Scientists at Southwest Research Institute have uncovered how solar activity influences the velocity distribution and behavior of helium pickup ions. The study was published in The Astrophysical Journal.
A new study led by Southwest Research Institute’s Dr. Keiichi Ogasawara indicates that helium pickup ions are a wellspring of solar energetic particles. These high-energy accelerated particles travel at speeds twice as fast as during times of quiet solar activity. But when they are boosted by solar shocks, such as coronal mass ejections, they can penetrate spacecraft and spacesuits, posing a radiation hazard to astronauts. Image Credit: Courtesy of NASA Scientific Visualization Studio/ANIL RAO/Univ. of Colorado/MAVEN/NASA GSFC.
Pickup ions are charged particles formed when neutral atoms from outside the solar system become ionized. This ionization occurs through solar ultraviolet radiation, after which the particles are captured by the interplanetary magnetic field.
A study led by SwRI’s Dr. Keiichi Ogasawara reveals that these pickup ions serve as a source of solar energetic particles (SEPs). These high-energy accelerated particles, including protons, electrons, and heavy ions, are generated by solar phenomena such as flares and coronal mass ejections (CMEs). By analyzing data from NASA’s Solar Terrestrial Relations Observatory, SwRI observed the early stages of helium pickup ion acceleration during multiple CME events.
We carefully identified the specific properties of the ions and used them to trace the physical energy transfer processes. We also considered the roles played by different types of interplanetary shocks, when fast-moving solar wind disturbances collide with slower-moving solar wind plasmas.
Dr. Keiichi Ogasawara, Southwest Research Institute
Understanding the timing and mechanisms behind SEP occurrences is crucial, as high-energy SEPs can penetrate spacecraft and spacesuits, creating significant radiation risks for astronauts.
SwRI researchers also analyzed the velocities of individual helium pickup ions in relation to the orientation of the local magnetic field, identifying distinct behaviors as these ions interact with various types of shocks linked to CMEs.
The velocity distribution of pickup ions is quite different from that of the solar wind. In fact, they can be twice as fast as the solar wind even during relatively quiet times. Because of this difference, pickup ions are more effectively accelerated to even higher energies than normal solar wind particles.
Dr. Keiichi Ogasawara, Southwest Research Institute
Unlike SEPs, which are high-energy and sporadic, the solar wind is a continuous, lower-energy stream of plasma emitted by the Sun’s outer atmosphere, the corona.
SwRI has developed a new method to track the evolution of pickup ions as they move through shock waves, turbulence, and large-scale magnetic structures. This technique enables researchers to distinguish between processes that increase or decrease particle energy and those that preserve energy levels.
This study examined particle behavior across a broad range of structures in the heliosphere including magnetic structures, interplanetary shocks, and the sheath region that forms in advance of a CME.
Dr. Keiichi Ogasawara, Southwest Research Institute
Journal Reference:
Ogasawara, K., et al. (2025) Helium Pickup Ion Velocity Distributions Observed in Interplanetary Coronal Mass Ejection Structures. The Astrophysical Journal. doi.org/10.3847/1538-4357/adb1b4.