Explicant el misteri darrere de la reconnexió magnètica ràpida

Erupcions solars i ejeccions de massa coronal

Les erupcions solars i les ejeccions de massa coronal al sol són causades per la “reconnexió magnètica”, quan les línies de camp magnètic de direccions oposades es fusionen, s’uneixen i es separen, creant explosions que alliberen quantitats massives d’energia. Crèdit: NASA Conceptual Image Laboratory

Els investigadors identifiquen la física que permet explosions magnètiques ràpides a l’espai.

Quan les línies de camp magnètic de direccions oposades es fusionen, creen explosions que poden alliberar enormes quantitats d’energia. La fusió de línies de camp oposades al sol crea erupcions solars i ejeccions de massa coronal, que són explosions massives d’energia que poden viatjar a la Terra en menys d’un dia.

Tot i que la mecànica general de la reconnexió magnètica s’entén bé, els investigadors han lluitat durant més de mig segle per explicar la física precisa darrere de l’alliberament ràpid d’energia que es produeix.

Un nou estudi de recerca de Dartmouth publicat ahir (28 d’abril de 2022) a la revista Física de les comunicacions proporciona la primera descripció teòrica de com un fenomen conegut com a “efecte Hall” determina l’eficiència de la reconnexió magnètica.

Diagrama de reconnexió magnètica

La reconnexió magnètica es produeix quan les línies de camp magnètic de direccions oposades es fusionen, s’uneixen i es separen, alliberant quantitats massives d’energia per escalfar plasmes i impulsar fluxos de sortida d’alta velocitat. Crèdit: Yi-Hsin Liu/Dartmouth College

“La velocitat a la qual es tornen a connectar les línies de camp magnètic és d’extrema importància per als processos a l’espai que poden afectar la Terra”, va dir Yi-Hsin Liu, professor ajudant de física i astronomia a Dartmouth. “Després de dècades d’esforç, ara tenim una teoria completa per abordar aquest problema de llarga data”.

La reconnexió magnètica existeix a tota la natura en els plasmes, el quart estat de la matèria que omple la major part de l’univers visible. La reconnexió té lloc quan les línies de camp magnètic de direccions oposades es dibuixen entre si, es trenquen, es reuneixen i després s’allunyen violentament.

En el cas de la reconnexió magnètica, el trencament de les línies magnètiques força a sortir magnetitzat[{” 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

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.