An invisible rain falls constantly, filtered by forces we cannot see

There is a rain that falls on Earth every moment of every day, but it is not made of water. It is made of particles—protons, atomic nuclei, electrons—accelerated to energies far beyond anything our machines can produce, arriving from the far reaches of the galaxy and beyond. These cosmic rays have been striking our atmosphere for billions of years, yet the question of how they enter remains a study in invisible architecture: the Earth’s magnetic field, the solar wind, and the air itself all play a role in a quiet filtration system that determines what reaches us and what does not.

The journey begins far away. Primary cosmic rays are mostly protons—about 88 percent—with roughly 10 percent helium nuclei and 2 percent heavier elements, following the abundance of elements found in stars . Their energies are staggering. While a typical particle might carry a few MeV, cosmic rays commonly reach 1 GeV, and some have been detected with energies up to 10^20 eV—far beyond what any human-built collider can achieve . Where exactly they originate remains an open question, though some are known to come from rare solar flares while the majority appear to have galactic or extragalactic sources .

Before these particles can reach the atmosphere, they must navigate a magnetic gate. Earth’s magnetic field interacts with charged particles, deflecting them into curved paths. At the equator, where cosmic rays approach perpendicular to the field, the deflecting force is strongest, and a particle must have at least 10 to 12 billion electron volts to penetrate to the surface. At the poles, particles travel parallel to the field lines and experience little deflection, allowing even lower-energy cosmic rays to enter . This latitude effect means that cosmic ray intensity is greatest at the poles and lowest at the equator, increasing by nearly 14 percent from the equator to about 40 degrees north at sea level .

The solar wind adds another layer of modulation. During periods of high solar activity, the sun’s magnetic field strengthens and shields the inner solar system from galactic cosmic rays, reducing their intensity. During quiet periods, more low-energy rays reach Earth. This 11-year cycle creates a corresponding rhythm in cosmic ray flux . Short-term events—coronal mass ejections, for instance—can block cosmic rays for several days, creating what are called Forbush decreases .

Once a primary cosmic ray penetrates the magnetic shield and reaches the upper atmosphere, its journey changes character. It collides with an atomic nucleus—oxygen, nitrogen, argon—and shatters into a cascade of secondary particles, mostly pions and kaons. These particles are energetic enough to produce further collisions, creating an air shower that spreads through the atmosphere . At sea level, about 70 percent of secondary cosmic rays are pions, 29 percent are electrons and positrons, and 1 percent are heavier particles including muons, neutrons, and gamma rays . Muons, in particular, are highly penetrating; unlike pions, they do not interact strongly with matter and can pass through the atmosphere and deep underground. A volume the size of a human head is traversed by a muon every second .

The study of cosmic rays has yielded discoveries far beyond the particles themselves—the positron, mesons, and insights into nuclear processes that would later be probed by accelerators . Yet the fundamental question of where they come from and how nature accelerates them to such extreme energies remains unsolved. For now, the rain continues, invisible and constant, carrying messages from distant stars that we are only beginning to read.

Cosmic rays enter Earth’s atmosphere by penetrating the planet’s magnetic field, which deflects lower-energy particles at the equator but allows more through at the poles. Once in the upper atmosphere, primary rays collide with atomic nuclei, creating cascading showers of secondary particles that reach the surface.

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Sources: Caltech, CERN, IOP Publishing, UNSCEAR, INAF