A new auroral phenomenon: The anti-black aurora

Abstract
Black auroras are small-scale features embedded in the diffuse background aurora that
show a significant reduction in optical brightness compared to the surrounding diffuse aurora.
We report, for the first time, an even more elusive small-scale optical structure has been
observed occurring paired with ~10% of black aurora patches. A patch or arc segment of
enhanced luminosity, distinctly brighter than the diffuse background, which we name the anti-
black aurora, may appear adjacent to the black aurora. The anti-black aurora always moves in
parallel to the black aurora, although it may suddenly switch sides for no apparent reason. The
paired phenomenon always drifts with the same average speed in an easterly direction. From the
first dual-wavelength observations of anti-black and black aurora pairs, we show that the anti-
black and black auroras have a higher and lower mean energy, respectively, of the precipitating
electrons compared to the diffuse background.

Methods
Optical data

For the 2016 campaign a iXon-888 EMCCD’s and an ALTA-U47 back-illuminated full frame megapixel CCD (DASI) was mounted next to each other. The EMCCD was attached with an Andover 430HC10 filter and the DASI with an Andover 844.6 nm filter with an 85% transmission and 10 nm FWHM, which were available for this campaign. Images produced by the EMCCD had 256 × 256 pixel resolution in a 30 ◦ FOV. The DASI produced the same resolution images in a FOV of 50 ◦ , with a maximum temporal resolution of 1 Hz. This is raw data recorded during the campaing, before the calibration steps were applied for this project (i.e., Dark subtraction, flat fielding, background subtraction, median filtering)

Radar data

Recorded using the monostatic UHF radar at Ramfjordmoen. The UHF radar has an 32 m antenna that is a mechanically fully steerable parabolic
dish used for transmission and reception. It has a transmitter peak power of 2.0 MW and operates in the 930 MHz band (radio spectrum). The beam itself has a full opening of 0.6 ◦ .