Data may challenge and reshape current models of solar wind evolution.
A recent study led by Dr. Michael Starkey of the Southwest Research Institute has delivered the first observational evidence from the Magnetospheric Multiscale (MMS) Mission of pickup ions (PUIs) and their related wave activity in the solar wind near Earth. NASA launched the MMS mission in 2015, deploying four spacecraft to study Earth’s magnetosphere, the magnetic field that protects the planet from harmful solar and cosmic radiation.
Formation and behavior of PUIs
PUIs are created when neutral particles traveling through the heliosphere become ionized by the solar wind. Once ionized, they are carried along with the solar wind and spiral around the local magnetic field, forming a plasma population with properties that differ from the standard solar wind particles.
In this study, PUIs displayed a characteristic velocity distribution, without the presence of other significant energetic ion or electron populations. Evidence of wave activity was identified using MMS magnetic field measurements, combined with theoretical predictions of expected wave growth modes based on models of the observed PUIs.
“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,” Starkey said. “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 plasma populations
By modeling the individual ion components (solar wind and PUIs), the authors identified which populations could be responsible for the observed wave activity. They concluded that the observed waves were likely generated by helium and/or hydrogen PUIs, but due to instrument limitations, they were unable to pinpoint the precise ion species responsible.
At farther distances from the Sun, the relative density of PUIs in the solar wind increases, which increases their contribution toward the heating and thermalization of the solar wind through wave-particle interactions. At the outer edges of the solar system, PUIs contribute significantly to the total dynamic pressure in the solar wind, which has large implications for physical processes taking place at the termination shock and in the heliosheath.
“Near Earth, the intensity of PUIs is relatively low, and so it is typically assumed that their contribution to wave-particle interactions in the solar wind is negligible,” Starkey added. “If this assumption is false, current theory and modeling of the solar wind and its evolution throughout the heliosphere would need to be updated.”
Reference: “First MMS Observations of Waves Possibly Generated by PUIs Near Earth” by Michael J. Starkey, Kyunghwan Dokgo, Justyna M. Sokół, Joey Mukherjee, Charles W. Smith, Stephen A. Fuselier and Roman Gomez, 25 May 2025, Journal of Geophysical Research: Space Physics.
DOI: 10.1029/2024JA033660
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