Coronal type III radio bursts and their X-ray flare and interplanetary type III counterparts

Abstract

Context. Type III bursts and hard X-rays are both produced by flare energetic electron beams. The link between both emissions has been investigated in many previous studies, but no statistical studies have compared both coronal and interplanetary type III bursts with X-ray flares. Aims. Using events where the coronal radio emission above 100 MHz is exclusively from type III bursts, we revisited some long-standing questions regarding the relation between type III bursts and X-ray flares: Do all coronal type III bursts have X-ray counterparts? What correlation, if any, occurs between radio and X-ray intensities? What X-ray and radio signatures above 100 MHz occur in connection with interplanetary type III bursts below 14 MHz? Methods. We analysed ten years of data from 2002 to 2011 starting with a selection of coronal type III bursts above 100 MHz. We used X-ray flare information from RHESSI >6 keV to make a list of 321 events that have associated type III bursts and X-ray flares, encompassing at least 28% of the initial sample of type III events. We then examined the timings, intensities, associated GOES class, and whether there was an associated interplanetary radio signature in both radio and X-rays. Results. For our 321 events with radio and X-ray signatures, the X-ray emission at 6 keV usually lasted much longer than the groups of type III bursts at frequencies >100 MHz. The selected events were mostly associated with GOES B and C class flares. A weak correlation was found between the type III radio flux at frequencies below 327 MHz and the X-ray intensity at 25-50 keV, with an absence of events at high X-ray intensity and low type III radio flux. The weakness of the correlation is related to the coherent emission mechanism of radio type IIIs which can produce high radio fluxes by low density electron beams. Interplanetary type III bursts (<4 MHz) were observed for 54% of the events. The percentage of association increased when events were observed with 25-50 keV X-rays. A stronger interplanetary association was present when 25-50 keV RHESSI count rates were above 250 counts/s or radio fluxes of around 170 MHz were large (>103 SFU), relating to electron beams with more energetic electrons above 25 keV and events where magnetic flux tubes extend into the high corona. We also find that whilst on average type III bursts increase in flux with decreasing frequency, the rate of this increase varies from event to event.