An advance in the understanding of cosmic particle accelerators

These shock waves are one of nature’s most powerful particle accelerators and have long intrigued scientists because of the role they play in producing cosmic rays — high-energy particles that travel vast distances in space.

The research, published today in Nature Communicationscombines satellite observations from NASA’s MMS (Magnetospheric Multiscale) and THEMIS/ARTEMIS missions with recent theoretical advances, offering a comprehensive new model to explain electron acceleration in collisionless environments.

Paper, ‘Discovery of an unexpectedly low electron injection threshold via enhanced shock acceleration’, it was written by a team of international academics, led by dr. Savvas Raptis from the Applied Physics Laboratory of the Johns Hopkins University, in the USA, and in collaboration with dr. Ahmad Lalti from Northumbria University.

This research solves a long-standing puzzle in astrophysics — how electrons reach extremely high, or relativistic, energy levels.

For decades, scientists have been trying to answer a key question in space physics: what processes allow electrons to be accelerated to relativistic speeds?

The main mechanism to explain the acceleration of electrons to relativistic energies is called Fermi acceleration or Diffusive Shock Acceleration (DSA). However, this mechanism requires that the electrons are initially powered up to a certain threshold energy before DSA can effectively accelerate them. Trying to solve how the electrons achieve this initial energy is known as the ‘injection problem’.

This new study provides key insights into the electron injection problem, showing that electrons can be accelerated to high energies through the interplay of different multilevel processes.

Using real-time data from the MMS mission, which measures the interaction of the 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, December 17, 2017.

During this event, electrons in Earth’s front shock region — the region where the solar wind is previously disturbed by its interaction with the bow shock — reached unprecedented energy levels, exceeding 500 keV.

This is a surprising result considering that the electrons observed in the forward shock region are usually found at energies ~1 keV.

This research suggests that these high-energy electrons are generated by a complex interplay of multiple acceleration mechanisms, including electron interaction with various plasma waves, transient structures in the front shock, and the Earth’s bow shock.

All these mechanisms work together to accelerate electrons from low energies ~ 1 keV to relativistic energies reaching the observed 500 keV, resulting in a particularly efficient electron acceleration process.

By refining the shock acceleration model, this study provides new insight into the operation of space plasma and the fundamental processes that govern energy transfer in space.

As a result, the research opens new avenues for understanding the creation of cosmic rays and offers insight into how phenomena within our solar system can lead us to understand astrophysical processes in the universe.

dr. Raptis believes that studying phenomena at different levels is essential to understanding nature. “Most of our research focuses on either small-scale effects, such as wave-particle interactions, or large-scale properties, such as the effects of the solar wind,” he says.

“However, as we have shown in this paper, by combining phenomena at different levels, we were able to observe their interplay that ultimately energizes particles in space.”

dr. Ahmad Lalti added: “One of the most effective ways to deepen our understanding of the universe we live in is to use our near-Earth plasma environment as a natural laboratory.

“In this paper, we use in situ observations from MMS and THEMIS/ARTEMIS to show how different fundamental plasma processes at different levels act in concert to drive electrons from low energies to high relativistic energies.

“These fundamental processes are not limited to our solar system and are expected to occur throughout the universe.

“This makes our proposed framework relevant to a better understanding of electron acceleration to cosmic-ray energies in astrophysical structures light-years away from our solar system, such as other star systems, supernova remnants, and active galactic nuclei.”