Plasma Outflows in Coronal Streamers

Abstract

In this Letter we show that it is possible to determine slow velocities (~20 km s^-1) in the extended solar corona by means of the ratio of the resonance doublet of O VI. We apply this technique to a quiescent streamer at solar minimum, observed by the Ultraviolet Coronagraph Spectrometer (UVCS) for 4 consecutive days, and determine the velocity pattern in it. We show that a rapid velocity increase occurs on the lateral sides of the streamer as the distance from the streamer axis increases. We also show that, probably, outflowing plasma exists also in the streamer core. We point out the interest of examining the possible relation of this finding with the reduction of the O VI emission observed by UVCS in the core of some streamers. We also point out the importance of studying the connection of the plasma just outside the v~=20 km s^-1 curve with the streamer cusp, to gain insight in the physics of the slow solar wind.