Sun

sun

History of observation The existence of features on the Sun was known from the records of sunspots observed by ancient astronomers with the naked eye; however, no systematic studies were made of such features until the telescope was invented in the early 17th century. Sir Isaac Newton set forth the role of the Sun as the centre of attraction of the known planetary system. Scheiner’s drawings in the Rosa Ursina are of almost modern quality, and there was little improvement in solar imaging until 1905. In the 1670s the British astronomer John Flamsteed and the French astronomer Gian Domenico Cassini calculated the distance to the Sun. The Italian scientist Galileo Galilei and the German mathematician Christoph Scheiner were among the first to make telescopic observations of sunspots.Hard X-ray data could be obtained without saturation. However, the task is not easy. By 1960 astronomers realized that these sites not only had to be clear but that they also had to have stable air. A huge advance in resolution came with Skylab, a manned U.S. They also were the first to observe gamma rays emitted from nuclear reactions in flares and to use an externally occulted coronagraph to view coronal mass ejections. In 1891, while he was a senior at the Massachusetts Institute of Technology in Cambridge, Massachusetts, George Ellery Hale invented the spectroheliograph, which can be used to take pictures of the Sun in any single wavelength. Moreover, satellites sampling the solar particles have the ability to monitor directly solar waves and particles that do not reach the ground. Prior to the construction of the Mount Wilson facility, all solar observatories were located in cloudy places, and long-term studies were not possible. Ultraviolet optics demanded special coatings and films (now charge-coupled devices) for observing. By locating observatories near lakes and by employing electronic imaging and vacuum telescopes, astronomers were able to make new, higher-resolution observations. With one or two exceptions, all of the important spectral lines from the chromosphere and corona are in the ultraviolet, and since the photosphere is relatively weak in the ultraviolet, it is easy to disentangle the images of the upper layer from the powerful visible radiation of the photosphere. Solar Maximum Mission (SMM), launched in 1980. After using the instrument on the great Yerkes refractor in Williams Bay, Wisconsin, U.S., Hale developed the Mount Wilson Observatory in California and built the first solar tower telescopes there. The development of instruments to study the Sun also benefited the creation of satellites that explored beyond the solar system.

The U.S. With the Lyot filter, cinematography of the solar activity of magnetic and velocity fields became a reality. Special solar trackers were required to keep the image steady, and good telemetry was needed for the large data flow. Skylab produced the first high-resolution images in the ultraviolet lines as well as the first X-ray images of the corona. For the corona, special coronagraphs were developed, with a series of occulting disks in front of an ultraclean lens. Hale discovered the magnetic fields of sunspots by observing the splitting of their spectral lines into a number of components; this splitting, known as the Zeeman effect, occurs in the presence of a strong magnetic field. New technological developments permitted greatly improved data, particularly on the solar-cycle dependence of the solar constant. In 1969 the movement began with the Aerospace Corporation Observatory (now the San Fernando Observatory) and the Big Bear Solar Observatory, both in California. In the 1960s the American astronomer Robert Leighton modified Hale’s spectroheliograph so that it could measure both velocities and magnetic fields and with it discovered solar oscillations. After 1950, new observatories were established in areas that were less cloudy. By continuously studying the spots for two cycles, he discovered, with the American astronomer Seth Barnes Nicholson, the law of sunspot polarities. In the 1930s the French astronomer Bernard Lyot introduced the coronagraph, which made possible spectral observations of the corona when the Sun is not in eclipse, and the birefringent filter, which permitted two-dimensional monochromatic images. space station that used leftover hardware from the Apollo project. Free of ground effects, these observatories achieved a new level of stable images and were soon followed by lake-sited solar observatories in India and China. An entirely new dimension of solar studies was initiated by the space age. The Skylab images displayed the coronal holes for the first time, and the timing of their disk passage showed their role as a source of high-speed solar wind streams and geomagnetic disturbances. The next important spacecraft was the U.S. In 1981 SMM’s attitude control system malfunctioned, and the SMM mission was suspended until 1984 when it was repaired by the space shuttle Challenger. Orbiting Solar Observatory series of satellites (OSO 1–8, launched from 1962 to 1975) made the first observations of X-rays and gamma rays from solar flares. For X-rays, a high rate of photon detections per unit time was required to avoid early problems with pulse pile-up. Later, in 1953, the American father-and-son team of astronomers Harold and Horace Babcock, working with the same instruments, developed the magnetograph, with which the polar field was detected.Yohkoh produced continuous images of the corona in soft X-rays, detected and located hard X-ray bursts, and produced important soft X-ray spectra. The European Space Agency spacecraft Ulysses, launched in 1990, was the first space probe to travel in a polar orbit around the Sun. Hinotori obtained the first measurements of a superthermal (30,000,000–40,000,000 K) cloud produced by solar flares, which is the source of the soft X-ray burst accompanying all solar flares. Japan launched two very successful satellites, Hinotori and Yohkoh, in 1981 and 1991, respectively. It discovered that the solar wind speed does not increase continuously toward the poles but rather levels off at high latitudes at 750 kilometres (450 miles) per second.The U.S. The two spacecraft of the U.S. Solar Terrestrial Relations Observatory (STEREO) mission, also launched in 2006, form a 90° angle with the Sun in order to make stereoscopic images of it. The Japanese spacecraft Hinode, launched in 2006, discovered magnetic waves in the solar chromosphere that drive the solar wind. There are several satellites actively observing the Sun. SOHO can observe the Sun continuously, and, among its many discoveries, it has found that sunspots are shallow and that the solar wind flows outward by waves in vibrating magnetic field lines. The U.S. TRACE is a powerful tool for exploring the chromosphere-corona interface and has found that much of the heating in the corona takes place at its base. satellite Solar Dynamics Observatory (SDO), launched in 2010, carries three instruments that observe the Sun every 10–50 seconds to study changes that previously launched satellites were not able to observe. satellites Solar and Heliospheric Observatory (SOHO) and Transition Region and Coronal Explorer (TRACE), launched in 1995 and 1998, respectively, have produced many important results.