Extremely long baseline interplanetary scintillation measurements of solar wind velocity

We present results of observations of interplanetary scintillation (IPS) made using the telescopes of the MERLIN and EISCAT networks in which the beam separation approached 2000 km, much larger than in any previous IPS experiments. Significant correlation between the scintillation patterns was observed at time lags of up to 8 s and fast and slow streams of solar wind were very clearly resolved. One observation showed clear evidence of two discrete modes of fast solar wind, which we interpret as originating in the crown of the northern polar coronal hole and in an equatorward extension of the polar hole. We suggest that experiments of this type will provide a new and important source of information on the temporal and spatial variation of small-scale turbulence in the solar wind. The improved velocity resolution available from extremely long baseline measurements also provides new information on the development of the large-scale velocity structure of the solar wind in interplanetary space.