Recent observations from NASA's Magnetospheric Multiscale (MMS) mission have revealed that pickup ions (PUIs) and their associated wave activity play a more substantial role in shaping the solar wind near Earth than previously understood.
The study, led by Dr. Michael Starkey of the Southwest Research Institute (SwRI), utilized data from the four spacecraft of the MMS mission, which was launched in 2015 to investigate Earth's magnetosphere. PUIs are formed when neutral particles in the heliosphere become ionized and are then "dragged" along by the solar wind. Once charged, these ions gyrate around the local magnetic field, creating a distinct plasma population.
While typically present in low concentrations near Earth, the MMS findings indicate that PUIs can actively generate waves within the solar wind. This discovery suggests that current models of solar wind dynamics throughout the heliosphere may need revision, potentially impacting our understanding of processes extending to the solar system's outer boundaries. Researchers analyzed magnetic data from the MMS mission, cross-referencing it with theoretical models to understand wave development in the presence of PUIs. They concluded that helium and/or hydrogen PUIs are the most probable sources of the observed wave activity, although instrument limitations prevented definitive identification of the specific ion species.
Dr. Starkey highlighted the potential impact of these findings, stating, "It may be that PUIs play a larger role in the heating and thermalization of the solar wind near Earth than previously thought, which would have large implications for models of the solar wind throughout the heliosphere." This suggests that the mechanisms by which the solar wind gains energy and reaches thermal equilibrium might be more complex than currently modeled. As the solar wind travels farther from the Sun, the relative density of PUIs increases, amplifying their contribution to heating and thermalization processes through wave-particle interactions.
At the heliosphere's outer reaches, PUIs significantly influence the solar wind's dynamic pressure, affecting phenomena at the termination shock and within the heliosheath. The findings, published in the Journal of Geophysical Research: Space Physics, underscore the continued importance of studying the solar wind and its interactions with Earth's magnetosphere for understanding space weather and protecting critical infrastructure. Further investigations are exploring how solar events, such as coronal mass ejections (CMEs), affect the velocity distribution and evolution of helium pickup ions, as accelerated energetic particles can pose radiation hazards to astronauts and sensitive equipment.