The eCallisto project is an international network of solar radio spectrometers designed to detect and measure radio emissions from the sun's corona.

The sun is a huge ball of hyrogen gas underoging nuclear fusion, creating enormous amounts of energy in the form of electromagnetic radiation and charged particles, many of them electrons and protons. It's the motion of these charged particles that gives rise to emissions of radio waves that we can detect on Earth. There are three main mechanisms for generating the radio waves. These are thermal Bremsstrahlung (braking radiation), Gyromagnetic Emission and Plasma Emission. (There is a fourth mechanism, Electron-Cyclotron Maser Emission, which is far less common.)

Bremstrahlung is the process by which electrons rapidly slow down due to collisions with ions. "Bremmsstrauhlung" is a German word meaning "braking" (brems) "radiation" (strahlung). This rapid deceleration generates radiowaves typically in the frequency range below 300 MHz and is responsible for a steady background radio noise from the sun's corona. 

Gyromagnetic Emission arises when a charged particle moves in the sun's magnetic field which causes it to follow a spiral path. This motion causes the charged particles to lose energy by emitting radio waves, typically in the GHz region. Like bremsstrahlung, this emission forms a continuous background noise.

Plasma Emission arises from instabilities in the "gas" of electrons within the sun. In a typical gas like air on Earth, the pressure and density remains about the same because the air molecules only interact by bouncing off one another. However, in an electrically charged gas, also known as a plasma, there are strong electric and magnetic fields that can effect the motion of the particles over great distances. But likewise, the positions and motion of the particles also determine the strength of the electric and magnetic fields. These relationships can result in a very complex interaction causing charge density fluctuations that result in the emission of radio waves. Plasma emission can be short lived but very strong and coherent and is an indication of a solar event, like a solar flare.

The eCallisto system consists of a log-periodic Yagi-type antenna which has a relatively broad frequency response to radio waves and a relatively narrow field of view (about 10 degrees). Our antenna is directed to the sun using a motor control system with the location of the sun calculated based on the local time. The controller is a Raspberry Pi that is linked to the main Radio Astronomy Laboratory by an ethernet link. The output from the antenna is fed to a preamplifier and detector based on a broadband cable TV tuner, enabling detection of solar radio emissions in the range 45 to 870 MHz.

Details about the international eCallisto collaboration can be found here.