‘What is the dust environment in the inner heliosphere?’
The visible emission at 1 AU, from heights above 4 Rs, is dominated by scattering from interplanetary dust, the F-corona. It is a nuisance for coronal studies in the visible as it obscures the signal from CMEs and coronal streamers. Accurate removal of the F-corona is The Wide-Field Imager for Solar Probe Plus (WISPR) essential for the derivation of coronal density structure (e.g., Hayes et al. 2001) but the current F-coronal models are unreliable, as LASCO/C3 observations have shown. The failure of the models stems from our incomplete understanding of the physical properties and distribution of the dust in the inner heliosphere. Most of what we know comes from coronagraph and eclipse observations from Earth and the in-situ and photometric observations from the Helios mission in the 1970’s (Leinert et al. 1998).
The F-corona brightness results from the line-of-sight integral of the scattering from 1–100 μm dust particles. These particles undergo efficient forward scattering at small angles. Hence dust located in the region about halfway between the Sun and the observer generates most of the F-corona brightness at small elongations (Mann et al. 2004) resulting in the very stable F-corona emission observed by LASCO. This complicates the inversion of the brightness observations and leads to unreliable determinations of the structure and density distribution of the near-Sun dust and its interplay with planets. For example, the existence of a dust-free zone in the inner corona (<4 Rs) due to sublimation, predicted by Russell (1929), has never been proven experimentally and there is only a marginal detection of a planetary dust ring from Helios observations in the Venus orbit, similar to that seen at Earth’s orbit (Leinert and Moster 2007; Jones et al. 2013). Such shortcomings have significant impact on our understanding of dust-plasma interactions and the interpretation of the evolution of circumstellar dust rings and planet formation.
WISPR will revolutionize the remote sensing study of the F-corona by going much closer to the Sun and with much higher sensitivity, spatial resolution and spatial coverage compared to the Helios photometers. Thanks to 18 years of LASCO/C3 observations, we have developed robust data analysis techniques to achieve F-corona model subtractions with accurate photometry. The same techniques are used for the removal of the F-corona from the SECCHI/HI images and the upcoming SoloHI instrument on the Solar Orbiter mission.
With WISPR we will extract quantitative measurements and record the first F-corona images from locations within 0.3 AU. During the perihelion pass, the region of dust contributing to the scattering will move closer to the Sun contributing to an increase in the brightness (due to the increased density of dust) until eventually it must start to roll over close to the Sun and finally disappear at the dust-free zone (Fig. 7). The high orbital velocities during the perihelion passages will result in brightness measurements of the F-corona from a multitude of vantage points relative to the dust cloud thus allowing us to derive much more accurate measurements of the dust density distribution within 0.3 AU. Thanks to the reduced line-of-sight effect, WISPR will be able to detect and measure the boundaries of the dust-free region and possibly verify the existence of dust enhancements in the orbits of Venus and Mercury.
Another unique science opportunity is the search for planetoids within theMercury orbit. A dynamically stable region interior to Mercury’s orbit is predicted to contain a population of small, asteroid like bodies called Vulcanoids from the early solar system and may be the source of impacts onto Mercury. Searches for the existence of Vulcanoids have not been successful. Durda et al. (2000), Merline (2008), and Steffl et al. (2013) have used LASCO, Messenger and SECCHI observations to search for Vulcanoid objects and have put upper limits on the number of objects above certain sizes. While asteroids have been detected within the Vulcanoid region (0.08–0.2 AU), none were Vulcanoids. With WISPR, we will be able to extend these searches to fainter objects and place new constraints on the formation and evolution of objects in this region.