  A treatment is given of the nonperturbative QCD vacuum in a magnetic field.
The low-energy equation for the trace of the energy-momentum tensor in a
magnetic field is derived. It is shown that the derivatives with respect to a
magnetic field of the quark and gluon contributions to the trace of the
energy-momentum tensor are equal. The dependence of the gluon condensate on the
magnetic field strength is derived both for strong and weak fields.
