Supra-arcade downflows
This article may be too technical for most readers to understand.(November 2015) |
Observations
Description
SADs are dark, finger-like plasma voids that are sometimes observed descending through the hot, dense plasma above bright coronal loop arcades during solar flares. They were first reported for a flare and associated coronal mass ejection that occurred on January 20, 1999 and was observed by the SXT onboard Yohkoh.[1] SADs are sometimes referred to as “tadpoles” for their shape and have since been identified in many other events (e.g.[2][3][4][5]). They tend to be most easily observed in the decay phases of long-duration flares,[2] when sufficient plasma has accumulated above the flare arcade to make SADs visible, but they do begin earlier during the rise phase.[6] In addition to the SAD voids, there are related structures known as supra-arcade downflowing loops (SADLs). SADLs are retracting (shrinking) coronal loops that form as the overlying magnetic field is reconfigured during the flare. SADs and SADLs are thought to be manifestations of the same process viewed from different angles, such that SADLs are observed if the viewer's perspective is along the axis of the arcade (i.e. through the arch), while SADs are observed if the perspective is perpendicular to the arcade axis.[7][8]
Basic properties
SADs typically begin 100–200
Instrumentation
SADs are typically observed using soft
Causes
SADs are widely accepted to be byproducts of magnetic reconnection, the physical process that drives solar flares by releasing energy stored in the Sun's magnetic field. Reconnection reconfigures the local magnetic field surrounding the flare site from a higher-energy (non-potential, stressed) state to a lower-energy (potential) state. This process is facilitated by the development of a current sheet, often preceded by or in tandem with a coronal mass ejection. As the field is being reconfigured, newly formed magnetic field lines are swept away from the reconnection site, producing outflows both toward and away from the solar surface, respectively referred to as downflows and upflows. SADs are believed to be related to reconnection downflows that perturb the hot, dense plasma that collects above flare arcades,[4] but precisely how SADs form is uncertain and is an area of active research.
SADs were first interpreted as