A New Look at Type-III Bursts and Their Use as Coronal Diagnostics

We present meter-wave solar radio spectra of the highest spectro–temporal resolution achieved to date. The observations, obtained with the first station of the Long Wavelength Array (LWA1), show unprecedented detail of solar emissions across a wide bandwidth during a Type-III/IIIb storm. Our flux calibration demonstrates that the LWA1 can detect Type-III bursts much weaker than 1 SFU, much lower than previous observations, and that the distribution of fluxes in these bursts varies with frequency. The high sensitivity and low noise in the data provide strong constraints to models of this type of plasma emission, providing evidence against the idea that Type-IIIb striae are generated from electrons trapped in Langmuir-wave sidebands. The continuous generation of electron beams in the corona revealed by the high density Type-III storm is evidence for ubiquitous magnetic reconnection in the lower corona. Such an abundance of reconnection events not only contributes to the total coronal energy budget, but also provides an engine by which to form the populations of seed particles responsible for proton-rich solar energetic-particle events. An active region (AR) with such levels of reconnection and the accompanying Type-III/IIIb storms is proposed here to be associated with an increase of SEP production if a CME erupts. The data’s constraints on existing theories of Type-IIIb production are used to make an association of the observed Type-IIIb storm to specific electron-beam paths with increased inhomogeneities in density, temperature, and/or turbulence. This scenario ties in the observed timing of Type-III and -IIIb storms, constrained theories of Type-III and -IIIb emission, and the ability of the emitting AR to produce a strong SEP event. The result requires but a single observable to cement these ideas, the statistical correlation of Type-III/IIIb activity with SEP-productive AR.