A solar flare that seemed to largely avoid our planet was actually concealing a section launched directly toward Earth.
The event studied occurred in December 2024. The Sun had expelled a large amount of matter into space. This type of event is called a coronal mass ejection. It is a vast cloud of charged particles, accompanied by a magnetic field, moving away from the Sun.
Three-dimensional reconstruction of a coronal mass ejection observed from the side.
Credit: NASA Goddard Space Flight Center / Scientific Visualization Studio
From Earth, most of this ejection appeared to be heading in another direction. An observation from our vicinity could therefore give the impression that the danger was limited. Yet the flare’s actual shape was far less regular than it appeared.
To understand what had happened, the researchers brought together measurements from 17 probes distributed throughout the inner Solar System. This exceptional configuration allowed them to observe the same flare from several locations. Among these spacecraft was Europa Clipper, which was then en route to Jupiter.
This NASA probe was carrying out instrument checks when it measured an unusually hot and low-density solar wind. The researchers then linked this signal to the flare that had occurred a few days earlier. Other probes made it possible to reconstruct its progression.
The decisive clue came from STEREO-A, which was observing the Sun from another angle. Its data revealed a distinct and faster section of the ejection, directed toward Earth. From our planet, this component was hidden by the event’s geometry. The flare therefore had a highly asymmetrical shape.
This detail matters for space weather. When such ejections reach Earth’s environment, they can cause geomagnetic storms. These can disrupt radio communications, satellite signals, and some power grids. For astronauts in space, energetic particles also pose a radiation-related risk.
In this specific case, the researchers had a rare advantage: numerous spacecraft were in useful positions at the same time. Interplanetary missions could therefore complement specialized satellites. Even when traveling toward another destination, their measurements can help detect parts of flares that are invisible from Earth.
The challenge now is to better integrate these scattered viewpoints into monitoring systems. With more missions to the Moon, Mars, and distant planets, some probes could form an additional network for observing the Sun and warning crews located far from Earth.