A Dec. 15, 2024 coronal mass ejection tracked by a record 17 spacecraft turned out to have two asymmetric lobes, including a faster Earth-bound component hidden behind a larger, slower lobe.
SOHO saw only the slower off-angle lobe, while the wider spacecraft spread across the solar system exposed the missed structure; Solar Orbiter’s non-detection at 0.94 AU also helped constrain the CME’s shape.
The fast lobe crossed the Earth-Mars sector at an average 840 km per second, while the slower lobe moved at 534 km per second and had slowed to about 400 km per second by STEREO-A.
Detections progressed from near Mercury and BepiColombo on Dec. 16 to Earth on Dec. 17, STEREO-A on Dec. 18, and Mars and Europa Clipper farther out, though the event was too weak to produce major aurorae.
Researchers said the unusually lopsided CME could improve space-weather forecasting for astronauts beyond Earth, where missing a fast CME can cut warning time and raise radiation risk.