11:15 AM - 11:30 AM
[PEM12-27] Orthoherium variability from the upper thermosphere to the lower exosphere observed at Longyearbyen, Svalbard (78.1°N, 16.0°E)
Keywords:Orthohelium, Airglow, Upper thermosphere, Lower exosphere, Svalbard, Ground-based spectroscopy
This study presents the time variability of metastable orthohelium, He(23S), on various time scales in the polar region. A dataset consisting of continuous observations of He(23S) airglow brightness at 1083 nm for 6 months was obtained from a short-wavelength infrared imaging spectrograph (NIRAS-2) at the KHO, Svalbard (78.1°N,16.0°E).
The NIRAS-2 is a brand new 2-D imaging spectrograph that is sensitive to radiation at wavelengths from 1.05 to 1.35 microns (Nishiyama et al., 2024). It was installed at the KHO, Longyearbyen in November 2022. The NIRAS-2 has 1-D field-of-view (FOV) along the geomagnetic meridional direction and an angular resolution of 55 degrees and 0.11 degrees per pixel. The sensor is an InGaAs 2-D array (640 pixel × 512 pixel) with 15-microns pitch size, and it can be cooled down to −80°C by using four Peltier stages. The He(23S) spectra observed by the NIRAS-2 were blended with other Q-branch lines in the OH(5,2) band because of its limited spectral resolution. Thus, the following simple analysis was performed for each spectra by using a synthetic spectrum of the OH(5,2) band. First, OH rotational temperature, Trot, was estimated only using P-branch lines by fitting the OH synthetic spectrum; then the synthetic spectrum of the OH Q-branch was calculated from the Trot. The OH spectrum was subtracted from the observed spectrum to obtain "pure" He(23S) emission spectra.
Time series of the estimated He(23S) airglow brightness from the ends of September 2024 to February 2025 clearly showed a seasonal variation known as helium winter bulge and solar zenith angle dependence. Semi-annual variations of He(23S) airglow brightness agreed to that of He density at 500-km altitude calculated by MSIS 2.1. It is also found that sudden increases in He(23S) airglow, which are thought to correspond to solar proton events, took place in a few times. Additionally, He(23S) airglow displayed responses to two G4 (severe) geomagnetic storm on different time scales. During the storm in October 2024, a depletion of He(23S) airglow brightness lasting a few days was observed, which is consistent with the previous study; this depletion was likely caused by enhanced Penning ionization due to upwelling N2 from the lower atmosphere. On the other hand, during the storm on new year's day of 2025, anomalous enhancements of He(23S) airglow brightness lasting a few hours were identified twice at duskside and morning side. It is highly likely that both enhancements were caused by particle precipitations (He+/He++ and electron) from the sapce, which are a phenomenon unique to the polar regions.
The NIRAS-2 measurements demonstrated that He(23S) from the upper thermosphere to the exosphere changed drastically as a result if forcing both from the lower atmosphere and from space. He(23S) measurements will improve our understanding of the thermosphere-ionosphere coupling system and extend the coverage of space weather forecasting up to the exobase.
The NIRAS-2 is a brand new 2-D imaging spectrograph that is sensitive to radiation at wavelengths from 1.05 to 1.35 microns (Nishiyama et al., 2024). It was installed at the KHO, Longyearbyen in November 2022. The NIRAS-2 has 1-D field-of-view (FOV) along the geomagnetic meridional direction and an angular resolution of 55 degrees and 0.11 degrees per pixel. The sensor is an InGaAs 2-D array (640 pixel × 512 pixel) with 15-microns pitch size, and it can be cooled down to −80°C by using four Peltier stages. The He(23S) spectra observed by the NIRAS-2 were blended with other Q-branch lines in the OH(5,2) band because of its limited spectral resolution. Thus, the following simple analysis was performed for each spectra by using a synthetic spectrum of the OH(5,2) band. First, OH rotational temperature, Trot, was estimated only using P-branch lines by fitting the OH synthetic spectrum; then the synthetic spectrum of the OH Q-branch was calculated from the Trot. The OH spectrum was subtracted from the observed spectrum to obtain "pure" He(23S) emission spectra.
Time series of the estimated He(23S) airglow brightness from the ends of September 2024 to February 2025 clearly showed a seasonal variation known as helium winter bulge and solar zenith angle dependence. Semi-annual variations of He(23S) airglow brightness agreed to that of He density at 500-km altitude calculated by MSIS 2.1. It is also found that sudden increases in He(23S) airglow, which are thought to correspond to solar proton events, took place in a few times. Additionally, He(23S) airglow displayed responses to two G4 (severe) geomagnetic storm on different time scales. During the storm in October 2024, a depletion of He(23S) airglow brightness lasting a few days was observed, which is consistent with the previous study; this depletion was likely caused by enhanced Penning ionization due to upwelling N2 from the lower atmosphere. On the other hand, during the storm on new year's day of 2025, anomalous enhancements of He(23S) airglow brightness lasting a few hours were identified twice at duskside and morning side. It is highly likely that both enhancements were caused by particle precipitations (He+/He++ and electron) from the sapce, which are a phenomenon unique to the polar regions.
The NIRAS-2 measurements demonstrated that He(23S) from the upper thermosphere to the exosphere changed drastically as a result if forcing both from the lower atmosphere and from space. He(23S) measurements will improve our understanding of the thermosphere-ionosphere coupling system and extend the coverage of space weather forecasting up to the exobase.