Push and Shove: Trajectories of Interstellar Neutral Hydrogen Subject to a Time-Dependent Radiation Pressure Force

Radiation pressure plays a significant role in the orbital motion of neutral hydrogen particles streaming in from the Local Interstellar Medium (LISM). The addition of a radiation pressure force alongside the usual gravitational force will inevitably change the characteristic velocities, and therefore energies, of hydrogen particles arriving at an explorer like IBEX or IMAP. In this study, we use a code that backtraces particle orbits from a selection of representative points located approximately 1 AU from the Sun in the ecliptic plane. The code uses an idealized oscillatory radiation pressure force with a period that imitates the solar cycle. The phase space data at each target point is calculated, assuming a Maxwellian at large distances from the Sun in the upstream direction. This data allows us to identify potential shifts of the direct and indirect components of the core deflected Maxwellian from those of the standard gravity-only Keplerian model at various points in time. We also observe structures within these phase space plots corresponding to pseudo-bound orbits, which arise due to energy losses as the radiation pressure switches between attractive and repulsive. Sample individual trajectories are shown, and their properties studied. Thanks to calculations for these orbits, we identify potential anomalous orbits that might contribute significantly to the observations of particles at the aforementioned explorers.