The spectacularly colored Northern Lights – or Northern Lights – that fill the skies in high latitude regions have fascinated people for thousands of years. Now, a team of scientists has solved one of the last mysteries surrounding its origin.
Scientists know that electrons and other energized particles that emanate from the sun as part of the “solar wind” travel down the Earth’s magnetic field lines and enter the upper atmosphere, where they collide with oxygen molecules. and nitrogen, propelling them into an excited state. These molecules then relax by emitting light, producing the beautiful green and red hues of dawn.
What has not been fully understood is precisely how groups of electrons accelerate through the magnetic field on the last leg of their journey, reaching speeds of up to 45 million mph. In a study published today in the journal Nature Communication, this question is answered by physicists from UCLA, Wheaton College, the University of Iowa, and the Space Science Institute.
One popular theory has been that electrons hitchhike on Alfvén waves – a type of electromagnetic wave that spacecraft have frequently identified traveling to Earth along magnetic field lines above the aurora. While space research has strongly supported the theory, limitations inherent in spacecraft measurements have prevented a definitive test.
To overcome these limitations, physicists conducted laboratory experiments on the Large Plasma Device at UCLA’s Basic Plasma Science Facility, a national collaborative research site jointly supported by the US Department of Energy and the National Science Foundation.
After reproducing conditions that mimicked those of Earth’s auroral magnetosphere, the team used specially designed instruments to launch Alfven waves into the 20-meter-long chamber of the plasma apparatus. Since the Alfvén waves are believed to collect only a small portion of the electrons in the plasma of space, physicists focused on determining whether there were any electrons that appeared to travel at a speed comparable to the electric field. waves.
“This difficult experiment required a measurement of the very small population of electrons moving through the chamber at almost the same speed as the Alfvén waves, counting less than one in a thousand electrons in the plasma,” said Troy Carter, professor of physics and director of UCLA Plasma Science and Technology Institute.
“The measurements revealed that this small population of electrons undergoes ‘resonant acceleration’ by the electric field of the Alfvén wave, similar to a surfer catching a wave and being continuously accelerated as the surfer moves with the wave.” said Gregory Howes, associate professor of physics at the University of Iowa.
Howes noted that these Alfvén waves appear as a result of geomagnetic storms, spatial phenomena triggered by violent events on the sun, such as solar flares and coronal mass ejections. These storms can cause what is called a “magnetic reconnection” in the Earth’s magnetic field, in which magnetic field lines are stretched like rubber bands, break, and then reconnect. These offsets launch Alfven waves along the lines towards Earth.
And because the regions of magnetic reconnection move during a storm, the Alfvén waves – and the surf electrons that accompany them – travel along different field lines during this period, ultimately leading to the twinkling glow of the aurora light curtains, Carter said.
In physics, electrons riding a wave’s electric field is a phenomenon known as Landau damping, in which the energy of the wave is transferred to accelerated particles. As part of their research, the team used an innovative analytical technique that combined measurements of the electric field of Alfvén waves and electrons to generate a unique signature of the electron acceleration by Landau damping. Using numerical simulations and mathematical modeling, the researchers demonstrated that the signature of the acceleration measured in the experiment matched the predicted signature for the Landau damping.
The agreement of experiment, simulation and modeling provides the first direct test showing that Alfven waves can produce accelerated electrons that cause auroras, Carter said.
“This experimental confirmation of the physics behind the aurora is due to the persistent ingenuity of research groups at the University of Iowa and UCLA,” said Vyacheslav (Slava) Lukin, director of the physics program of the plasmas at the National Science Foundation, which was not involved in the research. “From supporting students through an NSF Graduate Research Fellowship, to the NSF CAREER program for early career faculty, to the 25-year partnership between NSF and the Department of Energy which has enabled the unique capabilities of the Basic Plasma Science Facility, it is a success story of discovery made possible through the continued support of the academic research community. “