Why A geomagnetic storm is a temporary disturbance of Earth's magnetic field, usually triggered when a coronal mass ejection or a high-speed solar wind stream slams into the magnetosphere. The disturbance is measured by the K index (short term, 3-hour) and A index (daily); high values mean stormy conditions. During a storm HF paths, especially those crossing the polar regions, are absorbed and weakened, while auroral VHF propagation may improve.
Watch out The solar flux index tracks 10.7 cm solar radio noise, a measure of ionizing activity, not the state of Earth's magnetic field, and ordinary thunderstorms are weather in the lower atmosphere with no connection to the geomagnetic field.
Geomagnetic = Earth's magnetism. Storm = high A and K numbers, bad HF, good aurora.