Astronomers at NASA have discovered high-speed electrons that reside outside the Earth’s magnetic field which somehow get super-accelerated once they fall into a turbulent region called the foreshock. High above the surface, Earth’s magnetic field constantly deflects incoming supersonic particles from the sun, but these particles are disturbed in regions just outside the magnetic field and some that get reflected into the foreshock region accelerate almost at the speed of light.
Observation from from NASA’s THEMIS (Time History of Events and Macroscale Interactions during Substorms) mission revealed that this turbulent region is responsible for the creation of these ‘Megafast Electrons’. According to NASA, these extremely fast particles have been observed in near-Earth space and many other places in the universe, but the mechanisms that make the electrons accelerate have not been understood. The new results provide the first steps towards an answer, while opening up more questions, NASA says, adding that this strange discovery could change particle physics forever.
The image below represents one of the traditional proposed mechanisms for accelerating particles across a shock, called a shock drift acceleration. The electrons (shown in yellow) and protons ( in blue) can be seen moving in the collision area where two hot plasma bubbles collide (red vertical line). The cyan arrows represent the magnetic field and the light green arrows, the electric field.
Now that it has been discovered that electrons can be accelerated to extremely high speeds in a near-Earth region farther from Earth than previously thought possible, it opens up a whole new set of questions about what causes the acceleration. NASA thinks the findings may change the accepted theories on how electrons can be accelerated not only in shocks near Earth, but also throughout the universe. And having a better understanding of how particles are energized will help scientists and engineers in many ways.
“This affects pretty much every field that deals with high-energy particles, from studies of cosmic rays to solar flares and coronal mass ejections, which have the potential to damage satellites and affect astronauts on expeditions to Mars,” said Lynn Wilson, lead author of the paper.
The results of the study, published in the journal Physical Review Letters, describe how such particles may get accelerated in specific regions just beyond Earth’s magnetic field. When these particles stream toward Earth, they encounter a boundary region, the outermost layer of the magnetosphere, known as the bow shock and the magnetic field in the bow shock slows the particles causing most of them to be deflected away from earth. However some of these particles are reflected back towards the sun, forming a band of highly energetic, fast-moving electrons and ions just outside the magnetosphere called the foreshock region.
Previously, it was assumed that one way the electrons get to such high energies is by bouncing back and forth across the bow shock, and each time they collided with our magnetic field, they gained a little extra energy. However, the new observations from the THEMIS satellites have revealed that these particles can also gain energy through electromagnetic activity in the foreshock region itself, which no scientist can explain how.
“This is a puzzling case because we’re seeing energetic electrons where we don’t think they should be, and no model fits them,” said David Sibeck, co-author and THEMIS project scientist at NASA Goddard. “There is a gap in our knowledge, something basic is missing.”
If the electrons had originated from the bow shock and got accelerated in it, researchers say the electrons would have preferred movement direction and location in line with the magnetic field and moving away from the bow shock in a small, specific region. However, they found that the electrons were moving in all directions, not just along magnetic field lines and the bow shock could only produce energies at roughly one tenth of the observed electrons’ energies. Instead, the cause of the electrons’ acceleration was found to be from within the foreshock boundary region itself.
“It seems to suggest that incredibly small-scale things are doing this because the large-scale stuff can’t explain it,” Wilson said.
“High-energy particles have been observed in the foreshock region for more than 50 years, but until now, no one had seen the high-energy electrons originate from within the foreshock region, NASA says. “This is partially due to the short timescale on which the electrons are accelerated, as previous observations had averaged over several minutes, which may have hidden any event. THEMIS gathers observations much more quickly, making it uniquely able to see the particles.”
The team aims to gather more observations from THEMIS to understand the mechanism behind the acceleration of electrons at such incredible speeds.
- Source: NASA
- Reference: Relativistic Electrons Produced by Foreshock Disturbances Observed Upstream of Earth’s Bow Shock – Physical Review Letters