It seemed out of thin air, an auroral drapery of reds, greens, purples, and even ghostly blues rippling over dark night skies in northern U.S. states. A sudden G3-level geomagnetic storm burst on September 29, pushing the auroral oval well into the south and illuminating skies from Alaska to Maine. Such was not predicted by the National Oceanic and Atmospheric Administration’s Space Weather Prediction Center, but the Kp index a measure of geomagnetic disturbance rose to 7.33, a level linked to intense auroral activity.
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The timing was by no means coincidental. The storm came on the tail of the equinox in September, a time when geomagnetic storms actually increase because of the Russell-McPherron Effect. It is a process described first in 1973 by Christopher Russell and Robert McPherron and which happens when the south-pointing aspect of the Sun’s magnetic field points in the opposite direction of Earth’s own field. This alignment produces “cracks” in the magnetosphere, and charged particles from the sun flood towards the poles with uncommon efficiency. As British Geological Survey’s Dr. Ciaran Beggan described, “During the equinoxes, the orientation of the Earth’s poles is (almost) perpendicular to that of the Sun. This maximizes the ‘coupling’ between the solar wind and the Earth’s magnetic field.”
The impact is heightened this year by the continuing solar maximum, the Sun’s 11-year cycle of activity. Sunspots are plentiful now, and with them come solar flares and coronal mass ejections enormous clouds of superheated plasma capable of triggering geomagnetic storms days later. “We are currently in solar maximum, when the sun is most active, leading to higher chances of solar storms,” aurora hunter Wil Cheung said. NASA’s Jamie Favors illustrated that such heightened activity.
Space weather observatory networks, from the 30-minute aurora warnings provided by NOAA to smartphone applications such as My Aurora Forecast, are geared to detect these occurrences before they occur. But the September 29 storm challenges the limits of predictability. Solar wind conditions can change in the blink of an eye, particularly when coronal holes such as the 500,000-kilometer-wide feature earlier in the season spew rain of fast-moving particles toward our planet.
From the earth’s surface, the display is a function of both space weather and nighttime geography. The aurora borealis exists throughout the year, but is observable based on length of darkness and lack of light pollution. Within the region of the Arctic Circle, midsummer twilight will mask the lights even during geomagnetic storms common to the region; the September equinox sees a reduction from the long dark nights. Inside the 48 lower-48 contiguous states, backwoods scenery like Maine’s Katahdin Woods & Waters National Monument, Minnesota’s Gunflint Trail, and Michigan’s Upper Peninsula are most accessible when storms force the auroral oval south.
The color chemistry is equally alluring. Charged particles drifting along magnetic field lines and colliding with gases in the atmosphere excite atoms and molecules to emit. Oxygen at higher elevations emits greens and reds, nitrogen blues and purples colors Pilot Matt Melnyk so vividly painted as he flew over Hudson Bay: At times it felt like we were flying right inside of the Aurora.
Severe geomagnetic storms such as the present G3 event might have hidden effects, apart from the brilliant light display. Such storms, NOAA cautions, can bring about surface charging on satellites, enhanced drag on low-Earth-orbit spacecraft, and intermittent interference with GPS and radio communications. Such hazards must be monitored in real time, not just for amateur aurora enthusiasts, but for industries reliant on satellite facilities.
The next few weeks are still a time of increased potential. With equinox-related magnetic alignment still prevalent and solar maximum ongoing, aurora-witnessers north of the equator and even into the middle latitudes can once again be subject to the effects of another unexpected display. For restless spirits willing to venture outdoors beneath transparent, black skies, the alignment of magnetism, solar wind, and terrestrial atmosphere could provide additional nights of uncharacteristic and intense light.
