In the last 50 years measurements of the geomagnetic field configuration have been performed regularly with increasing precision, revealing a yearly weakening of the field intensity of 0.07% and a westward drift of ~0.2 degrees per year over the Earth 's surface. This field can be described, to first order, as a magnetic dipole tilted with respect to the rotation axis by ~11.5 degrees, displaced by ~400 km with respect to the Earth's center and with a magnetic moment M = 8.1E25 G cm3. The dipole orientation is such that the magnetic South pole is located near the geographic North pole, in the Greenland, at a latitude of 75 degrees N and a longitude of 291 degrees. The magnetic North pole is instead near the geographic South pole, on the border of the Antarctica. The intensity at the Earth's surface varies from a maximum of ~0.6 G near the magnetic poles to a minimum of ~0.2 G in the region of the South Atlantic Anomaly (SAA), between Brazil and South Africa. The complex behaviour of the equipotential field lines is mainly a consequence of the offset and tilt. In FLUKA the geomagnetic field is taken into account in two different stages of the simulation chain. 1) Effect of geomagnetic cutoff which modulates the primary spectrum: at a given location (point of first interaction of primary particles) and for a given direction a threshold in magnetic rigidity exists. The closer the injection point is to the geomagnetic equator, the higher will be the vertical rigidity threshold. The standard possibility offered to the user is to evaluate the geomagnetic cutoff making use of a dipolar field centered with respect to the centre of the Earth, adapted to give the "correct" vertical rigidity cutoff for the geographic location under examination. Under this approximation, an analytical calculation of the cutoff can be performed and the FLUKA source routine for galactic cosmic rays can apply the resulting geomagnetic cutoff. In case an off-set dipole (not provided at present) or more sophisticated approaches are deemed necessary, a spherical harmonic expansion model like the IGRF model is available [CLIMAX]. However no default mean is provided for making use of these higher order approximations for computing geomagnetic cutoff's, since no analytical calculation is possible, and a numerical (back)tracking of the primary particle from(/to) infinity is required. Please note that activating these more realistic options for the earth geomagnetic field by means of the GCR-SPE card has only the effect of using the resulting field while showering in the atmosphere (see next point), a minimal correction with respect to the dominant effect of the geomagnetic cutoff. 2) The local geomagnetic field can be taken into account during shower development in the atmosphere. The field is automatically provided by the default MAGFLD FLUKA user routine, in accordance to the option selected in the GCR-SPE card. For local problems, provided the coordinate system is consistently used (that is geomagnetic coordinates for the dipolar field, geographic ones for the multipolar field) there is no need to provide any orientation or intensity information about the field.
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