Planetary Radio Emissions

Many people have heard about auroral lights, but it is less well known that also radio emissions are created on top of them at high latitudes. Such auroral radio emissions have a typical frequency of a few hundred kHz, and they not only occur at Earth, but also at the gas and ice giant planets in our solar system. In a current project we focus on the study and classification of fine spectral structures of auroral radio emissions at Earth, Jupiter, and Saturn using data from the missions Cluster, Juno, and Cassini, respectively.

There are multiple other radio and plasma waves inside and outside the magnetosphere of a planet like Langmuir waves, upper and lower hybrid waves, whistler waves propagating along magnetic field lines, or continuum and narrowband emissions created typically at plasma density gradients. The figure shows a typical radio spectrum of Saturn recorded by Cassini. It contains the auroral Saturn Kilometric Radiation, narrowband emissions at 5 and 20 kHz, and radio emissions caused by Saturn lightning and dust impacts from the ring plane. Radio emissions are also an important tool to study natural discharge processes (lightning) inside the stormy atmosphere of a planet.

Radio emissions can remotely reveal us important properties of the plasma and the magnetic field at their source and along their path of propagation towards the receiving antennas. For a correct determination of the wave polarization and the incoming radio wave direction, it is necessary to calibrate spacecraft antenna systems. This was done at IWF for many missions (e.g., Cassini, Juno, STEREO, Solar Orbiter), and most recently for the radio wave antennas of the RPWI instrument on-board JUICE.

At Earth, earthquakes create electrical charges which generate seismo-electromagnetic waves that can be detected by satellites like China’s CSES-1 & CSES-2 missions and ground-based antennas such as the INFREP network. In this field of research, we characterize and investigate the dynamo mechanism which generates electric fields and currents in the electrically conducting ionosphere. It interacts at higher altitudes with the magnetosphere and at lower altitudes with the atmosphere. It can be considered as a key region because of its relationship to the space environment via the magnetosphere and the Earth's lithosphere layer through the atmosphere.