Abstract
Abstract The Wide-field Imager for Solar PRobe (WISPR) obtained the first high-resolution images of coronal rays at heights below 15 R ⊙ when the Parker Solar Probe (PSP) was located inside 0.25 au during the first encounter. We exploit these remarkable images to reveal the structure of coronal rays at scales that are not easily discernible in images taken from near 1 au. To analyze and interpret WISPR observations, which evolve rapidly both radially and longitudinally, we construct a latitude versus time map using the full WISPR data set from the first encounter. From the exploitation of this map and also from sequential WISPR images. we show the presence of multiple substructures inside streamers and pseudostreamers. WISPR unveils the fine-scale structure of the densest part of streamer rays that we identify as the solar origin of the heliospheric plasma sheet typically measured in situ in the solar wind. We exploit 3D magnetohydrodynamic models, and we construct synthetic white-light images to study the origin of the coronal structures observed by WISPR. Overall, including the effect of the spacecraft relative motion toward the individual coronal structures, we can interpret several observed features by WISPR. Moreover, we relate some coronal rays to folds in the heliospheric current sheet that are unresolved from 1 au. Other rays appear to form as a result of the inherently inhomogeneous distribution of open magnetic flux tubes.
Highlights
The appearance of coronal rays evolves dramatically on daily and yearly timescales
Wide-field Imager for Solar PRobe (WISPR) unveils the fine-scale structure of the densest part of streamer rays that we identify as the solar origin of the heliospheric plasma sheet typically measured in situ in the solar wind
The unprecedented proximity of Parker Solar Probe (PSP) to the Sun allows WISPR to capture in great detail coronal and streamer rays
Summary
The appearance of coronal rays evolves dramatically on daily and yearly timescales. Their daily evolution is mostly due to the effect of solar rotation and smallscale evolution of the magnetic field that brings rays located at different longitudes (and latitudes) into the plane of the sky of the observing telescope. A source of difficulty resides in the nature of the observations themselves; any WL image of the solar corona results necessarily from the integration of sunlight that has been scattered by electrons situated along each line of sight (LOS) of each pixel in the image This observational constraint complicates any interpretation of the 3D structure of streamer rays and the determination of their source closer to the surface of the Sun. An example relates to the nature of polar rays observed in coronagraphs and eclipse images to extend over the northern and southern polar coronal holes tens of solar radii in heliocentric radial distance An example relates to the nature of polar rays observed in coronagraphs and eclipse images to extend over the northern and southern polar coronal holes tens of solar radii in heliocentric radial distance WL plumes unambiguously related to coronal holes are clear features in WL coronagraphs; they can extend up to many solar radii above the limb of the Sun and contribute to the occurrence of polar rays (Wang 1994)
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