  We develop a model for the radio afterglow of the giant flare of SGR 1806-20
arising due to the interaction of magnetically-dominated cloud, an analogue of
Solar Coronal Mass Ejections (CMEs), with the interstellar medium (ISM). The
CME is modeled as a spheromak-like configuration. The CME is first advected
with the magnetar's wind and later interacts with the ISM, creating a strong
forward shock and complicated backwards exhaust flow. Using three-dimensional
magnetohydrodynamic simulations, we study various relative configurations of
the magnetic field of the CME with respect to the ISM's magnetic field. We show
that the dynamics of the forward shock mostly follows the Sedov-Taylor
blastwave, while the internal structure of the shocked medium is considerably
modified by the back flow, creating a multiple shock configuration. We
calculate synthetic synchrotron emissivity maps and light curves using two
assumptions: (i) magnetic field compression; (ii) amplification of the magnetic
field at the shock.We find that models with magnetic field amplification
account better for the observed radio emission.

High Energy Astrophysical Phenomena;Solar and Stellar Astrophysics