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New observations reveal pickup ions near Earth
A recent study led by Dr. Michael Starkey of the Southwest Research Institute reports the first direct observations by NASA’s Magnetospheric Multiscale (MMS) mission of pickup ions (PUIs) and associated wave activity in the solar wind near Earth. These findings suggest that a previously overlooked population of ions may influence solar wind heating and, ultimately, space weather models.
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This illustration shows Earth’s magnetosphere interacting with the solar wind. The magnetosphere shields the Earth from harmful solar and cosmic radiation.
The MMS mission, launched in 2015, operates four coordinated spacecraft that measure electric and magnetic fields and plasma populations in and around Earth’s magnetosphere. By combining high-resolution magnetic field data with particle measurements and theoretical instability analyses, the research team identified plasma waves consistent with growth driven by PUIs.
How pickup ions form and why they matter
Formation and velocity signatures
Pickup ions originate when neutral particles drifting through the heliosphere become ionized—by solar ultraviolet radiation, charge exchange, or collisions—and are then picked up by the solar wind flow. Once ionized, these particles are accelerated by the ambient electric and magnetic fields and form distinct velocity-space distributions that differ from the thermal solar wind protons and electrons.
In the MMS observations, PUIs showed a characteristic nonthermal velocity distribution and no concurrent presence of other prominent energetic ion or electron populations. Magnetic field fluctuations recorded by MMS matched theoretical predictions for wave growth driven by such nonthermal ions, indicating a direct PUI–wave coupling in near-Earth solar wind.
Dr. Starkey commented on the implications: "The results of this study indicate that PUIs can in fact generate waves in the solar wind near Earth and motivate the need for further statistical studies of these processes. 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."
Modeling, limitations and heliospheric consequences
Researchers modeled separate ion populations (core solar wind, helium PUIs, hydrogen PUIs) to determine which components could drive the observed wave activity. The modeling indicates that helium and/or hydrogen PUIs are likely sources, but instrument sensitivities and species discrimination limits prevented definitive identification of the specific ion species in this event.
Farther from the Sun the relative number density of PUIs increases, and their role in wave generation, plasma heating, and thermalization becomes more significant. In the outer heliosphere, PUIs contribute appreciably to solar wind dynamic pressure and affect processes at the termination shock and within the heliosheath. If PUIs are also influential closer to Earth, as these MMS observations suggest, current solar wind evolution models and space weather forecasts may require revision to account for PUI-driven wave–particle interactions.
Mission and instrumentation context: MMS consists of four tetrahedrally arranged spacecraft carrying high-time-resolution magnetometers and particle instruments designed to resolve small-scale reconnection and plasma processes. While optimized for magnetospheric studies, their sensitivity has enabled this new detection of PUI-driven waves in the upstream solar wind.
Expert Insight
Dr. Elena Morales, an astrophysicist specializing in heliospheric plasma at the University of Colorado, offered perspective: "This result is an important reminder that even low-density particle populations can shape plasma dynamics through wave generation. Confirming how often PUIs drive waves near Earth will tell us whether we need to revise models used for space weather prediction."
The team calls for expanded statistical surveys using MMS and complementary missions (e.g., Parker Solar Probe, Solar Orbiter) to map PUI occurrence, species composition, and their role in wave–particle heating across heliocentric distance.
Conclusion
MMS observations provide the first direct evidence that pickup ions can generate wave activity in the solar wind near Earth. Although PUIs are relatively sparse close to our planet, their ability to drive waves and contribute to plasma heating could require updates to solar wind and space weather models. Continued multi-spacecraft observations and targeted modeling are needed to quantify the prevalence and impact of PUIs across the heliosphere.
Source: scitechdaily
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