Filtration of Interstellar Hydrogen at the Heliosphere: Charge-Exchange Mean Free Paths and the Hydrogen Wall

Interstellar hydrogen atoms approaching the Sun must cross the heliosheath, where many swap electrons with slow protons and are deflected or lost, so the hydrogen density inside the heliosphere is lower than in the cloud outside, a reduction called filtration. In the piled-up plasma of the hydrogen wall, where the charge-exchange cross-section at thermal energies is much larger than at kiloelectronvolt energies, the mean free path falls to about 22 astronomical units, giving an optical depth of 0.90 across a twenty-astronomical-unit wall layer and a naive transmission of 0.41. Detailed models return a milder filtration near one half, because atoms exchanged in velocity space partly refill the beam, and the gap between the naive and modelled values measures the importance of that refilling. The mean free paths and optical depth are computed from stated published inputs with the deposited script.