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

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3. Discussions and conclusions

Historically the minimum of a sunspot cycle is generally the period when quantities like the smoothed sunspot number, monthly sunspot number, monthly number of spot groups, maximum number of spotless days, and 10.7-cm radio flux show a minimum. Equal numbers of old and new cycle regions is another indicator. Parameters like Mg II chromospheric index, Ca II K index, He I 1083-nm equivalent width, and total irradiance are also used to monitor the sunspot cycle. It is obvious that the inclusion of other measures of transition between cycles can lead to differences between the official minima and the minimum in the respective quantities. According to published reports, two periods of low activity were identified in 1996, one in the month of May and the other in October, for the solar cycle 22 (Joselyn et al. 1997; Harvey & White 1999). We performed a statistical study of the type I radio bursts observed during the period 1993-99 to verify the time of the minimum between the solar cycles 22 & 23. Our main conclusion is that October 1996 is the most probable time of sunspot minimum, since it was the period when no type I radio bursts were observed (As mentioned in the earlier section, the occurrence of radio noise storms requires the presence of sunspots or sunspot groups of size [FORMULA] 100 millionths of the solar disc, and magnetic field strength [FORMULA] 1000 Gauss). This agrees well with the earlier result that no sunspots were observed for more than one solar rotation (Carrington rotation 1914), during September-October of 1996. Also, October 1996 was the period when there was a minimum in the monthly sunspot number, monthly number of sunspot groups and 10.7-cm radio flux (Harvey & White 1999; De Toma et al. 2000).

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001