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Using simultaneous X-ray and radio observations from solar flares, weinvestigate the link between the type III radio burst starting frequency andhard X-ray spectral index. For a proportion of events the relation derivedbetween the starting height (frequency) of type III radio bursts and theelectron beam velocity spectral index (deduced from X-rays) is used to inferthe spatial properties (height and size) of the electron beam accelerationregion. Both quantities can be related to the distance travelled before anelectron beam becomes unstable to Langmuir waves. To obtain a list of suitableevents we considered the RHESSI catalogue of X-ray flares and the Phoenix 2catalogue of type III radio bursts. From the 200 events that showed both typeIII and X-ray signatures, we selected 30 events which had simultaneous emissionin both wavelengths, good signal to noise in the X-ray domain and > 20 secondsduration. We find that > 50 % of the selected events show a good correlationbetween the starting frequencies of the groups of type III bursts and the hardX-ray spectral indices. A low-high-low trend for the starting frequency of typeIII bursts is frequently observed. Assuming a background electron density modeland the thick target approximation for X-ray observations, this leads to acorrelation between starting heights of the type III emission and the beamelectron spectral index. Using this correlation we infer the altitude andvertical extents of the flare acceleration regions. We find heights from 183 Mmdown to 25 Mm while the sizes range from 13 Mm to 2 Mm. These values agree withprevious work that places an extended flare acceleration region high in thecorona. We analyse the assumptions required and explore possible extensions toour assumed model. We discuss these results with respect to the accelerationheights and sizes derived from X-ray observations alone.

The Low-High-Low Trend of Type III Radio Burst Starting Frequencies and Solar Flare Hard X-rays