Solar & Galactic Cosmic Rays

Primary Cosmic Rays

Total average cosmic ray flux at the Earth's surface is about 2 x 102 particles m-2 s-1 (neutrinos not included). This flux is highly variable as a result of variations in solar activity. The sun is capable of producing lethal doses of proton radiation for brief intervals and generally arise from solar flares. This poses a serious potential risk for astronauts in earth orbit and interplanetary travellers.

Astronauts in earth orbit can be quickly returned to Earth, but on a manned mission to Mars astronauts would need to be provided with adequate shielding in their spacecraft.

1. Protons

These are of two main types:

i) Solar protons. These are protons whose energies are so low (in the 0.5MeV to 200MeV range) that their secondary showers are not detected at ground level, but their energies are high enough to pose a potential hazard to astronauts. Although solar protons are somewhat less energetic than protons from outside the solar system, their number density and hence their impact rate (particles ·m-2 ·s-1) can be orders of magnitude higher than the background galactic cosmic ray flux.

The lower energy protons become deflected by the Earth's magnetic field and are directed towards the Earth's poles. At altitudes of 90km to 120km the solar protons in the 100keV range give up their energy creating an intense field of ionized nitrogen and oxygen and low energy protons.

The recombination spectra which are created are responsible for the colours we see in the aurora.

ii) Protons from outside the solar system. These are protons with energies ranging from 10 to 10 20 MeV. The protons with the highest energies collide with atoms of the atmosphere (notably nitrogen and oxygen) and produce spectacular secondary particle showers which cascade to the Earth's surface.

2. Alpha particles

These are helium nuclei which, because of their large size and large charge (+2), produce significant secondary cosmic ray showers.

3. Solar neutrinos

Neutrino radiation comes more or less directly from the core of the Sun where they arise from nuclear reactions with the Sun's core. Its flux at the Earth's surface is about 1015 m-2 · s-1 This is an enormous number! Neutrinos interact so weakly with matter that they arrive at the Earth (at the speed of light) directly from the core of the Sun. The neutrinos then pass straight through the Earth (and everyone and everything on and in it) as if the Earth were a totally transparent piece of crystal glass.

Secondary Cosmic Rays

A secondary shower arises whenever a highly energetic particle collides with an atom of N2 or O2 and creates a "spray" of collision fragments. These fragments in turn collide with more N2 and O2 and so on, until all the incident energy has been dissipated. The total momentum of the shower fragments is directed downwards. Eventually an intense burst of particles reaches the Earth's surface and is absorbed by the ground or other material

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1. Mesons (muons)

Highly energetic particles are created when primary cosmic rays collide with atoms of oxygen and nitrogen in the upper atmosphere. Mesons make up most of the "hard" (penetrating) cosmic radiation which reaches the Earth's surface. Mesons react weakly with matter. A (typical) 70MeV meson can penetrate 30m into the earth.

2. Neutrons and protons

All neutrons and protons that reach the Earth are produced in secondary showers. Neutrons and protons are produced in about equal numbers.

3. Electrons and positrons

These constitute the "soft" radiation component of the secondary cosmic radiation spectrum. A highly energetic electron (or positron) will usually trigger a chain of collisions spawning more electron-positron pairs. This is called a cascade shower

1. Solar Flares

Solar flares produce large amounts of radio and X-ray radiation. They also produce intense particle bursts which in some instances are "beamed" directly at the Earth; usually however it reaches the Sun-facing hemisphere of the Earth's orbit more or less isotropically from the general direction of the Sun as the Earth encounters a wide beam of incident radiation.

Transparency Master		 A flare is first detected by a significant brightening of a spot on the Sun's surface. This brightening occurs at many wavelengths, from radio to X-ray.

The flare also ejects a stream of high velocity ionized particles, notably, electrons, protons, and helium nuclei into interplanetary space. These streams are directed outwards and are generally in the plane of the planetary orbits.

The rotation of the Sun's magnetic field also imparts a slight angular velocity to the particle stream. The energy spectrum of the particles cause them to spread out as they move outwards from the Sun, the highest velocity particles move outwards ahead of the slower, lower energy particles.

Transparency Master		 If the alignment of the particle stream is in the plane of the Earth's orbit there is a probability that the Earth will intercept the particle stream within a few hours or days, depending upon the trajectory and velocity of the particles.

Because the particles are charged they not only contribute to the solar cosmic ray flux above the atmosphere, they also represent a huge electrical current which collides with the Earth's magnetosphere capable of causing major disturbances in the Earth's magnetic field.

These disturbances, called magnetic storms, are not only responsible for large scale interruptions in electrical power grids on a continental scale, but also the interruption of global radio and telecommunication services.

Particle Related Phenomena

  1. Aurora
  2. Particle Radiation from the Sun

Terrestrial Sources: Particles (and Waves)

All naturally occurring terrestrial sources arise from the radioactive decay of naturally occurring radioactive elements which are either chemically combined in the minerals that make up the soil and rock, or as gases which are radioactive daughter products of other elements, of which radon is a good example.

All radioactive materials are emitters of particle radiation and electromagnetic radiation. The particles emitted are either alpha particles or electrons. Every particle emission is accompanied by a tiny pulse of electrogmagnetic radiation which has a gamma ray signature.

For example 19K40 ( a radioactive isotope of potassium) which has a half life of 1.28 x109, is a beta (electron) emitter. Each nucleus will eject a beta particle whose energy is in the 1.35-1.50MeV range accompanied by a 1.46MeV gamma ray.

On the other hand, 88Ra226 (radium) whose half-life is about 1600a, is an alpha emitter. When a radium nucleus undergoes a radioactive decay, it emits an alpha particle whose energy is in the 4.5MeV range, accompanied by a 200keV gamma ray.

The particle (alpha and beta) components of radioactive decay

Geological Emissions

1. Natural radioactivity

Radon escapes from soils, rocks, and building materials as a gas. Because it is a relatively dense gas it tends to accumulate in unventilated basements of buildings. Higher concentrations occur in brick and stone structures because of the high mineral content of naturally occurring radioactive substances, especially radium.

Certain locations on the Earth have an unusually high natural background radiation from radioactive minerals in the local soil and rock.

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Prepared by the YES I Can! Science Team,