) Pergamon www.elsevier.com/locate/asr Adv. Space Res. Vol. 29, No. 3, pp. 349-354, 2002 © 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved. Printed in Great Britain 0273-I 177/02 $22.00 + 0.00 PII: S0273-1177(01)00596-8 DECIMETRIC DOT-LIKE STRUCTURES H.S. Sawant 1, F.C.R. Fernandes 1, J.R. Cecatto 1, H.O. Vats 12, J.A.C.F. Neri 1, V.A. Portezani 1, A.R.F. Martinon 1, M. Karlick:~3, K. JiiiSka 3, and H. M@sz~,rosov£ 3 1Astrophysics Division - INPE, Cx.P. 515, 12201-970, Sdo Josd dos Campos, Brazil 2Physical Research Laboratory - PRL, Ahmedabad, 380009, India 3Astronomical Institute of the Academy of Sciences, CZ-25165 Ond~ejov, Czech Republic ABSTRACT Various types of fine structures have been observed by the Brazilian Solar Spectroscope (BSS) in the fre- quency range of (1000 - 2000) MHz, since its regular operation from 1998. Here, we report for the first time statistical analysis of the 165 isolated dot-like emissions and their chains observed, from September 1999 to September 2000, in the decimetric band. Dots are observed in groups spread over the frequency - time plane. Rarely their chains are observed. Chains of dots cover the frequency range of ,-~ 150 MHz and hardly cover the entire frequency range of observations. Isolated dot-like emission has been observed to the limit of the BSS resolutions (3 MHz, 50 ms). In 50 % of the cases the observed instantaneous bandwidth and flux values are 5 MHz and -~ 20 sfu, respectively. Also, in 90 % of the cases total duration is of about 50 ms, equal to the limit of BSS time resolution. The groups of the dots have been found to be associated with the rising part of the impulsive flares and a radio-continuum. A model of dot-like emissions based on the anisotropic beam instability is suggested. © 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION Observations of the burst components and their interpretations in the decimetric range lead to better understanding of the processes of the acceleration/ejection of the particles (Bastian et al., 1998; Mel~ndez et al., 1999) including the process of chromospheric evaporation (Aschwanden and Benz, 1995). On the other hand, in decimetric range, many fine structures in frequency/time with varying intensity are observed offering challenges to plasma physicists. Recently, various high-resolution digital decimetric spectroscopes have been put into operation (Kriiger and Voight, 1995). A number of catalogues of the decimetric-metric observations have been published (Bernold, 1980; Slottje, 1981; Wiehl et al., 1985; Gfidel and Benz, 1988). However, there are few catalogues reporting high-resolution observations above 1000 MHz (Allaart et al., 1990; Isliker and Benz, 1994; Fernan- des et al., 2001). Examination of various catalogues suggests that short duration narrow bandwidth bursts exhibiting intensity variation with frequencies are observed from 30 MHz to ~ 10 GHz. All spectroscopes have observed bursts to the limit of their resolutions. Some of these observations have been classified as dot-like emissions. To our knowledge, dot type emission in the decimetric range associated with the rising part of the impulsive flare have been reported for the first time here. The observed short duration, ~ 50 ms, of the isolated dot-like emission puts a constraint on the process of acceleration. Here, we will discuss details of the characteristics of the dot-like emissions observed in the frequency range of (1000 - 2000) MHz and their association with other solar activity. Also, the possible emission mechanism is proposed. INSTRUMENTATION The Brazilian Solar Spectroscope - BSS - in conjunction with the 9 meter diameter polar mounted parabolic antenna (Sawant et al., 2001) is operating regularly, from 11 - 19 UT, over the frequency range of (1000 - 2000) MHz, with high time (50 ms) and frequency (3 MHz) resolutions. BSS is a digital spectroscope having 100 digital channels (Sawant et al., 2000), with the absolute timing accuracy of 3 ms. Minimum 349