OBSERVATIONS OF A SOLAR WIND DOMAIN BOUNDARY EXTENDING 1 AU FROM THE SUN

We present measurements of a spatially coherent structure that extended over 0.5 AU through the solar wind. This is the first observation of such a feature in white light, and it is rare, possibly unique. While we cannot present conclusive evidence of its origin and nature, we speculate, based on white-light observation and measurement, that it is a domain boundary between fast and slow solar wind streams, possibly arising from the flank of a coronal mass ejection that erupted some 10 hr prior to its appearance. The puzzling aspect of this feature is that it maintained its structural integrity for several days in the solar wind at distances near 1 AU, yet it showed no signs of turbulent break up. This is despite an expectation, which we derive from basic hydrodynamic theory, that turbulence induced by the Kelvin-Helmholtz instability should at least be present there. We present our observations, measurements, and speculations and conclude with an appeal to the wider community for suggestions as to the source, nature, and uniqueness of this feature.