PSP/WISPR Observations of Dust Density Depletion near the Sun. II. New Insights from within the Depletion Zone

Abstract

Visible light observations from the Wide-field Imager for Solar PRobe (WISPR) aboard the Parker Solar Probe (PSP) mission offer a unique opportunity to study the dust environment near the Sun. The existence of a dust-free zone (DFZ) around stars was postulated almost a century ago. Despite numerous attempts to detect it from as close as 0.3 au, observational evidence of a circumsolar DFZ has remained elusive. Analysis of WISPR images obtained from heliocentric distances between 13.3–53.7 R ⊙ over multiple PSP orbits shows a gradually decreasing brightness gradient along the symmetry axis of the F-corona for coronal heights between 19 and 9 R ⊙. Below 9 R ⊙, the gradient reverses its trend, approaching the radial dependence exhibited at heights above 19 R ⊙. After taking into account the effects of both the electron corona background and the nonresolved starlight, the WISPR observations down to 4 R ⊙ are consistent with forward-modeling simulations of the F-corona brightness within [−6, 5]% if a circumsolar region of depleted dust density between 19 and 5 R ⊙ enclosing a DFZ is considered. In addition, we show, for the first time, that the F-corona brightness inward of about 15 R ⊙ depends on the observer’s location for observing distances below 35 R ⊙.