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Researchers offer an unprecedented look at the “central engine” that powers the solar flare

Researchers offer an unprecedented look at

Observations of a large solar flare on September 10, 2017 in extreme ultraviolet light (gray background, NASA solar dynamics observatory) and a microwave oven (red to blue indicate the rising frequencies observed in the solar massif of the extended Owens Valley). The light orange curves are selected by magnetic field lines from the corresponding theoretical model of solar energy eruption. The flash moves by the eruption of a twisted magnetic flux rope (illustrated by a beam of colored curves). Microwave sources are observed throughout the central region, where a large-scale current connection sheet is located – the “central motor” of the flash and is used to measure its physical properties. Credit: CSTR / NJIT, B. Chen, S. Yu; NASA Solar Dynamics Observatory

In a study published in Nature astronomy, an international team of researchers presented a new, detailed look at the “central engine” of a large solar flare, accompanied by a powerful eruption, first captured on September 10, 2017 by the Owens Valley Solar Massif (EOVSA) – a solar radio telescope operated by the Solar Research Center. ) New Jersey, Institute of Technology (NJIT).

The new data, based on EOVSA’s observations of the event at the microwave wavelength, offer the first measurements characterizing the magnetic fields and particles at the base of the explosion. The results revealed a huge “sheet” of electric current that stretched for more than 40,000 kilometers through the core of the flash, where opposite magnetic field lines approach each other, break and reconnect, generating intense energy that feeds the flash.

Notably, the team’s measurements also point to a magnetic bottle-shaped structure located at the top of the flash-shaped loop (known as an arcade flash) at an altitude of nearly 20,000 kilometers above the Sun’s surface. The structure, according to the team, is probably the primary place where high-energy flash electrons fall into the trap and accelerate almost to the speed of light.

Researchers say a new understanding of the study of the central engine that drives such powerful eruptions could help future space weather forecasts for potentially catastrophic solar energy emissions – the most powerful solar system explosions that could severely disrupt technology on Earth, such as satellite operations, GPS navigation and communication systems, among many others.

“One of the main goals of this study is to better understand the fundamental physics of solar eruptions,” said Bin Chen, lead author and professor of physics at NJIT. “It has long been suggested that the sudden release of magnetic energy through the reconnection current sheet is the cause of these large eruptions, but its magnetic properties were not measured. With this study, we finally measured the details of the current sheet’s magnetic field for the first time. engine of the main flares of the Sun “.

“The place where all the energy is stored and released in solar flares has so far been invisible. To play on the term cosmology, this is the” dark energy problem “of the Sun, and we previously had to conclude indirectly that a magnetic flash recovery sheet existed,” – said Dale Gary, director of EOVSA at NJIT and co-author of the article. “EOVSA images taken at many microwave frequencies have shown that we can capture radio radiation to illuminate this important region. Once we had this data and analysis tools created by co-authors Gregory Fleischmann and Gel Nita, we were able to begin radiation analysis to include these measurements “.

Earlier this year in the magazine ScienceThe team said it could finally provide quantitative measurements of the magnetic field strength that develops immediately after the flash ignites.

Continuing his research, the team’s latest analysis combined numerical simulations performed at the Center for Astrophysics | Harvard and the Smithsonian (CfA) with EOVSA spectral observations and multiwave data – covering radio waves to X-rays – were collected from a solar flare of size X8.2. The outbreak is the second largest in the last 11-year solar cycle, which occurred during the rapid release of coronal mass (CME), which caused a large-scale impact in the upper solar corona.

Among the surprises of the study, the researchers found that the measured magnetic field profile along the current flash sheet has closely matched predictions from numerical team simulations based on the well-known theoretical model for explaining solar flare physics first proposed in the 1990s with analytical form.

“We were surprised that the measured magnetic field profile of the current sheet is in line with the theoretical forecast made a decade ago,” Chen said.

