Introduction
The first radio signals sent across the Atlantic Ocean by Guglielmo Marconi in 1905 would never have arrived had it not been for a layer of charged particles called the ionosphere. This layer, some 50 miles from the surface, acted like a reflecting mirror for radio waves. But what was causing this part of the atmosphere to remain charged in the first place? The answer had to await the advent of rocket technology following 1945.
This is the first x-ray image of the sun taken on April 19, 1960. Image courtesy: Naval Research Lab.
The partnership of Herbert Friedman, a solar physicist, and E. O. Hulbert, a specialist in the upper atmosphere, both at the Naval Research Laboratory in Washington DC, led to an amazing discovery.
Friedman’s first sounding rocket experiment was launched from White Sands, New Mexico using a V-2 rocket captured from Germany. It carried instruments able to detect ultraviolet and X-ray radiation – two forms of electromagnetic radiation that had enough energy to ionize atmospheric gas atoms. What was soon discovered was that the sun, itself, was the source of enough x-ray and ultraviolet radiation to keep the ionosphere fully charged during the daytime.
But these atmospheric studies quickly turned into studies of the sun itself. A method was soon developed to actually photograph the sun at ultraviolet wavelengths, and eventually x-rays too. Friedman eventually captured images of solar flares, demonstrating for the first time that these flares produce enormous amounts of x-ray energy.
Modern X-ray mage of the sun taken by the Yohkoh satellite.
Modern X-ray mage of the sun taken by the Yohkoh satellite.
Then during the October 12, 1958 total solar eclipse, a series of rocket launches proved that the sun’s x-ray light extended far above the sun’s surface into the corona, and was also concentrated into small, bright regions on the solar surface. Simultaneous studies of the upper atmosphere, meanwhile, proved that, as the x-ray light from the sun was occulted, measurable changes occurred in the heating of the upper atmosphere and ionosphere.
As the Space Age progressed, newer x-ray imagers were designed and carried into space. A recent pioneering satellite, Yohkoh, operated in space between 1991 and 2001 and returned thousands of images of the solar x-ray corona and surface activity. These have been made into actual movies with a cadence fast enough to let solar physicists investigate large-scale releases of magnetic energy from solar active regions.
External Links
- Yohkoh Solar Observatory - David McKenzie, Montana State University.
Preview Image
"Solar X-ray Cycle" - The solar corona is a region of magnetically confined gas surrounding the sun. The corona is incredibly hot, reaching temperatures above 2 million degrees, hot enough to produce X-rays. Scientists believe that the solar magnetic field heats the corona; because the solar magnetic fields undergoes an 11-year solar activity cycle, the temperature and brightness of the solar corona also undergoes variations with this activity cycle. This variation is beautifully illustrated in the above images of the solar corona, obtained by the Soft X-ray Telescope on the Yohkoh solar observatory. The corona looks bright, while the solar surface (which, at 6000 degrees, is too cool to produce X-rays) appears dark. The 12 solar X-ray images shown above were obtained between 1991 and 1995, and show the decrease in solar coronal brightness (by about a factor of 100) as the sun goes from an "active" state to a less active state. Yohkoh has just celebrated 10 years in orbit, observing the sun from space for nearly one complete solar cycle. (Source: NASA/HEASARC.)
Citation
Odenwald, Sten, Ph.D. (Contributing Author); Bernard Haisch (Topic Editor). 2009. "Solar Imaging: X-ray." In: Encyclopedia of the Cosmos. Eds. Bernard Haisch and Joakim F. Lindblom (Redwood City, CA: Digital Universe Foundation). [First published November 27, 2007].
<http://www.cosmosportal.org/articles/view/137540/>
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