A collision cross-section for interactions between magnetic diffuse clouds

A collision cross-section for magnetic diffuse clouds is derived. This accounts for both direct collisions between clouds that physically contact each other, and indirect collisions between remote clouds whose magnetic flux tubes become entangled. The effect of multiple cloud interactions is considered. The resulting collision cross-section equals approximately the geometric cross-section multiplied by $$(\rho_\text c\upsilon_\text A/\rho\upsilon)^{2/3}$$ for internal cloud density ρ_c, internal Alfvén velocity υ_A, average interstellar density ρ and cloud velocity υ. Magnetic fields enhance the exchange of momentum between clouds, so this effective collision cross-section is larger than the geometric cross-section by a factor of 2 to 5 for typical conditions.