  This work demonstrates the possibility of magnetic field topology
investigations using microwave polarimetric observations. We study a solar
flare of GOES M1.7 class that occurred on 11 February, 2014. This flare
revealed a clear signature of spatial inversion of the radio emission
polarization sign. We show that the observed polarization pattern can be
explained by nonthermal gyrosynchrotron emission from the twisted magnetic
structure. Using observations of the Reuven Ramaty High Energy Solar
Spectroscopic Imager, Nobeyama Radio Observatory, Radio Solar Telescope
Network, and Solar Dynamics Observatory, we have determined the parameters of
nonthermal electrons and thermal plasma and identified the magnetic structure
where the flare energy release occurred. To reconstruct the coronal magnetic
field, we use nonlinear force-free field (NLFFF) and potential magnetic field
approaches. Radio emission of nonthermal electrons is simulated by the GX
Simulator code using the extrapolated magnetic field and the parameters of
nonthermal electrons and thermal plasma inferred from the observations; the
model radio maps and spectra are compared with observations. We have found that
the potential magnetic field approach fails to explain the observed circular
polarization pattern; on the other hand, the Stokes $V$ map is successfully
explained by assuming nonthermal electrons to be distributed along the twisted
magnetic structure determined by the NLFFF extrapolation approach. Thus, we
show that the radio polarization maps can be used for diagnosing the topology
of the flare magnetic structures where nonthermal electrons are injected.
