tokens: Fluxes and fluences of solar energetic particles shall be assumed to be isotropic . NOTE Anisotropic distributions do exist in near - Earth space due to geomagnetic shielding ( see clause 9.2.4 ) and in the early stages of a SPE , where particles arrive along interplanetary field lines . The direction of the interplanetary magnetic field can be variable and not along the Earth - Sun direction . An isotropic distribution is assumed due to a lack of knowledge for specific events .