Broadband transverse waves below 1 Hz in the afternoon sector of the magnetosphere

We present an observation of broadband transverse waves (BTW) below 1 Hz in the afternoon sector of the magnetosphere at about L similar to 10. We fin d that the BTW correlate well with density enhancements and bidirectional e lectron flows. Characteristics of the BTW are summarized as follows. The wa ve amplitude varies within a few periods, which results in a broad frequenc y band in the Fourier spectra. Only perturbations transverse to the backgro und magnetic field are observed. The amplitude ratio of electric to magneti c field fluctuations is close to the local Alfven speed, so, that the wave mode is suggested to be Alfven waves. The Poynting flux is directed away fr om the equatorial region toward the ionosphere. We have assessed various re sonance mechanisms (Landau resonance, cyclotron resonance, and resonance wi th bouncing electrons) but find that none gives a compelling interpretation of the observation, which thus requires future study.