In Situ Observations of Interplanetary Dust Variability in the Inner Heliosphere

This work examines the variation of interplanetary dust count rates and directionality during the first three solar encounters made by the Parker Solar Probe spacecraft, covering distances between 0.65 au (similar to 140 solar radii, R-S) and 0.16 au (similar to 35 R-S). Dust detections are made by the FIELDS instrument via plasma clouds, produced by impact ionization of dust grains on spacecraft surfaces and resultant spacecraft potential perturbations. Dust count rates and inferred densities are found to vary by similar to 50% between the three solar encounters (similar to 5 months per orbit), with most of the variation concentrated below 0.23 au (similar to 50R(S)). Dust count rates and directionality, as well as the encounter-to-encounter variability in both quantities are found to be consistent with beta-meteoroids: dust grains exiting the solar system on hyperbolic trajectories. Interpretation of the the dust count rate data and the dust directionality data independently suggest (i) that the beta-meteoroid source region is more complex than preliminary models suggest, and (ii) that the primary beta-meteoroid source region is approximately located between 10 and 30 solar radii from the Sun. These data offer important clues as to the location and geometry of the beta-meteoroid source region, and consequently clues about the collisional and sublimation processing of interplanetary dust grains near the Sun.