First attempt of Doppler velocity measurement for molecular nitrogen ion through auroral 427.8-nm emission by a Fabry–Perot interferometer in Norway

Abstract Heavy ions originating from the ionosphere, such as molecular nitrogen ions ( $${N}_{2}^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>N</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mo>+</mml:mo> </mml:msubsup> </mml:math> ), have been observed in the magnetosphere during periods of geomagnetically active conditions. However, the mechanism that transfers heavy ions into the magnetosphere is not fully understood. Measurement of $${N}_{2}^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>N</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mo>+</mml:mo> </mml:msubsup> </mml:math> upflow in the Earth’s upper atmosphere would contribute to the understanding of the mechanism, although such measurements have not been done yet. In this study, we report a new attempt to measure the Doppler shift of auroral $${N}_{2}^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>N</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mo>+</mml:mo> </mml:msubsup> </mml:math> first negative band (1NG) emissions around 427.8 nm through a Fabry–Perot interferometer (FPI) in Norway. The CCD detector position was adjusted to the focusing point of the 427.8-nm emission, because it was originally located at the focusing point of the 632.8-nm calibration laser. An intense 427.8-nm emission was observed at dawn on 29th September 2024, probably due to resonant scattering of sunlight by high-altitude $${N}_{2}^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>N</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mo>+</mml:mo> </mml:msubsup> </mml:math> ions. Downward velocities of 100–300 m/s with a 90% confidence interval of $$\pm$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>±</mml:mo> </mml:math> 50–120 m/s were observed with a time resolution of 4 min 20 s at 02:00–03:00 UT (03:00–04:00 LT). Based on the comparison with the auroral Hall current estimated by ground geomagnetic variations, we consider that the observed ion velocities are significantly influenced by the perpendicular ion velocity due to ExB plasma drift. We also noted that systematic errors of the measured Doppler velocity may occur due to difference of $${N}_{2}^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>N</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mo>+</mml:mo> </mml:msubsup> </mml:math> temperatures at two directions in sky scanning, which varies the ratio of $${N}_{2}^{+}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mi>N</mml:mi> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mo>+</mml:mo> </mml:msubsup> </mml:math> (1NG) band emission lines that form the observed interference fringes. Based on model calculations, we estimated that 5% temperature difference in the two observation points possibly makes systematic errors of 5–10 m/s for 400 K and that of 8–13 m/s for a rotational temperature of 1000 K. In summary, we succeeded in detecting the Doppler shift even from band emission observations, but the current observations are strongly affected by variations in the electric field perpendicular to the magnetic field and N₂⁺ temperature. This problem can be solved by simply observing only a single field-aligned measurement using a frequency-stabilized laser at wavelengths near 427.8 nm. Graphical Abstract