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Southwest Research Institute scientists have discovered how solar activity affects the velocity distribution and evolution of helium pickup ions.

Pickup ions are charged particles created when neutral particles originating outside of our solar system are ionized. They are ionized by solar ultraviolet radiation and captured by the interplanetary magnetic field.

A new study led by SwRI's Dr. Keiichi Ogasawara indicates that these pickup ions are a wellspring of solar energetic particles (SEPs). These high-energy accelerated particles include protons, electrons and heavy ions produced by solar events like flares and coronal mass ejections (CMEs). Using data from NASA's Solar TErrestrial RElations Observatory, SwRI detected the initial characteristics of the helium pickup ion acceleration through several CME events.

The research is in The Astrophysical Journal.

"We carefully identified the specific properties of the ions and used them to trace the physical energy transfer processes," Ogasawara said. "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."

Understanding how and when SEPs occur is critical because, when they are accelerated to higher energies, they can penetrate spacecraft and spacesuits, posing a radiation hazard to astronauts.

SwRI also studied the velocities of individual helium pickup ions in relation to their local magnetic field orientations and identified their characteristic behaviors when interacting with different types of shocks associated with CMEs.

"The velocity distribution of pickup ions is quite different from that of the solar wind," Ogasawara said. "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."

In comparison to SEPs, the solar wind is a continuous lower-energy flow of plasma emitted by the corona, the sun's outer atmosphere.

SwRI developed a new method for tracking particle evolution as pickup ions travel through shock passages, turbulence and large-scale magnetic structures. This allows researchers to separate processes that increase or decrease energy from those that maintain 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," Ogasawara said.