Cosmic ray astronomy

File:Observatorio pierre auger.jpg in Argentina is the world's largest cosmic ray observatory]]

Astronomers use ground-based detectors, high-altitude research balloons, artificial satellites and other methods to detect cosmic rays. Ground-based detectors, often spread over large areas (for example, the Pierre Auger Observatory is an array of detectors spread over 3,000 square kilometers), identify and analyze the secondary particles (electrons, positrons, photons, muons, etc.) produced in a chain reaction of particle interactions triggered by the collision of cosmic rays and Earth's atmosphere. The properties of the original cosmic ray particle, such as arrival direction and energy, are inferred from the measured properties of the extensive air shower, which is the cascade of secondary particles collectively showering down through the atmosphere. There are two kinds of ground-based detectors: Surface detector arrays analyze the air shower at a unique altitude, whereas air fluorescence detectors record the shower development in the atmosphere, based on the interactions of air shower particles with nitrogen molecules in the atmosphere.{{Citation |title=The Fluorescence Detector of the Pierre Auger Observatory |author1=Petr Nečesal for the Pierre Auger Collaboration |journal=Journal of Physics: Conference Series |volume=293, XIV International Conference on Calorimetry in High Energy Physics (CALOR 2010) |doi=10.1088/1742-6596/293/1/012036 |year=2011 |issue=1 |pages=12–36|arxiv=1011.6523 |bibcode=2011JPhCS.293a2036N }} Modern "hybrid" detectors, such as the Pierre Auger Observatory in Argentina and the Large High Altitude Air Shower Observatory in Sichuan, China, take advantage of the complementary nature of these two. Moreover, scientific balloons (such as the one used in Cosmic Ray Energetics and Mass Experiment{{Cite web |url=https://www.nsf.gov/news/news_summ.jsp?cntn_id=118226 |title=NSF/NASA Scientific Balloon Launches From Antarctica |website=U.S. National Science Foundation |access-date=April 29, 2024 |date=December 21, 2010}}) and satellites (such as China's Dark Matter Particle Explorer or DAMPE telescope) can also be used to observe pure cosmic rays at very high altitudes and in outer space.

By studying the energy, direction, and composition of cosmic rays, scientists can uncover the sources and acceleration mechanisms behind these particles, which reveal astrophysical processes such as supernova explosions, black hole accretion, and galactic magnetic fields. Observations of cosmic rays led to the discovery of subatomic particles beyond the proton, neutron, and electron, including the positron and the muon, laying the groundwork for modern particle physics. It reveals the nucleosynthetic processes leading to the origin of the elements. By measuring cosmic rays, scientists discovered the presence of magnetic fields and radiation in the Solar System. Some cosmic rays originate from beyond the Solar System or galaxy, allowing scientists to estimate the amount and composition of matter in the universe, providing crucial information about its makeup. Cosmic rays are generated in extreme astrophysical environments such as exploding stars, black holes, and galactic collisions and provide a rare window into these processes. Energetic cosmic rays can interact with objects traveling through space, altering their isotopic composition. By studying these isotopes in meteorites, scientists can determine when they formed and fell on Earth, providing insights into the history of the Solar System. Cosmic rays have practical applications, including monitoring soil moisture for agriculture and irrigation practices and carbon-14 dating, which helps determine the ages of archaeological artifacts and geological formations.{{Cite web |title=Cosmic rays, explained |url=https://news.uchicago.edu/explainer/what-are-cosmic-rays#universe |author=Louise Lerner |date=December 5, 2023 |website=UChicago News |access-date=April 29, 2024}}

File:Hess.jpg first discovered cosmic rays in 1912 with airborne ballons.]]

