Geomagnetic Storm

A geomagnetic storm is the magnetosphere's large-scale response to sustained forcing by the solar wind — the days-long global disturbance that follows when a coronal mass ejection or fast wind stream with southward magnetic field couples into Earth's field via reconnection. Energy floods the system: the ring current — a belt of energised particles circling the planet — intensifies (measured by the Dst index's deep negative excursions), the auroral ovals brighten and bulge equatorward, and ionospheric and ground currents surge. Severity is graded by Kp and NOAA's G1–G5 scale, and the consequence list is the space-weather canon: transformer-threatening geomagnetically induced currents in power grids (Quebec's 1989 blackout the standard citation), GNSS range errors and scintillation, HF radio disruption on polar routes, elevated satellite charging, and a heated, swollen thermosphere that multiplies drag — dragging low orbits off prediction and, in the 2022 case that entered industry folklore, deorbiting a freshly launched satellite batch outright. Storms cluster around solar maximum, but fast-stream storms recur through the whole cycle, and the Carrington-class tail of the distribution is the scenario resilience planning is written for.