Intrinsic Instability of Coronal Streamers

Plasma blobs are observed to be weak density enhancements as radially-stretched structures emerging from the cusps of quiescent coronal streamers. In this paper, it is suggested that the formation of blobs is a consequence of an intrinsic instability of coronal streamers occurring at a very localized region around the cusp. The evolutionary process of the instability, as revealed in our calculations, can be described as follows, (1) through the localized cusp region where the field is too weak to sustain the confinement, plasmas expand and stretch the closed field lines radially outwards as a result of the freezing-in effect of plasma-magnetic field coupling; the expansion brings a strong velocity gradient into the slow wind regime providing the free energy necessary for the onset of the subsequent magnetohydrodynamic instability; (2) the instability manifests itself mainly as mixed streaming sausage-kink modes, the former results in pinches of elongated magnetic loops to provoke reconnections at one or multi locations to form blobs. Then, the streamer system returns to the configuration with a lower cusp point, subject to another cycle of the streamer instability. Although the instability is intrinsic, it does not lead to the loss of the closed magnetic flux, neither does it affect the overall feature of a streamer. The main properties of the modelled blobs, including their size, velocity profiles, density contrasts, and even their daily occurrence rate are in line with available observations. Subject headings: Sun: corona − Sun: streamer − Sun: MHD instability