PSP/WISPR observations of dust density depletion near the Sun

Abstract

Context. In 1929, Russell predicted that dust particles cannot survive in a region close to any star, hence giving justification for a dust free zone to exist inside a certain distance from the star. This theoretical prediction has not been confirmed, even with our Sun. Aims. We use the unique vantage points and new perspectives of the Parker Solar Probe (PSP) mission to study the dust environment close to the Sun with imaging observations from the Wide Field Imager for Solar Probe (WISPR) as PSP orbits, progressively closer to the Sun (PSP will ultimately reach a perihelion distance of 9.86 R⊙). Methods. We analyze the radial brightness profile of the axis of symmetry of the F-corona in the WISPR images obtained from heliocentric distances between about 0.350 AU (75 R⊙) and 0.129 AU (28 R⊙) to detect any change from earlier observations. Historically, at observer locations between 1 and 0.3 AU, the brightness of the axis of symmetry has been shown to fall off as a power law of solar distance, r−n, with an exponent of n = 2.3. Results. We show that as PSP approaches its perihelion distance of 28 R⊙ (orbits 4 and 5), the radial gradient of the brightness profile of the axis of symmetry of the F-corona gradually becomes less steep, starting at about 19 R⊙ down to the shortest elongations reached with current WISPR observations at about 7.65 R⊙. This observational signature is modeled with an ad hoc homogeneous dust density model (i.e., it is not based on any physical model) along the symmetry axis of the zodiacal dust cloud, which (1) varies as the historical density profile, r−1.3, down to 19 R⊙, then (2) stays approximately constant down to 10 R⊙, and finally (3) decreases exponentially to become zero at 3 R⊙. The density profile below 19 R⊙ is accomplished by using a multiplier on the historical density profile that decreases linearly down to 3 R⊙. The distance dependence and range of the multiplier were chosen to best match the brightness observations below 30 R⊙. Conclusions. The observed brightness decrease in the axis of symmetry is interpreted as the signature of the existence of a dust density depletion zone between about 19 R⊙ and 3 R⊙, which at the inner limit of WISPR’s field of view of 7.65 R⊙ has a dust density that is ~5% lower than the density at 19 R⊙, instead of the expected density which is three times if no depletion zone exists. No noticeable variations in the brightness of the F-corona axis of symmetry were observed from 2018 to 2020.