  The evolution of the spatial degrees of freedom of a photon propagating
through atmospheric turbulence is treated as a non-Markovian process. Here, we
derive and solve the evolution equation for this process. The turbulent medium
is modeled by a sequence of multiple phase screens for general turbulence
conditions. The non-Markovian perspective leads to a second-order differential
equation with respect to the propagation distance. The solution for this
differential equation is obtained with the aid of a perturbative analysis,
assuming the turbulence is relatively weak. We also provide another solution
for more general turbulence strength, but where we introduced a simplification
to the differential equation.
