Recent analysis of data from NASA’s Juno spacecraft reveals that Jupiter’s bow shocks not only deflect the solar wind but also serve as potent particle accelerators, propelling electrons to relativistic energies of at least 1 MeV.
As celestial bodies traverse streams of charged particles, their magnetic fields act as barriers. This interaction results in incoming particles being slowed and redirected, creating a ‘bow shock.’ Just beyond this boundary lies the foreshock, a dynamic area where magnetic forces can accelerate particles to speeds approaching that of light. Image credit: Ben C. Smith, Johns Hopkins Applied Physics Laboratory.
A shock is a disturbance caused by an object moving faster than the local speed of sound through a medium, leading to a sudden change in pressure at the interface.
Jupiter’s bow shock, for instance, arises where the planet’s magnetic field interacts with the solar wind, similar to how a ship’s bow creates waves in the water.
Most shocks in space plasma are collisionless due to low particle density, which prevents direct collisions and energy transfer through heat. Instead, electromagnetic forces govern these interactions.
Collisionless shocks are believed to be key sites where cosmic rays are accelerated to near-light speeds in a process termed relativistic electron acceleration.
Despite this understanding, scientists have faced challenges in directly observing and confirming the mechanisms behind these structures.
“Since the discovery of cosmic rays over a century ago, astronomers have been tracing their origins,” stated Dr. Savas Raptis from Johns Hopkins University Applied Physics Laboratory and his colleagues.
“These high-energy particles originate from various sources, including supernovae and solar eruptions.”
“When solar cosmic rays interact with Earth, they can induce space weather impacts that disrupt satellites, communications, and electricity grids.”
“The NASA mission illustrated how some electrons attain high energy levels in regions near Earth known as foreshocks, where solar particles first encounter Earth’s magnetic field.”
“Scientists have long suspected that this same acceleration process occurs in the foreshocks of other planetary bodies and astrophysical systems, but confirmation has been elusive until now.”
The research team analyzed data gathered by Juno during its approach to Jupiter on October 1, 2023.
Before passing through the bow shock, Juno traversed a foreshock, a turbulent area where the solar wind initially senses the planet’s magnetic field.
Over approximately 20 minutes, Juno detected a significant bubble-like disturbance identified as a foreshock transient.
The spacecraft employed three onboard instruments to measure electrons accelerated to energies reaching up to 1 MeV within this transient structure.
“By utilizing these observations and supplementary data from our solar system, we propose a universal scaling law for the Hyras limit, empirically linking the size of observable transients to maximum particle energy,” the authors concluded.
“Applying this model across diverse environments, from planetary bow shocks to protostellar jets and supernova remnants, suggests a range for maximum achievable particle energies from the MeV scale to tens of GeV and TeV, offering a method for constraining maximum cosmic ray energies in astrophysical shocks.”
The team’s paper was published in the journal Nature on June 3, 2026.
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S. Raptis et al. 2026. Relativistic electron acceleration in Jupiter’s bow shock and beyond. Nature 654, 47-51; doi: 10.1038/s41586-026-10473-z
Source: www.sci.news
