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GTRN: Geomagnetic Transmission Routine

Overview

Earth's magnetic field effectively shields systems from some of the energetic particles found in interplanetary space. GTRN evaluates this geomagnetic shielding effect.

GTRN is only intended for use inside of geosynchronous orbit. For orbits at or beyond geosynchronous, do not run GTRN. Instead, begin your CREME calculations with the FLUX module.

  • FLUX is also the recommended starting point for a highly elliptical orbit, with perigee at low altitude and apogee at or beyond geosynchronous, since most of the orbit will be exposed to interplanetary fluxes. However, this approach may be somewhat conservative, since it neglects the shielding provided by the solid Earth in the low-altitude portion of the orbit.

The output of GTRN is a geomagnetic transmission function, which is written to an output file ("something.gtf", where "something" is the rootname you specify; the extension ".gtf" is added automatically by GTRN.) This file is used as an input to the FLUX calculation. It must be created before you run FLUX.

IMPORTANT NOTES:

  • Whenever possible, you should use the pre-calculated transmission functions in GTRN, since they are based on a better model of Earth's magnetic field than is presently available to you if you specify the orbital parameters in GTRN yourself.
  • GTRN is only intended for use inside of geosynchronous orbit. For orbits at or beyond geosynchronous, do not run GTRN. Instead, begin your CREME calculations with the FLUX module.
    • FLUX is also the recommended starting point for a highly elliptical orbit, with perigee at low altitude and apogee at or beyond geosynchronous, since most of the orbit will be exposed to interplanetary fluxes. However, this approach may be somewhat conservative, since it neglects the shielding provided by the solid Earth in the low-altitude portion of the orbit.
  • The non-pre-calculated geomagnetic transmission functions produced by CREME when you input the above orbital parameters differ from those produced by the old CREME code (1986). These CREME GTF calculations include semi-empirical transmission enhancement at rigidities below 1 GV which approximately takes into account the effects of magnetospheric current systems. This approximate technique is based on "An Approach to Determination of Real Cosmic Ray Cut-Off Rigidities", by R. A. Nymmik, Proceedings of the 22nd International Cosmic Ray Conference (Dublin), 3, 652-655 (1991).
  • Improved geomagnetic transmission calculations for arbitrary orbits, using techniques similar to those now available only in the pre-calculated GTFs, are planned for the next CREME release.
  • The CREME software does not offer ballistic trajectories at present.
  • The CREME geomagnetic transmission calculations do account for the solid Earth's shadowing effect. This shadowing is a simple geometrical factor, calculated on a point-by-point basis during the orbit-averaging. In general, the net effect of this shadowing, when averaged over the altitudes sampled in the orbit, can be examined by looking at the asymptotic value of the transmission function, at ~20 GV.

Inputs

To run GTRN, you must:

  1. Choose one of the available pre-calculated transmission functions , OR, if a pre-calculated transmission function is not available for your orbit, specify your orbital parameters ;
  2. Specify a geomagnetic weather condition;
  3. Specify whether you want to calculate a single transmission function, averaged over the whole orbit, OR do separate calculations for different orbital segments.
    • The orbital segments option is not available if you have chosen the pre-calculated transmission functions.
    • In the orbital segments option, GTRN produces up to 10 output files, named "something.gt#", where # = 1,2,3,4,5,6,7,8,9,X.

You can use CREME's GTRN routine to calculate geomagnetic transmission functions for arbitrary orbits by specifying these orbital parameters:

Outputs

This file is produced by the GTRN routine and contains the geomagnetic transmission function. This output file is used as an input for the FLUX routine when calculating orbit-averaged ionizing particle fluxes inside Earth's magnetosphere.

The names of output files from GTRN must be as follows:

  • If you choose the whole orbit option in GTRN, the output file must have a name of the form "something .gtf ". (That is, a rootname which you specify; the extension ".gtf " is added automatically.)
  • If you choose the orbital segments option, GTRN can produce up to 10 output files. In this case, the output files are automatically assigned names of the form "something .gt#", where "something" is the rootname you specify and #=1,2,3,4,5,6,7,8,9,X. The output files correspond to different orbital segments, ordered according to increasing lower-bound on McIlwain L.

Without these extensions, the files cannot be located by some of the CREME pulldown menus and on-line directory features.

The GTRN output file also contains header information which documents the inputs and parameters which produced it. You can examine this header information (to make sure, for example, that you've selected the .gtf file you want).

GTRN output files are written in a two-column format, with rigidity (in GV) in the first column and fraction of the orbit (dimensionless) in the second column. You can download GTRN output files to your own PC for plotting with your own software. At the top of GTRN output files are several lines of header information, with a "%" in the second column of each header line. You may have to delete some or all of these lines before passing the file to your plotting software.

GTRN output files track the geomagnetic transmission function from 0 to 20 GV in 0.02 GV steps.

See also: Plotting CREME96 Spectra

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