A major geomagnetic storm is never simply a spectacular sky event; it is a stress test of the technologies that assume the Earth’s magnetic environment is quiet.
A fast coronal mass ejection, accompanying an X-class flare, can “ring” the magnetosphere, and as a result, aurorae will be extended far beyond the polar regions into areas of the U.S. that have never experienced aurorae before. In this particular case, the speeds of the coronal mass ejection were approximately 1,000-1,400 km/s, which is sufficiently high to compress the Earth’s magnetic field and result in strong storms. The bottom line is simple: the auroral oval will be expanded, and mid-latitude regions may be included in it for a short time if the energy transfer is efficient.
This efficiency depends on a surprisingly complex factor masked by a cloud of plasma: the polarity of the embedded magnetic field. This can be simplified by space weather enthusiasts to the following: the polarity of Bz, or the north-south component of the interplanetary magnetic field. If Bz is pointing south for an extended period of time, it can impact Earth’s magnetic field, which is pointing north, to create a pathway for particles and energy to pour into the upper atmosphere. If Bz is pointing north, Earth’s magnetic field will shield against this energy, and the best predictions will result in a lackluster show. In some cases, real CMEs will have a mixed magnetic field, so there may be some variabilityarcs and curtains of bright light, followed by periods of quiet when the sky is empty, only to come alive again.
The problem is that Bz cannot be determined with certainty from solar images.
Instead, the personality of the storm is evaluated upstream, by solar wind monitors situated near the Sun-Earth line. Satellites like DSCOVR and ACE analyze the approaching current at the L1 Lagrange point, 1.5 million miles away from Earth, measuring velocity, density, temperature, magnetic field strength, and magnetic field direction in real-time.
These data drive the models that bring the aurora maps that the public views, but it is also an alert system for operators who are much more concerned about induced currents and navigation than color. To answer the question of why a forecast will suddenly go from bad to good or from good to terrible, three things are most relevant: the angle of the magnetic field (again, Bz), the speed of the solar wind, and the strength of the magnetic field.
Negative Bz is the most important factor, speed is a measure of how much energy is transferred in a given time period, and strength is a component of how well that energy is transferred into the magnetosphere. The type of alert that is issued for public consumption will usually distill this down to a single headline number: the Kp index. The Kp index is a 0-9 number that shows the level of disturbance to the Earth’s magnetic field on a three-hourly basis, and it is what the decision to issue a geomagnetic watch or warning is based on.
If the Kp index is above 5, then the chances of viewing aurorae at lower latitudes are likely to be improved. The same energy from the storm that produces the coloring of the upper atmosphere can also affect systems that depend on a stable radio link and time. Geomagnetic storms are associated with effects on the communication of spacecraft, atmospheric drag, and GPS accuracyphenomena that can be subtle to the general public but important to the operator. For the sky enthusiast, the engineering message is implicit in the experience: the aurora is not a distant light show but a readable display of the coupled Sun-Earth system. When the system is locked in, the boundary between “high-latitude phenomenon” and “backyard surprise” “can shift hundreds of miles in a night.
