Editing Magnetic sail (section)

=== Plasma mass density and velocity terminology and units ===
A [[Plasma (physics)|plasma]] consists exclusively of charged particles that can interact with a magnetic or electric field. It does not include neutral particles, such as neutrons. atoms or molecules.The plasma mass density ρ used in magnetohydrodynamic models only require a weighted average mass density of charged particles that includes neutrons in the ion, while kinematic models use the values for each specific ion type and in some cases the parameters for electrons as well as detailed in the [[#Magnetohydrodynamic model|Magnetohydrodynamic model]] section.

The velocity distribution of ions and electrons is another important parameter but often analyses use only the average velocity for the aggregate of particles in a plasma wind for a particular plasma environment (pe) is <math>v_{pe}</math>. The apparent wind velocity <math>u_{sc}</math> as seen by a spacecraft traveling at velocity <math>v_{sc}</math> (positive meaning acceleration in the same direction as the wind and negative meaning deceleration opposite the wind direction) for a particular plasma environment (''pe'') is <math>u_{sc}=v_{pe}-v_{sc}</math>.