The Wide-field Imager for Solar Probe (WISPR) imaged in great details the solar corona in white-light (i.e. visible light) during the first approach to the Sun of the Parker Solar Probe (PSP) at 35.7 R_⨀. In fact, WISPR captured with an unprecedented precision coronal and streamer rays.
Streamer rays are elongated structures that extend from the low to the high corona. They are visibles in great detail by the naked eye during total solar eclipses, see for instance the total solar eclipse captured by Nicolas Lefaudeux in 2019 (figure 1). Streamer rays form over the tip of helmet streamers and are mostly made of dense plasma. It is well-known that the slow solar wind, which is denser than the fast solar wind, flows around these regions. Therefore, WISPR images are crucial to exploit and interpret in order to better understand the slow solar wind.
In this study we present a new technique to exploit and interpret images taken from a rapidly moving spacecraft. A lot of images have been taken by WISPR and we had to reduce the data before making any analysis. In figure 2, an animation shows how we built a summary (or synoptic) map from the many WISPR images.
Then we used an MHD model (called MULTI-VP) of the solar corona and a ray tracing code to simulate WISPR images. Even though there are discrepancies between the model and observations, the modeling was found to be greatly helpful to interpret WISPR observations. A preview of the 3-D modeled coronal magnetic field is shown in figure 3.
To conclude we showed that during the first PSP encounter, WISPR observed a very small folding of the heliospheric plasma sheet (HPS). Its signature was a thin and isolated streamer ray in the white-light images. We also emphasized the capability of WISPR to capture finer structures within the HPS than 1AU observatories. As PSP gets closer to the Sun, the WISPR « microscope » will zoom even further and provide a better insight of the slow solar wind.