 K (no weighting) at 2021 UT and T = 141 K → 144 K at 1150 UT. Thus, the weighting of Q1; 2 (3) is not the cause of the low temperatures of SATI. 3.2. Water vapor absorption The observed peaks at Q1; 2 (2) are always smaller than those of .tted peaks in Fig. 1. This might indicate the absorption of this particular line by water vapor. Smith and Newnham (2001) showed measurements of water vapor absorption lines in the Q-branch wavelength range on the basis of observations at the Rutherford Appleton Laboratory for several days. According to their Fig. 5, signi.cant absorption is not seen around the Q1; 2 (2) lines. However, in the tables of water vapor absorption lines by Toth (1994) and Flaud et al. (1997), there are several lines of absorptions in the vicinity of all Q1; 2 (1; 2; 3) lines. In our Fig. 1, the di;erences between the observed and .tted peaks at the Q1; 2 (2) lines are signi.cant in September and less in November and December. This fact is consistent with the idea of the water vapor absorption, because atmospheric humidity is high in summer and low in winter in Japan. Although we chose the clear sky interval for the present analysis, the curve of 1453 UT on September 27 was obtained with thin clouds in the sky for a brief period of ∼1 h (identi.ed from all-sky airglow images). This curve shows larger di;erence at Q1; 2 (2) peak compared with that of 1036 UT, suggesting the e;ect of water vapor absorption. However, even for the data of December 22, where the .tting is rather well, the estimated temperatures are very low compared with the results of MC4 (Fig. 3). We should note that considering this water vapor absorption, the .tting equation (1) may be applicable primarily for the fall season, where the present data set was obtained.
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3.3. Flat-=eld calibration Because SATI employs imaging optics, it needs a Kat-.eld calibration to determine the instrumental synthetic spectra and absolute sensitivity. The count N (counts) on a CCD pixel by the incident light I () (R/nm) can be denoted as N = ti G I ()Ttotal () d + DK ; (2) where G (count/R/s) is the sensitivity of the optics, ti (s) is the integration time, and DK is the dark count of the CCD detector. Ttotal () is the transmission of the total optics including .lter, lenses, and windows. Ttotal () can be expressed as Ttotal () = Toptics ()T.lter (), where Toptics () and T.lter () denote transmission of the optics and .lter, respectively. We assume that Toptics () does not depend on (Toptics () = Toptics ) and put Toptics out from the integration of (2). Namely, N = ti GToptics I ()T.lter () d + DK : (3) The function T.lter () is determined by the calibration of each .lter. For SATI, T.lter () is measured at two incident angles: 0◦ and maximum angle. For incident angles between these two values, we use theoretical transmission functions to obtain the transmission for di;erent incident angles and for di;erent wavelengths corresponding to the di;erent fringes. GToptics in Eq. (3) is determined for each CCD pixels by a Kat-.eld calibration using integrating sphere. Because the size of the front optics of SATI is rather large (232 mm), we need a large integrating sphere as a spatially uniform light source. For our SATI, we use a 2-m integrating-sphere at the National Institute of Polar Research, Japan, and a 1-m half-sphere with 8 cylindrical lenses in Canada. Both spheres use panchromatic light sources, such as tungsten lamp. Using the information of known intensity I () of the spheres and T.lter () obtained by the .lter calibration, we can determine the absolute sensitivity GToptics for each CCD pixel. It is not likely that Toptics has a wavelength dependence for such a short wavelength range of the three Q-branch lines (2 nm). However, Toptics decreases with increasing the radius, because of the decrease of optical transmission of camera lens at the edge of the image. As discussed in Section 3.1 the spectral .tting procedure changes the e;ective refractive index and center wavelength for each observed fringe image (procedure (1)). This procedure means that the peak location of synthetic spectra varies for each measurement, as shown by the example of 1028 UT on September 29 as shown in Fig. 1. This may cause a mismatch of the sensitivity GToptics determined by the Kat-.eld calibration for each pixel (each radius) and GToptics during the airglow measurement. The best way to determine the accurate synthetic spectra is to use monochromatic light source for the integrating sphere calibration, instead of using panchromatic light source and
