Science

Solar radio emission

The Sun is our closest star and an intense radio source. In addition to the strong thermal radiation of the quiet Sun there is intense radio emission from bursts that are associated with phenomena of solar activity like flares and coronal mass ejections (CMEs).

The theory of space plasmas provides the tools for investigating the physics of the source regions of solar radio emission as it is observed on Earth, e.g. by LOFAR in the frequency range 20-240 MHz. Thus, radio waves can be used to investigate the physical processes related with flares and CMEs. Basically, energetic electrons are needed for non-thermal radio emission. These electrons are produced by processes associated with flares and CMEs. They excite high frequency plasma waves, e.g. Langmuir and/or upper-hybrid waves, which convert into electromagnetic (radio) waves. Thus, the radio waves are emitted near the local electron plasma frequency. For instance, electrons can directly be accelerated in a flare and then be injected into magnetic field geometries that are open towards the higher corona or interplanetary space, leading to the well-known solar type III radio bursts.

The phenomena of the active Sun, like flares and CMEs influence the Earth and its environment and can lead to disturbances of our technical civilisation. Flares are accompanied with an enhanced emission of X-rays and enhanced fluxes of energetic particles, e.g. electrons, protons, and heavy ions, while CMEs also produce highly energetic particles, and can cause geomagnetic storms if they impact on Earth’s magnetosphere. These effects of the solar-terrestrial relations are called Space Weather.