SAR Arc Over Alqueva Observatory ᐉ

From the photographer: “After checking many elements with so many coincidences I have now almost 99,9% certain that two nights ago, from 23th to 24th April, when I was over night at the official Observatory of Dark Sky® Alqueva, in Cumeada village, Portugal, capturing my latest deep sky target in a region of the sky full of galaxies known as Markarian’s Chain, I captured the rare moment of a red aurora borealis display glowing over Alqueva, as low as 38º latitude. It happened during the strong burst of activity with a severe magnetic storm reaching a geomagnetic K-index level of 8 between 00h45 and 02h15 (local time +1UT), and then decreasing to a Level k5. I had one of my cameras pointing to west region on a 300mm lens, covering a short field of view. We had a dark starry night, the sky was clear as shown on radar images. Our SQM system at the Observatory was registering the peak measurement of the red glowing light exactly at the same time that I had it visible on my images and the report from Spaceweather alert. Unfortunately I was too tired after a few consecutive nights shooting and was napping a bit between tasks and while my telescopes were working. I didn’t had my fish-eye all sky lens with me, so I had installed only one extra camera with a wide angle, which was pointing to the Milky Way – opposite direction – and started around 2AM when the level decreased suddenly, being registered only at the beginning of the time lapse. But fortunately the close-up deepsky sequence that covers the galaxy field, was showing at the end of the video an incredible red display which last for about 1h. In a question of a few minutes all the sky was fulfilled by an intense red glow coming apparently from “nowhere”, which impels me to believe that this could only be the result of an extreme aurora activity that was visible from all over northern Europe. My friend and TWAN colleague Alan Dyer pointed me to most plausible cause, an even rare type of auroras called SAR arc (Stable Auroral Red) arcs which were only discovered in 1956. Alan mentioned that “in the northern hemisphere, SARs occur south of the main aurora, and would be seen before any aurora itself appears at lower latitudes. They are almost always sub-visual but cameras pick them up easily”. I never saw something like that in my life, and it was not visible with naked-eye, I only noticed after being notified by Spaceweather alerts about the undergoing storm, and after checking if my deepsky camera was focused, but I remember to look up and think on that “weird reddish” light visible on my lcd screen, and even checked if the white balance was ok, but everything was set up properly as expected. Finally, I looked up to visually confirme if the sky was clean, and it was totally clear. Normally from Alqueva, I usual get green airglow on my photos, as red airglow is normally well seen in high altitude places, like Atacama or La Palma, and is indeed faint comparing to the level of brightness I get on histogram. We have confirmed and radar images shows that the sky was clear too, but I even checked the images from two nights before when I was capturing the same region of the sky but had a few high clouds and the appearance on the image is completely different, the image tends to be more pale white and faint galaxies simply vanish while stars shows a diffuse halo or a glow around them. And during the red aurora display the galaxies were still sharp as before and keep revealing the faint dusty oblong shape. Plus after checking Spaceweather website, I saw that an NLC camera located in Southern Spain (37ºN) captured the Northern Lights too.”