If 1 / ω ω g ≪ ≪ T and R ≪ ≪ L , the two first terms will be much less than f since ∂ ∂ f / ∂ ∂ t ′ ∼ ∼ f , v ′ ≲ ≲ 1 and ∂ ∂ f / ∂ ∂ r ′ ∼ ∼ f due to the definitions of T, L, and V above. Since the last term is of the order of f , we can neglect the two first terms and write ( E V B + v ′ × × B B ) ⋅ ⋅ ∂ ∂ f ∂ ∂ v ′ ≈ ≈ 0 ⇒ ⇒ ( E + v × × B ) ⋅ ⋅ ∂ ∂ f ∂ ∂ v ≈ ≈ 0