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

2. Data set

The type I radio burst data reported were obtained from the spectral observations of solar radio emission published in the Solar Geophysical Data (March 1993 - February 2000). The reporting stations were Bleien (47N 9E), Potsdam (52N 13E), Ondrejov (49N 14E), San Vito (41N 18E), Izmiran (55N 37E), Learmonth (22S 114E), Hiraiso (36N 140E), Culgoora (30S 150E), Palehua (21N 204E) and Sagamore Hill (42N 289E). On occasions, data obtained with the Gauribidanur radioheliograph (13N 77E, Ramesh et al. 1998) was also used for the present study. One can notice that the longitudinal distribution of the different observing locations is such that there is almost a continuous observation of the Sun over a period of 24 hrs. Fig. 1 shows the total number of type I radio bursts observed each year during the period 1993-99. The lowest number of bursts observed was during 1996, the official minimum between the solar cycles 22 & 23 (Harvey & White 1999, and the references therein). In order to ascertain the exact time of minimum during 1996, we plotted a histogram of the total number of bursts observed each month during 1996 (Fig. 2). One can notice that no bursts were observed during the month of October 1996. The non-uniformity in the distribution of bursts in each of the above two figures (Fig. 1 & Fig. 2) might be due to the differences in the probability of their association with different classes of sunspots (Fokker 1960; Le Squeren 1963), according to the Zürich classification scheme. Fig. 3 & Fig. 4 shows the number of type I radio bursts observed each month during 1996 plotted against the monthly average of the 10.7-cm radio flux, and sunspots respectively (The data corresponding to the F10.7 monthly average and sunspot monthly average in Fig. 3 & Fig. 4 were taken from De Toma et al. [2000]). The variations in the number of observed bursts correlates well with that in the 10.7-cm flux and in sunspot number.

Fig. 1. Histogram showing the total number of type I radio bursts observed each year during the period 1993-99.

Fig. 2. Histogram showing the number of type I radio bursts observed each month during the year 1996.

Fig. 3. Variation of the number of type I radio bursts observed each month during 1996 with the monthly average of 10.7-cm radio flux during the same year.

Fig. 4. Variation of the number of type I radio bursts observed each month during 1996 with the monthly average of sunspots during the same year.

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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