Abstract
Inner source pickup ions (PUIs) are believed to be created by the interaction between the solar wind and interplanetary dust grains. The production mechanism, however, is not well understood. We use the Stopping Range of Ions in Matter and Energetic Particle Radiation Environment Module to simulate the production and transport of inner source C+ and O+ produced by five mechanisms: solar wind recycling, neutralization, backscattering, sputtering, and sputtering-induced recycling. This is the first study to consider backscattering and sputtering-induced recycling. We compare the velocity distribution function (VDF) and C+/O+ abundance ratio to observations from the charge-time-of-flight instrument on board the SOlar and Heliospheric Observatory. Observations reveal a new constraint: a broad VDF at 1 au with a possible cutoff near twice the solar wind speed—suggesting that inner source PUIs are injected into the solar wind at near-zero speeds. In light of this constraint and our model-data comparison, backscattering and sputtering-induced recycling satisfy the most production constraints. However, based on intensity, sputtering and sputtering-induced recycling are the dominant mechanisms.
