GEOMAG2: Geomagnetic Cutoff Transmission Function

Overview

GEOMAG2 is a long, complicated program that computes the geomagnetic cutoff transmission function corresponding to a given orbit. The program traces the orbit in small time steps for two days of the orbit, calculating the position and then the cutoff 200 times. These cutoffs are integrated into a "transmission function", see Adams et. al. (1983) and Appendix 2 of this report for details on this process.

Inputs

GEOMAG2 options are just those parameters of the orbit that are mentioned in Section 7.5. They should describe the orbit corresponding to the STASS.DAT file being used. GEOMAG2 does not need to be linked with any other program files; it uses the input file CUTOFF.DAT, a table of cutoffs at 20 km altitude (see Appendix 6).

The auxiliary program GEOMAG2 requires an exact description of the satellite's orbit, which the program traces. To describe the orbit, one must input

• the altitude at apogee
• the altitude at perigee
• the orbital inclination

To describe the satellite's position at the start of the calculation, one must specify

• the initial longitude of the ascending node
• the initial latitude of the ascending node
• the displacement of the perigee from the ascending node (not needed for a circular orbit)

In addition, GEOMAG2 asks whether there is a "magnetic storm" and whether to take into account the fact that the earth casts a shadow on the spacecraft in cosmic rays. See Adams, et. al. (1983), for details.

Outputs

GEOMAG2 outputs a two-column data file: the first column is cutoff in GeV/ec; the second column is the transmission function, which varies from 0 to 1. This file, called GTRANS.DAT, is used as input, along with a corresponding STASS.DAT file, for the main programs -- if the "geomagnetic cutoff and trapped protons" option is chosen.