Take an unprecedented look at the “core engine” fueling a massive solar flare

Extreme UV sun flare

Observation of a large solar flare on September 10, 2017 in extreme ultraviolet (grayscale background, by the NASA Solar Dynamics Observatory) and microwaves (red to blue indicate increasing frequencies, observed by the Owens Valley solar matrix expanded). The light orange curves are selected magnetic field lines from the corresponding theoretical theoretical eruptive solar flare model. The eruption is driven by the eruption of a twisted magnetic flux string (illustrated by a set of color curves). Microwave sources are observed throughout the central region where a large-scale reconnect current sheet, the torch’s “core engine,” is found and used to measure its physical properties. Credit: CSTR / NJIT, B. Chen, S. Yu; NASA Solar Dynamics Observatory

An international research team has unveiled a new appearance within the “central engine” of a solar flare accompanied by an eruption, revealing a huge “sheet” of electrical current, offering the first measurements that characterize the magnetic field.

In a study published in Astronomy of nature, an international team of researchers has presented a new detailed look inside the “central engine” of a large solar flare accompanied by a powerful eruption first captured on September 10, 2017 by the Owens Valley Solar Array (EOVSA), a radio solar telescope installation operated by New Jersey Institute of Technology‘s (NJIT) Solar-Terrestrial Research Center (CSTR).

The new findings, based on EOVSA’s observations of the event at microwave wavelengths, offer the first measurements that characterize the magnetic fields and particles at the heart of the blast. The results have revealed a huge “sheet” of electrical current that stretches over 40,000 kilometers through the core-burning region where opposing magnetic field lines approach, break and reconnect, generating the intense energy that powers the flare.

In particular, the team’s measurements also indicate a bottle-like magnetic structure located on top of the torch’s looped base (known as the torch arcade) at a height of nearly 20,000 kilometers above the surface. from the Sun. The structure, the team suggests, is probably the main site where the highly energetic electrons in the flare get trapped and accelerated at almost the speed of light.

Researchers say new insight from the core engine study that powers such powerful eruptions may help future predictions of space weather for potentially catastrophic energy releases from solar flares, the most powerful explosions in the solar system, capable of severely disrupting technologies on Earth, such as satellite operations, GPS navigation and communication systems, among many others.

“One of the main goals of this research is to better understand the fundamental physics of solar flares,” said Bin Chen, lead author of the article and professor of physics at NJIT. “It has long been suggested that the sudden release of magnetic energy through the reconnect current sheet is responsible for these large eruptions, however their magnetic properties have not been measured.” With this study we have finally measured the details of the magnetic field of a current leaf for the first time, giving us a new understanding of the central motor of the main flares of the Sun “.

“The place where all the energy is stored and released in solar flares has been invisible until now. … To play a term in cosmology, it’s the Sun’s “dark energy problem,” and we’ve previously had to indirectly infer that the flare’s magnetic reconnect sheet existed, “said Dale Gary, director of EOVSA at NJIT and co-author of the paper. “EOVSA images taken at many microwave frequencies showed that we can capture radio emissions to illuminate this important region. Once we got that data and the analysis tools created by co-authors Gregory Fleishman and Gelu Nita, we were able to start analyzing radiation to allow for these measurements. “

Earlier this year in the journal Science, the team reported that they could finally provide quantitative measurements of the intensity of the evolving magnetic field directly after the torch is lit.

Continuing with their research, the team’s latest analysis combined numerical simulations conducted at the Center for Astrophysics | Harvard and Smithsonian (CfA) with observations from EOVSA spectral images and multiple wavelength data, spanning radio waves to X-rays, collected from X8.2 size solar flare. The eruption is the second largest in the solar cycle in the past 11 years, and occurred with a rapid coronal mass ejection (CME) that caused a large-scale shock to the upper solar corona.

