Feb 27, 2014 geomagnetic storm and aurora

Solar Cycle 24 has been somewhat scarce in terms of mid-latitude aurora as there have been few very complex and active sunspots. On February 25th sunspot group AR 1990, returning for its third rotation (!), produced a strong X4.9 flare. It caused a fast full-halo coronal mass ejection, but as the sunspot was still near the solar limb, the majority of the mass was directed away from the Earth. I did not expect much in terms of auroral activity this far south.

As the CME hit the magnetosphere on February 27th, conditions still did not look particularly impressive (v~500 km/s, Bz -20 nT). Enough for a moderate geomagnetic storm, G1-G2 level, but nothing major. Expecting aurora to be seen about as far south as Scotland – Denmark, I was stunned when my colleague Javor Kac notified me that possible aurora was visible on Pic du Midi Observatory (Pyrenees!) webcam. In 15 minutes we were out on our observing spot – Žagarjev vrh in central Slovenia.

The very first photo I took showed clear auroral glows and rays up to about 20° high. Soon aurora also became visible visually, as a diffuse reddish glow about 15° high above the northern horizon with rapidly moving discrete vertical rays.  This activity lasted about 5 minutes and slowly dissipated thereafter. Aurora remained visible on photos for 21 minutes. Soon after last traces of it disappeared the sky clouded over. Lucky weather break for a change.

IMG_2404_pa_tn

IMG_2411_pa_tn IMG_2412_pa_tn IMG_2418_pa_tn IMG_2422_pa_tn IMG_2426_pa_tn

IMG_2433_pa_tn IMG_2437_a_tnIMG_2451_a_tn IMG_2456_a_tn IMG_2464_a_tn

All photos: Canon 6D, Sigma 35-mm f/1.4 (@f/1.4), 7 seconds at ISO 800.

Žagarjev vrh (Slovenia) is at corrected geomagnetic latitude 40° North. It is very noteworthy that aurora was seen this far south at low geomagnetic latitude, even though the geomagnetic storm reached only G2 level (Kp=6, moderate). Peak DST index was -101 nT. It is probably worth noting that Scandinavian magnetometers recorded higher local/regional activity with Kp=7 during the same time period.

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