Solar decimetric type III bursts in semi-closed magnetic field structures

¹ National Space Research Institute (INPE), São José dos Campos, SP, Brazil
² Astronomical Institute of the Academy of Sciences of the Czech Republic, 25165 Ondřejov, Czech Republic e-mail: hana@asu.cas.cz
³ Institute of Research and Development (IP&D – UNIVAP), São José dos Campos, SP, Brazil

Received: 12 April 2007
Accepted: 1 April 2008

Abstract

Aims. We investigate statistically seventeen groups of solar type III bursts, observed in the frequency range 950-2500 MHz using the Brazilian Solar Spectroscope (BSS) during the period 1999-2002.

Methods. Using specially-developed Interactive Data Language (IDL) software called BSSView, the spectral characteristic properties of the bursts were investigated. For illustration a semi-closed magnetic field structure with trapped electron beams was modelled using the particle-in-cell model.

Results. Most of the type III bursts studied in this paper have starting frequencies in the range 1100-1400 MHz and their average bandwidth, half-power duration, and interval between adjacent bursts are 116 ± 59 MHz, 108 ± 41 ms, and 647 ± 643 ms, respectively. The number of bursts with negative frequency drift, which is on average -914 ± 684 MHz s^-1, is greater than the number with positive frequency drift, which is on average +807 ± 675 MHz s^-1. The type III bursts have power-law distribution function for their half-power duration   , bandwidth   and   , and frequency drift . Most of the groups of type III bursts were also found to exhibit either positive or negative group frequency drifts of on average +53 ± 38 MHz s^-1 and -46 ± 42 MHz s^-1, respectively. The detailed statistical analysis suggests that in the frequency range studied there are two categories of type III bursts, the average parameters of which we provide in brackets for each type: a) numerous narrowband bursts (bandwidth ~100 MHz) with small frequency drift (~±500 MHz s^-1) in groups with visible group drift (~±50 MHz s^-1) and b) less numerous broader band bursts (bandwidth 171 MHz) with higher frequency drift (~±1800 MHz s^-1) in groups without group drift. The statistics of the parameters of these drifting groups are presented for the first time. The drifting groups of type III bursts are interpreted as due to electron beams trapped in moving plasmoids, which are semi-closed magnetic field structures.