A cosmic ray, nicknamed ‘Amaterasu,’ seems to have reached Earth from a seemingly empty region of the universe.
How much energy did Amaterasu have?
- Amaterasu cosmic ray had an energy of 240 exa-electron-volt (EeV).
- The electron-volt (eV) is a unit of energy, like joules, used to measure the energy of subatomic particles.
- Light particles in sunlight have an energy of about 1.6-3.1 eV.
- In fusion, when one deuterium nucleus and one tritium nucleus combine, they release one helium atom, one neutron, and 17.6 million eV of energy.
- The mass-energy of a single Higgs boson particle, considered ‘heavy,’ is 125.1 billion eV.
- Cosmic rays typically span an energy range from about one billion eV to approximately 100 billion billion eV.
- The Amaterasu cosmic ray stands out with an energy of 240 EeV, equivalent to 240 billion billion eV, making it exceptionally high.
What are Cosmic rays?
- Cosmic rays are streams of energetic particles and clusters of particles coming from outer space and the sun.
- They include protons and alpha particles (nuclei of helium atoms).
- Only low-intensity cosmic rays reach the earth’s surface.
- Only low-intensity cosmic rays reach Earth’s surface, losing most of their energy in the atmosphere through collisions with gas atoms.
- As they collide with atmospheric gas atoms, cosmic rays produce a shower of other particles.
- The sources of cosmic rays and the reason for their high energy levels remain a mystery.
What information Cosmic-ray energies provide?
- Ultra-high-energy cosmic rays (UHECRs) are subatomic particles from extragalactic sources with energies greater than 1 EeV.
- While scientists have observed UHECRs with energies surpassing 100 EeV, those with energies beyond approximately 60 EeV generally don’t persist over long distances in space.
- This is because of the cosmic microwave background (CMB), which is radiation in the microwave frequency leftover from the Big Bang, that now permeates the universe.
- This background radiation suppresses the flux of UHECRs above 60 EeV.
- The suppression of UHECRs increases the longer they travel through the CMB.
- Consequently, any observed UHECRs on Earth must have originated from a distance where this suppression was not complete.
- Scientists estimate this distance to be between 50 and 100 megaparsecs, equivalent to 1,500-3,000 billion billion kilometers.
- Moving near the speed of light, a cosmic ray will require 3-10 million years to travel this distance.
Types of cosmic rays:
- Cosmic rays can be categorized into two types: galactic cosmic rays (GCR), originating from beyond our solar system, and solar cosmic rays, high-energy particles emitted by the sun, primarily consisting of protons.
Solar cosmic rays:
- Solar cosmic rays, primarily originating from solar flares, are also referred to as solar energetic particles.
- Scientists, through tracking cosmic rays, have determined that the mass ratio of helium to hydrogen nuclei (representing the total masses of hydrogen and helium) is approximately 28:100.
- This implies that there are about 28 grams of alpha particles for every 100 grams of protons in cosmic rays.
- Notably, this ratio closely mirrors the abundance of helium and hydrogen in the early universe.
Galactic cosmic rays:
- GCRs are slowly changing streams of high-energy particles that constantly strike the earth.
- They are thought to originate outside the solar system in events such as supernovae.
- A supernova is an explosion that occurs when a massive star nears the end of its life after running out of matter that it can fuse.
- While approximately 89% of GCRs consist of hydrogen nuclei, the remaining portion includes the nuclei of all elements, extending down to trace amounts of uranium.
- These nuclei are fully ionized, indicating that all of their electrons have been stripped away.
- Consequently, these particles interact with and are influenced by magnetic fields.
- The strong magnetic fields of the sun have the effect of altering the energy levels of GCRs that reach Earth.
- When cosmic ray particles reach the earth’s atmosphere, they ionise air molecules that are at least about 3 km above the surface.
Beyond that, they will have lost most of their energy
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