The Lorentz Force is the force due to a Magnetic Field. When particle of charge q moves with velocity v in the presence of electric field E and magnetic field B, the force F is:
F = q (E + v × B)
This is derivable from Maxwell's Equations but is not immediately evident.
The Lorentz force is an element of Magnetohydrodynamics and is used in astrophysical models for situations including significant magnetism, such as solar and solar system phenomena. When modeling more distant events, where measurement of the magnetism is less clear-cut, magnetism may still be included as a means to produce a workable model, and inferred by the model's success.
Magnetic Flux Density (B)