An explanation for fast downflows on the Sun

Abstract

The discovery of coronal loops has provoked major revisions in the theory of solar and stellar atmospheres. The solar corona is now recognized as a highly structured plasma, organized by bundles of intense magnetic field into individual arch- and loop-like structures along which gas can flow¹. Each arch, rooted in the solar interior, extends through the chromosphere (T<10⁴ Κ), and generally contains coronal plasma (T>10⁶Κ) in its upper reaches. Early attempts at explaining the temperature and density structure of isolated static loops met with considerable success, despite the simplicity of the theory^2,3; however, attempts to generalize the theory to describe loops with mass flows have remained singularly unsuccessful^4–9. Here we point out that the puzzling observational properties of loop mass flows^10–15 can be naturally explained if the loop is sufficiently cool (T ≲ 10⁶ K)—that is, if it contains no coronal plasma.