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A new study led by Southwest Research Institute's Dr. Michael Starkey has provided observational evidence from the SwRI-led Magnetospheric Multiscale (MMS) Mission of pickup ions (PUIs) and associated wave activity in the near-Earth solar wind environment. The MMS mission, launched by NASA in 2015, placed four spacecraft in orbit to observe Earth's magnetosphere, a magnetic field around the planet that shields it from harmful solar and cosmic radiation.

The "First MMS Observations of Waves Possibly Generated by PUIs Near Earth" paper is in the Journal of Geophysical Research: Space 麻豆淫院ics

PUIs are formed when neutral particles flowing through the heliosphere are ionized in the solar wind. These PUIs are dragged along with the solar wind and gyrate around the local magnetic field, forming a distinct plasma population with different characteristics from the typical solar wind population.

The PUIs were observed to have a typical velocity distribution absent of any other significant energetic ion or electron populations. The wave activity was identified using magnetic field data from MMS combined with theoretical analysis of the 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."

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."