Supra-arcade downflows (SADs) are sunward-travelingplasma voids that are sometimes observed in theSun's outeratmosphere, orcorona, duringsolar flares. Insolar physics,arcade refers to a bundle ofcoronal loops, and the prefixsupra indicates that the downflows appear above flare arcades. They were first described in 1999 using the Soft X-ray Telescope (SXT) on board theYohkoh satellite.^[1] SADs are byproducts of themagnetic reconnection process that drives solar flares, but their precise cause remains unknown.

SADs are dark, finger-like plasma voids that are sometimes observed descending through the hot, denseplasma above brightcoronal loop arcades duringsolar flares. They were first reported for a flare and associatedcoronal mass ejection that occurred on January 20, 1999, and was observed by the SXT onboardYohkoh.^[1] SADs are sometimes referred to as “tadpoles” for their shape and have since been identified in many other events.^[2][3][4][5] They tend to be most easily observed in the decay phases of long-durationflares,^[2] when sufficientplasma 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 overlyingmagnetic field is reconfigured during theflare. 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]

SADs typically begin 100–200Mm above thephotosphere and descend 20–50Mm before dissipating near the top of the flare arcade after a fewminutes.^[7][9] Sunward speeds generally fall between 50 and 500 km s^−1[2][7] but may occasionally approach 1000 km s⁻¹.^[7][10] As they fall, the downflows decelerate at rates of 0.1 to 2 km s⁻².^[7] SADs appear dark because they are considerably less dense than the surroundingplasma,^[3] while their temperatures (100,000 to 10,000,000K) do not differ significantly from their surroundings.^[11] Theircross-sectional areas range from a few million to 70 million km^2[7] (for comparison, thecross-sectional area of theMoon is 9.5 million km²).

SADs are typically observed using softx-ray andextreme ultraviolettelescopes, or EUVs, that cover awavelength range of roughly 10 to 1500 Å, and are sensitive to thecoronalplasma, through which the downflows move. These emissions, however, are blocked byEarth's atmosphere, so observations are made usingspace observatories. The first detection was made in 1999 by a soft x-ray telescope (SXT) onboard theYohkoh spacecraft,^[1] and was soon followed by observations fromNASA’sTRACE satellite, and thespectroscopic SUMER instrument on board theSOHO Observatory.^[3][4] More recently, studies on SADs have used data from an SXT onboard theHinode, as well as from theSolar Dynamics Observatory.^[11] In addition to EUV and X-ray instruments, SADs may also be seen bywhite lightcoronagraphs such as theLarge Angle and Spectrometric Coronagraph onboardSOHO,^[12] though these observations are less common.

SADs are widely accepted to be byproducts ofmagnetic reconnection, the physical process that drivessolar flares by releasing energy stored in theSun's magnetic field. Magnetic reconnection changes the localmagnetic field surrounding theflare site from a higher-energy (non-potential,stressed) state to a lower-energy (potential) state. This process is facilitated by the development of acurrent sheet, often preceded by or similar to acoronal mass ejection. As the field is being reconfigured, newly formedmagnetic field lines are swept away from the reconnection site, producing new flows both toward and away from thesolar surface, respectively referred to as downflows and upflows. SADs are believed to have relation to reconnection downflows that disrupt the hot, denseplasma that collects aboveflare arcades,^[4] but precisely how SADs form is uncertain and is an area of active research.

SADs were first interpreted ascross sections of magneticflux tubes, which comprisecoronal loops, that retract down due tomagnetic tension after being formed at thereconnection site.^[1][7] This interpretation was later revised to suggest that SADs are insteadwakes behind much smaller SADLs,^[8] rather than cross sections of theflux tubes themselves. Another possibility, also related toreconnection outflows, is that SADs arise from an instability, such asRayleigh-Taylor instability^[13] or a combination of the tearing mode andKelvin-Helmholtz instabilities.^[14]

• Supra-Arcade Downflows - RHESSI Wiki (berkeley.edu)
• NASA: Closeup of Solar 'Tadpoles' (nasa.gov)
• Hinode/XRT: Supra-Arcade Downflows Post X-Flare (cfa.harvard.edu)
• Hinode/XRT: Supra-Arcade Downflowing Loops (cfa.harvard.edu)

• Astrophysics
• Magnetohydrodynamics
• Solar phenomena
• Space physics
• Sun

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