Temporal evolutions of N2+ Meinel (1,2) band near 1.5.μm associated with aurora breakup and their effects on mesopause temperature estimations from OH Meinel (3,1) band

Abstract

We have carried out ground-based NIRAS (Near-InfraRed Aurora and airglow Spectrograph) observations at Syowa station, Antarctic (69. 0 ∘S , 39. 6 ∘E) and Kiruna (67. 8 ∘N , 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 N2+ Meinel (1,2) band around 1.5μm was significant were identified. Focusing on two specific cases, detailed spectral characteristics with high temporal resolutions of 30 s are presented. Intensities of N2+ 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 P 1(2) line (∼1523nm), N2+ emissions were almost equal to or greater than the OH line intensity. On the other hand, at a wavelength of P 1(4) line (∼1542nm), the OH line was not seriously contaminated and still dominant to N2+ emissions. Furthermore, we evaluated N2+ (1,2) band effects on OH rotational temperature estimations quantitatively for the first time. Auroral contaminations from N2+ (1,2) band basically lead negative bias in OH rotational temperature estimated by line-pair-ratio method with P 1(2) and P 1(4) lines in OH (3,1) band. They possibly cause underestimations of OH rotational temperatures up to 40 K. In addition, N2+ (1,2) band contaminations were temporally limited to a moment around the aurora breakup. This is consistent with proceeding studies reporting that enhancements of N2+ (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 N2+ (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.[Figure not available: see fulltext.].