Solar Cycle Variability of Nanodust Fluxes in the Inner Solar System

Interplanetary nanometer-sized grains (nanodust) are believed to be produced as part of the zodiacal dust collisional cascade at relatively high fluxes in the inner solar system (<~1 au). Nanodust grains possess large charge-to-mass ratios and have theoretically been shown to strongly couple to electromagnetic fields. Thus, the morphology and variability of the interplanetary plasma and field environment dictates the resulting dynamics of nanodust grains. For example, the switch in the polarity of the solar dipole magnetic field yields focusing and defocusing periods, where the predominant direction of the interplanetary electric field either points towards or away from the heliospheric current sheet, respectively. To further quantify the nature and variability of nanodust fluxes in the inner heliosphere both at various points throughout a single solar cycle and across different solar cycles, we have used a combination of the WSA-Enlil model, which is a coupled solar corona-solar wind 3D MHD model driven by solar photospheric magnetic field observations, and a nanodust dynamics and charging model. These simulations build upon previous simulations (Poppe and Lee, JGR, 2020) that explored the nanodust dynamics and fluxes in the inner heliosphere for Carrington Rotations (CRs) 2052-2059, during the minimum phase of Solar Cycle 23/24 and just shortly after the launch of the STEREO mission. Our new results include nanodust fluxes within CRs at different periods within a single solar cycle (i.e., ascending phase, solar max, declining phase, solar min) as well as comparative CRs across different solar cycles. We compare the dynamics of interplanetary nanodust grains across these different CRs, emphasizing in particular the accessibility of nanodust fluxes to various observing locations/spacecraft (e.g., 1 au, Earth, STEREO A/B, Parker Solar Probe, etc.). Finally, by using Solar Cycle 23 as a proxy for Solar Cycle 25, which both share the same defocusing-to-focusing transition in heliospheric polarity, we make predictions for the timing of the return of nanodust impact signals on the STEREO-A WAVES instrument. This prediction can be tested against future S/WAVES measurements extending to or just after SC 25 maximum. A positive result in this prediction would lend strong support for the interpretation of the single-hit events on STEREO as impacts from high-velocity nanodust grains.