A new auroral phenomenon, the anti-black aurora

A E Nel,T Aslaksen,M J Kosch,D Whiter,B Gustavsson

doi:10.1038/s41598-021-81363-9

Abstract

Black auroras are small-scale features embedded in the diffuse background aurora, typically occurring post-substorm after magnetic midnight and with an eastward drift imposed. Black auroras show a significant reduction in optical brightness compared to the surrounding diffuse aurora, and can appear as slow-moving arcs or rapidly-moving patches and arc segments. We report, for the first time, an even more elusive small-scale optical structure that has always been observed occurring paired with sim 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 is of similar shape and size, and always moves in parallel to the drifting 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 (427.8 nm and 844.6 nm) optical observations of the phenomenon recorded on 12 March 2016 outside Tromsø Norway, 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.

Highlights

Black auroras are small-scale features embedded in the diffuse background aurora, typically occurring post-substorm after magnetic midnight and with an eastward drift imposed
We report on the first spectral optical observations of the anti-black auroras, recorded in 2016 at EISCAT near Tromsø, Norway, from which we infer the mean energy of the precipitating electrons simultaneously in the diffuse background, black and anti-black auroras for the first time
We report the discovery of a wholly new type of aurora, namely, the anti-black aurora which is always paired with a drifting black aurora patch

Summary

Introduction

Black auroras are small-scale features embedded in the diffuse background aurora, typically occurring post-substorm after magnetic midnight and with an eastward drift imposed. For the first time, an even more elusive small-scale optical structure that has always been observed occurring paired with ∼ 10% of black aurora patches. From the first dual-wavelength (427.8 nm and 844.6 nm) optical observations of the phenomenon recorded on 12 March 2016 outside Tromsø Norway, 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. We report on the first spectral optical observations of the anti-black auroras, recorded in 2016 at EISCAT near Tromsø, Norway, from which we infer the mean energy of the precipitating electrons simultaneously in the diffuse background, black and anti-black auroras for the first time

Methods

Results

Conclusion