Sunday, January 15, 2023

3828 - COSMIC RAYS - where do they come from?

 

     -  3828  -   COSMIC  RAYS  -  where do they come from?    Cosmic rays produce extensive particle showers that send a cascade of electrons, photons, and muons to Earth's surface.

           


            ---------  3828  -   COSMIC  RAYS  -  where do they come from?

            -    After the discovery of radiation by French physicist Henri Becquerel in 1896, scientists believed atmospheric ionization (where an electron is stripped from an air molecule) occurred only from radioactive elements found in ground rocks or from radioactive gases.

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            -    Austrian physicist Victor Hess found an additional source in 1912, when he strapped three electrometers into a balloon and measured atmospheric radiation at an altitude of about 15,000 feet during a total solar eclipse.

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            -    Physicists initially believed cosmic rays were gamma rays, high-energy radiation produced by radioactive decay. During the 1930s, however, experiments revealed that cosmic rays are mostly charged particles.

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            -    In 1937, French physicist Pierre Auger (1899–1993) found that extensive particle showers occur when cosmic rays collide with particles high in the atmosphere, producing a cascade of electrons, positrons, photons, muons (particles similar to electrons but 200 times as massive), and other particles that reach Earth’s surface.

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            -    Auger found an ionization rate about four times greater than at ground level. Hess could explain the variant observations only if a powerful source of radiation were penetrating the atmosphere from above. Much later, in 1936, Hess received the Nobel Prize in physics for the discovery of what we now call cosmic rays.

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            -    Cosmic rays traveling at speeds approaching that of light constantly pelt Earth’s upper atmosphere from the depths of space, creating high-energy collisions that dwarf those produced in even the most powerful particle colliders. The atmospheric crashes rain down gigantic showers of secondary particles to the surface of our planet.

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            -    But despite being discovered more than a century ago, physicists still don’t know where cosmic rays come from. The short answer to why we can’t trace cosmic rays back to their source: magnetic fields.   Charged cosmic-ray particles are redirected by the magnetic fields they pass through on their long journey through space.

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            -    One thing we certainly do know about cosmic rays is that they are comprised of extremely energetic charged particles — like protons, alpha particles, and atomic nuclei like helium and iron, with miniscule proportions of antiparticles.

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            -    The average energy of a solar photon is approximately 1.4 electron volts (eV). For reference, a flying mosquito has an energy of about 1 trillion eV, or 1x10^12 eV, but a mosquito is also much, much larger than a single particle. Meanwhile, an alpha particle emitted during the decay of Uranium-238 possess 4.27*10^6 eV of energy.  Compare that to a cosmic ray proton, which has an energy of some 1x10^20 eV.

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            -  This energy corresponds to that of a tennis ball smashed with a velocity of around 124 miles per hour. Only, the tennis ball 10^29 times heavier than the proton.

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            -   That means a proton can only reach [that] extreme, macroscopic energy by travelling at almost the speed of light. The universe must be able to accelerate particles to these energies, but we still do not know how it does it.

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            -    One of the best ways of accelerating particles is a shock front that occurs when a medium with a large velocity runs into a slower one, producing a shock— a sudden change in the properties of the medium.  In the case of the universe, the changed properties are velocity and density, and even magnetic fields. Luckily for the cosmic rays, the field becomes highly turbulent in that process. And the combination of a shock front with turbulence is a great particle accelerator.

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            -    But what could produce such a shock front? One likely suspect is supernovae. As a shell of shocked material blasted away from an exploding star, it hits the cool interstellar medium that lies between stars, almost like a cosmic tsunami. The phenomena of a travelling shock front can also be found in active galaxies, where huge plasma jet exist. 

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            -     It is ironic that science’s journey to discover the source of high-energy particles from space began in the upper atmosphere, and has since moved deep underground.

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            -    It was August 1912 when Austrian-American physicist Victor Hess began a series of flights to the upper atmosphere in a hydrogen-filled balloon equipped with an electroscope. His aim: to take measurements of ionizing radiation. At the time, it was widely believed that radiation from the Earth itself was responsible for this phenomenon of knocking electrons off atoms. Should this be the case, however ionization should be strongest near the planet’s surface.

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            -  That’s not what Hess found.   Hess discovered something startling. At a nosebleed-inducing altitude of 3.3 miles, ionization rates of the air were three times that measured at sea level. He concluded that the source of this ionization was not coming from below our feet, but instead from above. Further measurements made during a solar eclipse also showed the Sun wasn’t the source of this ionization radiation.

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            -    During the course of seven balloon trips, Hess discovered cosmic rays, confirmed and named by Robert Millikan in 1925 , coming from beyond our solar system.  To study these collisions caused by cosmic rays, particle physicists retreated below ground, employing increasingly monstrous particle accelerators to slam together particles in an attempt to replicate the collisions that cosmic rays spark in the upper atmosphere.

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            -    This quest has culminated with CERN’s Large Hadron Collider (LHC) with a 16-mile circumference deep beneath the French/Swiss border. Yet, despite its impressive size, power and utility, the LHC still can’t reach the energies produced by cosmic ray collisions.

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            -    The discovery of gravitational waves ,  ripples in space-time predicted by Einstein’s theory of general relativity,  has made a new form of astronomy possible, allowing us to investigate events and objects that we could never hope to observe in the electromagnetic spectrum alone.

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            -    This combination of electromagnetic or “traditional” astronomy and gravitational-wave detections (along with detecting neutrinos, which are ghost-like particles with virtually no mass or electric charge) is known as “multi-messenger astronomy”.

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            -    The gravitational-wave signal GW170817 came from the merger of two neutron stars and was observed in 2017. It was significant for both multi-messenger astronomy and identifying potential sources of cosmic rays. Not only did this violent merger become the first such event to be detected in both gravitational waves and electromagnetic radiation, but it also confirmed that the merger of compact stellar remnants can accelerate particles to great speeds, creating cosmic rays.

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            -    As multi-messenger astronomy is refined, observations such as this, as well as the information they unlock, should become more commonplace.  There is no other way than multi-messenger astronomy to understand the origin and impact of cosmic rays.

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            January 15, 2022        COSMIC  RAYS  -  where do they come from?           3828                                                                                                                             

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