The Sun & Space Weather

The Sun is a variable star on an eleven-year cycle, and the variability reaches us. Flares release X-rays that arrive in eight minutes and black out high-frequency radio on the daylit side. Coronal mass ejections throw a billion tonnes of magnetised plasma outward, and if one is aimed correctly it compresses Earth's magnetic field a day or two later, producing aurora far from the poles, extra drag on satellites in low orbit, and induced currents in power grids and pipelines.

Forecasting it is possible but only just. The warning comes from a handful of spacecraft parked at the L1 point, a million and a half kilometres upstream, where the solar wind passes them fifteen to sixty minutes before it reaches us. That is enough to trip a grid operator into a safe configuration and not much else; the direction of the field embedded in the cloud, which decides whether the storm couples to Earth's field at all or slides past it, is not reliably known until the cloud arrives. So a forecast is a probability rather than a schedule, and the operational products read like weather did in the 1950s. The Carrington event of 1859 set telegraph offices on fire. A repeat would be considerably more expensive, and the estimates for how expensive vary by an order of magnitude, which is itself the finding.

That is why a fleet does nothing but watch. SOHO has been at L1 since 1995; SDO images the disc every few seconds; Solar Orbiter is climbing out of the ecliptic to look at the poles, which no instrument had properly seen; and Parker Solar Probe flies through the corona itself at nearly 700,000 kilometres an hour, close enough that its heat shield does the work its instruments cannot. Parker answered one long-standing question by finding the switchbacks — sharp reversals in the magnetic field — that appear to be part of how the corona gets to a million degrees while the surface below it sits at six thousand. It has not answered why. Space weather remains the one branch of astronomy with an operational forecast office, and the only one where being wrong costs somebody money the same week.