Decimetric type-III Bursts as the Trapping of Superthermal Electrons in Plasmoids

H. Meszarosova$^1$, H.S. Sawant$^2$, M. Karlicky$^1$, F.C.R. Fernades$^3$, J.R. Cecatto$^2$, M.C. de Andrade$^2$

$^1$Astronomical Institute of the Academy of Sciences of the Czech Republic, 25165 Ondrejov, Czech Republic
$^2$National Space Research Institute (INPE), Sao Jose dos Campos, SP, Brazil
$^3$Institute of Research and Development (IP&D - UNIVAP), Sao Jose dos Campos, SP, Brazil

In the period 1999-2002, 17 groups of type III bursts, observed by Brazilian Solar Spectroscope (BSS) in the frequency range 950-2500 MHz, were investigated statistically. Most of the type III bursts have starting frequencies in the range 1100-1400 MHz and their average bandwidth, half-power duration and interval between adjacent bursts of 134 MHz, 123 ms and 797 ms, respectively. The number of bursts with negative frequency drifts (on average -1384 MHz/s) is greater than those with positive frequency drifts (on average 1428 MHz/s). In addition, we have found that the type III bursts have power-law distribution functions of their half-power durations, bandwidths and frequency drift of the bursts. Most of the groups of type III bursts were also found to exhibit either positive or negative group frequency drifts (on average 40 MHz/s). A detailed statistical analysis suggests that there are two categories of type III burst groups. One category is composed by broadband bursts without any group frequency drifts and the other one is composed of narrow band bursts showing the group frequency drift(s). In this work we suggest that the broadband bursts have their origins in open magnetic field lines and we have, for the first time, included statistics of various parameters of positive/negative drifting groups of narrow band type III bursts. These drifting groups are interpreted as the trapping of superthermal electrons in plasmoids (plasma in closed magnetic structures).