FP: A Fokker-Planck solver for modeling the transport of flare-accelerated particles

We present a new open-source computational model, FP, that solves the Fokker Planck equation to model the transport of flare-accelerated particles at the top of magnetic flux loops to their eventual thermalization in the footpoints. Our technique includes forces corresponding to Coulomb collisions including second order energy diffusion and pitch-angle diffusion, magnetic mirroring, synchrotron emission and, critically, energy loss due to the return current electric field. It is applicable to particles of arbitrary mass and charge. In the case of nonthermal electrons, FP predicts the bremsstrahlung produced as they collide with the ambient solar atmosphere. We have incorporated FP into the OSPEX X-ray spectral analysis tool. We use OSPEX+FP to fit X-ray spectra observed by RHESSI to constrain nonthermal electron distributions in the SOL2013-05-13T16:01 X-class solar flare. This flare shows a large spectral break, likely indicating the effect of the return current electric field on nonthermal electrons.