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the theoretical .lter transmission functions. However, because the size of the front optics of SATI is rather large, the size of the integrating sphere should be large (1–2 m). For such a large integrating sphere, monochromatic light source with suIcient brightness is not available. 3.4. Transition probabilities The fourth ambiguity of the temperature determination is the di;erence of the adopted transition probabilities. SATI uses Gattinger (1984) and MC4 uses Mies (1974). We roughly estimated the di;erence of rotational temperatures due to the di;erence of transition probabilities for both OH(6-2) Q- and P-branches. For the Q-branch at T = 200 K, Gattinger (1984) gives the line intensity ratio Q1 (2)=Q1 (1) = 0:381, while Mies (1974) gives Q1 (2)=Q1 (1) = 0:383. Thus, both probabilities give almost same temperature for the Q-branch temperature estimation. For the P-branch at T = 200 K, Gattinger (1984) gives the line intensity ratio P1 (4)=P1 (2) = 0:778, while Mies (1974) gives P1 (4)=P1 (2) = 0:831. For T = 190 K, Mies (1974) gives P1 (4)=P1 (2) = 0:78, which is nearly equal to the ratio at T = 200 K for Gattinger (1984). Thus, the two probabilities give temperature di;erence of ∼10 K for the P-branch temperature estimation. French et al. (2000) showed that the di;erences of temperatures estimated using three transition probabilities by Mies (1974), Langho; et al. (1986), and Turnbull and Lowe (1989) were ∼11 K. From these considerations, the temperature ambiguity due to the transition probabilities is not likely to be the major cause of the systematic low temperatures of SATI. 3.5. Use of Q-branch lines The wavelength scan of SATI to obtain several line intensities is done by changing incident angle of the light to the .lters, similarly to that of tilting-.lter photometers. The advantage of SATI is that it does not mechanically tilt the .lters, but the lights in di;erent incident angles focus on di;erent locations of the imaging CCD detector by the optics, so that there are no moving parts (Wiens et al., 1991). However, this method has a limitation of the range of incident angle (and thus, wavelength range) that can be monitored, because of the limit of the CCD size. The OH(6-2) Q-branch lines are suitable for SATI, because it has a narrower wavelength range (836:5 nm (Q1 (3)) –834:5 nm (Q1 (1)) = 2:0 nm) compared with that of frequently used OH(6-2) P-branch (846:5 nm (P1 (4))– 839:9 nm (P1 (2)) = 6:6 nm). Pendleton and Taylor (2002) pointed out, however, from a high-precision study of the rotational branching of the OH (6-2) band, that the rotational temperatures determined on the basis of OH Meinel (6-2) Q-branch intensities contain systematic errors, irrespective of the speci.c choice of A(j; j ) coeIcients used in the analysis. They suggested using either empirical or corrected coeIcients for accurate
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determination of rotational temperatures from Q-branch intensities. The high correlation of r = 0:87 between the SATI and MC4 temperatures is consistent with their suggestion. For the empirical coeIcients (line intensities) of Q-branch system, French et al. (2000) determined ratios of the line intensities empirically using airglow measurement by a Czerny–Turner scanning spectrophotometer at Davis, Antarctica. According to their Table 1, the ratios of Q1 (1)=P1 (2) and Q1 (3); =P1 (4) were 1.261 and 0.173, respectively, while those of Gattinger (1984) (adopted for SATI) were 1.476 and 0.260, respectively. Because our SATI determines the temperature essentially from the ratio of Q1 (1)=Q1 (3), this di;erence directly a;ects on the temperature calculation. We estimated this e;ect by modifying the line intensities of Q1 (1) and Q1 (3) using the result of French et al. (2000) (Q1 (1) = 1:261P1 (2) and Q1 (3) = 0:173P1 (4) with P1 (2) and P1 (4) line intensities by Gattinger, 1984). From the comparison of Q1 (1)=Q1 (3) between the original and modi.ed values, we estimate the temperature modi.cation by the change of the coeIcients. The temperatures signi.cantly increased (30–60 K) from the original values (120 → 148 K, 150 → 189 K, and 190 → 250 K). Thus, this e;ect of incorrect Q-branch coeIcient can be a major cause of the systematic low OH temperatures of SATI. The thin dotted curves in Fig. 3 indicate the temperatures estimated using this modi.cation. The temperature signi.cantly increases from the original values, though there are still some discrepancies (20–40 K) from the MC4 temperatures. 4. Concluding remarks From simultaneous measurements in September–December 2000 at midlatitude Shigaraki, we obtained a clear positive correlation of the OH rotational temperatures measured by SATI (through the OH(6-2) Q-branch) and MC4 (through the OH(6-2) P-branch). However, the temperature measured by SATI was unusually low compared with typical temperature in the mesopause region. Since SATI has been widely used at several locations in the world, we discuss possible causes of the systematic low temperature of our SATI, i.e., (1) spectral .tting, (2) water vapor absorption, (3) Kat-.eld calibration, (4) transition probabilities, and (5) use of Q-branch lines. (1)–(4) may cause systematic o;set of the temperature (5) can be a major cause of the low temperature of SATI. It should be noted that the low OH temperature of SATI is signi.cant not only for our SATI in Japan, but also for other SATIs in Spain (LLopez-GonzLalez et al., 2003), Saskatchewan (Chshyolkova, 2003), and Resolute Bay (Won et al., 2003). Detailed analyses of SATI data from Resolute Bay also suggests the importance of the empirically-determined Q-branch coeIcients (G.G. Shepherd, private communications, 2003). Measurement of
