The role of solar wind heavy ions in the space environment

Abstract

The physical processes in the impact of highly charged ions on gaseous or solid targets appears not to have had the attention of the community involved in spacecraft development, or of many others in the engineering community concerned with gas‐solid impacts. Orbiting spacecraft at altitudes above the magnetospheric bow‐shock, spacecraft traveling through interplanetary space, and any solar system body without a substantial intrinsic magnetic field are subjected to the impact of the solar wind flux. This is a highly variable phenomenon sourced by magnetohydrodynamic turbulence in the solar corona at plasma temperatures generally above 106 K. The mean velocity of the outflowing solar wind is about 450 km s−1, but is quite variable, with values as high as 800 km s−1 appearing fairly frequently. The solar wind density at 1 AU is about 10 cm−3. The ions are dominated by protons, with the inclusion of about 4% He++, and smaller mixing ratios of heavier ions. The effect of the heavy ions can be significant relative to the protons in spite of low mixing ratios because of the larger kinetic energy of heavy ions moving at the same velocity as the protons, and the substantial internal energy invested in multiple charge states created in the solar corona. Solar wind He++, for example, is much more effective for sputtering than protons. The characteristics of the heavy stripped ions lead to the following conclusions: Multiple charge capture near the solid surface leads to very efficient surface sputtering given that molecular ion recombination leads to dissociation with a probability very close to 1.0. The multiply charged heavy ion can therefore be 10 to 1000 times more efficient per ion than light singly charged ions in sputtering and damaging solid surfaces. In addition, the multiply charged ion produces soft X‐rays in the capture process, that can penetrate deeply into the solid to produce multiple damaged sites caused by secondary photoelectrons.