A rare observation from mid-latitude of a blue aurora

Aurora observations at mid-latitudes are rare but not exceptional. The aurorae are usually seen as diffuse red illuminations of the sky above the Northern (respectively Southern) horizon (respectively in the northern and southern hemispheres) because they take place at much higher latitude and their lower parts (green, purple, blue) fall below the horizon. However, while high-latitude sightings of blue aurorae are frequent, sighting at mid-latitudes have rarely been reported. During the night of September 24–25, 2023, a series of aurorae were seen from a viewpoint at 48.3° geographic north and 1.2° geographic east (49.88°N, 84.55°E in geomagnetic coordinates). These aurorae appeared above the horizon in northern to north-eastern direction. At around 23 UT (1 LT), the aurora appeared red. Three hours later, a blue aurora was seen in the northeast direction from the observation site and photographed using a wavelength-calibrated canon 6D camera. No colors other than blue were present on the images. While the red aurora is common and its excitation mechanism understood, the origin of the blue aurora is more difficult to determine. We argue that the observed blue aurora cannot be attributed to electron or proton precipitation. The excitation of the Vegard-Kaplan and first positive bands of N2 by low energy electrons cannot account for the lack of red and green colors in the images although they cannot be fully ruled out. The resonant scattering of solar light on N2+ at ionospheric F-region heights appears to be the most likely explanation. More frequent systematic multi-instrument multi-point observations could provide additional insight into the origin of these aurorae and help understand how often N2+ is uplifted to F-region heights. For such kind of work, amateur astrophotographers could provide valuable support.