Why in the News?

A recent study published by Johns Hopkins University (USA) and Northumbria University (UK) reveals how Collisionless Shock Waves act as cosmic accelerators, helping subatomic particles gain ultra-high energy and travel vast distances in space.

What are Collisionless Shock Waves?

• Collisionless shock waves are disturbances in plasma (ionized gas) where energy transfer occurs without direct particle collisions, relying instead on electromagnetic forces.
• They are found in supernova remnants, black hole disks, pulsars, magnetars, and planetary magnetospheres.
• They act as natural cosmic accelerators, boosting electrons and other charged particles to ultra-high speeds.

Key Findings from the Study

• NASA’s MMS, THEMIS, and ARTEMIS missions observed an electron acceleration event near Earth’s bow shock on December 17, 2017.
• Electrons in Earth’s foreshock region gained 500 keV of energy, reaching 86% of the speed of light, a huge increase from their usual 1 keV.
• Diffusive shock acceleration (known for producing high-energy cosmic rays) requires electrons to already be moving at 50% of light speed before further acceleration can occur.
• The study identifies how electrons receive this initial boost, a long-standing astrophysical mystery.
• Scientists have long assumed that supernova explosions are the primary source of cosmic rays.
• The recent study suggests that planetary magnetospheres interacting with stellar winds could also contribute to high-energy cosmic rays.

How Shock Waves accelerate Particles without Collisions?

• Unlike in solids, liquids, or gases, where energy is transferred via molecular collisions, plasma particles interact through electromagnetic fields.
• This allows shock waves to accelerate electrons without direct contact.
• Multi-Stage Acceleration Process:
  1. Plasma waves interact with electrons, imparting initial energy.
  2. Magnetic turbulence in the shock front causes electrons to spiral, further increasing their speed.
  3. Repeated interactions with plasma waves push electrons to relativistic speeds.
• Role of Earth’s Bow Shock & Foreshock:

• • When the solar wind collides with Earth’s magnetosphere, it forms a shock wave.
  • The foreshock region ahead of this wave is highly turbulent, enabling efficient electron acceleration.

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