A new ground-based diagnostic of reconnection at the magnetopause using ULF waves is presented, and we give an example day in which the dynamics of the open-closed boundary (OCB) are clearly observed. The diagnostic is based on data from two close (~110 km), magnetic azimuthally spaced magnetometer sites (Davis and Zhongshan, Antarctica). During average IMF conditions Davis passes just equatorward of the last closed field lines in the dayside magnetosphere, and cross-phase measurements indicate propagation away from local magnetic noon (LMN) as expected for the Kelvin-Helmholtz Instability (KHI) acting at the magnetopause. During times when IMF Bz < -2 nT intervals of broadband activity appear, with the azimuthal component of propagation directed towards noon. Statistical analysis shows a correlation between the negative magnitude of IMF Bz and cross-phase measurements ¿2--6 hours from LMN. We conclude that under the right conditions, reconnection signatures can be identified in magnetometer data superimposed on the KHI-like propagation signatures of the last closed field lines.