Wake formation behind positively charged spacecraft in flowing tenuous plasmas

Spacecraft in tenuous plasmas become positively charged because of photoelectron emission. If the plasma is supersonically drifting with respect to the spacecraft, a wake forms behind it. When the kinetic energy of the positive ions in the plasma is not sufficient to overcome the electrostatic barrier of the spacecraft potential, they scatter on the potential structure from the spacecraft rather than get absorbed or scattered by the spacecraft body. For tenuous plasmas with Debye lengths much exceeding the spacecraft size, the potential structure extends far from the spacecraft, and consequently in this case the wake is of transverse dimensions much larger than the spacecraft. This enhanced wake formation process is demonstrated by theoretical analysis and computer simulations. Comparison to observations from the Cluster satellites shows good agreement.