Observations of convection in the dayside magnetosphere by the beam instrument on Geotail

We report observations of magnetospheric convection by the beam instrument, EFD-B. on Geotail. The region analyzed in this study is mainly the afterno on sector of the magnetosphere between L = 9.7 - 11.5. When the instrument is operated, electron beams are emitted from guns and some of them return t o detectors attached to the main body of the satellite. However, we find th at the return beams are often spread over a wide range of satellite spin ph ase angles, so that the calculated convection is unreliable. In order to re move noisy data, we set up suitable selection criteria. We infer that the c onvection strength is of the order of 20 km/s. The convection has generally westward and outward components. This indicates that the plasma located at the satellite positions is being convected toward the magnetopause. Moreov er, the obtained convection is highly variable because standard deviations are comparable to the strength. We then compare the convection estimated by the beam instrument with that by the particle instrument, LEP. We find tha t the convections derived from the two instruments are positively correlate d, with correlation coefficients above 0.7. The analysis reported here is e xpected to be useful in the interpretation of the multi-spacecraft data fro m the Cluster II mission.