airglow rotational temperature is important and useful for the investigation of the mesopause dynamics, but is not a simple matter because of various inherent diIculties of the measurement, as discussed above. Comparison of di;erent techniques, such as that in this paper, will help to solve these diIculties. Acknowledgements SATI and MC4 at the Shigaraki MU Observatory were operated in collaboration with the Radio Science Center for Space and Atmosphere, Kyoto University. The MU radar at Shigaraki belongs to and is operated by the Radio Science Center for Space and Atmosphere, Kyoto University. We are grateful to G.G. Shepherd for his kind assistance on the comparison of the four SATI instruments and making constructive comments on the revision of the manuscript. We thank R.H. Wiens for his helpful comments on the revision of the manuscript. We are also grateful to the referee for making important suggestions. This work was supported by Grant-in-Aid of the Ministry of Education, Culture, Sports, Science, and Technology of Japan (11440145). References Clemesha, B.R., Takahashi, H., Batista, P.P., Sahai, Y., Simonich, D.M., 1991. The temperature dependence of airglow emissions from the upper mesosphere and lower thermosphere. Planetary Space Science 39, 1397–1404. Chshyolkova, T.E., 2003. Spectral airglow temperature imager: operation and data processing. Master Thesis, Department of Physics and Engineering Physics, University of Saskatchewan. Flaud, J.-M., Camy-Peyret, C., Bykov, A., Naumenko, O., Petrova, T., Scherbakov, A., Sinitsa, L., 1997. The water vapor linestrengths between 11 600 and 12 750 cm−1 . Journal of Molecular Spectroscopy 185, 211–221. French, W.J.R., Burns, G.B., Finlayson, K., Greet, P.A., Lowe, R.P., Williams, P.F.B., 2000. Hydroxyl (6,2) airglow emission intensity ratios for rotational temperature determination. Annales of Geophysics 18, 1293–1303. Gattinger, R.L., 1984. Synthetic spectra of airglow emitters. Herzberg Institute of Astrophysics, Ottawa. Langho;, S.R., Werner, H.J., Rosmus, P., 1986. Theoretical transition probabilities for the OH Meinel system. Journal of Molecular Spectroscopy 118, 507–529. LLopez-GonzLalez, M.J., RodrLMguez, E., Wiens, R.H., Shepherd, G.G., Sargoytchev, S., Brown, S., Shepherd, M.G., Aushev, V.M., LLopez-Moreno, J.J., Rodrigo, R., Cho, Y.-M., 2003. Seasonal variations of O2 atmospheric and OH (6-2) airglow and temperature at mid-latitudes from SATI observations. Annales of Geophysics, in press. Mies, F.H., 1974. Calculated vibrational transition probabilities of OH(X2II). Journal of Molecular Spectroscopy 53, 150–188. Pendleton Jr., W.R., Taylor, M.J., 2002. The impact of L-uncoupling on Einstein coeIcients for the OH Meinel (6,2) band: implications for Q-branch rotational temperatures. Journal of Atmospheric Solar-Terrestrial Physics 64, 971–983.
K. Shiokawa et al. / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 891 – 897 She, C.Y., Chen, S., Hu, Z., Sherman, J., Vance, J.D., Vasoli, V., White, M.A., Yu, J., Krueger, D.A., 2000. Eight-year climatology of nocturnal temperature and sodium density in the mesopause region (80 to 105 km) over Fort Collins, CO (41◦ N, 105◦ W). Geophysical Research Letters 27, 3289–3292. Shiokawa, K., Katoh, Y., Satoh, M., Ejiri, M.K., Ogawa, T., Nakamura, T., Tsuda, T., Wiens, R.H., 1999. Development of optical mesosphere thermosphere imagers (OMTI). Earth Planets Space 51, 887–896. Smith, K.M., Newnham, D.A., 2001. High-resolution atmospheric absorption by water vapor in the 830–985 nm region: evaluation of spectroscopic databases. Geophysical Research Letters 28, 3115–3118. Takahashi, H., Clemesha, B.R., Sahai, Y., 1974. Nightglow OH(8,3) band intensities and rotational temperature at 23◦ S. Planetary Space Science 22, 1323–1329. Toth, R.A., 1994. Measurements of H216 O line positions and strengths: 11610 to 12861 cm−1 . Journal of Molecular Spectroscopy 166, 176–183.
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Turnbull, D.N., Lowe, R.P., 1989. New hydroxyl transition probabilities and their importance in airglow studies. Planetary Space Science 37 (6), 723–738. Wiens, R.H., Zhang, S.-P., Peterson, R.N., Shepherd, G.G., 1991. MORTI: a mesopause oxygen rotational temperature imager. Planetary Space Science 39, 1363–1375. Wiens, R.H., Moise, A., Brown, S., Sargoytchev, S., Peterson, R.N., Shepherd, G.G., LLopez-GonzLalez, M.J., LLopez-Moreno, J.J., Rodrigo, R., 1997. SATI: a spectral airglow temperature imager. Advances in Space Research 19, 677–680. Won, Y.-I., Wu, Q., Cho, Y.M., Shepherd, G.G., Killeen, T.L., Espy, P.J., Kim, Y., Solheim, B., 2003. Polar cap observations of mesospheric and lower thermospheric 4-hour waves in temperature. Geophysical Research Letters 30 (7), 1377. doi:10.1029/2002GL016364.
