Beam Speeds and Source Longitudes for Type III Solar Radio Bursts from Magnetic Mapping Analyses

Energetic electrons accelerated during solar flares can form an electron beam and generate type III solar radio bursts while streaming along open magnetic field lines. The electron beams are rarely measured in situ and estimating the beam speed from the frequency drift rate of the type III emission is indirect and has several issues. A new method is presented that combines the electron paths obtained from maps of the large-scale magnetic field lines with the release of electrons at the onset times t₀ of type III bursts. Electron energy losses are neglected, so the beam speed at a distance L along a field line from the source varies with time as v_b(t) = L / (t - t₀). The method is applied to 21 type III bursts for which the STREREO spacecraft observed beam-driven Langmuir wavepackets in situ and the beam speeds were independently estimated from observations and theory of the nonlinear electrostatic decay process for Langmuir waves. The two methods yield estimated beam speeds that: (1) range from 0.03c to 0.28c, decreasing with time within each event; (2) agree within better than ≈ 10% for the first and last wavepackets in all 21 events, and (3) agree better than 10% for each of over 14 groups of Langmuir wavepackets during an unusually long-lived event that lasted almost 75 minutes and clearly demonstrates the hyperbolic relation above for v_b(t). Finally, tracing the field lines from STEREO back to the Sun yields source longitudes that are within 10° and 20° of the associated flare's active region for 6 and 17, respectively, of the 21 type IIIs considered. These results provide strong arguments in favour of the model and against this paper's Langmuir wavepackets being driven by beams faster than 0.3c.