Abstract
We have carried out ground-based NIRAS (Near-InfraRed Aurora and airglow Spectrograph) observations at Syowa station, Antarctic (69.0^{circ }text {S}, 39.6^{circ }text {E}) and Kiruna (67.8^{circ }text {N}, 20.4^{circ }text {E}), Sweden for continuous measurements of hydroxyl (OH) rotational temperatures and a precise evaluation of auroral contaminations to OH Meinel (3,1) band. A total of 368-nights observations succeeded for 2 winter seasons, and 3 cases in which text {N}_2^+ Meinel (1,2) band around 1.5,mu text {m} was significant were identified. Focusing on two specific cases, detailed spectral characteristics with high temporal resolutions of 30 s are presented. Intensities of text {N}_2^+ band were estimated to be 228 kR and 217 kR just at the moment of the aurora breakup and arc intensification during pseudo breakup, respectively. At a wavelength of text {P}_1(2) line (sim 1523 ,text {nm}), text {N}_2^+ emissions were almost equal to or greater than the OH line intensity. On the other hand, at a wavelength of text {P}_1(4) line (sim 1542 ,text {nm}), the OH line was not seriously contaminated and still dominant to text {N}_2^+ emissions. Furthermore, we evaluated text {N}_2^+ (1,2) band effects on OH rotational temperature estimations quantitatively for the first time. Auroral contaminations from text {N}_2^+ (1,2) band basically lead negative bias in OH rotational temperature estimated by line-pair-ratio method with text {P}_1(2) and text {P}_1(4) lines in OH (3,1) band. They possibly cause underestimations of OH rotational temperatures up to 40 K. In addition, text {N}_2^+ (1,2) band contaminations were temporally limited to a moment around the aurora breakup. This is consistent with proceeding studies reporting that enhancements of text {N}_2^+ (1,2) band were observed associated with International Brightness Coefficient 2–3 auroras. It is also suggested that the contaminations would be neglected in the polar cap and the sub-auroral zone, where strong aurora intensification is less observed. Further spectroscopic investigations at these wavelengths are needed especially for more precise evaluations of text {N}_2^+ (1,2) band contaminations. For example, simultaneous 2-D imaging observation and spectroscopic measurement with high spectral resolutions for airglow in OH (3,1) band will make great advances in more robust temperature estimations in the auroral zone.
Highlights
The mesosphere and lower thermosphere (MLT), from 80 to 120 km altitude in the terrestrial atmosphere, is affected by general wind circulation and atmospheric waves with various scales and solar radiation and energetic particle precipitations from the space
OH 3,1 band measurements and temperature estimations Typical nightly mean spectrum obtained from NIRAS measurements on May 29 and May 6, 2018 are shown in Fig. 2; Fig. 2 a is a spectra with the 600-lpmm grating and a center wavelength of 1504 nm
It should be noted that data near the edge on a short wavelength side is not reliable due to low sensitivity of the NIRAS and water vapor absorption, and not used for quantitative discussions
Summary
The mesosphere and lower thermosphere (MLT), from 80 to 120 km altitude in the terrestrial atmosphere, is affected by general wind circulation and atmospheric waves with various scales and solar radiation and energetic particle precipitations from the space. Main scientific purposes of the NIRAS are as follows: an updating spectral features and absolute intensities in near infrared-short wavelength infrared aurora (0.9–1.6 μm), a precise evaluation of auroral contaminations to OH Meinel (3,1) band, and continuous measurements of OH rotational temperatures.
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