Authors: Ali Rahmati, Davin Larson, Roberto Livi, Phyllis Whittlesey, Michael McManus, Orlando Romeo, Jaye Verniero, Justin Kasper, Stuart Bale, Marc Pulupa, Keith Goetz, and the SWEAP and FIELDS teams

During two of Parker Solar Probe’s Venus flybys (VGA2 and VGA3), the SWEAP/SPAN-Ion instrument
detected proton distributions that resembled those of pickup protons, with ring-beam distributions and
energies as high as four times the solar wind proton energy. The source of such pickup protons at Venus
is thought to be the ionization of the extended neutral exosphere of the planet.
The trajectories of the pickup ions can be analytically modeled and convolved with the instrument Field of
View (FOV) to construct modeled proton distributions, allowing us to conduct model comparisons with
the measured pickup ions fluxes. Using these model-data comparisons, the hydrogen neutral densities
upstream of Venus can be reconstructed at distances on the order of a few proton gyro-radii, which would
help constrain the hydrogen escape rates from Venus.
In this work, we use a pickup ion model that employs the measured vector components of the solar wind
velocity and interplanetary magnetic field (IMF) to simulate the trajectories of pickup ions. A key data
input in calculating the trajectories of pickup ions is the three components of the IMF, measured in the
solar wind by the MAG instrument on PSP. Due to the sensitivity of pickup ion trajectories to the
direction and magnitude of the magnetic field, the accuracy of the measured components of the IMF plays
a key role in determining what part of the pickup ion ring-beam distribution would be in the FOV of
SPAN-Ion.
Preliminary analysis indicates that the measured proton 3D distributions do not perfectly match the
modeled distributions. In this work, a few theories that would potentially explain this offset between the
data and the model results are proposed and will be explored in detail, which include:
• The possibility that the measured pickup protons are no longer in a perfect ring-beam distribution
due to pitch-angle scattering into shell-like distributions.
• Pickup protons that were born in a different magnetic environment and drifted into the one
measured by PSP/MAG.
• The possibility of the presence of an offset in the measured magnetic field.
• Shock-drift acceleration of upstream solar wind protons.
• Solar wind proton beams.