“The strength of the Sun’s magnetic field plays a key role in accelerating plasma during an eruption. Our model was used to calculate the physics of magnetic forces during this eruption, which is a strongly twisted” rope “of magnetic field lines, or magnetic flux,” said Katie Reeves. , astrophysicist CfA and co-author of the study. “It is noteworthy that this complex process can be captured by a simple analytical model, and that the predicted and measured magnetic fields correspond so well.”

Simulations performed by Chenhai Sheng in CfA were developed to numerically solve control equations to quantify the behavior of electrically conductive plasma throughout the magnetic field of the flash. Using advanced computing techniques known as “adaptive grid enhancement,” the team was able to solve a thin thin recovery sheet and capture its detailed physics on ultra-thin spatial scales below 100 kilometers.

“Our simulation results match both the theoretical prediction of the magnetic field configuration during a solar eruption and reproduce a set of observable features of this particular flash, including magnetic force and plasma inflow / outflow around the recovery sheet,” Shen said.

Shocking measurements

The measurement results and the corresponding team simulation results showed that the flash current has an electric field that produces a shocking 4,000 volts per meter. Such a strong electric field is present in an area of ​​40,000 kilometers, greater than the length of the three Earths, located next to each other.

The analysis also showed that a huge amount of magnetic energy is pumped into the current sheet at an estimated rate of 10-100 billion trillion (1022-1023) joules per second – that is, the amount of energy processed in the flash engine during each second is equivalent to the total energy released by the explosion of about one hundred thousand of the most powerful hydrogen bombs (50-megaton class) at the same time.

“Such a huge emission of energy on the current sheet is impressive. The strong electric field generated there can easily accelerate the electrons to relativistic energies, but the unexpected fact we found was that the electric field profile in the current area of ​​the sheet did not match the spatial distribution of relativistic electrons that we measured, “Chen said. “In other words, we had to play something else to speed up or redirect these electrons. What our data showed was a special location at the bottom of the current sheet – a magnetic bottle – crucial for obtaining or limiting relativistic electrons.”

“Although there seems to be a place on the current sheet where energy is released to make the ball roll, most of the acceleration of the electrons takes place in this other place, the magnetic bottle. … Similar magnetic bottles are designed to limit and accelerate particles in some laboratory thermonuclear reactors “. Gary added. “Others have suggested such a structure in solar flares before, but we can really see it in numbers now.”

Approximately 99% of the relativistic flash electrons were observed to gather near the magnetic bottle for the entire duration of the radiation for five minutes.

Currently, Chen says the group will be able to use these new measurements as a comparative baseline for studying other solar flares, as well as to study the exact mechanism that accelerates particles, combining new observations, numerical simulations and advanced theories. Due to EOVSA’s breakthrough capabilities, NJIT was recently selected to participate in the NASA / NSF DRIVE Joint Solar Flash Energy Research Center (SolFER).

“Our goal is to develop a full understanding of solar flares, starting with their initiation, until they finally spray high-energy particles into the solar wind, and finally into the Earth’s space environment,” said Jim Drake, a professor of physics at the University of Maryland. principal investigator SolFER, who did not participate in this study. These first observations already suggest that relativistic electrons can be trapped in a large magnetic bottle formed when the corona’s magnetic fields are “reconnected” to release their energy. EOVSA observations will continue to help us figure out how the magnetic field moves. energy electrons “.

“Further study of the role of the magnetic bottle in the acceleration and transport of particles will require more advanced modeling for comparison with EOVSA observations,” Chen said. “We certainly have great prospects for studying these issues that relate to these key issues.”

Measuring the structure of a giant solar flare

More information:
Bin Chen et al. Measurement of the magnetic field and relativistic electrons along the solar flare current sheet, Nature astronomy (2020). DOI: 10.1038 / s41550-020-1147-7

Provided by the New Jersey Institute of Technology

Citation: Researchers offer an unprecedented look at the “central engine” that powers the solar flare (2020, July 27), obtained on July 29, 2020 from https://phys.org/news/2020-07-unprecedented-central-powering-solar- flare.html

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