Investigating Galactic Cosmic Ray Ionization’s Contribution to Aerosol Formation on Neptune and a Potential Link to the Relationship Between Brightness and Solar Activity

Abstract

In this work we investigate how ionization from Galactic Cosmic Rays (GCRs) can affect aerosol nucleation on Neptune and discuss potential implications for the observed brightness variations that appear to follow the solar cycle. We calculate the planetary vertical cutoff rigidity for Neptune based on the magnetic field model and a modified version of the particle trajectory code the Geomagnetic Cutoff Rigidity Computer Program. By combining the cutoff rigidity with the known GCR spectrum we calculate the proton flux into the atmosphere of Neptune at the 49 km level above 1 bar (0.082 bar, 53 K) at both solar minimum and maximum and find it to vary between a factor 1.2 and 2.2 in the areas of high proton flux, depending on the cosmic ray spectrum used. Using the Infrared Space Observatory nominal atmospheric profile and methane supersaturation scenarios based upon observations of the methane fraction, we then calculate vertical profiles of neutral homogeneous and ion‐induced nucleation rates to identify potential altitudes sensitive to ion‐induced nucleation induced by GCRs. We construct a planetary map of potential ion‐induced nucleation hotspots. Using this map and the variation of sub‐Earth latitude of Neptune, we then investigate a potential bias in brightness observations of Neptune. We find that it could potentially help explain the shift between anti‐correlation and direct correlation of solar activity and the brightness of Neptune, but note that there are currently too many variables to conclude with confidence to what degree our explanation contributes.