Spiegare il mistero della rapida riconnessione magnetica

Le emissioni di brillamenti solari di massa coronarica sono causate dalla “riconnessione magnetica” quando le linee del campo magnetico di direzioni opposte si uniscono, si uniscono e si rompono, provocando esplosioni che rilasciano enormi quantità di energia. Prestito: Laboratorio di imaging concettuale della NASA

I ricercatori stanno scoprendo la fisica che consente rapide esplosioni magnetiche nello spazio.

Quando le linee in direzioni opposte di un campo magnetico si fondono, creano esplosioni che possono rilasciare enormi quantità di energia. La fusione di linee di campo opposte sul Sole provoca brillamenti solari – emissioni di massa della corona – enormi esplosioni di energia che possono viaggiare sulla Terra in meno di un giorno.

Sebbene il meccanismo generale della riconnessione magnetica sia ben compreso, i ricercatori hanno lottato per più di mezzo secolo per determinare l’esatta fisica dietro il rapido rilascio di energia.

Il nuovo studio di ricerca di Dartmouth è stato pubblicato ieri (28 aprile 2022) sulla rivista Fisica della comunicazione fornisce la prima descrizione teorica di come il fenomeno noto come “effetto Hall” determini l’efficacia della riconnessione magnetica.

Schema di riconnessione magnetica

La riconnessione magnetica si verifica quando linee di campi magnetici in direzioni opposte si fondono, si ricongiungono e si rompono, rilasciando enormi quantità di energia per riscaldare il plasma in modo che fuoriesca ad alte velocità. Prestito: Yi-Hsin Liu / Dartmouth College

“La velocità con cui le linee del campo magnetico si riconnettono sono estremamente importanti per i processi spaziali che potrebbero influenzare la Terra”, ha affermato Yi-Hsin Lu, assistente di fisica e astronomia di Dartmouth. “Dopo decenni di sforzi, ora abbiamo una teoria completa per risolvere questo problema di vecchia data”.

La riconnessione magnetica è presente in tutta la natura, nel plasma, il quarto stato della materia che riempie la maggior parte dell’universo visibile. La riconnessione si verifica quando le linee del campo magnetico in direzioni opposte vengono allungate, rotte, ricollegate e quindi gravemente strappate.

Nel caso di riconnessione magnetica, la rottura delle linee magnetiche fa sì che queste escano magnetizzate[{” attribute=””>plasma at high velocities. The energy is created and displaced to plasmas through a tension force like that which ejects objects from slingshots.

Hall Effect and Magnetic Reconnection

Around the region where reconnection occurs, the departure of the ion motion (blue streamlines in (a)) from the electron motion (red streamlines in (a)) gives rise to the “Hall effect,” which results in the electromagnetic energy transport pattern illustrated by yellow streamlines in (b). This transport pattern limits the energy conversion at the center, enabling fast reconnection. Credit: Yi-Hsin Liu/Dartmouth College

The Dartmouth research focused on the reconnection rate problem, the key component of magnetic reconnection that describes the speed of the action in which magnetic lines converge and pull apart.

Previous research found that the Hall Effect— the interaction between electric currents and the magnetic fields that surround them—creates the conditions for fast magnetic reconnection. But until now researchers were unable to explain the details of how exactly the Hall effect enhances the reconnection rate.

The Dartmouth theoretical study demonstrates that the Hall effect suppresses the conversion of energy from the magnetic field to plasma particles. This limits the amount of pressure at the point where they merge, forcing the magnetic field lines to curve and pinch, resulting in open outflow geometry needed to speed up the reconnection process.

Xiaocan Li, Yi-Hsin Liu, and Shan-Chang Lin

Dartmouth’s Xiaocan Li, postdoctoral researcher (left); Yi-Hsin Liu, Assistant Professor of Physics and Astronomy (center); Shan-Chang Lin, PhD candidate (right). Credit: Robert Gill/Dartmouth College

“This theory addresses the important puzzle of why and how the Hall effect makes reconnection so fast,” said Liu, who serves as deputy lead of the theory and modeling team for NASA’s Magnetospheric Multiscale Mission (MMS). “With this research, we also have explained the explosive magnetic energy release process that is fundamental and ubiquitous in natural plasmas.”

The new theory could further the technical understanding of solar flares and coronal mass ejection events that cause space weather and electrical disturbances on Earth. In addition to using the reconnection rate to estimate the time scales of solar flares, it can also be used to determine the intensity of geomagnetic substorms, and the interaction between the solar wind and Earth’s magnetosphere.

Yi-Hsin Liu

Yi-Hsin Liu, Assistant Professor of Physics and Astronomy, Dartmouth College. Credit: Robert Gill/Dartmouth College

The research team, funded by the National Science Foundation (NSF) and NASA, is working alongside NASA’s Magnetospheric Multiscale Mission to analyze magnetic reconnection in nature. Data from four satellites flying in tight formation around Earth’s magnetosphere as part of the NASA mission will be used to validate the Dartmouth theoretical finding.

“This work demonstrates that fundamental theory insights reinforced by modeling capabilities can advance scientific discovery,” said Vyacheslav Lukin, a program director for plasma physics at NSF. “The technological and societal implications of these results are intriguing as they can help predict impacts of space weather on the electrical grid, develop new energy sources, and explore novel space propulsion technologies.”

The new study can also inform reconnection studies in magnetically confined fusion devices and astrophysical plasmas near neutron stars and black holes. Although there is no current applied use, some researchers have considered the possibility of using magnetic reconnection in spacecraft thrusters.

Reference: “First-principles theory of the rate of magnetic reconnection in magnetospheric and solar plasmas” by Yi-Hsin Liu, Paul Cassak, Xiaocan Li, Michael Hesse, Shan-Chang Lin and Kevin Genestreti, 28 April 2022, Communications Physics.
DOI: 10.1038/s42005-022-00854-x

This work is funded by the NSF’s PHY and AGS Divisions, NASA’s Magnetospheric Multiscale (MMS) mission, and the U.S. Department of Energy.

Co-authors of the study are Paul Cassak, West Virginia University; Xiaocan Li, Dartmouth; Michael Hesse, NASA’s Ames Research Center; Shan-Chang Lin, Dartmouth; and Kevin Genestreti, Southwest Research Institute.

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