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When the 1999 Yuhkoh flare was found, astrophysicists wondered what caused the down motion. They called these dark finger-like phenomena “downward-moving dark spaces.” Astrophysicists have a more accurate term for them now: supra-arcade downflows (SADs.).
A group of scientists led by Chengcai Shen have an explanation for SADs. Shen is an astronomer at the CfA, the Harvard and Smithsonian Center for Astrophysics. The groups paper is “The origin of underdense plasma downflows connected with magnetic reconnection in solar flares.” The journal Nature Astronomy published the work.
A solar game is an active location with multiple coronal loops. Coronal loops are magnetic structures that extend from the Suns photosphere out into the corona, looping pull back to reconnect with the photosphere again. Coronal loops trap plasma magnetically, which makes them visible.
Image Credit: By NASA Public Domain.
A supra-arcade is a solar arcade with included functions. In addition to the loops, there are downflows above the arcade. Researchers believed supra-arcades are in some way linked with the magnetic reconnection behind solar flares, but the specifics were unidentified.
” We needed to know how these structures happen,” says lead author and CfA astronomer Chengcai Shen. “Whats driving them, and are they genuinely connected to magnetic reconnection?”.
Solar flares are intricate phenomena. Spacecraft like NASAs Solar and Heliophysics Observatory (SOHO) and the Parker Solar Probe shed new light on the Suns solar flares.
It was a Japanese-led objective called Yohkoh that spotted an uncommon solar flare in 1999. Scientists believed supra-arcades are in some way linked with the magnetic reconnection behind solar flares, however the specifics were unknown.
“In eruptive solar flares, dark, finger-shaped plasma downflows moving towards the flare arcade have been frequently regarded as the principalobservational evidence for such reconnection-driven outflows.”.
Solar flares are complex phenomena. They involve plasma, electro-magnetic radiation across all wavelengths, activity in the Suns environment layers, and particles taking a trip at near light speed. Spacecraft like NASAs Solar and Heliophysics Observatory (SOHO) and the Parker Solar Probe shed brand-new light on the Suns solar flares.
However it was a Japanese-led mission called Yohkoh that spotted an unusual solar flare in 1999. This flare showed a down streaming movement towards the Sun in addition to the normal outward circulation. What caused it?
A group of researchers believe theyve figured it out.
The Sun has complex electromagnetic fields that can end up being compressed and damaged. They can break, releasing fast-moving and effective radiation along magnetic lines, then reconnect to form loops.
” On the Sun, what occurs is you have a great deal of magnetic fields that are pointing in all various instructions. Ultimately, the electromagnetic fields are pushed together to the point where they reconfigure and release a lot of energy in the type of a solar flare,” says study co-author and CfA astronomer Kathy Reeves.
” Its like extending an elastic band and snipping it in the middle. Its stressed out and extended thin, so its going to snap back,” Reeves added.
That understanding is firmly developed, so its affordable to conclude that the same mechanism directed SADs. “A particular feature of magnetic reconnection is the production of quick reconnection outflow jets near the plasma Alfven speeds,” the authors compose in their paper. “In eruptive solar flares, dark, finger-shaped plasma downflows approaching the flare arcade have actually been typically considered the principalobservational evidence for such reconnection-driven outflows.”.
Observations didnt completely back that explanation. It comes down to speed.
” However, they typically show a speed much slower than that anticipated in reconnection theories, challenging the reconnection-driven energy release scenario in standard flare designs,” they compose.
In the rubber band example, the snap-back is rapid. But when researchers enjoyed these SADs, many of them didnt snap back. Instead, they reconnected more gradually with the Sun. If the very same thing happened with a rubber band, we d believe somebody increased our drink.
” This is not anticipated by classic reconnection models, which show the downflows should be much quicker. Its a dispute that requires some other description,” lead author Shen said.
Heres where NASAs Solar Dynamics Observatory (SDO) comes in. The AIA provides us continuous full-disk protection of the Suns chromosphere and corona.
This image from the SDOs AIA is an outstanding example of the instruments power. Its an extreme UV light image of the electromagnetic fields and the loops they produce, which are invisible to our eyes. These features dwarf the Earth. Image Credit: Solar Dynamics Observatory/NASA.
Shen and the other authors created 3D simulations of solar flares and compared them to solar flares the SDO observed. They found that magnetic reconnection isnt the source of SADs.
Rather, fluid dynamics are at the heart of SADs. When they interact in the unstable environment above the game, 2 fluids of various densities produce the SADs.
This image reveals some of the simulations created by the team of researchers. It reveals how the density of the plasmas changes in time, leading to the development of SADs. Image Credit: Shen et al. 2022.
The authors say that the rough environment in this “user interface region” where downward reconnection outflows impinge on closed flare arcades hasnt received much attention in previous research study. “This interface region hosts a myriad of turbulent circulations, electron currents, and shocks, crucial for flare energy release and particle velocity,” the authors discuss in their paper.
This is the Crab Nebula, a well-studied supernova remnant. It shows the rough flows and shock waves that create the same structures seen in SADs. The fingers in residues like these are Rayleigh-Taylor fingers. The unstable interface in between fluids of different densities creates the fingers, similar to how SADs are formed on the Sun. Image Credit: By NASA, ESA, J. Hester and A. Loll (Arizona State University) Public Domain.
In this case, the two fluids are both plasmas. They have different densities, leading to the unusual behaviour of SADs.
” Those dark, finger-like voids are really an absence of plasma. The density is much lower there than the surrounding plasma,” co-author Reeves discussed.
The authors mean to keep studying SADs and other solar functions utilizing observations and simulations. They think their work may cause better tools for predicting area weather condition.
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