5:15 PM - 7:15 PM
[PEM12-P04] NLC Imager for observations of noctilucent clouds in the high latitude region. -Observation plan in Kiruna, Sweden.
Keywords:Noctilucent clouds (NLCs), Polar mesospheric clouds (PMCs), Mesopause, Upper atmosphere, Polar region
Images of noctilucent clouds (NLCs) often contain fine wave structures, ranging in size from a few kilometers to tens of kilometers. These are thought to reflect local, small-scale atmospheric disturbances in the upper mesosphere. Satellite imagery cannot resolve such fine structures, so ground-based imaging of NLCs is the effective method to study small-scale disturbances in the upper mesosphere.
NLCs are often observed by satellites (e.g. Himawari-8, SNPP, MATS, AIM satellites) in high-latitude regions. However, this area is under the influence of the midnight sun, which makes it difficult to detect NLCs from the ground due to the bright background sky condition. Therefore, the opportunities for NLC observations from the ground in high latitude regions are limited. On the other hand, there is an opportunity for NLC observations during the NLC waning period (August) even in the high-latitude region. In addition, slow variations in the solar elevation angle allow continuous observations of NLCs with similar geometric conditions throughout the nights of August. Thus, there is an obvious benefit in studying NLC morphology when the probability of NLC observations increases in high-latitude regions.
We have investigated the feasibility of overcoming this "bright background sky problem" by developing an optical imager dedicated to noctilucent cloud observations. Noctilucent clouds are known to have a spectral peak in their radiance at 400-500 nm. On the other hand, the background spectrum in the twilight sky is attenuated at wavelengths shorter than 680 nm. Therefore, there should be an optimal wavelength band for noctilucent cloud observations that gives a better signal-to-noise ratio (SNR) in shorter wavelength regions. We have developed a new imaging technique based on a cooled CMOS camera equipped with a bandpass filter with a center wavelength of 371 nm (near UV range) and a bandwidth of 40 nm [Endo et al., 2024a]. With this new technique, an effective SNR (SNR>1.80) for NLC under a bright sky condition corresponds to a local solar zenith angle of ˜91°. The effectiveness of this technique has been verified by the first test observation of the NLC in Kiruna, Sweden (N 67.8, E20.4) in August 2024 [Endo et al., 2024b]. We have further optimized the throughput of the imager by using a new objective lens with high transmittance in the NUV region. In this study, we present technical characteristics of the final version of the NLC imager and the observation plan which will be conducted in Kiruna in the summer of 2025.
[1] Endo et al. (2024) “Development of NLC Imager for observation in the high latitude region-The test observation in Kiruna, Sweden”, SGEPSS 2024 Fall meeting, R005-P05
[2] Endo et al. (2024) “NLC Imager for observations in the high latitude region. -The test observation in Kiruna, Sweden”, The 15th Symposium on Polar Science, OSp4
NLCs are often observed by satellites (e.g. Himawari-8, SNPP, MATS, AIM satellites) in high-latitude regions. However, this area is under the influence of the midnight sun, which makes it difficult to detect NLCs from the ground due to the bright background sky condition. Therefore, the opportunities for NLC observations from the ground in high latitude regions are limited. On the other hand, there is an opportunity for NLC observations during the NLC waning period (August) even in the high-latitude region. In addition, slow variations in the solar elevation angle allow continuous observations of NLCs with similar geometric conditions throughout the nights of August. Thus, there is an obvious benefit in studying NLC morphology when the probability of NLC observations increases in high-latitude regions.
We have investigated the feasibility of overcoming this "bright background sky problem" by developing an optical imager dedicated to noctilucent cloud observations. Noctilucent clouds are known to have a spectral peak in their radiance at 400-500 nm. On the other hand, the background spectrum in the twilight sky is attenuated at wavelengths shorter than 680 nm. Therefore, there should be an optimal wavelength band for noctilucent cloud observations that gives a better signal-to-noise ratio (SNR) in shorter wavelength regions. We have developed a new imaging technique based on a cooled CMOS camera equipped with a bandpass filter with a center wavelength of 371 nm (near UV range) and a bandwidth of 40 nm [Endo et al., 2024a]. With this new technique, an effective SNR (SNR>1.80) for NLC under a bright sky condition corresponds to a local solar zenith angle of ˜91°. The effectiveness of this technique has been verified by the first test observation of the NLC in Kiruna, Sweden (N 67.8, E20.4) in August 2024 [Endo et al., 2024b]. We have further optimized the throughput of the imager by using a new objective lens with high transmittance in the NUV region. In this study, we present technical characteristics of the final version of the NLC imager and the observation plan which will be conducted in Kiruna in the summer of 2025.
[1] Endo et al. (2024) “Development of NLC Imager for observation in the high latitude region-The test observation in Kiruna, Sweden”, SGEPSS 2024 Fall meeting, R005-P05
[2] Endo et al. (2024) “NLC Imager for observations in the high latitude region. -The test observation in Kiruna, Sweden”, The 15th Symposium on Polar Science, OSp4