What makes one solar outburst a global headline while another barely registers on the ground?
The contrast may not be in the brightness of the flare itself, but in the invisible geometry of the magnetic field that follows along. A flare of X-class strength may momentarily saturate radio communications on the sunlit side of the planet, while a follow-on coronal mass ejection (CME) may or may not couple well with the Earth’s magnetic shield. The strength of the coupling, rather than the speed, will determine whether the impact will be a mere tremor of the magnetosphere or a storm strong enough to reach into latitudes that rarely if ever see an aurora.
In this latest occurrence of solar activity, the intensity of the flare was X1.9 and was associated with sunspot AR4341. The direct impact of the flare was observed in the form of R3 radio blackout regions on the dayside of the Earth, which is a reminder that the flare’s impact takes the speed of light to reach the Earth. On the other hand, the CME, which is a phenomenon that is sometimes but not always associated with solar flares, moves out as a magnetized cloud of plasma. According to NOAA, CMEs are large amounts of plasma and magnetic fields that are ejected from the sun’s corona, which can increase in size as they move out into interplanetary space and, in the fastest cases, can reach Earth in 15-18 hours.
Although the models can estimate the arrival time and probability of a hit, the most crucial ingredient is still very hard to come by: the magnetic orientation of the CME. One of the most popular subjects of conversation in space weather talk is the north-south component of the embedded interplanetary magnetic field, Bz. If Bz turns south for a sufficiently long period of time, Earth’s magnetosphere will find a way to link up with the incoming magnetic field, pouring energy into the Earth’s environment and triggering geomagnetic storms. But if Bz turns north, the same CME can be largely skirted around a “big one” that brings little more than a quick shock.
It is exactly this uncertainty that causes the confidence level of the prediction to suddenly increase just before the impact. The key data is supplied by solar wind observation satellites placed upstream, near the L1 point between the Sun and Earth. NOAA states that in 2016, the real-time solar wind satellite in operation was DSCOVR, with ACE as the secondary source of data as required. The sensors at L1 can detect the shock front and magnetic field as the CME arrives, normally providing a 15 to 60-minute warning of the arrival of the disturbance in the Earth’s magnetic environment. This is when the door is about to open, as in southward Bz, or mostly closed. When the coupling is strong, the intensity of the geomagnetic storm can be translated from space physics to operational danger.
NOAA’s G-scale goes from G1 to G5, with G3 and G4 corresponding to problems that are important to today’s infrastructure: satellite charging and tracking issues, increased drag on satellites in low Earth orbit, and sometimes problems with navigation and radio communications. G4 specifies that aurorae can be observed as far south as Alabama and northern California, which is a geographic shorthand that also implies the degree of magnetosphere disturbance. The lesson for engineers, operators, and anyone whose business involves timing signals, radio propagation, or the integrity of spacecraft is this: The showstoppers get all the publicity, but the real punch of the storm is encoded in the magnetic field that arrives at the very end measured, not guessed.