Historical milestones in cosmic ray astronomy include Victor Hess's discovery of cosmic rays during balloon flights in 1912; the identification of new subatomic particles like the positron and muon in the 1930s, expanding our understanding of particle physics;{{Cite web |title=Cosmic rays: particles from outer space |url=https://home.cern/science/physics/cosmic-rays-particles-outer-space |access-date=April 29, 2024 |website=CERN|date=10 April 2024 }} Pierre Victor Auger's discovery of extensive particle showers from cosmic ray collisions high in the atmosphere;{{Cite web |website=Astronomy |date=July 1, 2019 |author=David J. Eicher |title=Where do cosmic rays come from? |access-date=April 29, 2024 |url=https://www.astronomy.com/science/where-do-cosmic-rays-come-from/}} ground-based detectors measuring cosmic ray flux and energy spectrum in the 1940s-1950s;{{Citation |journal=Advances in Space Research |volume=44 |issue=10 |date=November 16, 2009 |pages=1124–1137 |title=Long-term (50 years) measurements of cosmic ray fluxes in the atmosphere |author1=Yu.I. Stozhkov |author2=N.S. Svirzhevsky |author3=G.A. Bazilevskaya |author4=A.N. Kvashnin |author5=V.S. Makhmutov |author6=A.K. Svirzhevskaya|doi=10.1016/j.asr.2008.10.038 |bibcode=2009AdSpR..44.1124S }} the establishment of the Volcano Ranch cosmic ray observatory in the 1960s, initiating large-scale experiments;{{Cite news |url=https://www.latimes.com/archives/la-xpm-1995-12-03-me-9743-story.html |title=Defunct N.M. Site Starred in Cosmic Ray Research From '58 to '72 : Physics: Scientists hope to revive Volcano Ranch architecture and add latest technology for two new facilities in quest to understand universe |author=Matt Mygatt |date=December 3, 1995 |newspaper=LA Times |access-date=April 29, 2024}} the discovery of cosmic ray anisotropy (the fact that cosmic rays do not arrive uniformly from every region of the sky) in the 1960s, unveiling variations in flux and direction; the emergence of high-energy gamma-ray telescopes in the 1980s-1990s, enabling observations of gamma rays produced by cosmic ray interactions; the advent of space-based detectors like AMS-02 on the International Space Station in the 2000s, providing insights from space;{{Cite web |title=AMS, a decade of cosmic discoveries |date=May 19, 2021 |author=Ana Lopes |url=https://home.cern/news/news/experiments/ams-decade-cosmic-discoveries |access-date=April 29, 2024 |website=CERN}} and recent progress in multi-messenger astronomy in the 2010s, integrating cosmic ray observations with other astrophysical signals for a more complete view of cosmic phenomena.{{Citation |author1=Halzen Francis |author2=Kheirandish Ali |title=Multimessenger Search for the Sources of Cosmic Rays Using Cosmic Neutrinos |journal=Frontiers in Astronomy and Space Sciences |volume=6 |year=2019 |page=32 |doi=10.3389/fspas.2019.00032 |doi-access=free |bibcode=2019FrASS...6...32H }}

With advancements in technology and the development of more sensitive detection systems, astronomers anticipate making new discoveries about the sources, acceleration mechanisms, and propagation of cosmic rays. These insights will contribute to a deeper understanding of the underlying physics governing the cosmos. Future cosmic ray observatories, such as the Cherenkov Telescope Array, will use advanced techniques to detect gamma rays produced by cosmic ray interactions in Earth's atmosphere. Since these gamma rays will be the most sensitive means to study cosmic rays near their source, these observatories will enable astronomers to study cosmic rays with unprecedented precision.{{cite web |url=https://www.ctao.org/emission-to-discovery/science/study-themes/ |website=Cherenkov Telescope Array Observatory (CTAO) |title=The Venturing Beyond the High-Energy Frontier |access-date=April 29, 2024}}

Cosmic ray astronomy faces difficulty in identifying the exact sources of cosmic rays because charged particles are deflected by magnetic fields in space, and as a result tracing the paths of cosmic rays back to their origins require sophisticated modeling techniques and multi-messenger observations to infer their source locations. Moreover, due to the high-energy nature of these rays, the need for full-sky exposure,{{Citation |title=Cosmic ray anisotropy analysis with a full-sky observatory |author=P. Sommers |year=2001 |journal=Astroparticle Physics |volume=14 |issue=4 |pages=271–286|doi=10.1016/S0927-6505(00)00130-4 |arxiv=astro-ph/0004016 |bibcode=2001APh....14..271S }} minimization of deflection by magnetic fields and elimination of background from distant sources present technical challenges.

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