Earth, Planets and Space (Jan 2021)
Temporal evolutions of $$\text {N}_2^+$$ N 2 + Meinel (1,2) band near $$1.5.\,\upmu \text {m}$$ 1.5 . μ m associated with aurora breakup and their effects on mesopause temperature estimations from OH Meinel (3,1) band
Abstract
Abstract We have carried out ground-based NIRAS (Near-InfraRed Aurora and airglow Spectrograph) observations at Syowa station, Antarctic ( $$69.0^{\circ }\text {S}$$ 69 . 0 ∘ S , $$39.6^{\circ }\text {E}$$ 39 . 6 ∘ E ) and Kiruna ( $$67.8^{\circ }\text {N}$$ 67 . 8 ∘ N , $$20.4^{\circ }\text {E}$$ 20 . 4 ∘ 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 $$\text {N}_2^+$$ N 2 + Meinel (1,2) band around $$1.5\,\mu \text {m}$$ 1.5 μ m was significant were identified. Focusing on two specific cases, detailed spectral characteristics with high temporal resolutions of 30 s are presented. Intensities of $$\text {N}_2^+$$ 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 $$\text {P}_1(2)$$ P 1 ( 2 ) line ( $$\sim 1523 \,\text {nm}$$ ∼ 1523 nm ), $$\text {N}_2^+$$ N 2 + emissions were almost equal to or greater than the OH line intensity. On the other hand, at a wavelength of $$\text {P}_1(4)$$ P 1 ( 4 ) line ( $$\sim 1542 \,\text {nm}$$ ∼ 1542 nm ), the OH line was not seriously contaminated and still dominant to $$\text {N}_2^+$$ N 2 + emissions. Furthermore, we evaluated $$\text {N}_2^+$$ N 2 + (1,2) band effects on OH rotational temperature estimations quantitatively for the first time. Auroral contaminations from $$\text {N}_2^+$$ N 2 + (1,2) band basically lead negative bias in OH rotational temperature estimated by line-pair-ratio method with $$\text {P}_1(2)$$ P 1 ( 2 ) and $$\text {P}_1(4)$$ P 1 ( 4 ) lines in OH (3,1) band. They possibly cause underestimations of OH rotational temperatures up to 40 K. In addition, $$\text {N}_2^+$$ 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 $$\text {N}_2^+$$ 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 $$\text {N}_2^+$$ 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.
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