A team including scientists from the University of Tokyo has now produced the first high-quality, multipoint measurements of how a cloud of solar ejecta evolves as it moves away from the sun. They used an ad hoc network of spacecraft instruments that were never originally designed to study solar eruptions, turning routine engineering monitors into scientific sensors that can track these events.
The eruptions, known as coronal mass ejections, or CMEs, are large blobs of magnetized plasma that erupt from the solar atmosphere and race outward through interplanetary space. When a CME passes near Earth, operators sometimes switch satellites into safe mode until the disturbance subsides, but better forecasting is needed because unexpected events can still cause serious damage.
The new method relies on a phenomenon called a Forbush decrease, where the intensity of background cosmic rays temporarily drops when a CME passes by. The CME's strong magnetic field deflects many of the high-energy charged particles that constantly bombard the solar system, making the CME appear partially opaque to cosmic rays. By measuring how the cosmic ray flux changes in time and space, the team can infer the structure of the CME and how it evolves during its journey.
In March 2022, three spacecraft were fortuitously aligned to observe the same solar eruption from different vantage points: ESA's Solar Orbiter, ESA and JAXA's BepiColombo mission, and a NASA near-Earth spacecraft. This rare configuration allowed researchers to compare the event simultaneously along different directions and distances from the sun, building a more complete three-dimensional picture of the CME.
By combining the cosmic ray data with in situ measurements of the solar wind and magnetic field, the scientists linked variations in particle intensity directly to the physical structure of the eruption. This showed how the CME's shape and strength changed as it propagated outward, revealing details that are essential for improving models of solar storms and their potential impact on Earth.
A key result of the study is the demonstration that simple housekeeping instruments can be repurposed for frontline space weather science. On BepiColombo, the team carefully calibrated a system-monitoring detector, originally intended only to check spacecraft health, and transformed it into a sensor capable of registering Forbush decreases associated with CMEs. Long-ignored engineering data thus became a valuable scientific resource.
Unlike specialized instruments that may operate only intermittently, these general-purpose monitors tend to run continuously, providing an almost unbroken stream of measurements. When data from several such instruments on multiple spacecraft are combined, researchers can monitor CMEs from several locations at once and reconstruct their evolution in both space and time.
Because these instruments were not designed with this science in mind, the team first had to characterize their behavior from the ground up. They developed new calibration procedures and analysis techniques to extract reliable cosmic ray signals from the engineering telemetry, ensuring that the reconstructed particle variations truly reflected the passage of the CME.
With a growing fleet of spacecraft scattered between the sun and Earth and more missions planned, opportunities for routine multipoint observations are increasing. By integrating measurements from all available platforms, including repurposed engineering sensors, scientists expect to build much more accurate models of how solar ejections propagate and how they might affect planetary environments.
The work is reported in The Astrophysical Journal in a paper titled "Spatiotemporal Evolution of the 2022 March Interplanetary Coronal Mass Ejection Revealed by Multipoint Observations of Forbush Decreases". The authors describe how their approach links the observed cosmic ray decreases to the detailed structure of the CME and outline how similar methods could underpin future operational space weather forecasting systems.
Research Report:Spatiotemporal Evolution of the 2022 March Interplanetary Coronal Mass Ejection Revealed by Multipoint Observations of Forbush Decreases
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