5:15 PM - 7:15 PM
[PCG19-P11] Development of a near-infrared laser heterodyne spectrometer for thermospheric helium observation
Keywords:Thermosphere, Helium, Heterodyne, Spectrometer, Development, Near-Infrared
The Earth's upper thermosphere (300-500 km altitudes) is located at the boundary between space and the Earth's atmosphere. This region exhibits a complex region of interaction, affected by both space and the lower atmosphere. Since the upper atmosphere is a reservoir of escaping atmosphere to space, thermospheric variations are essential for understanding the evolution of planetary atmospheres. Recently, optical observations of 1083 nm emission lines of metastable He in the upper thermosphere have revealed that the lower atmosphere can significantly affect the thermosphere, as faster than expected even during moderate geomagnetic storm events (Nishiyama et al., 2024). Meanwhile, ground-based observational tools remain limited, and in particular, there are no continuous observations of wind speeds and temperature fields.
Metastable He is produced by the impact of electrons on grand state He and by recombination of He ions precipitating as solar wind particles. The 1083 nm emission lines are difficult to separate from the OH emission lines due to their proximity with conventional spectrometers, so the observation of He still has uncertainties. The purpose of this study is to develop a new near-infrared laser heterodyne spectrometer to improve the wavelength resolution by three orders of magnitude or more (up to 107) compared to conventional instruments and to isolate the He emission line from the OH emission lines. In addition, we aim to derive wind velocity and temperature fields directly from the Doppler shift and Doppler width, respectively by fully resolving the He emission line profile for the first time through ultra-high wavelength-resolution observations. According to the previous study by Dynamics Explorer 2 and other themospheric observation satellites, which reported the typical vertical wind speeds in the upper thermosphere of up to 100 m/s, mostly within 30 m/s, the required accuracy of wind speed can be set to 30 m/s.
Infrared laser heterodyne spectroscopy is a method of superimposing an infrared light from an observation target with a laser source as the local oscillator (LO) that causes the intermediate frequency component in the radio wavelength. The background radio spectrometer can achieve a resolution of 107 or better for the 1083 nm signal. In this study, we report the results of an LO characterization test and a preliminary heterodyne detection test for the observation of He emission lines.
In the LO characterization test, the wavelength tunable range of the laser, and the wavelength stability, which determines the wavelength resolution, were evaluated. As a result, the wavelength tunable range of the laser was 1082.907-1083.806 nm, which sufficiently covers the target He emission lines (emission line positions: 1082.908 nm, 1083.025 nm, 1083.034 nm) and OH emission lines (emission line positions: 1082.918 nm, 1082.933 nm, 1083. 123 nm,1083.139 nm). The wavelength stability was found to have a standard deviation of 2.6 × 10-5 nm (95.45 % within 0.106 pm) over a 10-minutes integration time. This implies the wavelength resolution of the instrument is approximately 107. This result demonstrates that the spectral resolution is sufficient for isolating the He emission line from OH emission lines. The expected accuracy of the wind speed derivation is about 40 m/s, which is comparable to the expected wind speed from previous satellite observations. Further improvement can be expected by further wavelength stabilization through feedback using a gas cell.
As a next step, continuous observation from the ground will be realized in cooperation with conventional spectroscopy, to elucidate the dynamics of the upper atmosphere.
Metastable He is produced by the impact of electrons on grand state He and by recombination of He ions precipitating as solar wind particles. The 1083 nm emission lines are difficult to separate from the OH emission lines due to their proximity with conventional spectrometers, so the observation of He still has uncertainties. The purpose of this study is to develop a new near-infrared laser heterodyne spectrometer to improve the wavelength resolution by three orders of magnitude or more (up to 107) compared to conventional instruments and to isolate the He emission line from the OH emission lines. In addition, we aim to derive wind velocity and temperature fields directly from the Doppler shift and Doppler width, respectively by fully resolving the He emission line profile for the first time through ultra-high wavelength-resolution observations. According to the previous study by Dynamics Explorer 2 and other themospheric observation satellites, which reported the typical vertical wind speeds in the upper thermosphere of up to 100 m/s, mostly within 30 m/s, the required accuracy of wind speed can be set to 30 m/s.
Infrared laser heterodyne spectroscopy is a method of superimposing an infrared light from an observation target with a laser source as the local oscillator (LO) that causes the intermediate frequency component in the radio wavelength. The background radio spectrometer can achieve a resolution of 107 or better for the 1083 nm signal. In this study, we report the results of an LO characterization test and a preliminary heterodyne detection test for the observation of He emission lines.
In the LO characterization test, the wavelength tunable range of the laser, and the wavelength stability, which determines the wavelength resolution, were evaluated. As a result, the wavelength tunable range of the laser was 1082.907-1083.806 nm, which sufficiently covers the target He emission lines (emission line positions: 1082.908 nm, 1083.025 nm, 1083.034 nm) and OH emission lines (emission line positions: 1082.918 nm, 1082.933 nm, 1083. 123 nm,1083.139 nm). The wavelength stability was found to have a standard deviation of 2.6 × 10-5 nm (95.45 % within 0.106 pm) over a 10-minutes integration time. This implies the wavelength resolution of the instrument is approximately 107. This result demonstrates that the spectral resolution is sufficient for isolating the He emission line from OH emission lines. The expected accuracy of the wind speed derivation is about 40 m/s, which is comparable to the expected wind speed from previous satellite observations. Further improvement can be expected by further wavelength stabilization through feedback using a gas cell.
As a next step, continuous observation from the ground will be realized in cooperation with conventional spectroscopy, to elucidate the dynamics of the upper atmosphere.