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Space physics, also known asspace plasma physics, is the study of naturally occurringplasmas within Earth'supper atmosphere and the rest of theSolar System. It includes the topics ofaeronomy,aurorae, planetaryionospheres andmagnetospheres,radiation belts, andspace weather (collectively known assolar-terrestrial physics[1]). It also encompasses the discipline ofheliophysics, which studies thesolar physics of theSun, itssolar wind, thecoronal heating problem,solar energetic particles, and theheliosphere.
Space physics is both apure science and anapplied science, with applications inradio transmission,spacecraft operations (particularlycommunications andweather satellites), and inmeteorology. Important physical processes in space physics includemagnetic reconnection,synchrotron radiation,ring currents,Alfvén waves andplasma instabilities. It is studied using directin situ measurements bysounding rockets and spacecraft,[2] indirectremote sensing ofelectromagnetic radiation produced by the plasmas, and theoreticalmagnetohydrodynamics.
Closely related fields includeplasma physics, which studies more fundamental physics and artificial plasmas;atmospheric physics, which investigates lower levels of Earth's atmosphere; andastrophysical plasmas, which are natural plasmas beyond the Solar System.
Space physics can be traced to the Chinese who discovered the principle of thecompass, but did not understand how it worked. During the 16th century, inDe Magnete,William Gilbert gave the first description of theEarth's magnetic field, showing that the Earth itself is a great magnet, which explained why a compass needle points north. Deviations of the compass needlemagnetic declination were recorded on navigation charts, and a detailed study of the declination near London by watchmakerGeorge Graham resulted in the discovery of irregular magnetic fluctuations that we now call magnetic storms, so named byAlexander Von Humboldt. Gauss andWilliam Weber made very careful measurements of Earth's magnetic field which showed systematic variations and random fluctuations. This suggested that the Earth was not an isolated body, but was influenced by external forces – especially from theSun and the appearance ofsunspots. A relationship between individual aurora and accompanying geomagnetic disturbances was noticed byAnders Celsius andOlof Peter Hiorter in 1747. In 1860,Elias Loomis (1811–1889) showed that the highest incidence of aurora is seen inside an oval of 20 - 25 degrees around the magnetic pole. In 1881,Hermann Fritz published a map of the "isochasms" or lines of constant magnetic field.
In the late 1870s,Henri Becquerel offered the first physical explanation for the statistical correlations that had been recorded: sunspots must be a source of fast protons. They are guided to the poles by the Earth's magnetic field. In the early twentieth century, these ideas ledKristian Birkeland to build aterrella, or laboratory device which simulates the Earth's magnetic field in a vacuum chamber, and which uses a cathode ray tube to simulate the energetic particles which compose the solar wind. A theory began to be formulated about the interaction between the Earth's magnetic field and the solar wind.
Space physics began in earnest with the first in situ measurements in the early 1950s, when a team led byVan Allen launched the first rockets to a height around 110 km. Geiger counters on board the second Soviet satellite,Sputnik 2, and the first US satellite,Explorer 1, detected the Earth's radiation belts,[3] later named theVan Allen belts. The boundary between the Earth's magnetic field andinterplanetary space was studied byExplorer 10. Future space craft would travel outside Earth orbit and study the composition and structure of the solar wind in much greater detail. These includeWIND (spacecraft), (1994),Advanced Composition Explorer (ACE),Ulysses, theInterstellar Boundary Explorer (IBEX) in 2008, andParker Solar Probe. Other spacecraft would study the sun, such asSTEREO andSolar and Heliospheric Observatory (SOHO).
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