  The main result presented here is that the flow associated with a riemannian
metric and a non zero magnetic field on a compact oriented surface without
boundary, under assumptions of hyperbolic type, cannot have the same length
spectrum of topologically corresponding periodic orbits as the geodesic flow
associated with another riemannian metric having a negative curvature and the
same total volume. The main tool is a regularization inspired by U.
Hamenst\"adt's methods.
