High Energy Cosmic Rays and the Sun
There are two categories of cosmic rays: primary and secondary cosmic rays. Real (or "primary") cosmic rays can generally be defined as all particles that come to earth from outer space. These primary cosmic rays generally do not make it through the earth's atmosphere, and constitute only a small fraction of what we can measure using a suitable set of particle detectors at the earth's surface.
As we shall see, we do measure particles at sea level in such detectors. What we measure, however, are mostly the remains from interactions of primary cosmic rays with the upper atmosphere. These remnants are also particles, referred to as "secondary" cosmic rays. Often, however, the specification "primary" or "secondary" is omitted.
From the above we see that secondary cosmic rays are neither "rays" nor "cosmic": they are particles rather than rays, and they come from the upper atmosphere rather than outer space. On the other hand, they are produced by real cosmic rays! So where do primary cosmic rays come from?
Low Energy Cosmic Rays
The sun is a source of cosmic rays: the solar wind consists of protons and electrons ejected from the sun's corona and from solar flares. Almost all these solar cosmic rays, however, have a very low energy and except for a minute fraction they are all deflected by the earth's magnetic field and absorbed in the atmosphere. They have enough energy to ionize the various gasses in the upper atmosphere, which then causes beautiful displays known as the Aurora. More specifically, in the northern hemisphere it is called the Aurora Borealis, also known as Northern Lights, while in the southern hemisphere it is called Aurora Australis.
The picture above is of the Aurora Borealis, taken by Dick Hutchinson. For more photographs, see his web site. Big solar flares produce so much solar wind that it can cause radio interference and sometimes even damage to satellites.
These low-energy cosmic rays cannot be detected with particle detectors at the earth's surface. For that reason, we will not discuss them further.
High Energy Cosmic Rays
The sun does, however, also have an effect on high-energy cosmic rays. High-energy cosmic rays come from interstellar space and are sometimes called Galactic Cosmic Rays (GCRs), even though it is thought that some of them come from beyond our galaxy. The solar wind mentioned above consists of a continuous stream of plasma, loose protons and electrons. The region of space in which the influence of the solar wind is felt, called the heliosphere, extends far beyond the orbit of Pluto. Because the solar wind is a plasma, it is electrically conducting and transmits a part of the sun's magnetic field. When GCRs approach the sun they encounter the heliosphere and the magnetic field within it. Because of the shape of the magnetic field, the GCRs lose some of their energy, and the lower-energy ones never reach the vicinity of the earth. In times of high solar activity (high levels of solar wind) this effect is stronger and fewer GCRs reach the earth.
The sun has an 11-year cycle in its activity. One of the ways in which the cycle is made visible is in the number of sun spots that can be seen through telescopes. During a solar maximum the number of sun spots is high, and during a solar minimum the number of sun spots is low. During a solar maximum, the solar wind is also stronger and the sun is a tiny bit brighter (about 0.1%), even though sun spots are places on the sun's surface that are cooler than their surroundings - there are other features that more than compensate.
Measurements of cosmic rays go back to the year 1935 and show clearly that high solar activity corresponds to lower cosmic ray fluxes. At sea level, the flux is about 2% lower during a solar maximum than the average and vice versa.
But the 11-year cycle is not the only type of variation of the sun. Sun spots have been counted for centuries, ever since the telescope was invented (or at least made famous) by Galileo Galilei in 1609. This was right in the middle of the time known as "the little ice age" which lasted from about 1400 to about 1800. During the "Maunder minimum", which lasted from 1645 to about 1715, sun spots were particularly scarce. There is indirect evidence from radioactive carbon records that the cosmic ray flux reaching the earth was especially high during that time. We will get back to this in the section on cosmic rays and the atmosphere.