We discuss the possibility to measure the sign of dm2_{31} from
matter-induced charge asymmetries in atmospheric neutrino oscillations. The
main conclusion is that an impact on the nu_mu survival probability requires
the action of the MSW resonance, which becomes visible for baselines above
\~7000 km.Comment: 6 pp, 1 fig. Talk given at the 7th International Workshop on Topics
in Astroparticles and Underground Physics (TAUP2001), Laboratori Nazionali
del Gran Sasso, Italy, September 8-12, 200
