code: def vdm_b(vdm, lat):
      
      rad = old_div(np.pi, 180.)
      # changed radius of the earth from 3.367e6 3/12/2010
      fact = ((6.371e6)**3) * 1e7
      colat = (90. - lat) * rad
      return vdm * (np.sqrt(1 + 3 * (np.cos(colat)**2))) / fact
docs: Converts a virtual dipole moment (VDM) or a virtual axial dipole moment
      (VADM; input in units of Am^2) to a local magnetic field value (output in
      units of tesla)
  
      Parameters
      ----------
      vdm : V(A)DM in units of Am^2
      lat: latitude of site in degrees
  
      Returns
      -------
      B: local magnetic field strength in tesla