Statistical analysis of the onset temperature of solar flares in 2010–2011

Abstract

ABSTRACT Understanding the physical processes that trigger solar flares is paramount to help with forecasting space weather and mitigating the effects on our technological infrastructure. A previously unknown phenomenon was recently identified in solar flares: the plasma temperature, derived from soft X-ray (SXR) data, at the onset of four flares, was revealed to be in the range 10–15 MK, without evidence of gradual heating. To investigate how common the hot-onset phenomenon may be, we extend this investigation to solar flares of B1.2–X6.9 classes recorded by the X-ray Sensor (XRS) onboard the GOES-14 and GOES-15 satellites between 2010 and 2011. For this statistical study, we employed the same methodology as in recent work, where the pre-flare SXR flux of each flare is obtained manually, and the temperature and emission measure values are obtained by the flux ratio of the two GOES/XRS channels using the standard software. From 3224 events listed in the GOES flare catalogue for 2010–2011, we have selected and analysed 745 events for which the flare heliographic location was provided in the list, to investigate centre-to-limb effects of the hot-onset phenomenon. Our results show that 559 out of 745 flares (75 per cent) exhibit an onset temperature above 8.6 MK (the first quartile), with respective log10 of the emission measure values between 46.0–47.25 cm−3, indicating that small amounts of plasma are quickly heated to high temperatures. These results suggest that the hot-onset phenomenon is very common in solar flares.