Anomalous Interplanetary Dust Populations and Dust Impact Directionality Observed by Parker Solar Probe

Theory predicts several populations of zodiacal dust in the inner heliosphere, including -meteoroids, grains on bound elliptical orbits, and -meteoroids on hyperbolic escape trajectories governed largely by their size and composition. Yet there may be other populations present. The Parker Solar Probe (PSP) spacecraft provides our first opportunity for in-situ observation of interplanetary dust populations sunward of 0.25 AU, the densest and most dynamic part of the zodiacal cloud. Dust is detected using the FIELDS electric field instrument, which responds to impact ionization of hypervelocity collisions between dust grains and the spacecraft body. Over PSPs first seven orbits, dust count rates are found to be well organized by the orbital parameters of PSP's shrinking orbit. In particular, the first three orbits and the sixth orbit have a single, pre-perihelion count rate peak with a gradual drop off post-perihelion, while Orbits 4, 5 and 7 have two distinct count rate peaks on either side of perihelion. The post-perihelion peaks in Orbits 4, 5, and 7 are inconsistent with current zodiacal dust models that consider only - and -meteoroids. Dust directionality can be estimated based on the location of the impact on the spacecraft body. During Orbit 4, dust impact locations can be estimated using monopole data from all four antennas in the plane of the heat shield. All other orbits have dipole data, requiring a more complex algorithm to determine impact location. First, a pseudo-dipole data set was constructed to compare the Orbit 4 dipole data with the monopole results. Once confidence in this comparison was established, the dipole algorithm was then applied to the data from Orbits 1-7. Examination of dust directionality reveals an anti-ram impactor anomaly post-perihelion during Orbit 4. The anomaly is found to persist in other orbits, indicating the presence of a dust population separate from the nominal - and -meteoroids. The origin of the anomaly may be related to the Geminids meteoroid stream associated with the asteroid 3200-Phaethon.