A not so borealis aurora

The cameras that monitor permanently the Calar Alto sky have been able to record an aurora borealis on the night of Sunday, April 23, 2023, an exceptional sighting in Andalusia. The visibility of the phenomenon at the latitude of the observatory confirms the power of the solar eruption that took place a couple of days before and the exquisite transparency of the skies over Calar Alto.

During an aurora borealis (and australis), the night sky gets illuminated from red to blue and green colors. This is due to the solar wind, a variable stream of charged particles (protons and electrons) emitted by our star, which interacts with the upper atmosphere following Earth's magnetic field lines that pass through the poles.

They are thus common phenomena at high latitudes, getting even more spectacular during the maximum of the solar activity which shows a cycle of 11 years. But northern lights are very uncommon in the Iberian Peninsula. The last sighting of an aurora in Spain took place twenty years ago and was photographed as far South as the province of Valencia and sighted by two astronomers from Calar Alto, Nicolás Cardiel and Ana Guijarro.

At the typical latitude of Andalusia, northern lights are exceptional phenomena, that may only occur during the most powerful solar flares, as it happened in January 1938, in a country then lost in the horror of the Spanish Civil War...

On Sunday, April 23, 2023, just after dusk and for an hour or so, the surveillance cameras installed at Calar Alto were able to capture, for the first time, an aurora borealis and to follow its subtle evolution until its evanescence, around 11 pm local time (CEST).

The aurora took place in the time range predicted by the American Space Weather Prediction Center (NOOA), with a high Kp index of 8 (on a scale of 9) of geomagnetic storms.

The phenomenon is clearly visible in the CAHA cameras pointing north and north-northwest, between approx. 20 and 21 h UT, as a reddish “curtain” visible very low over the NNW horizon. This exceptional not-so-northern lights was captured independently by the camera dedicated to the study of the upper atmosphere of the Leibniz Institute for Astrophysics in Potsdam (AIP), located on the dome of the CAHA 3.5 m telescope.

Also visible a little further north, from Extremadura and from southern France, the recording by the Calar Alto cameras, located at a latitude of 37° 13', represents the southernmost confirmed sighting of an aurora borealis in the last decades.

Calar Alto Observatory is one of the infrastructures that belong to the national map of Unique Scientific and Technical Infrastructures (Spanish acronym: ICTS), approved on March 11th, 2022, by the Science, Technology and Innovation Policy Council (CPCTI)


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