Cosmic rays are energetic subatomic particles that arrive in the Earth’s atmosphere from outer space. It is possible that these particles are accelerated by surfing on the shock waves produced supernovae.

→ The vast majority, $89 \%$, of cosmic rays are protons. Of the remainder, $9 \%$ are alpha particles, $1\%$ are carbon, oxygen, and nitrogen nuclei and the remainder are electrons and gamma rays.

When cosmic rays hit the top of the Earth’s atmosphere, they produce cosmic air showers of secondary particles, including electrons, photons, neutrinos, and muons.

Cosmic rays have a wide range of energies — low energy cosmic rays are thought to come from the Sun, intermediate energy cosmic rays are thought to come from the Milky Way, and the highest energy cosmic rays are thought to come from galactic centres.

Particles which enter the Earth’s magnetic field at an angle experience the force $F=qvBsin\theta$. The component of their speed parallel to the magnetic field does not product a force, causing the particle’s motion in this plane to be constant speed. The component perpendicular to the magnetic field causes the particle to move with uniform circular motion in the plane perpendicular to the magnetic field. The overall result is a helical path.

→ As a particle approaches the Earth’s poles, the radius and the pitch of its motion will decrease.

The following relationships can be used to solve problems involving the force on a charged particle, its charge, its mass, its velocity, the radius of its path, and the magnetic induction of a magnetic field.

$$ F=qvB $$

$$ F=\dfrac{mv^2}{r} $$

Solar wind consists of a stream of charged particles which escape from the upper atmosphere of the Sun, consisting mainly of electrons, protons and alpha particles. Solar wind flows outwards as a plasma at great speeds and, as a result of the high energy of the particles, they are able to escape the Sun’s gravity.

→ The Earth’s magnetic field, forming a magnetosphere, protects its atmosphere from the impacts of Solar wind as it deflects the Solar wind around the planet.

→ Aurorae are produced at the Earth’s poles where the Solar wind is able to reach. This is due to the high energy Solar wind particles interacting with particles in the atmosphere, e.g. nitrogen and oxygen, which become excited and then emit photons when they return to their ground states.