  Let $(M,g)$ be a closed Riemannian manifold and $\sigma$ be a closed 2-form
on $M$ representing an integer cohomology class. In this paper, using
symplectic reduction, we show how the problem of existence of closed magnetic
geodesics for the magnetic flow of the pair $(g,\sigma)$ can be interpreted as
a critical point problem for a Rabinowitz-type action functional defined on the
cotangent bundle $T^*E$ of a suitable $S^1$-bundle $E$ over $M$ or,
equivalently, as a critical point problem for a Lagrangian-type action
functional defined on the free loopspace of $E$. We then study the relation
between the stability property of energy hypersurfaces in $(T^*M,dp\wedge
dq+\pi^*\sigma)$ and of the corresponding codimension 2 coisotropic
submanifolds in $(T^*E,dp\wedge dq)$ arising via symplectic reduction. Finally,
we reprove the main result of [9] in this setting.
