ExPRES: a tool to simulate planetary and exoplanetary radio emissions

Context. All magnetized planets are known to produce intense nonthermal radio emissions through a mechanism known as Cyclotron Maser Instability (CMI), that requires the presence of accelerated electrons generally arising from magnetospheric current systems. In return, radio emissions are a good probe of these current systems and acceleration processes. But the CMI generates highly anisotropic emissions, and thus leads to important visibility effects in the observations, which have to be taken into account when interpreting the data. Several studies have been previously performed that showed that modeling the radio source anisotropy effect can reveal a wealth of physical information about the planetary or exoplanetary magnetospheres that produce the radio emissions. Aims. We present a numerical tool, called ExPRES (Exoplanetary and Planetary Radio Emission Simulator), which is able to reproduce the observations of planetary and exoplanetary CMI-generated radio emissions in the time-frequency plane. Special attention is given to the computation of the radio emission beaming at and near its source. Methods. We explain what physical information about the system can be drawn from such radio observations, and how it can be obtained. Depending on the system studied, this information may include the location and dynamics of the radiosources in the magnetosphere, the type of current system leading to electron acceleration, the energy of accelerated electrons and, for exoplanetary systems, the magnetic field strength and the rotation period of the emitting body (planet or star – the latter corresponds to emissions induced by the planet in the stellar magnetic field), the planetary orbital period, the inclination of its orbit, and – if emission comes from the planet – the tilt of the planetary magnetic field relative to the rotation axis and its offset relative to the center of the planet. Most of these parameters can be measured only via radio observations. Results. Our results should provide the proper framework of analysis and interpretation for past (Voyager, Galileo. . . ), present (Cassini, ground-based radiotelescopes) and future (Juno, Juice) observations of solar system planetary radio emissions, as well as for future detections of radio emissions from exoplanetary systems (or from magnetic white dwarf–planet or white dwarf–brown dwarf systems). Such detections are expected to occur soon as the outcome of large observation programs carried on with giant radiotelescopes such as LOFAR, UTR2 or the GMRT. Our methodology can be easily adapted to simulate specific observations, once effective detection is achieved.