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Comparison of OH rotational temperatures measured by the spectral airglow temperature imager (SATI) and by a tilting-.lter photometer K. Shiokawaa;∗ , Y. Otsukaa , T. Ogawaa , H. Takahashib , T. Nakamurac , T. Shimomaid a Solar-Terrestrial
Environment Laboratory, Nagoya University, 3-13, Honohara, Toyokawa, Aichi 442-8507, Japan b Instituto Nacional de Pesquisas Espaciais, S. J. dos Campos, SP, Brazil c Radio Science Center for Space and Atmosphere, Kyoto University, Uji, Japan d Interdisciplinary Faculty of Science and Engineering, Shimane University, Matsue, Japan Received 29 June 2001; received in revised form 4 June 2002; accepted 3 March 2004
Abstract Measurements of OH rotational temperatures have been carried out at Shigaraki, Japan (34:8◦ N, 136:1◦ E), for 8 nights in September–December 2000, using the Spectral Airglow Temperature Imager (SATI) and a tilting-.lter zenith photometer (MC4), simultaneously. Clear positive correlation was obtained between the two temperatures. However, the absolute temperatures obtained by SATI were unusually low (∼130–180 K) with di;erences of ∼60 K from the MC4 temperatures. We discuss possible causes of the temperature di;erences, particularly for SATI, which newly uses OH(6-2) Q-branch lines (MC4 uses OH(6-2) P-branch lines) and imaging optics for the temperature measurement. c 2004 Elsevier Ltd. All rights reserved. Keywords: Airglow; Rotational temperature; Mesopause region; Spectral measurement; SATI; Aeronomy
1. Introduction The Spectral Airglow Temperature Imager (SATI) has been developed by Wiens et al. (1997), as a revision of the Mesospheric Oxygen Rotational Temperature Imager (MORTI, Wiens et al., 1991). Both instruments employed a new optical system, which uses cooled-CCD detectors to image airglow lines as concentric circles (spectral scanning in the radial direction and sky azimuth in the azimuthal direction). The instruments use a narrow-band .lter, which works as a Fabry–Perot etalon. MORTI measures only the O2 lines, while changing the .lter allows the revised SATI to measure O2 and OH bands. SATI has been widely introduced in Spain, Japan, and Canada during the Planetary Scale Mesopause Observing System (PSMOS) period. ∗
Corresponding author. Fax: +81-533-89-1539. E-mail address: [email protected] (K. Shiokawa).
Although the great capability of spectral and azimuthal imaging of SATI, the OH temperatures obtained by SATI at mid-latitude Shigaraki, Japan (34:8◦ N, 136:1◦ E), are systematically low at ∼130–180 K. Typical temperature in the OH emission altitudes (∼86 km) at midlatitudes is 170–220 K (She et al., 2000). To investigate the ambiguity in SATI OH temperatures, we simultaneously measured OH rotational temperatures at Shigaraki using SATI and a zenith tilting-.lter photometer (MC4). Large di;erences (∼60 K) were found in the temperatures measured by the two di;erent techniques. In this paper, we discuss possible causes of the temperature ambiguities particularly for SATI.
2. Observations Using an automatic system controlled by a personal computer, SATI has measured O2 and OH temperatures at Shigaraki since December 1997 as part of the Optical
c 2004 Elsevier Ltd. All rights reserved. 1364-6826/$ - see front matter doi:10.1016/j.jastp.2004.03.003
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SATI Shigaraki
Q1 (3) Dec.22, 2021UT Q2 (3) 20 T=150K 30
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Fig. 1. Examples of the observed (thick solid curves, averaged for all azimuth) and .tted synthetic (thin solid curves) spectra of OH(6-2) Q-branch by SATI at Shigaraki on September–December, 2000. The values of temperature T , which give the minimum di;erence between the observed and .tted spectra, are shown in the left. The .tted synthetic spectra for T + 20 K is also shown by the dotted curves for comparison. In the .tting procedure, 50 times higher signi.cance is weighted for the radius smaller than 76 pixel (indicated by the vertical dashed line).
Mesosphere Thermosphere Imagers (OMTIs, Shiokawa et al., 1999). SATI measures these bands alternately with an exposure time of 2 min. The time resolution of the O2 and OH temperatures obtained by SATI is thus ∼4 min. SATI has an annular .eld of view in the sky, with a radius of 30◦ ± 3:5◦ zenith angle centered on the zenith. The temperature and intensity of each emission are obtained for 12 azimuthal sectors. In this paper, we use azimuthally averaged data for comparison with the zenith-looking photometer. The rotational temperature was calculated using rotational transition probabilities given by Gattinger (1984). Fig. 1 shows examples of the observed (thick solid curve) and .tted synthetic (thin solid curve) spectra of OH(6-2) Q-branch by SATI. Three Q-branch lines (Q1 (1): 834:5 nm, Q1 (2): 835:3 nm, and Q1 (3): 836:5 nm) are identi.ed. Minor contributions from the second Q-branch lines (Q2 (1): 834:4 nm, Q2 (2): 835:0 nm, and Q2 (3): 836:1 nm) should
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observed calculated calculated (T+20K)
1500 1000 500 0 836.0
838.0
840.0
842.0
844.0
846.0
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Fig. 2. An example of the OH(6,2) P branch spectrum obtained by MC4 at 1000 UT (1900 LT) on September 19, 2000.
