LUNAR webinar, 12th October 2010

Nat Gopalswamy, NASA/GSFC

Low-frequency Solar Radio Bursts



The Sun is a radio emitter in the millimeter to kilometer wavelengths (100s of GHz to a few kHz). The radio emission occurs via various mechanisms: free-free, gyro-resonance, gyro-synchrotron, and coherent plasma emissions. Nonthermal radio emission is due to electrons accelerated from keV to MeV energies. The low frequency solar radio bursts are of particular importance, because they are associated with large-scale solar eruptions and the associated interplanetary disturbances. Coronal mass ejections (CMEs) have important connections to various types of radio emissions from the Sun. To observe these bursts, one has to go to space because of the ionospheric cutoff at ~20 MHz. In this talk, I focus on radio bursts that can be observed only from space: (i) Type III bursts, (ii) type II bursts, and (iii) type IV bursts. These bursts are typically the brightest features in the radio dynamic spectra and are several orders of magnitude brighter than the galactic background. Long-lasting type III bursts are associated with CME eruption, thought to be due to the reconnection process taking place beneath the erupting CME. Type II bursts are indicative of electron acceleration in the CME-driven shocks and hence considered to be the direct response of the CME propagation in the corona and interplanetary medium. Type IV bursts indicate large-scale post-eruption arcades containing trapped electrons that produce radio emission. These bursts provide valuable information on the particle acceleration mechanisms in the coronal and interplanetary medium and on the properties of the ambient medium.