First MMS Observations of Waves Possibly Generated by PUIs Near Earth

Abstract We report the first simultaneous observations of wave activity and pickup ions (PUIs) in the pristine solar wind upstream of Earth's bow shock (i.e., at 1 au) from the Magnetospheric Multiscale (MMS) mission. Low‐frequency electromagnetic waves induced by newborn interstellar PUIs have been confirmed as the dominant energy source that drives turbulence and thermal ion heating in the solar wind beyond 1 au. However, only a few observations of PUI‐generated waves exist near 1 au. Near 1 au, these waves are relatively weak and are best observed in the pristine solar wind, absent of any other wave activity or energized ion populations. In this work, we analyze a ∼3 min interval during which Earth was within the He focusing cone and MMS was in the pristine solar wind. We identify H + and He + PUI ring distributions provided by the Hot Plasma Composition Analyzer and compare their velocity space characteristics, which reveals that these PUIs were likely born from different neutral source populations (e.g., geocornal hydrogen and interstellar helium). We also identify potential signatures of distinct helium and hydrogen wave modes in the magnetic field power spectrum and perform a linear instability analysis which identifies the distinct wave growth rates. The peak growth rates coincide with enhancements in the magnetic power spectrum, suggesting that these waves could be generated by the observed H + and He + PUIs. These observations motivate the need for a systematic study of PUI‐generated waves near 1 au, which can be achieved using MMS data.