Magnetic reconnection has a crucial role in a variety of plasma environments 1-3 in providing a mechanism for the fast release of stored magnetic energy. During reconnection the plasma forms a 'magnetic nozzle', like the nozzle of a hose, and the rate is controlled how fast plasma can flow out of the nozzle. But the traditional picture of reconnection has been unable to explain satisfactorily the short timescales associated with the energy release, because the lfow is mediated by hevavy ions with a alow resultant velocity. Recent theoretical work 4-6 has suggested that the energy release is instead mediated by electrons in waves called 'whistlers', which move much faster for a given perturbation of the magnetic field because of their smaller mass. Moreover, th whistler vlocity and associated plasma velocity both increase as the 'nozzle' becomes marrower. A marrower nozzle therefore no longer reduces the total plasma flow-the outflow is independent of the size of the nozzle. Here we report observations demonstrating that reconnection in the magnetosphere is driven by whistlers, in good agreement with the theoretical predictions.