  Measurement of the coronal magnetic field is a crucial ingredient in
understanding the nature of solar coronal dynamic phenomena at all scales. We
employ STEREO/COR1 data obtained near maximum of solar activity in December
2012 (Carrington rotation, CR 2131) to retrieve and analyze the
three-dimensional (3D) coronal electron density in the range of heights from
$1.5$ to $4\ \mathrm{R}_\odot$ using a tomography method and qualitatively
deduce structures of the coronal magnetic field. The 3D electron density
analysis is complemented by the 3D STEREO/EUVI emissivity in 195 \AA \ band
obtained by tomography for the same CR period. We find that the magnetic field
configuration during CR 2131 has a tendency to become radially open at
heliocentric distances below $\sim 2.5 \ \mathrm{R}_\odot$. We compared the
reconstructed 3D coronal structures over the CR near the solar maximum to the
one at deep solar minimum. Results of our 3D density reconstruction will help
to constrain solar coronal field models and test the accuracy of the magnetic
field approximations for coronal modeling.
