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Electrodynamics

GITM uses an external library for high-latitude electrodynamics. This library is automatically cloned into ext/Electrodynamics when running Config.pl, though an update is not attempted.

GITM has the ability to use external auroral and/or potential models. These are specified independently so one could, for instance, use AMIE (file-based) aurora and Weimer potentials.

Specifying Electrodynamics Drivers

The Electrodynamics models are chosen in the #ELECTRODYNAMICS section of UAM.in. By default the values are set to:

#ELECTRODYNAMICS
zero        AuroralModel
60.0        DtAurora
zero        PotentialModel
60.0        DtPotential

This will cause warnings to be printed if running on Earth, as we often wish to provide high-latitude electrodynamics drivers when modeling the Earth. However, the run will complete.

If, for example, one wishes to perform a scientific run using a commonsense configuration, the recommended settings are:

#ELECTRODYNAMICS
fta         AuroralModel
60.0        DtAurora
weimer05    PotentialModel
60.0        DtPotential

This will use FTA1 and Weimer052 for the auroral and potential models, respectively.

The aurora and electric field model names are parsed in Electrodynamics/src/interpret_names.f90. See this file for the most up-to-date list of available modules and the acceptable names.

Aurora

The following Auroral models are available:

  • FTA
  • FRE
  • PEM
  • OVATION
  • AMIE

From the auroral module, GITM expects to receive Average Energy and Energy Flux, for all of the selected auroral types. At the moment these all must be from the same module, so one cannot use FTA for diffuse electron precipitation and AMIE for monoenergetic electron precipitation.

Aurora Types

Auroral types are specified in the #AURORATYPES section of UAM.in, and only electron diffuse aurora are included by default:

#AURORATYPES
T         UseDiffuseAurora (logical)
F         UseMonoAurora (logical)
F         UseWaveAurora (logical)
F         UseIonAurora (logical)

Some notes on the different auroral types:

  • NormalizeAuroraToHP is only recommended to be used in conjunction with FRE,
  • #AURORATYPES are not supported by all auroral models. Presently, only OVATION & MAGNIT (AMIE) can provide other than electron diffuse aurora.
  • AllowAurWODiffuse was added for stability with OVATION-Prime; it restricts mono/wave/ion aurora to only exist in locations which also contain electron diffuse aurora. This can be set in #AURORAMODS

Internally, GITM represents Monoenergetic and Wave/broadband aurora with a gaussian centered at the average energy.

Aurora Mods

The diffuse aurora can be represnted by either a Maxwellian or Kappa distribution using the #AURORAMODS section of UAM.in:

#AURORAMODS
F               NormalizeAuroraToHP     (logical)
1.0             AveEFactor    (real)
F               IsKappaAurora     (logical)
1.0             AuroraKappa    (real)
F               AllowAurWODiffuse (logical)
50.0            MaxAveEAurora    (real)

Potentials

The following electric field models can be used:

  • Weimer05
  • Millstone-Hill
  • Heppner Maynard
  • AMIE

Required Input Files

Each electrodynamics module has different inputs. At initialization, a verification check will be performed where GITM ensures that all the required input data are present and that the data file covers the entire simulation time range. If an input file ends before the requested stop time for a run, errors will be raised.

The check for valid data is located within Electrodynamics/src/indices_subroutines.f90. Some required input file-types are listed below:

Model IMF AE HP Kp
Weimer Yes No No No
FRE Yes No Yes No
HepMay Yes No No Yes
FTA No Yes No No
PEM No No Yes No
Ovation Yes No No Yes

HP can be derived from AE, and is not necessarily required to be in a standalone file. See here for more details.

File-based Electrodynamics

AMIE (Assimilative Mapping of Ionospheric Electrodynamics) is the name chosen for the type of files which can be interpreted by Electrodynamics. A number of Python routines can be found in Electrodynamics which may be useful when generating these inputs.

AMIE files can be used, for instance, to input custom auroral patterns into GITM. Once could generate an AMIE file with nominal diffuse electron aurora and several intense monoenergetic beams at any number of locations.

Running Electrodynamics Only

By using the #STATISTICALMODELSONLY option in UAM.in, it is possible to run any configuration of Electrodynamics models without GITM's physics, making the runs faster.

By setting the desired output type to 2DGEL, and an appropriate Dt for #STATISTICALMODELSONLY and #OUTPUT, GITM will read in the necessary input files and output precipitation & potential patterns using the specified electrodynamics modules. An example of this is located in srcTests/auto_test/UAM.in.04.ElectrodynamicsGeoCoords.test.

Additionally, one can output data on a magnetic grid instead of geographic, which is often desired when plotting outputs from the electrodynamics modules. To do this, one must manually control the magnetic field configuration through the use of #APEX and #DIPOLE in UAM.in.

By setting #APEX to F, GITM will use a tilted, offset dipole for the magnetic field. The tilt and offset are normally set automatically, however with the use of the #DIPOLE option, it is possible to force zero offset and tilt, effectively aligning the geographic and magnetic poles. The output files will then be in magnetic coordinates, on a magnetic grid, rather than geographic.

A complete example file for this can be found in srcTests/auto_test/UAM.in.05.ElectrodynamicsMagCoords.test, where the following sections are what differs this test from the previous:

#APEX 
F     Apex is turned off (so a dipole is used)

#DIPOLE
0.0         Magnetic Pole rotation
0.0         Magnetic pole tilt
0.0         x Dipole Center
0.0         y Dipole Center
0.0         z Dipole Center

  1. Wu, C., Ridley, A. J., DeJong, A. D., & Paxton, L. J. (2021). FTA: A Feature Tracking Empirical Model Of Auroral Precipitation. Space Weather, 19, e2020SW002629. https://doi.org/10.1029/2020SW002629. ↩

  2. Weimer, D. R. (2005), Improved ionospheric electrodynamic models and application to calculating Joule heating rates, J. Geophys. Res., 110, A05306, https://doi.org/10.1029/2004JA010884. ↩