Solar Type III Radio Bursts Observed by STEREO/Waves

Type III radio bursts are a consequence of suprathermal electrons accelerated during solar flares and/or Coronal Mass Ejections. As electron beams propagate outward from the Sun, they can excite intense Langmuir waves at the local plasma frequency fp. These waves can be partly converted via plasma emission processes into the electromagnetic radiation: type III radio bursts either at fp and/or at 2fp. We have performed a statistical study of 156 type III radio bursts observed by STEREO/Waves between May 2007 and September 2011. We have included only simple and isolated events. We have investigated the flux density between 125 kHz and 16 MHz. The maximum flux density occurs at 1 MHz on both spacecraft being in agreement with results of simulations of the resonant interaction of an electron beam with Langmuir waves. The apparent source size of observed type III radio bursts is very extended (40 degrees) for frequencies below 500 kHz while it remains almost constant (25 degrees) for higher frequencies.