We present an observation of broadband transverse waves (BTW) below 1 Hz in the afternoon sector of the magnetosphere at about L~10. We find that the BTW correlate well with density enhancements and bidirectional electron flows. Characteristics of the BTW are summarized as follows. The wave amplitude varies within a few periods, which results in a broad frequency band in the Fourier spectra. Only perturbations transverse to the background magnetic field are observed. The amplitude ratio of electric to magnetic field fluctuations is close to the local Alfv¿n speed, so that the wave mode is suggested to be Alfv¿n waves. The Poynting flux is directed away from the equatorial region toward the ionosphere. We have assessed various resonance mechanisms (Landau resonance, cyclotron resonance, and resonance with bouncing electrons) but find that none gives a compelling interpretation of the observation, which thus requires future study. ¿ 2001 American Geophysical Union