| Jan 13, 2025 |
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(Nanowerk News) Scientists have come a step closer to understanding how collisionless shock waves – found throughout the universe – are able to accelerate particles to extreme speeds.
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These shock waves are one of nature’s most powerful particle accelerators and have long intrigued scientists for the role they play in producing cosmic rays – high-energy particles that travel across vast distances in space.
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The research, published in Nature Communications (“Revealing an Unexpectedly Low Electron Injection Threshold via Reinforced Shock Acceleration”), combines satellite observations from NASA’s MMS (Magnetospheric Multiscale) and THEMIS/ARTEMIS missions with recent theoretical advancements, offering a comprehensive new model to explain the acceleration of electrons in collisionless shock environments.
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| Composite image of the Tycho Supernova remnant. Shock waves from such explosive events are believed to be the main drivers behind cosmic rays. (Image: MPIA/NASA/Calar Alto Observatory)
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This research addresses a long-standing puzzle in astrophysics – how electrons reach extremely high, or relativistic, energy levels.
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For decades, scientists have been trying to answer a crucial question in space physics: What processes allow electrons to be accelerated to relativistic speeds?
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The main mechanism to explain acceleration of electrons to relativistic energies is called Fermi acceleration or Diffusive Shock Acceleration (DSA). However, this mechanism requires electrons to be initially energized to a specific threshold energy before getting efficiently accelerated by DSA. Trying to address how electrons achieve this initial energy is known as ‘the injection problem’.
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This new study provides key insights into the electron injection problem, showing that electrons can be accelerated to high energies through the interaction of various processes across multiple scales.
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Using real-time data from the MMS mission, which measures the interaction of Earth’s magnetosphere with the solar wind, and the THEMIS/ARTEMIS mission, which studies the upstream plasma environment near the Moon, the research team observed a large scale, time dependent (i.e. transient) phenomenon, upstream of Earth’s bow shock, on December 17, 2017.
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During this event, electrons in Earth’s foreshock region – an area where the solar wind is predisturbed by its interaction with the bow shock – reached unprecedented energy levels, surpassing 500 keV.
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This is a striking result given that electrons observed in the foreshock region are typically found at energies ~1 keV.
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This research suggests that these high-energy electrons were generated by the complex interplay of multiple acceleration mechanisms, including the interaction of electrons with various plasma waves, transient structures in the foreshock, and Earth’s bow shock.
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All of those mechanisms act together to accelerate electrons from low energies ~ 1keV up to relativistic energies reaching the observed 500 keV, resulting in a particularly efficient electron acceleration process.
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By refining the shock acceleration model, this study provides new insight into the workings of space plasmas and the fundamental processes that govern energy transfer in the universe.
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As a result, the research opens new pathways for understanding cosmic ray generation and offers a glimpse into how phenomena within our solar system can guide us to understand astrophysical processes throughout the Universe.
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Dr. Raptis believes that studying phenomena across different scales is crucial for understanding nature. “Most of our research focuses on either small-scale effects, like wave-particle interactions, or large-scale properties, like the influence of solar wind,” he says.
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“However, as we demonstrated in this work, by combining phenomena across different scales, we were able to observe their interplay that ultimately energize particles in space.”
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Dr Ahmad Lalti added: “One of the most effective ways to deepen our understanding of the universe we live in is by using our near-Earth plasma environment as a natural laboratory.
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“In this work, we use in-situ observation from MMS and THEMIS/ARTEMIS to show how different fundamental plasma processes at different scales work in concert to energize electrons from low energies up to high relativistic energies.
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“Those fundamental processes are not restricted to our solar system and are expected to occur across the universe.
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“This makes our proposed framework relevant for better understanding electron acceleration up to cosmic-ray energies at astrophysical structures light-years away from our solar system, such as at other stellar systems, supernovae remnants, and active galactic nuclei.”
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