External Magnetic Field Namelist

The namelist &B_external provides options for imposing an external magnetic field.

Externally imposed magnetic field

  • n_imp (default n_imp = 0) is an integer controling the type of external field applied.

    n_imp=0

    No external magnetic field

    n_imp=1

    Follows idea of Uli Christensen of external field compensating internal field such that radial component of magnetic field vanishes at \(r/r_{cmb}=rrMP\) where rrMP is the ‘magnetopause radius’ input by the user (see below)

    n_imp=2

    Uniform axisymmetric magnetic field of geometry given by l_imp (see below)

    n_imp=3

    Uniform axisymmetric magnetic field which changes direction according to the direction of the axial dipole of the internal magnetic field

    n_imp=4

    Same as n_imp=3 but the amplitude of the external field is scaled to the amplitude of the axial dipole of the internal field

    n_imp=7

    External field depends on internal axial dipole through Special Heyner feedback functions

  • rrMP (default rrMP = 0.0) is a real which gives the value of ‘magnetopause radius’. In other words, it gives the radius (as a fraction of r_cmb) at which the radial component of the magnetic field vanishes due to cancelling out of external and internal magnetic field components. Used only when n_imp = 1.

  • amp_imp (default amp_imp = 0.0) is a real which gives the amplitude of the external magnetic field.

  • expo_imp (default expo_imp = 0.0) is a real which gives the exponent of dependence of external magnetic field on the axial dipole of the internal magnetic field. Used for n_imp=7.

  • bmax_imp (default bmax_imp = 0.0) is a real which gives the location of the maximum of the ratio of the poloidal potentials \(g_{ext}/g_{int}\).

  • l_imp (default l_imp = 1) is an integer which gives the geometry (degree of spherical harmonic) of the external magnetic field. The external field is always axisymmetric, hence m = 0 always. This option is used when n_imp = 2,3 or 4.

Current carrying loop

To simulate experiments, an external current carrying loop, concentric to the sphere and in the equatorial plane, has been implemented in the code. It’s radius is fixed at a distance \(a = r_{cmb}/0.8\) to match conditions of the Maryland 3 metre experiment.

  • l_curr (default l_curr = .false.) is a logical that controls switching on or off of the current carrying loop.

  • amp_curr (default amp_curr = 0.0) is a real that gives the amplitude of magnetic field produced by the current carring loop.

Warning

Note that an external magnetic field is incompatible with a region of low conductivity inside the spherical shell (i.e, if r_LCR < r_cmb). Thus, while imposing an external magnetic field, make sure r_LCR > r_cmb (which is the default case). For details on r_LCR, have a look at the section on electrical conductivity in the namelist for physical parameters.