Gannon Storm Compressed Earth's Plasmasphere to One-Fifth Its Radius

August 31, 2026 at 08:30 PM2 min

This digest was compiled by AI from multiple sources — links to the originals are below.

The May 2024 Gannon geomagnetic storm, the strongest in over two decades, compressed Earth's plasmasphere dramatically. Japan's Arase satellite measured the plasmapause moving from an L-shell near 7 to near 1.5 within nine hours. The contraction placed the boundary at roughly one-fifth of its earlier radial distance from Earth's centre.

Key Facts

The Gannon storm of May 2024 was the first G5 geomagnetic storm since October 2003, with a planetary Kp index of 9.
Japan's Arase satellite observed the plasmapause move from an L-shell near 7 to near 1.5 within nine hours during the storm.
NASA's event record lists a peak Dst of -412 nanoteslas, while the SYM-H index fell to -518 nanoteslas.
Active region 13664 released repeated powerful flares and Earth-directed coronal mass ejections starting 8 May 2024.

Storm Origins

Active region 13664 on the Sun released repeated powerful flares and several Earth-directed coronal mass ejections from 8 May 2024. The first interplanetary shock reached Earth's magnetic environment at about 17:05 UTC on 10 May. Solar-wind density and speed jumped, while the magnetic field carried by the solar wind repeatedly pointed south. That southward orientation couples efficiently with Earth's oppositely directed field, transferring energy into the magnetosphere through magnetic reconnection.

Plasmasphere Contraction

Earth's plasmasphere is a doughnut-like region filled with relatively cool, dense plasma drawn largely from the upper ionosphere. The plasmapause is a sharp density transition that bounds this region and is often described using an L-shell. During the Gannon storm, the Arase satellite measured the plasmapause moving from an L-shell near 7 to one near 1.5 within nine hours. That change placed the boundary at roughly one-fifth of its earlier radial distance from Earth's centre.

Storm Intensity

The planetary Kp index reached 9, corresponding to G5, the highest category on NOAA's geomagnetic-storm scale. It was the first G5 event since the Halloween storms of October 2003. NASA's event record lists a peak Dst of -412 nanoteslas, while the higher-time-resolution SYM-H index used in the later plasmasphere study fell to -518 nanoteslas. Those numbers are related measures of the disturbance, not interchangeable scores, and help explain why descriptions vary from 'strongest in two decades' to comparisons with the 1989 storm.