F.C.R. Fernandes, J.R. Cecatto, M.C. de Andrade, H.S. Sawant
National Institute for Space Research - INPE, San Jose dos
Campos, SP, Brazil
The Brazilian Solar Spectroscope (BSS), operating in the frequency range of 1.0-2.5 GHz is in regular operation at National Institute of Space Research (INPE), Brazil, since 1998. Various types of fine structures are observed with high resolution since then. Here, we concentrate in the analysis of radio emissions associated with 8 solar flares, observed in the period of March, 1999 to July, 2002, showing groups of decimeter fine structures with high frequency edge slowly drifting from high to low frequencies, with rates less than 100 MHz/s. The main morphological aspects of each fine structure are narrow-band of about 5-10 MHz and small duration of the order of 50 milliseconds. The majority of these fine structures are observed over a time interval of a couple of minutes before the maximum of the associated flare, in the impulsive phase. However, some observations during the gradual decay phase are also reported. Those fine structures were interpreted as a possible signature of chromospheric evaporation process in decimeter wavelengths. From the frequency drift rates of the high frequency edge, we estimated the front velocity of the evaporation shock, as in the range of 30-1700 km/s. The velocities of the exciter estimated from the drifting structures suggest that the shock-like exciter is propagating in the higher solar chromosphere. Details of these observations and their interpretation in terms of the plasma emission produced by accelerated particles and the chromospheric evaporation front is presented.