be also included in the observed spectrum. The peak heights were mostly reproduced by the synthetic curves (which includes both the Q1 and Q2 lines), particularly for Q1; 2 (1) and Q1; 2 (3). Detailed discussion of the spectral .tting procedures will be cited in Section 3. The zenith-looking tilting-.lter photometer (MC4) has measured airglow emission intensities of OI (557:7 nm), OI (630:0 nm), NaD (589:3 nm), OH(6-2), and O2 b (0-1) bands at the same place in Shigaraki since September 2000. The .eld of view of MC4 is 1:2 × 5:7◦ , equivalent to a 2 × 9 km rectangular area at around 90 km altitude in the zenith direction. The spectral resolutions of the OH(6-2) P lines are 0:96 nm for P1 (2) at 839.9 and 0:85 nm for P1 (4) at 846:6 nm. The background light level was measured at 847:7 nm. An example of the observed OH(6-2) P branch spectrum is shown in Fig. 2. The background is subtracted in this plot. The P1 (2) (839:9 nm), P1 (3) (843:0 nm) and P1 (4) (846:5 nm) lines can be seen clearly. The P2 series (small secondary peaks) are also well separated from the P1 series. In the present study intensity ratio between P1 (4) and P1 (2) was used to calculate the rotational temperature. The P1 (3) line was not used because there are P1 (12) and P2 (12) lines of the OH(5,1) band near to the P1 (3) line at 843:0 nm. It is diIcult to estimate the contamination of these lines. In the temperature calculation of MC4, we used .tting of synthetic spectra, similarly to the procedure for SATI, considering the e;ect of these lines and contamination of minor Q-branch lines in this wavelength region. The MC4 photometer was calibrated using a laboratory light standard, Eppley ES 8315, and a MgO white di;user screen. Overall, sensitivity was around 20 counts/Rayleigh/s, which provides a statistical signal-to-noise ratio of better than 30. Estimated errors originating from .lter transmission functions and absolute sensitivity for temperature calculation are ±2 K, and there is a random error range of ±3 K. In the present study, the intensity ratio between P1 (4) and P1 (2) was used to calculate the rotational temperature. The temperatures depend on the rotational transition probability, A(j; j ), where j is rotational quantum number. At
K. Shiokawa et al. / Journal of Atmospheric and Solar-Terrestrial Physics 66 (2004) 891 – 897
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Fig. 3. Comparison of OH rotational temperatures measured by SATI (thin solid curves) and MC4 (thick solid curves) at Shigaraki on December 22 and September 29, 2000. The SATI temperatures .tted to the MC4 temperatures by Eq. (1) are shown by thick dashed curves. The thin dotted curves are the SATI temperatures modi.ed by using Q-branch parameters of French et al. (2000).
present three published tables are available for the OH(6-2) band (Mies, 1974; Langho; et al., 1986; Turnbull and Lowe, 1989). According to French et al. (2000), there are significant di;erences between them. We used the A(j; j ) values given by Mies (1974). In this case, the di;erence from Turnbull and Lowe (1989) was −7 K, and from Langho; et al. (1986), +6 K. Discussion of the OH rotational transition probability is still open. Therefore, we have an uncertainty of ±6 K in the absolute temperature. The time resolution of the observation sequence is 1:5 min; for the rotational temperature determination, we took a running average of 5 data points, which is suIcient to compare the two data sets. In this paper, we use data on OH rotational temperatures obtained simultaneously by the two instruments for the 8 clear-sky nights of September 26, 27, and 29, October 26, November 22–24, and December 22. Fig. 3 shows an example of the measured OH rotational temperatures on September 29 and December 22, 2000. On both nights, the temperatures obtained by SATI were signi.cantly lower than those obtained by MC4. Looking at the other data set, we noticed that the di;erence in temperature between the two instruments is not constant, varying from 50 to 80 K on average. Concerning the short time variations, however, the relative
Fig. 4. Correlation of OH rotational temperatures measured by SATI and MC4. Each data point is indicated by colors, which show OH band intensity measured by SATI. The line shows the linear least-squares .tting (Eq. (1) in the text). The number of data points, average and standard deviation (sigma) of the temperatures, and the correlation coeIcient are shown at the top of the .gure.
amplitudes of variation obtained by both instruments was mostly similar. Based on 8-years observation by a sodium lidar at Fort Collins (41◦ N, 105◦ W), She et al. (2000) determined average temperatures in the mesopause region at midlatitudes. According to their results, the average temperatures in September and December at the OH emission altitude (86 km) are ∼195 and ∼210 K, respectively. The temperatures of SATI in Fig. 3 are in the range of ∼135–185 K, which is far lower than the average temperatures in the mesopause region. Fig. 4 shows the correlation between the OH rotational temperatures obtained by SATI and MC4 for the 8 nights at Shigaraki. The 5-point running-averaged MC4 temperatures are interpolated to obtain values at the time when SATI temperatures were measured. Thus, the time resolution of the data plotted in Fig. 4 is ∼4 min. A clear positive correlation is seen in Fig. 4. The correlation coeIcient r is 0.87. By .tting a linear function to this plot, we obtained the correction equation for converting from the SATI temperature TOH (SATI ) (K) to the MC4 temperature TOH (MC4) (K), i.e., TOH (MC4) = 1:59 × TOH (SATI ) − 24:3:
(1)
The dashed curves in Fig. 3 show SATI temperatures corrected by Eq. (1). They .t the MC4 temperatures with a di;erence of mostly less than 20 K. The di;erence in the amplitude of short-period variation (less than 1 h) could be
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due to the di;erence in the .eld of view. The SATI data are an average of a 30◦ circular ring view, while the MC4 data are from a small area on the zenith, as mentioned above. For the data of December 22, there seems to be a systematic di;erence of the .tted SATI temperatures, which is higher from the MC temperatures in the early evening, and lower from midnight to morning, suggesting that the parameters in Eq. (1) slightly vary depending on nights. The color scales showing the data points in Fig. 4 indicate the OH band intensity obtained by SATI. Clearly, both TOH (SATI ) and TOH (MC4) increase with increasing OH airglow intensity. A similar positive correlation has been discussed by previous researchers (e.g., Takahashi et al., 1974; Clemesha et al., 1991). This positive correlation may be due to a temperature-dependent reaction rate. If so, it is interesting to note that both the P- and Q-branch temperatures show the clear positive correlation in Fig. 4. However, this positive correlation can also be attributed to correlated temperature and density changes in the upper atmosphere, as suggested by Clemesha et al. (1991) using nocturnal mean data.
