Quickstart

This section presents a concise user’s guide to ExPRES.

Configuration File

An ExPRES run is configured through a configuration file (in JSON format). Examples of configuration files (and the associated results files) are available from the ExPRES section of the MASER data repository for Io-, Europa- and Ganymede-controlled emissions, and for various observers.

These routine simulation files are configured for using the JRM09 [CKO+18] magnetic field model (or ISaAC [HBZG11] for some of them), the CAN81 [CAN81] current sheet model, and an electron energy of 3 keV. The position of the Active Flux Tube (AFT) for Io is based on the (corrected) lead angle model of [HCZ08], whereas for Europa and Ganymede the AFT is the same as the flux tube connected to the moon. The file names are built automatically by ExPRES. The easiest option to build your configuration file is to update an existing one.

Updating a Configuration File

You can download a configuration file to get a template (please be sure to take one using the JRM09 magnetic field model [CKO+18], for up-to-date description). There are many items and options in this file. Here are the main ones.

Setting the temporal axis

The temporal axis is configured in the TIME section. The start time (MIN keyword) and end time (MAX keyword) should be provided in minutes relative to the simulation run time origin. The time sampling step is computed from the number of time steps (NBR keyword). The absolute time reference of the simulation run given in the OBSERVER section, with the SCTIME keyword. Hence, in most cases, MIN should be set to 0.

More detailed temporal axis setting description

Setting the spectral axis

The spectral axis is configured in the FREQUENCY section. The lower and upper bounds (MIN and MAX keywords) are given in MHz. The sampling interval is cimputed from the number of spectral step (NBR keyword). The spectral axis can use either a Linear or Log scale (TYPE keyword values). It is also possible to use a customised spectral axis.

More detailed spectral axis setting description

Setting the observer

The observer defines the place, from which the observation will be done. Basic users only need to use a limited set of parameters. In this short guide, we present the Pre-Defined type of observer (set in the TYPE keyword). If you need to change the central body (PARENT keyword), it is recommended to use a configuration file using the desired central body. The name of the observer (SC keyword) should then be a name known by ExPRES. The current list of known observers is: Cassini, Juno, Earth, Voyager1, Voyager2. The time origin of the simulation run is set with the SCTIME keyword, with the format: YYYYMMDDhhmm, with YYYY is the year, MM the month, DD the day, hh the hour, mm the minute, all 0-padded. The other parameters are not used in this case.

More detailed observer’s setting description

Setting the output parameters

The CDF sub-section of SPDYN defines the parameters that will be provided in the resulting CDF file. Each parameters can be selected/deselected setting its value to true/false. In most cases, setting Theta keyword (opening angle of the emission cone in the direction of the observer) to true is the minimal recommended setup. Note that the more options are set, the bigger is the output file.

More detailed output parameter’s description

Setting the plasma model parameters

The main set of parameters that can be adjusted is the plasma density model at the source. This is done through the DENS sub-section of BODY. The default model parameters, in case of the Io-controlled emissions, are an Ionospheric model (based on [HTK98]) and an Io torus model (based on [Bag94]).

More detailed the plasma density model’s description

Setting the radio source parameters

The SOURCE section defines the radio source parameters. There may be several sources in the configuration file. The parameters are:

  • TYPE: here, attached to a satellite, which means that the magnetic field lines used will be those connected to a moon.

  • SAT: if TYPE="attached to a satellite", then provide the name of the moon (which also needs to be defined as a BODY)

  • aurora_alt: sets the altitude (in Planetary radius) of the UV aurora (altitude below which electrons are lost by collision with the atmosphere)

  • NORTH: emission will be produced in the northern hemisphere

  • SOUTH: emission will be produced in the southern hemisphere

  • Width: width of the beaming hollow cone (in degrees)

  • current:

    • In most cases it should be set to Transient (Alfvenic), which calculates self-consistently the beaming angle using the Cyclotron maser Instability (CMI) and a loss cone distribution function

    • It can also be set to Constant, so that the beaming angle will not be calculated using the CMI, but will be set at a chosen values (see next parameters)

  • Constant: if Current="Constant" then provide here the value in degree (80.0 for example)

  • Accel: the energy of the resonant electrons (in keV)

  • Refraction: to take into account refraction in the source’s vicinity

Running ExPRES Online

The code is available for Run-on-Demand at Observatoire de Paris: https://voparis-uws-maser.obspm.fr/client/

Short workflow to use this interface:

  • Click on Job List (top left)

  • In Job List for, select ExPRES

  • Click on + Create New Job (top right)

  • In config choose the configuration file (.json) you want to run. The other parameters (runId, slurp_mem and Add control parameters have to be left as there are)

  • Click on Submit, and wait for a response. It will first marked as Queued and then as Executing. It will last a few tens of second to a few minutes (depends on how many time/frequency steps and how many cdf-output parameters you asked for).

  • Then it will be marked as Completed,

  • In > Job Results you will be able to download resulting files.

  • If the Job is marked as Error, something went wrong during the simulation. Then, look at the > Job Details, and check the stdout and stderr sections.

For more details see the Run-on-Demand page.