Astroboffins map engine of a solar flare: Magnetic mega-fields and Earth-dwarfing blankets of electric current
One of our Sun's large, recent solar flares was formed from the release of 10 to 100 billion trillion joules per second of magnetic energy through gigantic sheets of near-light-speed electrons, scientists say.
Those sheets of electric current stretched more than 40,000 kilometres across, or more than three times the diameter of the Earth, and sat at the base of the familiar loops of plasma seen bursting from our nearest star. This is according to a paper published in Nature Astronomy on Monday.
"During large eruptions on the Sun, particles such as electrons can get accelerated to high energies," said Kathy Reeves, who cowrote the study and is an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in the US. "How exactly this happens is not clearly understood, but it is thought to be related to the Sun's magnetic field."
Professor Bin Chen, of the New Jersey Institute of Technology (NJIT) and lead author on the study, added: "It has long been suggested that the sudden release of magnetic energy through the reconnection current sheet is responsible for these major eruptions, yet there has been no measurement of its magnetic properties.
"With this study, we’ve finally measured the details of the magnetic field of a current sheet for the first time, giving us a new understanding of the central engine of the Sun’s solar flares."
Here's a sense of how small we are ... The light orange curves show the solar flares' modeled magnetic field lines. The flare was driven by the twisted magnetic flux rope. The colored blobs are the observed current sheets. Image Credit: NJIT-CSTR, B. Chen, S. Yu; NASA Solar Dynamics Observatory. Click to enlarge
For the study, Chen and his colleagues took a closer look at a powerful solar flare that was recorded in September 2017 by ground-based telescopes at the NJIT's Owens Valley Solar Array (EOVSA) and NASA’s Solar Dynamics Observatory in space. The team studied the outburst across various wavelengths, including microwave and ultraviolet, and the data was used to build a realistic computer simulation of the eruption.
They found that, as the Center for Astrophysics put it, "the intense energy powering the flare is the result of an enormous electric current 'sheet' ... through the core flaring region, where opposing magnetic field lines approach, break, and reconnect." The center added:
Measurements taken during the study also indicate a magnetic, bottle-like structure located at the top of the flare's loop-shaped base, or flare arcade, at a height of nearly 20,000 kilometers above the surface of the Sun. The study suggests that this is the primary site where a solar flare’s highly energetic electrons are trapped and accelerated to nearly the speed of light.
“While the current sheet seems to be the place where the energy is released to get the ball rolling, most of the electron acceleration appears to be occurring in this other location, the magnetic bottle," said Dale Gary, director of EOVSA and co-author on the study.
"Others have proposed such a structure in solar flares before, but we can truly see it now in the numbers," Chen added, "What our data showed was a special location at the bottom of the current sheet – the magnetic bottle – appears to be crucial in producing or confining the relativistic electrons."
While the team reckons they have solved the question of from where the magnetic energy is released in solar flares, Chen said there are still many questions about the physics of solar flares and coronal mass ejections left to answer. "There are certainly huge prospects out there for us to study that address these fundamental questions," he concluded. ®