An impending collision between a "cannibal" coronal mass ejection (CME) and Earth is expected to occur on July 18, potentially triggering a significant geomagnetic storm. The unique CME is the result of two coronal mass ejections merging into a massive cloud of magnetized plasma.
A cannibal CME occurs when a second, faster CME catches up to an initial CME, engulfing it and creating a single, larger wave of plasma. In this case, the collision is the result of a surprise "dark eruption" and another CME that originated from different sunspots.
Although individual CMEs are of moderate strength, their combined size and speed increase the likelihood of a significant geomagnetic storm, potentially classified as a G1 or G2 level disturbance—the highest classes for a geomagnetic storm.
Cannibal CMEs are rare phenomena, requiring specific alignment and speeds of successive CMEs. However, there have been several instances of cannibal CMEs in recent years, including one in November 2021 that triggered a major geomagnetic storm.
Earth has already experienced five G1 or G2 geomagnetic storms this year, including the most potent storm in over six years. These storms have elevated the temperature of the thermosphere - the second-highest layer of Earth's atmosphere, to its highest levels in more than two decades. Additionally, the number of sunspots has been steadily increasing, reaching the highest total in nearly 21 years in June.