A Comparative Study of Confined and Eruptive Solar Flares using Microwave Observations

Abstract:

It is well known that about 10% X-class solar flares are not associated with coronal mass ejections (CMEs). These flares are referred to as confined flares, which are not associated with mass or energetic particles leaving the Sun. However, electrons are accelerated to MeV energies as indicated by the presence of microwave emission with a turnover frequency of ~15 GHz (Gopalswamy et al. 2009, IAU Symposium 257, p. 283). In this paper, we extend the study of confined flares to lower soft X-ray flare sizes (M and above) that occurred in the time window of the Nobeyama Radioheliograph (NoRH). We also make use of the microwave spectral information from the Nobeyama Radio Polarimeters (NoRP). During 1996 – 2014, NoRH and NoRP observed 663 flares with size M1.0 or larger. Using the CME observations made by SOHO/LASCO and STEREO/SECCHI, we found 215 flares with definite CME association (eruptive flares) and 202 flares that definitely lacked CMEs (confined flares). The remaining 146 flares whose CME association is unclear are excluded from the analysis. We examined the peak brightness temperature and the spatial size obtained by NoRH. Although there is a large overlap between the two populations in these properties, we found that microwave sources with the largest spatial extent and highest brightness temperature are associated with eruptive flares. Spectral analysis using NoRP data showed a tendency that more confined flares had higher turnover frequency (≥17 GHz). We also compare the NoRH images with the photospheric magnetograms to understand the difference in the magnetic structure of the two types of flare sources.