Type III Radio Bursts at Long Wavelengths: STEREO/Waves Observations and Future Prospects for Inner Heliospheric Missions

We investigate solar type III radio bursts observed by the S/Waves instruments on-board the STEREO spacecraft. These instruments provides us with goniopolarimetric (GP, also referred to as direction-finding) measurements between 125 kHz and 1975 kHz while amplitudes of electric field fluctuations are recorded up to 16 MHz. We have investigated large number of type III radio bursts from May 2007 till July 2010. Some of them have been associated with solar flares within the NOAA directory of active regions. That allows us to determine a source position of bursts when the electron density model of LeBlanc et al. (1998) has been considered. We have also located a region of type III radio bursts by triangulating the position using GP measurements. Observed type III radio bursts generally propagate in the solar equatorial plane. Our results indicate that the maximum flux density occurs at ~ 800 kHz. Future solar missions (e.g., Solar Orbiter and Solar Probe Plus) will provide new insights into properties of type III radio bursts as for instance sampling the region where this latter maximum occurs.