Positive Topological Entropy for Magnetic Flows on Surfaces

We study the topological entropy of the magnetic flow on a closed riemannian surface. We prove that if the magnetic flow has a non-hyperbolic closed orbit in some energy set T^cM= E^{-1}(c), then there exists an exact $C^\infty$-perturbation of the 2-form $\Omega$ such that the new magnetic flow has positive topological entropy in T^cM. We also prove that if the magnetic flow has an infinite number of closed orbits in T^cM, then there exists an exact C^1-perturbation of $\Omega$ with positive topological entropy in T^cM. The proof of the last result is based on an analog of Franks' lemma for magnetic flows on surfaces, that is proven in this work, and Ma\~n\'e's techniques on dominated splitting. As a consequence of those results, an exact magnetic flow on S^2 in high energy levels admits a C^1-perturbation with positive topological entropy. In the appendices we show that an exact magnetic flow on the torus in high energy levels admits a $C^\infty$-perturbation with positive topological entropy.