Investigating the Chromospheric Footpoints of the Solar Wind

Coronal holes present the source of the fast solar wind. However, the fast solar wind is not unimodal—there are discrete, but subtle, compositional, velocity, and density structures that differentiate different coronal holes as well as wind streams that originate within one coronal hole. In this Letter we exploit full-disk observational "mosaics" performed by the Interface Region Imaging Spectrograph (IRIS) spacecraft to demonstrate that significant spectral variation exists within the chromospheric plasma of coronal holes. The spectral differences outline the boundaries of some—but not all—coronal holes. In particular, we show that the "peak separation" of the Mg ii h line at 2803 Å illustrates changes in what appear to be open magnetic features within a coronal hole. These observations point to a chromospheric source for the inhomogeneities found in the fast solar wind. These chromospheric signatures can provide additional constraints on magnetic field extrapolations close to the source, potentially on spatial scales smaller than from traditional coronal hole detection methods based on intensity thresholding in the corona. This is of increased importance with the advent of Parker Solar Probe and Solar Orbiter and the ability to accurately establish the connectivity between their in situ measurements and remote sensing observations of the solar atmosphere.