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Combined EISCAT radar and optical multispectral and tomographic observations of black aurora

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<mark>Journal publication date</mark>25/06/2008
<mark>Journal</mark>Journal of Geophysical Research
Issue numberA06308
Publication StatusPublished
<mark>Original language</mark>English


Black auroras are recognized as spatially well-defined regions within a uniform diffuse auroral background where the optical emission is significantly reduced. Black auroras typically appear post-magnetic midnight and during the substorm recovery phase, but not exclusively so. We report on the first combined multimonochromatic optical imaging, bistatic white-light TV recordings and incoherent scatter radar observations of black aurora by EISCAT of the phenomenon. From the relatively larger reduction in luminosity at 4278 Å than at 8446 Å we show that nonsheared black auroras are most probably not caused by downward directed electrical fields at low altitude. From the observations, we determine this by relating the height and intensity of the black aurora to precipitating particle energy within the surrounding background diffuse aurora. The observations are more consistent with an energy selective loss cone. Hence the mechanism causing black aurora is most probably active in the magnetosphere rather than close to Earth.

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Copyright (2008) American Geophysical Union. Further reproduction or electronic distribution is not permitted