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Astron. Astrophys. 364, 873-875 (2000)

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1. Introduction

It is well known that the most frequently observed solar phenomenon at meter and decimeter wavelengths are disturbed periods usually of hours' or days' duration; these periods consist of a long series of bursts called storm bursts, and the radiation at such times is called the noise storm or enhanced radiation (see Kundu 1965; Elgaroy 1977; Krüger 1979; Kai et al. 1985, for a review on the topic). On single frequency records obtained with high time resolution ([FORMULA]100 msec), it appears as a steady or slowly varying backgound level with frequent short-lived bursts. The background level (without the bursts) is commonly referred to as the background continuum, and the bursts superimposed upon it are known as the storm bursts or type I bursts (Wild et al. 1963). It is generally accepted that the noise storm radiation originates in the solar atmosphere above a group of sunspots, and reaches a sharp maximum near the central meridian passage of the associated spot group (Hey 1946; McCready et al. 1947). The chances that a sunspot is associated with a noise storm increases with the area of the spot. Payne-Scott & Little (1951) found that sunspots whose area is [FORMULA] 400-millionths of the Sun's disc are more likely associated with noise storms. A similar result was arrived at independently by Le Squeren (1963). However, later observations indicate that even spots of size [FORMULA] 100 millionths of the solar disc are associated with type I storms (Dodson & Hedeman 1957; Dulk & Nelson 1973). Strong magnetic fields are also necessary for the generation of type I storms since the latter are 100% circularly polarised at most times (Ryle & Vonberg 1946; Martyn 1946; Appleton & Hey 1946). Based on a statistical study of solar activity observed during the sunspot minimum period in 1954, Dodson & Hedeman (1957) concluded that the spots associated with noise storms have a field strength [FORMULA] 1300 Gauss. This is expected because there is a close relationship between the size of a sunspot and its associated magnetic field (Payne-Scott & Little 1951). Elgaroy (1982) pointed out that the observed characteristics of the noise storms varies with sunspot cycle, i.e. they are more frequent and intense during the maximum of the cycle, and are rare and weak during the minimum. Brueckner (1983) showed that all type I noise storms observed during the Skylab period were caused by changes in the coronal magnetic field structure, and all coronal magnetic field changes observed on the disc were correlated with newly emerging flux. According to Kai et al. (1985), it is possible that the source of a type I storm lies in a closed loop of strong magnetic field just above the associated active region. Therefore, it is clear that the existence of a sunspot or a group of sunspots is a necessary condition for the generation of noise storms. In view of this close association between the two, we use the observations of type I radio bursts to independently verify the time of the minimum between the solar cycles 22 & 23.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001