By Nathan Case and Kasha Patel
On Tuesday, March 17, 2015, as people adorned themselves with green clothing and infused their livers with green beer, Earth was experiencing the biggest geomagnetic storm of the last decade—leading to beautiful, widespread aurora around world.
The red, white and green St. Patrick’s Day aurora was created through a rather fortuitous combination of factors. An explosion on the Sun occurred two days earlier on Sunday, March 15, 2015. The explosion, called a coronal mass ejection (CME), is a blast of large bubbles of gas (plasma) that can create strong geomagnetic disturbances as they collide with the Earth’s magnetic field (known as the magnetosphere). More importantly, the magnetic field of this CME was directed southward, opposite to the Earth’s terrestrial field, and the solar wind– a stream of charged particles that flows from the Sun past the Earth — was quite fast due the effects of an earthward pointing coronal hole.
The video shows the CME as captured by the Solar and Heliospheric Observatory. The CME then traveled over 90 million miles from the Sun to Earth to prompt the severe St. Patrick’s Day geomagnetic storm. Caption provided by Discover magazine.
The storm’s conditions created a perfect environment for aurora hunting. On a scale of G1 (minor) to G5 (extreme), this storm reached up to G4, or “severe” level, as classified by the National Oceanic and Atmospheric Administration. The storm’s Kp index, a global geomagnetic storm index, registered between the 6-8 range (9 is the highest on the scale). The solar wind power remained in the 3000GW + range for over 24 hours, meaning that the aurora was visible in the central to southern United States—a rare occurrence. The CME arrived earlier than expected with arrival estimates at 12:00 UTC and actual arrival occurring at 04:05 UTC.
The first aurora sightings appeared from New Zealand early on Tuesday morning (UTC). The peak of this storm hit during daytime over most of the United States and Europe, but the storm persisted into the night and gave both Americans and Europeans alike a beautiful light show. Reports came in from regions that rarely see the aurora such as the south of England, Germany and Poland. In the US, people reported seeing the aurora from the northern-mid US, including Pennsylvania, Virginia and Colorado.
Aurorasaurus gathered over 35,000 aurora-related tweets and reports. You guys and gals— our users— reported over 160 auroral sightings during the St. Patrick’s Day storm, and you verified over 250 tweets as being auroral sightings. (Thank you!) From midnight on March 17 to 1p.m. UTC on March 18, we saw a 50% increase in the number of registered users. Welcome aboard! (Visitors can, of course, still participate in the project by reporting and verifying tweets anonymously. Although if you do not register with Aurorasaurus then we are unable to send you location-based alerts telling you when people report an aurora sighting in your area.)
Aurorasaurus received a number of user reports via Twitter, aurorasaurus.org and the Aurorasaurus phone apps (iOS and Android). This time-lapse how the number of reports increased throughout the day as auroral activity peaked and diminished.
From the team here at Aurorasaurus, we wanted to say a huge thank you for all of your efforts! Your citizen science observations and your time spent in verifying tweets are sincerely appreciated. As you know, we combine your reports to provide our users with real-time alerts of when an aurora might be visible near them. During this storm alone we issued 361 such notifications! We will also use the data you provide to improve auroral oval models and develop a better notification system using both the traditional satellite-based data and your citizen science data.