A Relook on the Effects of Coronal Mass Ejections (CMEs) on Solar Radio Emissions

Coronal mass ejections (CMEs) are associated with a whole host of radio bursts caused by nonthermal electrons accelerated during the eruption process. Radio bursts at low frequencies (< 15 MHz) are of particular interest because they are associated with energetic CMEs that travel far into the interplanetary (IP) medium and affect Earth’s spacial environment if they are Earth-directed. Low frequency radio emission needs to be observed from space because of the ionospheric cutoff, although some radio instruments permit observations down to a few MHz [Erickson 1997; Melnik et al., 2008]. Three types of radio bursts are prominent at low frequencies: type II, type III, and type IV bursts, all due to nonthermal electrons accelerated during solar eruptions. The radio emission is thought to be produced by the plasma emission mechanism [Ginzburg and Zheleznyakov, 1958], involving the generation of Langmuir waves by nonthermal electrons. Langmuir waves scattered off of ions or low-frequency turbulence result in radiation at the fundamental of the local plasma frequency. Two Langmuir waves can also coalesce to produce electromagnetic waves at twice the local plasma frequency. The radio bursts thus provide important diagnostics of the solar eruption and the ambient medium through which the solar disturbances propagate.