Evidence for a Circumsolar Dust Ring at about 0.4 AU

In preparation for the Parker Solar Probe (PSP) and Solar Orbiter (SO) we devised a technique to create empirical F-coronal models from STEREO SECCHI Heliospheric Imagers. The results of a systematic study of the resulting F-coronal models obtained from 6+ years of HI-1A data have been published in a series of papers. The HI-1A instrument observes the interplanetary space between 4^o and 24^o elongation from the Sun, which is equivalent to about 16 to 96 Rsun. This region has not been analyzed heretofore so the properties of the F-corona as seen from 1 AU have been determined for the first time. In particular, we determined the ecliptic longitude dependence of the intensity distribution along its symmetry axis (Stenborg et al, 2018). To first order, the radial intensity profiles were fit with a single power law at each angle of the spacecraft orbital longitude. As a test of a technique to be used in the upcoming missions, we performed a numerical differentiation of the radial intensity profiles and found a consistent pattern in the derivatives between about 18^o and 23^o elongation that peaked at about 21^o, the pattern being observed from all along the ST-A orbit. The findings indicate the presence of a circumsolar density enhancement that peaks at about 23^o. A straightforward integration of the excess signal in the derivative space indicates that the intensity increase over the background F-corona is on the order of 2%, implying a dust density increase of about 3-4% at the center of the ring. We also found a large-scale modulation of the inner boundary of the pattern, in clear association with Mercury's orbit. We also found a more localized modulation of the inner boundary attributable to the dust trail of Comet 2P Encke, which occurs near ecliptic longitudes corresponding to the crossing of Encke's and Mercury's orbital paths. Moreover, evidence of dust near the S/C in a restricted range of ecliptic longitudes has also been revealed by this technique (attributable to the dust trail of comet P73/Schwassmann-Wachmann). We will discuss these findings. We acknowledge the support of the NASA STEREO project.