Observing auroral 427.8-nm emission using a Fabry-Perot interferometer in Norway for measurement of N2+ upflow: Initial analysis

Previous observations from artificial satellites have suggested that ionized nitrogen molecules (N2+) in the polar upper atmosphere are transported upward along the magnetic field line in association with auroral heating. However, the heating mechanism responsible for the vertical transport of these heavy short-lived molecules into the magnetosphere is not fully understood yet. The Fabry-Perot Interferometer (FPI) could possibly be utilized to measure the velocity of a specific ion in the lower ionosphere on a consecutive basis. This performance potentially represents a significant advantage over incoherent scatter radars and low-orbiting satellites. This is because the former is unable to identify specific atmospheric species and the latter faces considerable challenges in conducting measurements over extended periods in the lower ionosphere. For example, O+ emission at a wavelength of 732 nm has also been used for spectroscopic measurements of the ion flow. However, few observations are available due to the low emission intensity, and thus, the measurement area is limited to the dayside cusp region. In this study, for the first time in the world, we observed the N2+(1NG) band emissions near 427.8 nm using an FPI installed in Norway. Two principal mechanisms to cause the N2+ emission are excitation by auroral particle precipitation and resonant scattering of sunlight. We measured the upward velocity of the N2+ along the magnetic field line from the FPI Doppler spectrum. This groundbreaking work holds the potential to improve our understanding of ion dynamics in the Earth’s upper atmosphere and magnetosphere.

We conducted a campaign FPI measurements of the 427.8-nm emissions at Skibotn, Norway (69.3ºN, 20.4ºE) from March 8th to 14th, 2024. The likelihood of resonant scattering aurora is high in the equinox season due to the sun’s proximity to the horizon during the night. Based on our previous observations, de-focused images at a wavelength of 427.8 nm in the multi-wavelength FPI measurement lead to a small number of fringes and significant measurement uncertainty. The fringe image resolution was improved by lowering the camera position by 0.75 mm for an optimal focus condition, resulting in a significant increase in the number of fringes from 3 to 14. A clear sky was available on the night of March13th -14th, 2024 (min Dst = -19 nT, max AE = around 700 nT). We succeeded in estimating the N2+ velocity along the magnetic field line for both particle precipitation aurora and resonant scattering aurora. The estimated field-aligned velocities were upward, ranging from 170 to 340 m/s, with the time resolution of 4 minutes and a standard deviation of 200 – 300 m/s during appearange of particle precipitation aurora and resonant scattering aurora at 19:50 – 20:00 UT. During our observation through tropospheric clouds at 0:20-1:30 UT on March 9th (min Dst = -21 nT、max AE = around 700 nT), the estimated N2+ field-aligned velocities were between -70 and 150 m/s (1σ of 150 – 590 m/s) with the time resolution of 4 minutes. This cloudy-sky result is consistent with the characteristics of FPI measurement that the Doppler velocity approaches to zero due to cloud scattering of auroral emission. According to the results, optimizing the FPI for the 427.8-nm observation could yield data that would be valuable for future analysis. In this presentation, we will report these results and discuss the validity of the N2+ velocity measurements with the FPI.