20.4 Cosmic Rays

Cosmic Rays

Cosmic rays are high-energy particles that travel through space at nearly the speed of light. Understanding them matters for astronomy because they carry information about some of the most violent events in the universe, from supernova explosions to the environments around black holes. They also interact with Earth's atmosphere and magnetic field in ways that affect our planet.

Composition of cosmic rays

The vast majority of cosmic rays are protons (hydrogen nuclei), making up about 90% of the total. The rest is a mix of heavier particles and trace components:

• Alpha particles (helium nuclei) account for roughly 9% of cosmic rays
• Heavier atomic nuclei like carbon, oxygen, and iron make up most of the remaining 1%
• Trace quantities of electrons, positrons (the antimatter counterpart of electrons), and other subatomic particles round out the mix

What makes cosmic rays remarkable is their energy. Some cosmic rays carry energies exceeding $10^{20}$ eV, far beyond anything produced in human-made particle accelerators like the Large Hadron Collider. Ultra-high-energy cosmic rays (UHECRs) are defined as those with energies above $10^{18}$ eV. To put that in perspective, a single UHECR particle can carry roughly the same kinetic energy as a baseball pitched at 90 mph, all concentrated in a single subatomic particle.

Studying the isotope ratios found in cosmic rays also gives astronomers clues about where these particles originated and how long they've been traveling through space.

Theories of cosmic ray origins

Tracing cosmic rays back to their sources is genuinely difficult. Because cosmic rays are charged particles, they get deflected by magnetic fields throughout interstellar and intergalactic space. By the time they reach us, their paths have been scrambled, so you can't just point a detector and trace them back to a source the way you can with light. On top of that, the highest-energy cosmic rays are extremely rare, which limits the data astronomers have to work with.

Several candidate sources have been identified:

• Supernova remnants are considered the most likely source for the majority of cosmic rays. The expanding shock waves from supernova explosions can repeatedly accelerate particles to very high energies through a process called diffusive shock acceleration.
• Active galactic nuclei (AGN), which contain supermassive black holes at the centers of galaxies, may accelerate particles as matter spirals inward. Quasars are an especially energetic type of AGN.
• Pulsars, rapidly spinning neutron stars with extremely strong magnetic fields, can also accelerate particles to high energies.
• Gamma-ray bursts, produced by events like merging neutron stars or the collapse of massive stars, could generate the most extreme cosmic rays.

In all of these sources, the acceleration involves complex interactions between charged particles, magnetic fields, and shock waves.

Cosmic ray interactions with Earth

When a high-energy cosmic ray strikes a molecule in Earth's upper atmosphere, it triggers a cascade of secondary particles called an air shower. A single cosmic ray can produce millions of secondary particles that spread out as they descend. Ground-based observatories like the Pierre Auger Observatory in Argentina detect these showers to study the original cosmic ray's energy and direction.

Cosmic rays also affect the atmosphere in subtler ways:

• They ionize molecules in the upper atmosphere, which influences atmospheric chemistry (for example, contributing to the formation of nitrogen oxides) and the atmosphere's electrical properties.
• Earth's magnetic field deflects lower-energy cosmic rays, acting as a shield. This is why cosmic ray intensity at the surface is lower near the equator (where the field lines run parallel to the surface) and higher near the poles (where field lines funnel particles downward).
• Over geological timescales, changes in Earth's magnetic field strength and orientation, including magnetic reversals, have altered how many cosmic rays reach the surface.

Space environment and cosmic rays

Cosmic rays don't travel through empty space undisturbed. Several factors shape their journey through the solar system:

• The solar wind, a continuous stream of charged particles flowing outward from the Sun, pushes against incoming cosmic rays. During periods of high solar activity, the solar wind is stronger and fewer cosmic rays reach the inner solar system. This effect is called solar modulation.
• Earth's radiation belts (the Van Allen belts) are regions where charged particles from cosmic rays and the solar wind get trapped by Earth's magnetic field, creating zones of intense radiation surrounding the planet.
• Interplanetary magnetic fields carried by the solar wind further deflect and scatter cosmic rays as they move through the solar system, adding another layer of complexity to tracking their origins.