‘Detrimental effects to some of our critical infrastructure technology are possible, but mitigation is possible,’ the Space Weather Prediction Center said, summarizing the practical side of the same space weather conditions that could also produce an aurora visible well south of usual high-latitude locations.
When aurora becomes a possibility over a broad area of the United States, including viewlines that could stretch towards locations as far south as Alabama, it is not the lights that are the driver but rather the magnetic environment that the Earth is in. When it comes to operations, an aurora forecast is essentially a proxy for the degree to which the energy from the Sun is coupling into the magnetosphere.
A geomagnetic storm is a major disturbance of Earth’s magnetosphere due to an efficient energy transfer from the solar wind to the near-Earth space. The most intense events usually follow coronal mass ejections, in which enormous amounts of plasma and magnetic fields are directed towards Earth, compressing and disturbing the magnetosphere. The most favorable geometry for the storm development is characterized by high-speed solar wind and, most importantly, a southward-pointing interplanetary magnetic field, which favors efficient energy transfer on the dayside. This leads to an enhancement of magnetospheric and ionospheric currents, which is why the space weather scales are designed around the magnetic disturbance rather than the aurora visibility.
These public alerts may mention the NOAA geomagnetic storm scale, which goes from G1 (minor) to G5 (extreme). The G4 watch is significant because it shows a level of alert where the auroral oval may expand and become bright enough to be seen outside the polar states, as well as increasing the risk for technology systems that rely on steady radio communication and accurate timing. In addition, the heating and density variations that occur in the upper atmosphere during geomagnetic storms can cause drag on satellites in low Earth orbit, and ionospheric conditions can cause errors in satellite positioning systems. On the other hand, geomagnetically induced currents can be caused by storm currents on the ground.
The most important single value for observers will be the planetary K index. The Kp index varies from 0 to 9, and it is roughly proportional to the distance of the auroral oval from the magnetic poles. When the Kp index is low, the auroral oval remains well away from the magnetic poles, and the aurora is likely to be weak and remain well north. As the Kp index rises into the 6-7 range, the auroral oval will expand to the point where U.S. sites on the northern edge of the country may become feasible. When the Kp index reaches its highest levels (8-9), the auroral oval will extend well toward the equator and brighten considerably, giving more people a chance to view the aurora as more than a faint glow.
Intensity is not the only factor, but timing and geometry are also important. According to the Space Weather Prediction Center, the best viewing times are usually within an hour or two of midnight, often between 10 p.m. and 2 a.m. local time, although major activity may extend the viewing period. It is also necessary to have a clear view of the northern horizon because aurora does not have to be directly overhead to be observed; under optimal conditions, it can be observed from as far away as 1000 km. Darkness is, of course, a non-negotiable requirement because city lights, and even the bright Moon, can limit what can be detected by the eye even if the aurora is physically strong.
Practical map products, also included in aurora forecasts, are designed for non-experts. The operational “viewline” on North American graphics shows the southernmost latitude where aurora can be viewed on the horizon under the maximum activity for the night, according to the OVATION model. The images are updated continuously and are meant to assist observers in relating a local sky to a regional-scale disturbance.
In photography, the current benefit is the sensitivity of image sensors. Night modes in smartphones are capable of detecting weak arcs and glows of the aurora that are not visible to the naked eye, particularly when the exposure time is longer and the image is stabilized. In some instances, the camera acts as a scouting tool, not only as a recorder, to determine where the aurora is forming before it is visible to the naked eye.
