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 aurora contaminations to OH Meinel (3,1) band. A total of 368-nights observations succeeded for two winter seasons, and three cases in which N+2 Meinel (1,2) band around 1.5 μm was significant were identified. Focusing on two specific cases, detailed spectral characteristics with high temporal resolutions of 30 seconds are presented. Intensities of N+2 band were estimated to be 228 kR and 217 kR just at the moment of the aurora breakup and arc intensifications during pseudo breakup, respectively. At a wavelength of P1(2) line (∼ 1523 nm), N+2 emissions were almost equal to or greater than the OH line intensity. On the other hand, at a wavelength of P1(4) line (∼ 1542 nm), the OH line was not seriously contaminated and still dominant to N+2 emissions. Furthermore, we evaluated N+2 (1,2) band effects on OH rotational temperature estimations quantitatively for the first time. Aurora contaminations from N+2 (1,2) band basically lead negative bias in OH rotational temperature estimated by line-pair-ratio method with P1(2) and P1(4) lines in OH (3,1) band. They possibly cause underestimations of OH rotational temperatures up to 40 K. In addition, N+2 (1,2) band contaminations were temporally limited to a moment around aurora breakup. This is consistent with proceeding studies reporting that enhancements of 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 polar cap and sub-aurora zone, where strong aurora intensifications are less observed. Further spectroscopic investigations at this wavelength are needed especially for more precise evaluations of to 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.
Highly localized cooling in daytime mesopause temperature over the dip equator during counter electrojet events: First results
