63. The enhanced radiation from sunspot-regions

Extract

The so-called enhanced radiation appears at all frequencies in connexion with sunspot-regions. For λ < 25 cm. it may be interpreted satisfactorily as thermal radiation of the coronal condensations (Waldmeier, 1955) [1]. For λ > 50 cm. the mechanism responsible for the production of this emission is still unknown. The enhanced radiation from 62 to 200 Mc./s. shows a strong concentration in the radial direction and, therefore, becomes observable only when the source of radiation is close to the centre of the sun's disk. For sin θ = 0·3, i.e. at an angular distance of θ = 17° from the centre of the solar disk, the intensity has dropped to 50 % of that for θ = 0° and for sin θ = 0·5 to about 25 %. For sin θ = 0·8 the radiation intensity is certainly < 10%. This property has recently been investigated again by Machin and O'Brien (1954) [2] as well as by H. Müller[3], using the entire material of the years 1947–53 published in the Quarterly Bulletin on Solar Activity. According to Müller the limb darkening is very similar for the frequencies 62 to 200 Mc./s., the radiation intensity having decreased at sin θ = 0·29 to half of its value for sin θ = 0. A diminution to 10% may be found for 62 Mc./s. at sin θ = 0·54, for 80 Mc./s. at sin θ = 0·59, for 98 Mc./s. at sin θ = 0·62, for 175 Mc./s. at sin θ = 0·72 and for 200 Mc./s. at sin θ = 0·77. For 600 and 1200 Mc./s. the limb darkening is much weaker, the intensity decreasing to 50% at sin θ = 0·57 and 0·75 respectively.