First precise altitude distribution observation of blue aurora using hyperspectral camera

Research Background

 Auroras: Auroras are natural phenomena where electrons from space collide with Earth's atmosphere (oxygen and nitrogen) and emit light. The various colors — red, green, purple, etc. —depend on "which atoms or molecules emit light" and "how the energy changes." This light contains hidden information about the "velocity of particles falling down" and "conditions of the atmosphere."

 At What Height Do They Shine?: While auroras appear to spread across the sky when viewed from the ground, determining their actual altitude has been challenging. Conventional methods involved setting up multiple cameras at different locations to capture stereoscopic images, enabling height estimation. It was believed that a single camera alone could not determine altitude.

 New Idea: Researchers drew inspiration from plasma studies in laboratories. There, a long-established technique involved firing a "particle beam" and determining depth by the intersection of the light excited by that beam and the line of sight of the observation. This time, applied to the aurora, sunlight-excited auroral emissions (resonant scattered light) were used. By utilizing the intersection of this light and the camera's line of sight, height estimation became possible with just one camera.

 Strength of the hyperspectral camera: With ordinary cameras or filtered observations, during an astronomical twilight of dawn, sunlight reflection and resonant scattered light mix, making them difficult to distinguish. However, the hyperspectral camera can "observe light color (wavelength) information in extremely fine detail," enabling it to accurately separate and capture both components.

 Research Findings

 Regarding the blue aurora observed in Kiruna, Sweden, in the early morning of October 21, 2023, our research team analyzed data using a hyperspectral camera installed by the National Institute for Fusion Science. We successfully estimated the precise altitude distribution of nitrogen molecular ions (N₂⁺) responsible for the aurora's emissions.

 It is well known that during nighttime auroral emissions, nitrogen molecular ion emission is strongest at an altitude of approximately 130 km. However, this observation during dawn (astronomical twilight) revealed that the rate of increase in emission intensity peaks at an altitude of 200 km. This directly indicates that, at least during twilight, the emission at the high altitude of 200 km is exceptionally strong, suggesting the possibility that nitrogen molecular ions exist at such high altitudes.

This result confirms previous observations suggesting that the density of nitrogen molecular ions at high altitudes may be higher than previously thought, while also enabling verification of theoretical models concerning the physical processes involved in aurora formation. High-precision observations using hyperspectral cameras open new avenues for aurora research.

 Significance of Research Findings and Future Developments

 Aurora observations using the hyperspectral camera accurately captured the temporal and altitude variations of resonance-scattered light during astronomical twilight, which had previously been difficult to quantify. Compared to conventional cameras using interference filters, it broadened the observation domain and introduced a new altitude estimation method.

 This is expected to contribute to solving the long-standing problem of nitrogen molecular ion generation and outflow in the ionosphere.

 Going forward, it is expected that this interdisciplinary research will advance through collaboration with universities and research institutes both domestically and internationally, contributing to the development of global aurora research.

 【Glossary】

 ※1 Hyperspectral Camera: A camera capable of finely spectroscopically resolving light. While conventional cameras divide light into three bands (red, green, blue), hyperspectral cameras can divide light into hundreds of finer bands. Commercially available hyperspectral cameras are designed for daytime imaging and cannot observe faint auroras. High-sensitivity cameras are required for aurora observation.

 ※2 Astronomical twilight: The period before sunrise or after sunset when the sky does not become completely dark but remains faintly bright due to sunlight scattered by Earth's atmosphere. Specifically, this refers to the time when the solar sub-elevation angle (the angle between the horizon and the center of the Sun) is between 12 and 18 degrees.

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