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16.5} Geomagnetic field


 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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