Sky & Telescope July 1982 [antikvár]

Probing the Radio SunM. R. Kundu. University of MarylandSINCE J. S. Hey's discovery of solar radio waves in 1942 our understanding of how the Sun emits such radiation has advanced steadily. This improvement is the result of our increased comprehension of radiation theor)' and fully ionized gases (plasmas), along with the development of new obser\a(ional techniques. The continual struggle for ever better angular resolution. in particular, has resulted in the development of both new instruments and new ways to use them.There are three distinct types of radio emission from the Sun. These are from its atmosphere as a whole, from active regions associated with sunspots. and from flares. The radiation from the entire atmosphere (the "quiet Sun") is due to thermal emission from hot gas. The so-called slowly varj'ing component is related to active regions and arises from radiation by hot. dense gas as well as from electrons moving at speeds of a few kilometers per second and gyrating in a magnetic field. Finally, flare-induced radio "bursts" are produced by electrons moving near the speed of light in the solar magnetic field and also by means of plasma-wave interactions.It has long been realized that radio waves of different wavelengths originate at different levels in the Sun's atmosphere. This is a direct consequence of the criticalplasma frequency associated with a given electron density in a gas. At frequencies above this value solar plasma is transparent and does not emit significant radiation, while below it radio waves do not escape. Since the electron density falls with increasing height in the solar atmosphere, lower frequency radio waves originate atgreater altitudes. The choice of a particular wavelength, therefore, offers the possibility of exploring a specific layer of our star's outer envelope, as the upper illustration on the facing page shows.The chromosphere and lower corona can be studied best at centimeter wavelengths. This is also the spectral region in whichThis white-light photograph shows an active region of the Sun on February 20, 1956. The most prominent features are sunspots, but bright faculae are also visible. The slowly varying component of solar radio emission is strongly correlated with the number and size of active areas on the visible disk of the Sun. Mount Wilson and Palomar Observatories photograph.A strong solar flare as seen in the light of the hydrogen-alpha line. Note that the structure is basically filamentary. Radio bursts are closely connected with flares. Hale Observatories photograph taken November 21, 1969.6 Sky and Telescope, July. 1982