The energy of the cosmic rays nuclei covers a wide range from one million electron volt (MeV) to about 1020 eV, which is more than a thousand million times the energy of particles found in the largest particle accelerator in the world! Cosmic ray particles and heavy nuclei of energy less than 100 MeV per nucleon are absorbed in the upper atmosphere. Balloon-borne detectors can therefore register those with higher energy.
The Tata Institute of Fundamental Research has been engaged in astrophysical research including cosmic ray studies for several years. TIFR has set up a balloon facility for studies close to the top of the Earth’s atmosphere. While extensive investigations have been done at high latitudes (covering North America and Europe), data on the equatorial regions are meager, partly because the geomagnetic equator traverses mostly through oceans and not many countries possess the facility for scientific investigation. TIFR’s balloons are launched from Hyderabad. The southern sky is conveniently seen from there. The location also takes advantage of the strong magnetic field in the region, that allows only high-energy radiation (above 1010 eV) believed to have originated from outside the solar system and filters out low-energy components, thereby discarding unwanted background radiation.
The natural advantages are realized in regular programmes of balloon launching. Plastic balloons of about 84,900 cu m are fabricated and launched for the purpose. There are plans to make bigger balloons of 283,000 cu m. The balloons reach an altitude of about 36 km and as only a very small remnant of the atmosphere still exists at that height, the detectors are able to study cosmic radiation. The payload are recovered and often used again. Some of the pioneering observations of high energy electrons in cosmic radiations have been made by TIFR. The studies are important to understand the magnetic fields in space and the acceleration of cosmic rays.
The Unknown Origin of Cosmic Rays
Though the focus today is on discovering Earth-like objects, one mystery has remained a challenge, though it does not tend to hit the headlines. That is the origin of cosmic rays. On average, a square kilometer on the top of the Earth’s atmosphere is hit by an ultrahigh energy cosmic ray particle only once in a century! The Pierre Auger Cosmic Ray Observatory in Argentina covers 3000 Sq km and catches one ray every fortnight.
In 2008, the University of Utah’s High-Resolution Fly’s Eye Cosmic Ray Observatory has confirmed that ultra high energy cosmic rays come from great distances and most of them collide with the radiation left over from the birth of the Universe. In November 2007 the Auger Observatory also confirmed the so-called cut-off rate for the high energy cosmic rays that manage to reach the Earth.
By their very nature, cosmic rays are truly messengers from space. When tapped properly, they would tell a fascinating story of cosmic evolution. Cosmic rays are strange samples of matter from outside the solar system to reach the Earth. Their origin is still uncertain but it is known that they wander in interstellar space for millions of years before going past the Earth on their seemingly endless journey. It is now believed that the ultra high cosmic rays emanate from active galactic nuclei called blazers.
Being electrically charged particles, cosmic rays are subject to electric and magnetic fields in space. Since the electrons of cosmic rays have comparatively low mass, they would easily lose energy when confronted even by weak magnetic and radiation fields on their way. Their interaction results in new types of matter. Indeed, many elementary particles have been initially identified in cosmic rays. The particles include positrons, pi mesons, muons and K mesons which are the strange names given by those who seek the composition of the atom in particle accelerators on Earth. Basically, cosmic rays are composed of 90 per cent of high-energy protons, with 9 per cent of helium and 1 per cent of heavier elements such as carbon, oxygen and even uranium.
As the atmosphere absorbs cosmic rays, their discovery had to wait till a scientist ventures up in a balloon in search of celestial radiation. In 1912 an Austrian physicist, Victor Hess (1883-1964), made a series of balloon flights with instruments and found that the background radiation from the sky increased with altitude. The radiation was later known as cosmic rays. For many years researchers thought of them as a local phenomenon, confined to the surroundings of the Sun. in 1938, a group of French scientist headed by Pierre Auger showed that cosmic rays of high intensity occasionally reach the top of the atmosphere and following this ‘primary’ ray, ‘ secondaries’ spread out like a shower. A Swedish physicist, Hanes Alfven, suggested that cosmic rays were confined by the magnetic field of our galaxy. New ideas came with the use of radio telescopes. More powerful radiation is studied through ‘secondary’ air showers with the help of large collecting arrays.
Another interesting feature was noticed as cosmic rays were seen deflected by solar activity. The solar wind is a steady stream of particles, mostly hydrogen and boils off the corona and travels radially. The solar wind deflects cosmic rays and whenever there is a high solar activity, cosmic rays reaching the Earth would considerably be less. The Sun also emits particles of low energy during solar flares, but it has been verified that the Sun or stars like it cannot produce cosmic rays.
Analyses of meteorites have shown that cosmic rays retain their intensity over millions of years. Astronomers offer several answers to the mystery of cosmic rays. Some experts trace the origin of cosmic rays to exploding stars, where charged particles can be speeded up by magnetic fields. Others have suggested pulsars, quasars, the galactic centre and flare-type of stars as their origin. While there is no final answer to the source of cosmic rays, it is generally agreed that high-energy cosmic rays are extra-galactic in their origin, while low-energy rays start their journey from near the solar system.