Cosmic ray

Cosmic rays, also called astroparticles, are tiny bits of matter moving through space at nearly the speed of light. Most of these particles are protons or small groups of atoms that come from far beyond our Solar System. They travel from our own Milky Way galaxy, from other galaxies, and even from the Sun.

When cosmic rays reach Earth, they hit our planet's atmosphere and create new particles. Some of these particles reach the ground, but many are blocked by Earth's magnetic shield, called the magnetosphere.

We have known about cosmic rays since 1912, when a scientist named Victor Hess discovered them using balloons high above Earth. Today, we study them with special tools on satellites and space probes. Recent discoveries suggest that many cosmic rays might come from exploding stars called supernovas, as well as from powerful objects in distant galaxies known as active galactic nuclei.

Etymology

The word "ray" comes from an old idea that these particles were like light beams. People thought this because the particles could go through things. We now know they are tiny bits of matter with weight. They are similar to other particles we study, such as cathode rays, canal rays, alpha rays, and beta rays. Other types of light, like gamma rays or X-rays, are called by their special names because they don’t have weight.

Composition

Most cosmic rays that come from outside Earth are tiny parts of atoms. About 99% of these are just the centers of atoms. About 90% of these centers are simple protons, like hydrogen atoms without their outer parts. Another 9% are like helium atoms, and 1% are centers of heavier atoms.

When cosmic rays hit Earth's air, they break atoms apart and create new particles. Some of these particles can go all the way through the air and even underground. About one of these particles passes through a space the size of a person's head every second.

Main article: beta particle
Main articles: HZE ions, antimatter, positrons, antiprotons, anti-alpha particles

Energy

Cosmic rays are very interesting because they can affect tiny parts in electronics and living things when they are not protected by Earth's air and magnetic field. Scientists study them because some cosmic rays have very high energy—so high that they can reach about 3 × 10²⁰ eV. This is millions of times more energy than particles created in the largest machine on Earth, called the Large Hadron Collider. These high-energy cosmic rays might get their energy from powerful objects in space called active galactic nuclei.

Most cosmic rays do not have such high energy. The most common ones have an energy of around 300 megaelectronvolts.

History

After finding radioactivity in 1896, scientists thought electricity in the air came only from radioactive materials on the ground or gases in the air. But measurements showed that ionization increased with height, suggesting another source.

Discovery

In 1909, Theodor Wulf made a tool to measure radiation and found more at the top of the Eiffel Tower than at the bottom. His work was not widely accepted. In 1911, Domenico Pacini found that ionization changed over water and land, suggesting sources beyond Earth.

In 1912, Victor Hess flew high in a balloon with tools to measure radiation. He found it increased with height, even during a solar eclipse, suggesting radiation came from space. In 1913–1914, Werner Kolhörster confirmed these results from even higher altitudes.

Hess won the Nobel Prize in Physics in 1936 for this discovery.

Identification

In the 1920s, the term cosmic ray was created by Robert Millikan, who measured ionization from deep underwater to high altitudes. He thought cosmic rays were gamma rays from space. But later finds showed they were charged particles.

In 1927, Jacob Clay found cosmic ray intensity changed with latitude, showing they were charged. In 1929, Bothe and Kolhörster found particles that could go through thick gold, proving they were not photons.

In 1930, Bruno Rossi predicted differences in cosmic ray intensity from east to west, which was later confirmed, showing most were positive. By 1948, it was found that most cosmic rays are protons, with some helium nuclei and heavier elements.

Energy distribution

In 1954, experiments measured the energy of cosmic rays, finding they could go beyond 10²⁰ eV. The Auger Project in Argentina studies these high-energy rays to learn more about their origins and effects on our understanding of the universe.

Solar modulation

Solar modulation shows how the strength of cosmic rays changes as they travel through the space around our Sun, called the heliosphere. This happens because of the solar wind and the Sun's magnetic field. The Sun has cycles of activity about every 11 to 22 years, and these cycles change how many cosmic rays reach Earth. Because of this, the number of cosmic rays going to Earth goes up and down over time.

Main article: solar cycle
Main articles: solar wind

Parker transport equation

The Parker transport equation is named after Eugene Parker. It is a special math rule that helps us study how energetic particles move in space. These particles are found in places with lots of hot, moving gas, called astrophysical plasmas.

The equation helps scientists learn how these particles speed up and travel. It shows how particles spread out in space and change speed over time. This helps us understand big cosmic events.