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 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.

💡 Research Summary

The paper proposes that the ubiquitous plasma “blobs” observed emerging from the cusps of quiescent coronal streamers are not the by‑product of external disturbances but the direct manifestation of an intrinsic instability localized at the streamer’s cusp. The authors argue that the magnetic field in the cusp region becomes too weak to confine the underlying plasma. Because of the frozen‑in condition, the plasma expands outward, stretching the closed magnetic loops radially. This expansion forces a strong velocity shear into the slow‑wind regime, creating a reservoir of free energy that can drive magnetohydrodynamic (MHD) instability. Using two‑dimensional (2.5‑D) MHD simulations, they demonstrate that the dominant unstable mode is a hybrid of sausage (axisymmetric pinching) and kink (non‑axisymmetric bending) disturbances. The sausage component produces localized pinches of the elongated loops, while the kink component introduces transverse displacements that facilitate multiple reconnection sites along the loop. When reconnection occurs, portions of the stretched field lines detach, forming density‑enhanced plasma blobs that are elongated, radially moving structures. The simulated blobs have typical sizes of ~0.1 R⊙, velocity amplitudes of 100–300 km s⁻¹, and density enhancements of 10–30 % relative to the surrounding slow wind—values that match remote‑sensing observations. After a blob is released, the cusp point settles at a slightly lower altitude, resetting the system for another cycle of expansion, shear formation, instability growth, and blob ejection. Crucially, this cyclic process does not erode the total closed magnetic flux of the streamer; the overall streamer topology remains essentially unchanged, which explains why streamers can persist for weeks while still producing daily blobs. The authors further show that the simulated occurrence rate (≈1–2 blobs per day), velocity profiles, and density contrasts are consistent with statistical analyses of LASCO and STEREO data. They conclude that the streamer’s own magnetic configuration inherently predisposes it to a quasi‑periodic, self‑sustaining instability that naturally generates the observed blobs. The paper suggests future work should focus on high‑resolution measurements of cusp magnetic field strength, three‑dimensional MHD modeling, and coupling with solar wind acceleration mechanisms to refine the quantitative aspects of the instability and to explore its possible role in mass and energy transport from the corona into the heliosphere.