First Detection of PUIs in the near-Earth Solar Wind
In addition to electromagnetic radiation, the Sun emits an intense stream of charged particles including protons, electrons and alpha particles. This stream is known as the solar wind and permeates the heliosphere, a sparse region of space carved out within the interstellar medium. Current models suggest much of solar wind dynamics are dictated by the presence and motion of pickup ions (PUIs), initially neutral particles that become singly ionised by interactions with the solar wind.
Once ionised, they are captured within the interplanetary magnetic field and forced into a spiral path. This sudden motion produces instabilities within the plasma, which radiate as low-frequency electromagnetic waves via cyclotron radiation. These have been shown to drive turbulence and contribute significantly to the heating of the solar wind. Yet near Earth’s orbit, these waves have been notoriously challenging to detect, primarily as they are overshadowed by other local activity within the Earth’s magnetosphere.
Now, a new study led by Michael J. Starkey presents the first simultaneous detection of both PUIs and associated wave activity in the near-Earth solar wind. This was made possible by NASA’s Magnetospheric Multiscale (MMS) mission, carrying two primary instruments. These are the Hot Plasma Composition Analyzer (HPCA), which measures the velocity distribution of particles within the plasma, as well as the FIELDS instrument suite, to measure the electric and magnetic fields within the region. The observations were taken on the 19th December 2022, when Earth passed through the so-calledhelium focusing cone. This is a specific region of the heliosphere where interplanetary helium atoms are gravitationally concentrated by the Sun, enhancing the production of He⁺ PUIs.
By analysing three minutes of data captured by MMS, the team identified a clear circular distribution of particle velocities, which is characteristic of the spiral motion seen in recently formed PUIs. They managed to confirm such a distribution for both hydrogen (H⁺) and helium (He⁺) PUIs. Furthermore, the MMS data revealed low intensity, but statistically significant wave power near the characteristic cyclotron frequencies of H⁺ and He⁺. These are the natural frequencies at which charged particles of a certain charge-to-mass ratio gyrate within a magnetic field.
To strengthen their case, the authors modelled the observed velocity distributions as Maxwellian plasmas and computed linear analyses of how these instabilities would evolve. These demonstrated that the measured PUI populations could indeed generate wave growth in precisely the same frequency ranges where MMS detected enhanced magnetic power.
These results represent the first evidence from MMS of PUI-associated waves at one astronomical unit. However, the brief observation interval of only three minutes limited the frequency resolution, preventing a clean separation of H⁺ and He⁺ signatures in the spectrum. In addition, the wave power was modest, reflecting the delicate balance between PUI-driven instabilities and competing plasma processes so close to the Sun. Further investigations into PUIs, and their effects on the near-Earth environment, are needed.
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Journal Source: M. Starkey et al., First MMS Observations of Waves Possibly Generated by PUIs Near Earth, Advancing Earth and Space Sciences, (2025), DOI: https://doi.org/10.1029/2024JA033660
Cover Image Credit: NASA Science / https://www.youtube.com/watch?v=twB62NYsaIg