# Vlasov-Fokker-Plank(VFP) equation from Boltzmann equation

Dear community,
I am studying some equations related to the acceleration of cosmic rays(CRs) in magnetized plasma and I have seen a couple the equations I am not able to understand.
First, I see that it is used as time-dependent Boltzmann equation for the CRs

∂ƒ/∂t + (vx + u)∂ƒ/∂x - ∂u/∂xpx∂ƒ/∂ px + FL/a ∂ƒ/∂p = C(f)
, where u is the background plasma velocity, FL is the Lorentz force and C(f) is the collision term.
The problem is that I have always seen this equation as ∂ƒ/∂t + (vx + u)∂ƒ/∂x + FL/a ∂ƒ/∂p = C(f), this is, without the term - ∂u/∂x px ∂ƒ/∂px that I am not able toderive.

Then, this equation is linearized supposing that the distribution function, f = f0 + f1 p/p to obtain the VFP equation:

u∂ƒ0/∂x + c/3 ∂ƒ/∂x -1/3 ∂u/∂x p ∂ƒ0/∂px
How do we arrive to this expression? It is clear for me that the steady state solution is taken, so ∂ƒ/∂t = 0, but I do not even understand why the factor fx appears there (fx is the x component of the vector f1).

Could someone explain me the first "modified" boltzmann eq. and how to reach the expression of the VFP eq?

Related High Energy, Nuclear, Particle Physics News on Phys.org
jim mcnamara
Mentor
It might help if you tell us where you read this, which would help with context, e.g., a paper or textbook. Thanks.

It might help if you tell us where you read this, which would help with context, e.g., a paper or textbook. Thanks.
The paper in which I have seen that is: A. R. Bell, Cosmic ray acceleration (https://www.sciencedirect.com/science/article/pii/S0927650512001272)
And what I have called "modified" Boltzmann equation is the equation 1 of the paper.

vanhees71
• 