Why is Br a constant in the files?
The short answer is that Div B = 0. Remember we are doing a 1-D propagation and are therefore
assuming that there are no gradients in the direction perpendicular to the direction of propagation.
Think about this in Cartesian coordinates, taking the direction of propagation to be the z direction,
Div B = dBx/dx + dBy/dy + dBz/dz = dBz/dz = 0
So, the 1-D approximation results in the requirement that Bz cannot vary spatially and therefore
Bz also cannot change in time. In spherical coordinates, this means that Br falls off like 1/r^2
but beyond that there cannot be any spatial or time variation. This is an inherent limitation of 1D MHD.