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|موضوع: Cosmic Rays الأربعاء أكتوبر 27, 2010 5:05 pm|| |
Cosmic Rays and LiBeB: An Introduction to Cosmic Rays Cosmic rays were discovered in 1912 by Victor Hess whenhe found that an electroscope discharged more rapidly as he ascended ina balloon. He attributed this to a source of radiation entering theatmosphere from above. In 1936 he was awarded the Nobel Prize. Take a peek at the picture at right (click on itto enlarge) to see some of the heavier elements that can make up cosmicrays.
Cosmic rays are subatomic particles that travelat speeds very close to the speed of light. They include essentiallyall of the elements in the periodic table. About 89 percent of cosmicrays are hydrogen (single protons), 9-10 percent He, and about onepercent heavier nuclei. In fact, all of the elements in the periodic table-- at least through uranium, and possibly beyond -- are present in cosmicrays. Elements which are easily ionized, however, are systematicallymore abundant, by a factor of about 5, then those which are difficult toionized. Interestingly enough, electrons constitute only about 1percent of cosmic rays. It is not known why electrons are apparentlyless efficiently accelerated than nuclei.
The common heavier elements (such as carbon, oxygen,magnesium, silicon, and higher) are present in about the same relativeabundances as in our solar system. However, there are important differencesin elemental and a isotopic composition that provide us with informationon the origin and the history of these galactic cosmic rays -- this willbe discussed in more detail later. It is important to keep in mind that cosmic raysare, for the most part, ionized atoms. Any electrons that a cosmicray nucleus has are rapidly stripped away from the atom as it speeds throughthe interstellar medium. Thus, all cosmic rays are charged, and thereforeare affected by magnetic fields in the universe, including the Earth'smagnetic field. To maintain the observed intensity of cosmicrays over millions of years requires that a few percent of the 10^51 (andhigher) ergs released in a typical supernova explosion be converted tocosmic rays. The abundance and energies of these particles is suchthat the energy contributed to the Galaxy by cosmic rays is about 1 eVper cubic cm -- about equal to that contained in galactic magneticfields, and in the thermal energy of the interstellar medium! Galactic cosmic rays play important roles in thedynamics of the interstellar medium, including stabilization of interstellargas against gravitational collapse, and apparently help to regulate thecollapse of proto-stellar clouds. A recent study even asserts that galactic cosmic rays play a role in thedetermination of the scale height of gas in the galaxy, and also may playan important role in forming Giant Molecular Complexes -- one million solar-massclouds which are the sites of active star formation, such as the Orioncomplex (see photo at left). These are the basics of the all important cosmicray. Let's now examine some of the distinctions made between differentkinds of cosmic rays. Galactic Cosmic Rays
Usually the term "cosmic rays" refers to galactic cosmicrays, or GCRs. These are cosmic rays which originate in sources outsidethe solar system but generally inside the Milky Way Galaxy. Most GCRs have energies between 100 MeV (correspondingto a velocity for protons of 43% of the speed of light) and 10 GeV (99.6%of the speed of light). The number of cosmic rays with energies beyond1 GeV decreases by about a factor of 50 for every factor of 10 increasedenergy. Over a wide energy range the number of particles with energygreater than E (measured in GeV) is given approximately by the followingformula:
...where k is about 5000 per square meter per steradian per second. The highest energy cosmic rays measured to date havehad more than 10^20 eV, equivalent to the kinetic energy of a baseballtraveling at about 100 mph!Solar Energetic Particles
The Sun isa sporadic source of cosmic rays -- these are termed solar energetic particles,or SEP's. Nuclei and electrons are accelerated by shock waves travelingthrough the Corona and by magnetic energy released in solar flares. During such occurrences,the intensity of energetic particles in space can increase by a factorof 100 to a million for a period of time lasting from hours to days. The maximum energy reached in solar particle events is typically 10 to100 MeV, occasionally energies reach 1 GeV (roughly once a year) to 10GeV (roughly once a decade). Solar energetic particles can be usedto measure the elemental and isotopic composition of the Sun, complementingmeasurements made in spectroscopic studies of solar material.Anomalous Cosmic Rays
Anomalous cosmic rays (ACR) are cosmic rays originating from the interstellarspace beyond the heliopause (where the solar wind slowsas a result of plowing into the interstellar gas, creating a shock wave-- see graphic at left). They are most likely produced by neutralatoms in the interstellar medium which leak into the heliosphere and becomeionized by either solar UV-radiation or by charge-exchange with the solarwind. They are then picked up by the solar wind and carried backto the outer heliosphere. Lastly, they are somehow accelerated, e.g.by the solar wind termination shock, and drift into the inner heliosphereas cosmic rays. It is possible that the Voyager I spacecraft, whichshould reach 100 AU (thought to be the distance of the solar terminationshock) by 2007 will have the opportunity to observe an example of an SEPacceleration directly.
The image above and to the right depicts this process. Click on either one of the above graphics to enlarge that image.
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|موضوع: رد: Cosmic Rays الأحد أكتوبر 31, 2010 9:25 am|| |
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