B.8 Geomagnetic shielding (Størmer theory)

At a given location in the field there are minimum cutoff energies necessary for ions to penetrate to that point. Størmer’s theory gives a cutoff rigidity, P_c, for particle arrival at a point, depending on the point’s geomagnetic R, λ coordinates and the angle of ion arrival from east, γ [RN.20].

(B-13)

M is the normalized dipole moment of the Earth. From this equation, it can be seen that cosmic rays penetrate the geomagnetic field more easily from the west (γ = 180°) than from the east (γ = 0). The R, λ coordinates can be computed from B and L according to the method of Roberts [RN.28]. For vertical arrival, the expression simplifies to:

(B-14)

since γ =90° and R = L cos² (l)

An approximate value of 16 for the constant M/4 is used to fit with observed effective cutoffs. Magnetospheric disturbances, which often follow solarflares or CMEs, can result in a lowering of cutoff; this has been described by Adams et al. [RN.20] as:

(B-15)

with P_c in units of GV.