Exoplanetary and Planetary Radio Emission Simulator

Tuesday 14 February 2017, by Baptiste Cecconi, Philippe Zarka

ExPRES (Exoplanetary and Planetary Radio Emission Simulator), in French SERPE (Simulateur d’Emission Radio Planétaires et Exoplanétaire), is a tool that allows to simulate auroral radio emission dynamic spectra.

Physical Grounds of the code

The ExPRES code is assuming that auroral radio waves are emitted through the Cyclotron Maser Instability (CMI). This emission mechanism can transfer free energy present in the electron distribution function in the source, into the ambient electromagnetic fluctuation background, thus amplifying waves at a frequency close to the local electron cyclotron frequency, as a resonator. A very simplistic but more practical view is to see the source as an helicoidal antenna, the current of which would be the electrons spiraling in the ambient magnetic field.

Depending on the details of the electron distribution function in play, the CMI predicts the beaming pattern of the radio source consisting of a hollow cone, with a thin layer of emission (2-5 degrees) and a hall-cone opening varying between 90 degrees and 50 degrees. The radio source beaming pattern is thus very anisotropic. The visibility of a radio source strongly depends on the viewing geometry. The ExPRES code is computing this visibility, taking as input the location of the source and that of the observer, together with the selected CMI model parameters.

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CMI beaming pattern
The CMI produces a hollow-cone shaped beaming pattern. An observer must be in line with its thin layer of emission to be able to see the radio waves.

In summary, ExPRES is a numerical tool computing synthetic dynamic spectra of radio sources based on a few assumptions:

  • The type of the radio sources is similar to that of planetary auroral emissions
    • Radio amplification by CMI along a magnetic field line
  • The intensity of the radio sources is constant along a given magnetic field line
    • The intensity in the synthetic dynamic spectrum is only due to the source visibility by the specified observer
  • The beaming pattern of the source can be deduced from a restricted set of parameters
    • These parameters are sufficient to model most of the known planetary auroral emissions.
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ExPRES results and real data
Io-controlled radio emissions from Jupiter. (a,b,c) observations at the Nancay Decameter Array; (d,e,f) and (g,h,i) synthetic dynamic spectra of the above observations, under different assumptions. Hess, Cecconi and Zarka, GRL, 2008

ExPRES outputs

The ExPRES code has various output options. The main ones are:

  • Time-Frequency spectrograms (or dynamic spectra) that can be directly compared with actual measurements;
  • 2D movies of the visible radio source locations as seen from the observer;
  • 3D movies of the visible radio source locations in the planetary system.

Other output options are available including spectrograms versus planetary parameters such as the latitude, distance and longitude of the observer. Images are produced as PDF documents, and movies as MP4 video streams. It can also provide data in CDF format, that can be sent to external tools using SAMP.

Development Status

The current version of ExPRES is 5.0.

It has been developed since 2008, mainly by S. Hess [1] at LESIA. P. Zarka, L. Lamy and B. Cecconi participated to the development of the various versions on the code. C. Louis (PhD at LESIA) is now enhancing the code features and uses it for scientific studies, while the JUNO and Cassini missions are going to explore the auroral regions of Jupiter and Saturn, respectively.

Furthermore, the ExPRES service is being transferred to PADC, in order to improve the underlying technologies. In that scope, the plans are:

  • [2017] use UWS to manage jobs;
  • [2017] use the Paris Observatory computing cluster for job executions;
  • [2018] use GDL instead of IDL for job executions;
  • [2018] rebuild the user interface with a framework such as Django.


[1] now at ONERA, Toulouse.