2:30 PM - 2:45 PM
[PEM10-16] Spatio-Temporal Evolution of Solar Plasma Ejecta Approaching from a Multi-Point Comparison of Galactic Cosmic Ray Variations
Keywords:Interplanetary Coronal Mass Ejection (ICME), Galactic Cosmic Rays, Forbush Decrease (FD)
Understanding the propagation process of Interplanetary Coronal Mass Ejections (ICMEs) in the inner heliosphere is a crucial topic, especially from arrival predictions. To achieve more accurate space weather forecasting, it is essential to improve propagation models using in-situ ICME observations from spacecraft (e.g., JLF von Forstner et al., 2020). The Forbush Decrease (FD) (Forbush, 1937), which shields background galactic cosmic rays as an ICME passes an observer, is a powerful tool for tracking ICME evolution. While FD can be detected with relatively simple particle instruments, its depth, shape, and gradient reflect changes in the magnetic and geometrical structure of the propagating ICME. Therefore, by elucidating the physical relationship between ICMEs and FDs and leveraging low barriers to entry for observations of FD, we aim to extract ICME characteristics from a large dataset of FD events and ultimately contribute to improving ICME propagation models.
To investigate the relation between FD and ICME evolution, this study analyzes a multi-spacecraft ICME observation event in March 2022. The Mercury explorer BepiColombo (Bepi) and the solar observation spacecraft Solar Orbiter (SolO) were positioned at nearly the same heliocentric distance. Still, they differed in azimuth by approximately 50 degrees, while the lunar orbiter LRO and Solar Orbiter were separated radially by 0.5 AU and were nearly aligned in a straight line. This configuration provided an ideal opportunity to examine the large-scale structure of a single ICME. Using particle instrument data from these spacecraft, we extracted FD events through a calibration method established in this study (e.g., Kinoshita et al., 2025). We compared them with magnetic field and solar wind data.
First, a decrease in FD depth and gradient was observed in the radial direction between SolO and LRO, reflecting ICME expansion and weakening. FD exhibits a two-step decline (Cane, 2000), corresponding to the shock sheath and magnetic cloud regions of an ICME. While both spacecraft detected two distinct ICME regions in the magnetic field and solar wind data, the FD exhibited a two-step decrease in SolO but only a one-step reduction in LRO. This discrepancy is likely due to the expansion of the shock sheath, which increased the passage duration, and the weakening of the magnetic cloud, which reduced its shielding effect—leading to a significant drop in cosmic ray counts only in the sheath region. Our hypothesis—that the number of FD steps decreases due to ICME expansion and weakening—provides a new perspective on this issue, and we aim to further investigate this by increasing the sample size.
Next, an azimuthal comparison was then attempted between Bepi and SolO. However, a previous ICME had already passed Bepi, preventing the planned direct comparison. However, SolO simultaneously observed the second ICME of interest, which was clearly visible in the magnetic field and solar wind data, and had no effect on the recovery of the FD caused by the first ICME at Bepi. In SolO, where the ICME passed close to its center and exerted a relatively isotropic shielding effect, the FD showed a decrease of about 30%. In contrast, Bepi passed the outer edge of the ICME, and the shielding effect was anisotropic and insufficient, resulting in only about a 10% decrease - not enough to exceed the first FD event. This suggests that the shielding effect of ICMEs is azimuthally dependent.
Observational examples of FDs in the inner heliosphere remain limited, and our understanding of how recently erupted solar plasma generates FDs and their characteristics is still incomplete. In this study, we established methods for calibrating particle instruments and selecting comparable detector channels on other spacecraft, and through event studies, we showed that FDs can be used as an indicator of the attenuation and expansion of ICME and the longitudinal position relationship between ICME and the observer. We aim to achieve a statistically robust understanding of FD characteristics and their relationship with ICME evolution by applying these techniques to additional samples.
To investigate the relation between FD and ICME evolution, this study analyzes a multi-spacecraft ICME observation event in March 2022. The Mercury explorer BepiColombo (Bepi) and the solar observation spacecraft Solar Orbiter (SolO) were positioned at nearly the same heliocentric distance. Still, they differed in azimuth by approximately 50 degrees, while the lunar orbiter LRO and Solar Orbiter were separated radially by 0.5 AU and were nearly aligned in a straight line. This configuration provided an ideal opportunity to examine the large-scale structure of a single ICME. Using particle instrument data from these spacecraft, we extracted FD events through a calibration method established in this study (e.g., Kinoshita et al., 2025). We compared them with magnetic field and solar wind data.
First, a decrease in FD depth and gradient was observed in the radial direction between SolO and LRO, reflecting ICME expansion and weakening. FD exhibits a two-step decline (Cane, 2000), corresponding to the shock sheath and magnetic cloud regions of an ICME. While both spacecraft detected two distinct ICME regions in the magnetic field and solar wind data, the FD exhibited a two-step decrease in SolO but only a one-step reduction in LRO. This discrepancy is likely due to the expansion of the shock sheath, which increased the passage duration, and the weakening of the magnetic cloud, which reduced its shielding effect—leading to a significant drop in cosmic ray counts only in the sheath region. Our hypothesis—that the number of FD steps decreases due to ICME expansion and weakening—provides a new perspective on this issue, and we aim to further investigate this by increasing the sample size.
Next, an azimuthal comparison was then attempted between Bepi and SolO. However, a previous ICME had already passed Bepi, preventing the planned direct comparison. However, SolO simultaneously observed the second ICME of interest, which was clearly visible in the magnetic field and solar wind data, and had no effect on the recovery of the FD caused by the first ICME at Bepi. In SolO, where the ICME passed close to its center and exerted a relatively isotropic shielding effect, the FD showed a decrease of about 30%. In contrast, Bepi passed the outer edge of the ICME, and the shielding effect was anisotropic and insufficient, resulting in only about a 10% decrease - not enough to exceed the first FD event. This suggests that the shielding effect of ICMEs is azimuthally dependent.
Observational examples of FDs in the inner heliosphere remain limited, and our understanding of how recently erupted solar plasma generates FDs and their characteristics is still incomplete. In this study, we established methods for calibrating particle instruments and selecting comparable detector channels on other spacecraft, and through event studies, we showed that FDs can be used as an indicator of the attenuation and expansion of ICME and the longitudinal position relationship between ICME and the observer. We aim to achieve a statistically robust understanding of FD characteristics and their relationship with ICME evolution by applying these techniques to additional samples.