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This is one of the most persistent mysteries of the Sun: why the superheated surface of this large ball of incandescent plasma is actually colder than its outer atmosphere, called the crown.
Scientists now have a new, highly controversial explanation about this, and the answer was hidden in a strange solar phenomenon never seen before: a rain of plasma rain falling into newly discovered magnetic structures called Raining Null Point topologies.
On Earth, when it's hot, the water evaporates, turns into vapor that rises in the atmosphere, before the cooling actually reverses the process: the water molecules condense inside the clouds resulting in precipitation falling on land, oceans and rivers.
On the burning surface of the Sun, a similar cycle of events controls what is called coronal rain: an overheated plasma that rises from the Sun, often during solar flares, along invisible magnetic loops.
When this plasma cools away from the Sun, it forms a sort of fiery rainbow, condensing and then descending back into the photosphere in the footsteps of these invisible magnetic tracks.
Basically, the Earth and the Sun are also subject to showers, consisting solely of different types of condensation and cooling.
"The physics is literally the same," said solar physicist Emily Mason of the Catholic University of America Scientific news last year, describing his preliminary research.
Mason's research has just been published and teaches us something about coronal rain that we have never known: it occurs mainly in an unexpected place and is associated with a new phenomenon for solar physics.
Plasma rain in the RNTP (NASA / Emily Mason Solar Dynamics Observatory)
As part of his daily work at NASA's Goddard Space Flight Center, Mason was investigating coronal rain in gigantic magnetic structures called streamers, which can trace up to a million kilometers from the Sun's surface before they back in loop.
After months of searching for coronal rain in these epic arches, Mason failed. However, with the help of data from NASA's Solar Dynamics Observatory (SDO), she spied much smaller magnetic loops, closer to the photosphere, which seemed to show Plasma Rain.
"They were really bright and they continued to catch my eye," says Mason.
"When I finally watched them, they had dozens of rain hours at a time."
Despite the flood discovered, it was only when Mason shared the data with fellow NASA researchers that the smallest magnetic structures, called Raining Null Point Topologies (RNPT) topologies, were identified as a new solar phenomenon.
"I said:" Wait … wait, where do you see that? ", Explains Nicholeen Viall, NASA's solar energy scientist." I do not do not think anyone has ever seen this before! "
According to the researchers, RNPTs are at altitudes up to about 50,000 kilometers (30,000 miles) above the sun's surface. This seems like a huge scale, but compared to the helmet streamers that Mason originally examined, these structures were tiny – barely two percent of the height of the streamers.
This small size could also explain some things about the intense heat of the crown.
"These loops were much smaller than we were looking for," says one of the team members, NASA's solar physicist, Spiro Antiochos.
"So it tells you that the heating of the crown is much more localized than we thought."
The new discoveries do not tell us exactly how RNPTs could heat the corona – which remains hypothetical at the moment – but the considerable amount of this plasma phenomenon, as well as its current duration, suggests that it could be a big piece of the puzzle.
"The ease with which these structures were identified and the frequency of rainfall over all observations provides a compelling argument for concluding the presence of an ubiquitous phenomenon," explain the authors in their article.
"In any case, the rain continues for days on what appears to be the same magnetic loops, so it's clearly not a punctual phenomenon like flaring."
Strangely, however, all the plasma involved in this rain cycle does not seem to be returned to the sun.
The researchers also detected in the data a glimpse of a phenomenon called magnetic reconnection or exchange, in which the plasma of the closed magnetic loop could come out of the loop, possibly even contributing to solar wind forces generated by the Sun.
Much future research and observations will be needed to identify these possibilities, but the new discovery of RNPTs may be an important factor in helping scientists understand these aberrational discrepancies regarding the intense heat and plasma fluxes of the Sun.
"Since the understanding of coronal heating is undoubtedly the most important unresolved problem of solar physics," the researchers write, "detailed measurements of coronal rain are of crucial importance".
The results are reported in The letters of the astrophysical journal.
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