3. Discussion As shown in the previous section, there are large discrepancies (50–80 K) of the observed temperatures between SATI and MC4. Although the MC4 temperatures (180– 260 K in Fig. 4) may be slightly higher than the typical midlatitude temperatures from September to December (∼195–210 K), the SATI temperatures are much lower (130 –180 K in Fig. 4). Because SATI employed a new imaging optics with a CCD detector and is widely used at several places in the world, we particularly discuss the causes of the low temperatures of SATI in this section. 3.1. Spectral =tting Examples of spectral .tting of SATI are shown in Fig. 1. The following procedures are employed to .t the instrumental synthetic spectra S(T; k) to the observed spectra Io (k) for SATI (k and T denote the radius of the fringe in pixel number and temperature, respectively). (1) determination of e;ective refractive index and center wavelength for each fringe image to convert the synthetic spectra as a function of wavelength to those as a function of radius in pixels, (2) least-squares .tting of S(T; k) to Io (k) to determine Em (band intensity) and Gd (background continuum intensity) that convert S(T; k) to the intensity Is (T;k)(=Em S(T; k) + Gd ), and (3) calculation of residuals R = (Io (k) − Is (T; k))2 for each temperature to .nd out the temperature that gives minimum R. Because the Q1; 2 (3) peak is much smaller than the other two peaks, 50 times higher signi.cance is weighted for the radius smaller than 76 (indicated by vertical dashed line in Fig. 1) in the .tting procedures of (2) and (3). Thus,
the temperature is determined primarily from the intensity ratio of Q1; 2 (1) and Q1; 2 (3). The procedure (1) sometimes gives slightly inaccurate conversion from wavelength to radius, so that the peak locations of the observed and synthetic spectra become slightly di;erent, as shown in the spectra at 1028 UT of September 29 in Fig. 1. However, the observed intensity ratio is well reproduced by the best-.t synthetic spectra at T = 147 K (indicated by the dotted curve) particularly for Q1; 2 (1) and Q1; 2 (3). The curves of Is (T; k) for 20 K higher temperatures are also shown in Fig. 1 as dotted curves, which is clearly separated from the observed curves. Thus, it seems that the ambiguity of the spectral .tting cannot explain the systematic low temperatures of SATI. The 50 times higher weighting at the Q1; 2 (3) peaks below 76 pixel may slightly change the result of the least-squares .tting. We compared the results with the case of no weighting. For the case of September 29, 2000, T = 164 K (with weighting) → 133 K (no weighting) at 1912 UT and T = 147 K → 131 K at 1028 UT. For the case of December 22, 2000, T = 150 K (with weighting) → 157 K (no weighting) at 2021 UT and T = 141 K → 144 K at 1150 UT. Thus, the weighting of Q1; 2 (3) is not the cause of the low temperatures of SATI. 3.2. Water vapor absorption The observed peaks at Q1; 2 (2) are always smaller than those of .tted peaks in Fig. 1. This might indicate the absorption of this particular line by water vapor. Smith and Newnham (2001) showed measurements of water vapor absorption lines in the Q-branch wavelength range on the basis of observations at the Rutherford Appleton Laboratory for several days. According to their Fig. 5, signi.cant absorption is not seen around the Q1; 2 (2) lines. However, in the tables of water vapor absorption lines by Toth (1994) and Flaud et al. (1997), there are several lines of absorptions in the vicinity of all Q1; 2 (1; 2; 3) lines. In our Fig. 1, the di;erences between the observed and .tted peaks at the Q1; 2 (2) lines are signi.cant in September and less in November and December. This fact is consistent with the idea of the water vapor absorption, because atmospheric humidity is high in summer and low in winter in Japan. Although we chose the clear sky interval for the present analysis, the curve of 1453 UT on September 27 was obtained with thin clouds in the sky for a brief period of ∼1 h (identi.ed from all-sky airglow images). This curve shows larger di;erence at Q1; 2 (2) peak compared with that of 1036 UT, suggesting the e;ect of water vapor absorption. However, even for the data of December 22, where the .tting is rather well, the estimated temperatures are very low compared with the results of MC4 (Fig. 3). We should note that considering this water vapor absorption, the .tting equation (1) may be applicable primarily for the fall season, where the present data set was obtained.
