The Electromagnetic Signature of Outward Propagating Ion-scale Waves

Abstract

Abstract First results from the Parker Solar Probe (PSP) mission have revealed ubiquitous coherent ion-scale waves in the inner heliosphere, which are signatures of kinetic wave–particle interactions and fluid instabilities. However, initial studies of the circularly polarized ion-scale waves observed by PSP have only thoroughly analyzed magnetic field signatures, precluding a determination of solar wind frame propagation direction and intrinsic wave polarization. A comprehensive determination of wave properties requires measurements of both electric and magnetic fields. Here, we use full capabilities of the PSP/FIELDS instrument suite to measure both the electric and magnetic components of circularly polarized waves. Comparing spacecraft frame magnetic field measurements with the Doppler-shifted cold plasma dispersion relation for parallel transverse waves constrains allowable plasma frame polarizations and wavevectors. We demonstrate that the Doppler-shifted cold plasma dispersion has a maximum spacecraft frequency for which intrinsically right-handed fast-magnetosonic waves propagating sunwards can appear left-handed in the spacecraft frame. Observations of left-handed waves with are uniquely explained by intrinsically left-handed, ion-cyclotron waves (ICWs). We demonstrate that electric field measurements for waves with are consistent with ICWs propagating away from the Sun, verifying the measured electric field. Applying the verified electric field measurements to the full distribution of waves suggests that, in the solar wind frame, the vast majority of waves propagate away from the Sun, indicating that the observed population of coherent ion-scale waves contains both intrinsically left- and right-hand polarized modes.