  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 outward 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 a 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 many locations to form blobs. Then, the streamer system
returns to the configuration with a lower cusp point, subject to another cycle
of 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 modeled blobs,
including their size, velocity profiles, density contrasts, and even their
daily occurrence rate, are in line with available observations.
