On the Origin of Inner Source Pickup Ions

Inner source pickup ions are thought to originate from the interaction of solar wind ions with interplanetary dust grains in the inner heliosphere. Processes which produce inner source pickup ions, and which have been considered so far are implantation of solar wind on grains and subsequent desorption, charge exchange of solar wind ions during transit through submicron dust grains, sputtering and backscattering of ions. A large fraction if not all of the dust crossing the sphere of the Earth’s orbit must end up as pickup ions as is evidenced from the comparable order of magnitude of dust flux inward and pickup ion flux outward at 1 AU. This suggests that the ultimate fate for a large fraction of small interplanetary dust particles after evaporation or sputtering is conversion into pickup ions. Sputtering becomes particularly efficient when dust particles—after fragmentation by collisions with each other—have diminished to sizes comparable to the range of solar wind ions in dust material. The sputter products, charged or neutral molecules, atoms or ions, ultimately will undergo photodissociation, photoionization, ionization by charge exchange with solar wind ions, and/or ionization by electron collisions. We investigate the relative importance of various processes on pickup ions on their way out of the inner heliosphere and the relevance of inner source pickup ions for diagnostics of dust particles near the Sun.