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3.3. Flat-=eld calibration Because SATI employs imaging optics, it needs a Kat-.eld calibration to determine the instrumental synthetic spectra and absolute sensitivity. The count N (counts) on a CCD pixel by the incident light I () (R/nm) can be denoted as N = ti G I ()Ttotal () d + DK ; (2) where G (count/R/s) is the sensitivity of the optics, ti (s) is the integration time, and DK is the dark count of the CCD detector. Ttotal () is the transmission of the total optics including .lter, lenses, and windows. Ttotal () can be expressed as Ttotal () = Toptics ()T.lter (), where Toptics () and T.lter () denote transmission of the optics and .lter, respectively. We assume that Toptics () does not depend on (Toptics () = Toptics ) and put Toptics out from the integration of (2). Namely, N = ti GToptics I ()T.lter () d + DK : (3) The function T.lter () is determined by the calibration of each .lter. For SATI, T.lter () is measured at two incident angles: 0◦ and maximum angle. For incident angles between these two values, we use theoretical transmission functions to obtain the transmission for di;erent incident angles and for di;erent wavelengths corresponding to the di;erent fringes. GToptics in Eq. (3) is determined for each CCD pixels by a Kat-.eld calibration using integrating sphere. Because the size of the front optics of SATI is rather large (232 mm), we need a large integrating sphere as a spatially uniform light source. For our SATI, we use a 2-m integrating-sphere at the National Institute of Polar Research, Japan, and a 1-m half-sphere with 8 cylindrical lenses in Canada. Both spheres use panchromatic light sources, such as tungsten lamp. Using the information of known intensity I () of the spheres and T.lter () obtained by the .lter calibration, we can determine the absolute sensitivity GToptics for each CCD pixel. It is not likely that Toptics has a wavelength dependence for such a short wavelength range of the three Q-branch lines (2 nm). However, Toptics decreases with increasing the radius, because of the decrease of optical transmission of camera lens at the edge of the image. As discussed in Section 3.1 the spectral .tting procedure changes the e;ective refractive index and center wavelength for each observed fringe image (procedure (1)). This procedure means that the peak location of synthetic spectra varies for each measurement, as shown by the example of 1028 UT on September 29 as shown in Fig. 1. This may cause a mismatch of the sensitivity GToptics determined by the Kat-.eld calibration for each pixel (each radius) and GToptics during the airglow measurement. The best way to determine the accurate synthetic spectra is to use monochromatic light source for the integrating sphere calibration, instead of using panchromatic light source and
895
the theoretical .lter transmission functions. However, because the size of the front optics of SATI is rather large, the size of the integrating sphere should be large (1–2 m). For such a large integrating sphere, monochromatic light source with suIcient brightness is not available. 3.4. Transition probabilities The fourth ambiguity of the temperature determination is the di;erence of the adopted transition probabilities. SATI uses Gattinger (1984) and MC4 uses Mies (1974). We roughly estimated the di;erence of rotational temperatures due to the di;erence of transition probabilities for both OH(6-2) Q- and P-branches. For the Q-branch at T = 200 K, Gattinger (1984) gives the line intensity ratio Q1 (2)=Q1 (1) = 0:381, while Mies (1974) gives Q1 (2)=Q1 (1) = 0:383. Thus, both probabilities give almost same temperature for the Q-branch temperature estimation. For the P-branch at T = 200 K, Gattinger (1984) gives the line intensity ratio P1 (4)=P1 (2) = 0:778, while Mies (1974) gives P1 (4)=P1 (2) = 0:831. For T = 190 K, Mies (1974) gives P1 (4)=P1 (2) = 0:78, which is nearly equal to the ratio at T = 200 K for Gattinger (1984). Thus, the two probabilities give temperature di;erence of ∼10 K for the P-branch temperature estimation. French et al. (2000) showed that the di;erences of temperatures estimated using three transition probabilities by Mies (1974), Langho; et al. (1986), and Turnbull and Lowe (1989) were ∼11 K. From these considerations, the temperature ambiguity due to the transition probabilities is not likely to be the major cause of the systematic low temperatures of SATI. 3.5. Use of Q-branch lines The wavelength scan of SATI to obtain several line intensities is done by changing incident angle of the light to the .lters, similarly to that of tilting-.lter photometers. The advantage of SATI is that it does not mechanically tilt the .lters, but the lights in di;erent incident angles focus on di;erent locations of the imaging CCD detector by the optics, so that there are no moving parts (Wiens et al., 1991). However, this method has a limitation of the range of incident angle (and thus, wavelength range) that can be monitored, because of the limit of the CCD size. The OH(6-2) Q-branch lines are suitable for SATI, because it has a narrower wavelength range (836:5 nm (Q1 (3)) –834:5 nm (Q1 (1)) = 2:0 nm) compared with that of frequently used OH(6-2) P-branch (846:5 nm (P1 (4))– 839:9 nm (P1 (2)) = 6:6 nm). Pendleton and Taylor (2002) pointed out, however, from a high-precision study of the rotational branching of the OH (6-2) band, that the rotational temperatures determined on the basis of OH Meinel (6-2) Q-branch intensities contain systematic errors, irrespective of the speci.c choice of A(j; j ) coeIcients used in the analysis. They suggested using either empirical or corrected coeIcients for accurate