Among the study’s surprises, the researchers found that the measured profile of the magnetic field along the current flare sheet characteristic closely matched the predictions from the team’s numerical simulations, which were based on a well-known theoretical model. to explain the physics of solar flare, first proposed in the 1990s in an analytical way.

“We were surprised that the measured profile of the magnetic field on the current sheet beautifully matched the theoretical prediction made decades ago,” Chen said.

“The strength of the Sun’s magnetic field plays a key role in acceleration plasma during an eruption Our model was used to calculate the physics of magnetic forces during this eruption, which manifests as a highly twisted ‘string’ of magnetic field lines or magnetic flux string, ”explained Kathy Reeves, CfA astrophysicist and co-author. of the study. . “It is remarkable that this complicated process can be captured by a direct analytical model, and that the predicted and measured magnetic fields match so well.”

The simulations, performed by Chengcai Shen at CfA, were developed to numerically solve governing equations to quantify the behavior of the electrically conductive plasma through the torch’s magnetic field. By applying an advanced computational technique known as “adaptive mesh refinement,” the team was able to resolve the thin reconnect current sheet and capture its detailed physics on superfine spatial scales below 100 kilometers.

“Our simulation results match the theoretical prediction about the configuration of the magnetic field during a solar flare and reproduce a set of observable characteristics of this particular flare, including the magnetic force and plasma inputs / outputs around the current sheet reconnection, “Shen noted.

Shocking measurements

The team’s measurements and matching simulation results revealed that the current torch blade has an electric field that produces a shocking 4,000 volts per meter. Such a strong electric field is present in a region of 40,000 kilometers, greater than the length of three Earths placed side by side.

The analysis also showed that a large amount of magnetic energy is pumped to the current sheet at an estimated rate of 10-100 billion trillion (1022-1023) joules per second, that is, the amount of energy being processed in The torch motor, within every second, is equivalent to the total energy released by the explosion of approximately one hundred thousand of the most powerful hydrogen bombs (50 megaton class) at the same time.

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

“While the current blade appears to be the place where energy is released to roll the ball, most of the electron acceleration appears to be occurring at this other location, the magnetic bottle. … Similar magnetic bottles are being developed to confine and accelerate particles in some laboratory fusion reactors. “Gary added.” Others have proposed such a structure in solar flares before, but we can really see it now in numbers. ” .

Approximately 99% of the relativistic electrons in the flare were observed to congregate in the magnetic bottle throughout the five-minute emission.

For now, Chen says the group will be able to apply these new measurements as a comparative baseline to study other solar flare events, as well as explore the exact mechanism that speeds up particles by combining the new observations, numerical simulations, and advanced theories. Due to EOVSA’s innovative capabilities, NJIT was recently selected to participate in a joint meeting POT/ NSF DRIVE Science Center Collaboration in releasing solar flare energy (SolFER).

“Our goal is to develop a comprehensive understanding of solar flares, from their inception until they finally spray highly energized particles into the solar wind and eventually into Earth’s space environment,” said Jim Drake, professor of physics at the University from Maryland and SolFER principal investigator who was not involved in this study. “These early observations already suggest that relativistic electrons could be trapped in a large magnetic bottle produced as the corona’s magnetic fields are ‘reconnected’ to release their energy. … EOVSA’s observations will continue to help us unravel how the magnetic field drives these energetic electrons. “

“Further investigation of the role of the magnetic bottle in acceleration and particle transport will require more advanced modeling to compare with EOVSA’s observations,” Chen said. “Certainly, there are great prospects for us to study that address these fundamental questions.”

Reference: “Relativistic Electron and Magnetic Field Measurement Along a Solar Flare Stream Sheet” by Bin Chen, Chengcai Shen, Dale E. Gary, Katharine K. Reeves, Gregory D. Fleishman, Sijie Yu, Fan Guo, Säm Krucker, Jun Lin, Gelu M. Nita and Xiangliang Kong, July 27, 2020, Astronomy of nature.
DOI: 10.1038 / s41550-020-1147-7