  A widely accepted paradigm is that equipartition (eqp) between the energy
density of cosmic rays (CRs) and the energy density of the magnetic field
cannot be sustained in supernova remnants (SNRs). However, our 3D hydrodynamic
supercomputer simulations, coupled with a non-linear diffusive shock
acceleration model (NLDSA), provide evidence that eqp may be established at the
end of the Sedov phase of evolution in which most SNRs spend the longest
portions of their lives. We introduce term "constant partition" for any
constant ratio between the CR energy density and the energy density of magnetic
field in an SNR, while term "equipartition" should be reserved for the case of
approximately the same values of the energy density (also, it is {constant
partition} in the order of magnitude) of ultrarelativistic electrons only (or
CRs in total) and the energy density of magnetic field. Our simulations suggest
that this approximate constant partition does exist in all but the youngest
SNRs. We speculate that since evolved SNRs at the end of the Sedov phase of
evolution can reach eqp between CRs and magnetic fields, they may be
responsible for initializing this type of eqp in the interstellar medium.
Additionally, we show that eqp between the electron component of CRs and
magnetic field may be used for calculating the magnetic field strength directly
from observations of synchrotron emission from SNRs. The values of magnetic
field strengths in SNRs given here are approximately 2.5 times lower than
values calculated by Arbutina et al. (2012, 2013).
