Kilometre-scale structures in the Sun's corona

KNOWLEDGE of the structure of the Sun's corona is important for our understanding of how this high-temperature plasma is heated, and of the processes involved in the acceleration of the solar wind(1,2). The structure can be investigated directly by imaging at optical and shorter wavelengths, or indirectly through the effects of changing electron density on the propagation of radio waves (scattering and scintillation). Radio measurements have established many of the characteristics of the density fluctuations in the corona and solar wind, but the fundamental nature of these structures is not yet fully understood(3,4). Two specific features that have proved difficult to explain are an abrupt increase in anisotropy of the irregularities close to the Sun(5-7), and a break in the power-law spectrum describing the density fluctuations(8,9). Here I argue that these features are the manifestation of a transition from small ray-like or filamentary structures in the corona that rotate with the Sun to turbulent density irregularities convecting with the solar sind. I estimate the size of the smallest filamentary structure within coronal holes to be about 1 km at the Sun, approximately three orders of magnitude smaller than the smallest filamentary structures observed in images of different wavelengths(2,10-12).