The Effects of Solar Wind Structure on Nanodust Dynamics in the Inner Heliosphere

Several interplanetary spacecraft have inferred the presence of nanometer‐sized dust grains in the inner heliosphere accelerated to near‐solar wind velocities based on observations by radio wave antennas. These “nanodust” grains exhibit unique behavior in interplanetary space as they are strongly affected by both the solar gravitational force and the electromagnetic Lorentz force from the convecting interplanetary magnetic field. Here, we study the dynamics of nanodust grains in the inner heliosphere via a combination of background electromagnetic fields from the coupled Wang‐Sheeley‐Arge/Enlil heliospheric model and a nanodust grain charging and dynamics model. The model results predict strong time variability in the nanodust velocity and flux distributions at 1 au within a single Carrington rotation (CR) driven by a combination of nanodust grain size, heliospheric current sheet tilt, and varying heliospheric plasma parameters. In contrast, the general character of nanodust dynamics does not drastically change from one CR to the next. Despite including observationally driven heliospheric plasma conditions, the modeled nanodust impact rates do not agree with in situ observations by the STEREO A and B WAVES instruments, especially with regard to CR‐to‐CR variability. We interpret this data‐model disagreement as evidence that additional time variability must be present in the production rate of nanodust grains in the inner heliosphere. We discuss possibilities for this time variability, including non‐steady‐state collisional cascades, variability in nanodust trapping regions, and the impulsive disruption of inner solar system comets and/or asteroids.