Given a bounded domain G ⊂ ℝd, d ≥ 3, we study smooth solutions of a linear parabolic equation with non-constant coefficients in G, which at the boundary have to C1-match with some harmonic function in ℝd \ Ḡ vanishing at spatial infinity.

This problem arises in the framework of magnetohydrodynamics if certain dynamo-generated magnetic fields are considered: for example, in the case of axisymmetry, or for non-radial flow fields, the poloidal scalar of the magnetic field solves the above problem.

We first investigate the Poisson problem in G with the boundary condition described above as well as the associated eigenvalue problem and prove the existence of smooth solutions. As a by-product we obtain the completeness of the well-known poloidal ‘free decay modes’ in ℝ3 if G is a ball. Smooth solutions of the evolution problem are then obtained by Galerkin approximation based on these eigenfunctions.