By the early seventies, after a decade of monitoring the solar emission in the ultraviolet and X electromagnetic domains, the main characteristics of the structure and dynamics of the corona were fairly well established thanks to the Skylab mission, preceded by observations with the Orbiting Solar Observatories and by a series of sub-orbital rocket flights carrying scientific instrumentation.
Friedman reports the adventurous account of the first pioneering observations of the solar X-ray sources. The solar corona appeared as a highly inhomogeneous X-ray source with 80% of the emission coming from 5% or less of the area of the solar disk (Blake et al. The X-ray image of the Sun, obtained by the Naval Research Laboratory scientists during a sub-orbital rocket flight on April 19, 1960, in the form of a crude low-resolution photograph, revealed for the first time the hottest layers of the solar atmosphere against the bright disk. These events are characterized by the emission of highly energetic electromagnetic radiation and particle acceleration, frequently associated with huge plasma ejections, inducing transient changes and a reorganization of the structure of the corona. When the level of activity increases, magnetic energy is sporadically released in sudden and powerful reconnection events occurring at coronal heights. Coronal holes and quiescent streamers, reflecting long-lived large-scale patterns, are prevalent in the declining to minimum phase of solar activity when the magnetic field becomes organized according to lower order multipoles. Active regions are dominant in the ascending phase of the solar cycle and at solar maximum when the relatively simple poloidal configuration of the solar magnetic field typically observed at solar minimum is disrupted by the emergence of the toroidal field of the new cycle. It would take temperatures near a million degrees C to make iron atoms behave this way, but how could the corona possibly be that hot? Discovering the source of the Coronium line only opened up another perplexing question that astronomers were not able to answer until the start of the 21st Century.The picture of the solar corona derived from ground-based and space observations is that of a hot inhomogeneous magnetized atmosphere, which is constantly evolving and consisting of features developing on different scales in the various phases of the solar activity cycle. A weaker red line also seen in the corona at 6374 Angstroms, was produced by iron atoms stripped of 9 electrons (Fe X). One of the remaining 13 electrons, when excited, gives up its energy and produces this green line. The atom had lost 13 of its 26 electrons (a state that physicists call call ‘Iron-14’ or Fe XIV). It wasn’t until 60 years later that Swedish astronomer Bengt Edlin(1906-1993)finally determined that the Coronium lines were caused by the element iron seen under very high temperatures.
Young even went so far as to identify it as Iron Line Number 1474, though it didn’t fit the expected pattern of lines from this element. Like Lockyer’s helium, this new atomic line in the solar corona was considered to be from a new element unlike anything seen under laboratory conditions, so they called it Coronium. Norman Lockyer was the first astronomer to attach a spectroscope to a telescope to study the sun. This green line is so intense that, when eclipses are photographed (like the image above) the green color can easily be seen. When they pointed their telescopes at the solar corona during the Augeclipse, they saw several bright lines, including a particularly strong green line at a wavelength of 5303 Angstroms (1 Angstrom unit = 10 -8 centimeters). Meanwhile, the solar corona was revealing itself as an equally mysterious region.Ĭharles Augustinus Young (1834-1908) and William Harkness (1837-1903) independently discovered a new bright (emission) line in the spectrum of the Sun's corona. This element was not detected on Earth until about 25 years later. In 1868, Sir Norman Lockyer (1836-1920) discovered a new line in the solar spectrum called Helium. By splitting light into its component wavelengths much as prism splits light into its colors, astronomers soon detected familiar elements like calcium and sodium in the light from distant stars and nebulae. Thanks to the advent of the spectroscope by two German scientists, Robert Wilhelm Bunsen (1811-1899) and Gustav Robert Kirchoff (1824-1887) in 1859, astronomers began experimenting almost immediately with this new way of analyzing light. The green color of the solar corona seen during a total solar eclipse was once thought to be cause by the element coronium.
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