Particles from Space Cosmic Rays The term cosmic

Particles from Space

Cosmic Rays. The term cosmic ray is not precisely defined, but a generally accepted description is ‘high energy particles arriving at the Earth which have originated elsewhere’. It was thought this radiation was coming from the Sun, but measurements done by Austrian, Victor Hess, during a nearly complete solar eclipse (12 April 1912), gave the same results thus ruling out the Sun as the main source of this radiation.

Composition of cosmic rays Nature Protons Approximate % of all cosmic rays 89 Alpha Particles 9 Carbon, Nitrogen and Oxygen nuclei 1 Electrons Less than 0. 1 Gamma radiation Less than 0. 1

Energy of cosmic rays The energies of cosmic rays cover an enormous range, with the most energetic having energies much greater than those capable of being produced in our present day particle accelerators. The highest energies produced in particle accelerators are of the order of 1012 e. V. (1 e. V = 1. 6 x 10 -19 J) Cosmic rays have been observed with energies ranging from 109 to 1020 e. V. Those with energies above 1018 e. V are referred to as ultra -high-energy cosmic rays (UHECRs). The OMG particle with energy of 3 × 1020 e. V was recorded in Utah in 1991. Converting to joules (J), 3 × 1020 e. V = 3 × 1020 × 1. 6 × 10– 19 J = 48 J, ie ~50 J.

Origin Of cosmic rays The lowest energy cosmic rays come from the Sun and the intermediate energy ones are presumed to be created within our galaxy, often in connection with supernovae. Supernovae are believed to be the main source of cosmic rays. The origin of the highest energy cosmic rays is still uncertain. Active galactic nuclei (AGN) are thought to be the most likely origin for Ultra High Energy Cosmic Rays (UHECRs).

Interaction with the Earth’s atmosphere When cosmic rays reach the Earth, they interact with the Earth’s atmosphere, producing a chain of reactions resulting in the production of a large number of particles known as a cosmic air shower as shown. Primary cosmic ray p p+ p- p+ n n g g m+ n e+ e– p, proton; e–, electron; e+ positron; n neutrino; p+, p- pions; m+, m- muons; g gamma m-

When cosmic rays from space strike particles in the atmosphere they produce secondary particles, which go on to produce more collisions and particles, resulting in a shower of particles that is detected at ground level. The primary cosmic rays can usually only be detected directly in space by detectors on satellites. On rare occasions, very high energy cosmic rays can penetrate to ground level.

The Solar Wind The continual flow of charged particles from the Sun, caused by the high temperature providing particles with kinetic energy to escape from the Sun’s gravity, is called the solar wind. It is plasma composed of protons and electrons, alpha particles and a trace of heavy ions. Speed ranges from 300 to 800 kms– 1, (gusts of 1000 kms– 1 = 2. 25 million miles per hour). Comet tails interact with this “wind” and always point away from the Sun regardless of the direction of the comet itself.

Interaction of the solar wind with the Earth’s magnetic field When the solar wind interacts with the Earth’s magnetic field some high-energy particles leak into the Earth’s magnetosphere and become trapped to form the Van Allen belts. When charged particles move in magnetic fields they experience a central force moving them in a circle or helical path (see Unit 3). This traps them within the field.

Charged particles trapped in the belts move along magnetic field lines: oscillating back and forth between “mirror” points. Particles in the inner belt may interact with atoms in the upper atmosphere producing aurorae. (Northern/Southern lights). The magnetic field acts as a barrier to these harmful radiations protecting organisms on Earth from biological harm.

“Mirror points” Magnetic field lines Helical path of charged particle Note that the geographic North is, at present, a magnetic south pole etc.

Solar flares are explosive releases of energy from the Sun over virtually the entire e. m. spectrum; gamma rays to radio waves. They also emit high-energy particles or solar cosmic rays; mostly protons and alpha particles. The flares usually occur near sunspots and are thought to be related to magnetic fields in the Sun. Tension in these field lines cause them to “snap” and release huge quantities of energy from the Sun. This heats plasma to ~106 K generating x-rays and accelerating particles to almost the speed of light.

Particles streaming from the Sun after solar flares can disrupt communications and power delivery on Earth. A major solar flare in 1989 caused the US Air Force to lose communication with over 2000 satellites and induced currents in underground circuits of the Quebec hydroelectric system which shut it down. Coronal mass ejections, CME. This is a release of a large amount of radiation and matter, in the form of plasma, from the surface of the Sun.

Solar cycle All solar activities show a cyclic variation with a period of about 11 years. When an image of the sun is focussed on a screen, dark, cooler regions called sunspots are visible. The three main features of the solar cycle are: – the number of sunspots – the mean latitude of sunspots – the magnetic polarity pattern of sunspot groups. The number of sunspots increases and decreases with the 11 -year cycle.