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determination of rotational temperatures from Q-branch intensities. The high correlation of r = 0:87 between the SATI and MC4 temperatures is consistent with their suggestion. For the empirical coeIcients (line intensities) of Q-branch system, French et al. (2000) determined ratios of the line intensities empirically using airglow measurement by a Czerny–Turner scanning spectrophotometer at Davis, Antarctica. According to their Table 1, the ratios of Q1 (1)=P1 (2) and Q1 (3); =P1 (4) were 1.261 and 0.173, respectively, while those of Gattinger (1984) (adopted for SATI) were 1.476 and 0.260, respectively. Because our SATI determines the temperature essentially from the ratio of Q1 (1)=Q1 (3), this di;erence directly a;ects on the temperature calculation. We estimated this e;ect by modifying the line intensities of Q1 (1) and Q1 (3) using the result of French et al. (2000) (Q1 (1) = 1:261P1 (2) and Q1 (3) = 0:173P1 (4) with P1 (2) and P1 (4) line intensities by Gattinger, 1984). From the comparison of Q1 (1)=Q1 (3) between the original and modi.ed values, we estimate the temperature modi.cation by the change of the coeIcients. The temperatures signi.cantly increased (30–60 K) from the original values (120 → 148 K, 150 → 189 K, and 190 → 250 K). Thus, this e;ect of incorrect Q-branch coeIcient can be a major cause of the systematic low OH temperatures of SATI. The thin dotted curves in Fig. 3 indicate the temperatures estimated using this modi.cation. The temperature signi.cantly increases from the original values, though there are still some discrepancies (20–40 K) from the MC4 temperatures. 4. Concluding remarks From simultaneous measurements in September–December 2000 at midlatitude Shigaraki, we obtained a clear positive correlation of the OH rotational temperatures measured by SATI (through the OH(6-2) Q-branch) and MC4 (through the OH(6-2) P-branch). However, the temperature measured by SATI was unusually low compared with typical temperature in the mesopause region. Since SATI has been widely used at several locations in the world, we discuss possible causes of the systematic low temperature of our SATI, i.e., (1) spectral .tting, (2) water vapor absorption, (3) Kat-.eld calibration, (4) transition probabilities, and (5) use of Q-branch lines. (1)–(4) may cause systematic o;set of the temperature (5) can be a major cause of the low temperature of SATI. It should be noted that the low OH temperature of SATI is signi.cant not only for our SATI in Japan, but also for other SATIs in Spain (LLopez-GonzLalez et al., 2003), Saskatchewan (Chshyolkova, 2003), and Resolute Bay (Won et al., 2003). Detailed analyses of SATI data from Resolute Bay also suggests the importance of the empirically-determined Q-branch coeIcients (G.G. Shepherd, private communications, 2003). Measurement of
airglow rotational temperature is important and useful for the investigation of the mesopause dynamics, but is not a simple matter because of various inherent diIculties of the measurement, as discussed above. Comparison of di;erent techniques, such as that in this paper, will help to solve these diIculties. Acknowledgements SATI and MC4 at the Shigaraki MU Observatory were operated in collaboration with the Radio Science Center for Space and Atmosphere, Kyoto University. The MU radar at Shigaraki belongs to and is operated by the Radio Science Center for Space and Atmosphere, Kyoto University. We are grateful to G.G. Shepherd for his kind assistance on the comparison of the four SATI instruments and making constructive comments on the revision of the manuscript. We thank R.H. Wiens for his helpful comments on the revision of the manuscript. We are also grateful to the referee for making important suggestions. This work was supported by Grant-in-Aid of the Ministry of Education, Culture, Sports, Science, and Technology of Japan (11440145). References Clemesha, B.R., Takahashi, H., Batista, P.P., Sahai, Y., Simonich, D.M., 1991. The temperature dependence of airglow emissions from the upper mesosphere and lower thermosphere. Planetary Space Science 39, 1397–1404. Chshyolkova, T.E., 2003. Spectral airglow temperature imager: operation and data processing. Master Thesis, Department of Physics and Engineering Physics, University of Saskatchewan. Flaud, J.-M., Camy-Peyret, C., Bykov, A., Naumenko, O., Petrova, T., Scherbakov, A., Sinitsa, L., 1997. The water vapor linestrengths between 11 600 and 12 750 cm−1 . Journal of Molecular Spectroscopy 185, 211–221. French, W.J.R., Burns, G.B., Finlayson, K., Greet, P.A., Lowe, R.P., Williams, P.F.B., 2000. Hydroxyl (6,2) airglow emission intensity ratios for rotational temperature determination. Annales of Geophysics 18, 1293–1303. Gattinger, R.L., 1984. Synthetic spectra of airglow emitters. Herzberg Institute of Astrophysics, Ottawa. Langho;, S.R., Werner, H.J., Rosmus, P., 1986. Theoretical transition probabilities for the OH Meinel system. Journal of Molecular Spectroscopy 118, 507–529. LLopez-GonzLalez, M.J., RodrLMguez, E., Wiens, R.H., Shepherd, G.G., Sargoytchev, S., Brown, S., Shepherd, M.G., Aushev, V.M., LLopez-Moreno, J.J., Rodrigo, R., Cho, Y.-M., 2003. Seasonal variations of O2 atmospheric and OH (6-2) airglow and temperature at mid-latitudes from SATI observations. Annales of Geophysics, in press. Mies, F.H., 1974. Calculated vibrational transition probabilities of OH(X2II). Journal of Molecular Spectroscopy 53, 150–188. Pendleton Jr., W.R., Taylor, M.J., 2002. The impact of L-uncoupling on Einstein coeIcients for the OH Meinel (6,2) band: implications for Q-branch rotational temperatures. Journal of Atmospheric Solar-Terrestrial Physics 64, 971–983.
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