Wave power dropouts associated with radial field intervals in high speed solar wind

Examination of 770 days of heliospheric magnetic field data recorded within polar solar wind by the Ulysses spacecraft revealed a number of occasions when the wave power decreased by more than an order of magnitude for severa l hours. These power dropouts were found to be associated with intervals wh en the magnetic field was closely aligned with the radial direction. A case by case study showed that there was no obvious signature in bulk plasma pa rameters. A statistical study of 24 intervals when the field was close to r adial for at least 6 hours shelved that the wave power in those intervals w as significantly lower than that outside. There were no significant power c hanges when the field lay close to either the Parker spiral or an arbitrary direction. Although they are likely to be of solar origin, the exact sourc e for these events is not clear.