An international research team has made significant strides in forecasting solar storms by examining cosmic rays, utilizing data collected by the BepiColombo mission. Led by Gaku Kinoshita from the University of Tokyo, this pioneering study reveals how coronal mass ejections (CMEs) affect cosmic-ray intensity at varying distances from the Sun. This innovative approach aims to enhance the accuracy of space weather predictions, particularly following CMEs.
CMEs are massive bursts of plasma that emerge from the Sun’s outer atmosphere. When severe, these plasma clouds can traverse interplanetary space, occasionally interacting with Earth’s magnetic field to generate powerful geomagnetic storms. Such storms can lead to stunning auroras in polar regions and pose risks to satellite electronics and even terrestrial electrical grids. To mitigate potential damage, astronomers seek to predict the trajectory and intensity of CME plasma, allowing for timely protective measures to safeguard vulnerable systems.
Kinoshita’s team has identified a largely overlooked source of information: the interaction between CME plasma and cosmic rays. Cosmic rays, which are energetic charged particles from beyond the solar system, maintain a relatively stable flow through interplanetary space. When an interplanetary CME (ICME) passes, it temporarily displaces these cosmic rays, resulting in a localized reduction in their intensity, a phenomenon termed the Forbush decrease effect.
“This phenomenon can be detected even with relatively simple particle detectors and reflects the properties and structure of the passing ICME,” Kinoshita notes. Although cosmic-ray observations can yield insights into the characteristics of an ICME, previous observations of Forbush decreases had not been recorded simultaneously at multiple distances from the Sun. This gap left scientists uncertain about how the distance from the Sun influenced the severity of these decreases.
Kinoshita’s team has now filled this gap using the BepiColombo mission, a collaborative effort between Europe and Japan set to begin orbiting Mercury in November 2026. While the mission primarily focuses on Mercury’s surface and magnetosphere, it also includes non-scientific equipment designed to monitor cosmic rays and solar plasma in its vicinity.
“Such radiation monitoring instruments are commonly installed on many spacecraft for engineering purposes,” Kinoshita explains. “We developed a method to observe Forbush decreases using a non-scientific radiation monitor onboard BepiColombo.” By combining this data with information from specialized radiation monitoring missions, including the ESA’s Solar Orbiter, which is exploring the inner heliosphere from within Mercury’s orbit, the researchers constructed a detailed, distance-dependent profile of an ICME that occurred in March 2022.
The findings confirmed the anticipated relationship between the Forbush decrease and distance from the Sun. “As the ICME evolved, the depth and gradient of its associated cosmic-ray decrease changed accordingly,” Kinoshita states. This method now opens avenues for utilizing non-scientific radiation monitors on various missions throughout the solar system, providing a more comprehensive understanding of ICME effects based on distance.
“An improved understanding of ICME propagation processes could contribute to better forecasting of disturbances such as geomagnetic storms, leading to further advances in space weather prediction,” Kinoshita adds. This research could allow astronomers to model the paths and intensities of solar plasma shortly after a CME erupts, thus enhancing preparedness for potentially disruptive events.
The research findings are published in The Astrophysical Journal, marking a notable advancement in space weather forecasting methodology. As scientists aim to refine their predictive capabilities, this innovative approach could play a crucial role in safeguarding technology and infrastructure from the impacts of solar storms.
