The Dust Environment in the Inner Heliosphere

Abstract:

The Parker Solar Probe (PSP) mission has completed five encounters through the solar corona. For the first three encounters the perihelion distance was about 36 Rsun and the last two were at 26 Rsun. The sixth encounter is planned for about 20 Rsun on Sep 27, 2020. While PSP does not have a dedicated dust detector, measurements by the various instruments can provide insights into the dust environment in the inner heliosphere. The density of the interplanetary dust in orbit about the Sun, called the Zodiacal cloud, increases toward the Sun, and gives rise to the F-corona/Zodiacal light (ZL). This ZL is produced by photospheric light scattered by the dust particles. However, Russell (1929) predicted that no dust can survive the intense heat from any star and thus there should be a dust free zone (DFZ) at some distance from our Sun. No DFZ has been detected from the ground during solar eclipses or from space. This has been attributed to the large quantity of dust in between the observer at 1 AU and the DFZ hiding the presence of a DFZ. An increase in the ZL has been predicted to occur in circumsolar bands near the distance where dust species would sublimate. However, the particles could also be ground down to a size that then gets removed by radiation pressure as Beta-meteroids. Howard et al (2019) presented first results from the Wide Field Imager for Solar Probe (WISPR) on the PSP spacecraft after the first two orbits, which indicated that the gradient of the ZL brightness was very similar to the 1 AU gradient, but decreased beginning at just under 20 Rs. They interpreted the slope decrease as caused by a linear decrease in the dust density from 17 Rsun down to a distance less than 10 Rsun, which was the inner limit of the PSP/WISPR field of view. Here we give an update to the observations from WISPR at the closer perihelion distance of 26 Rs, at which the inner field limit is about 8 Rsun. The authors acknowledge support from the NASA Parker Solar Probe program for this effort.