Asymmetric access of energetic solar protons to the Earth's north and south polar caps

During the energetic solar particle event that began on January 24, 1969, the ratio N/S of the intensity of protons E p >0.3 Mev over the earth's north (N) polar cap (Λ>80°) to that over its south (S) polar cap (−Λ>80°) varied from a value greater than 20 to about 1, as observed with satellite Injun 5 in a low‐altitude polar orbit. The interplanetary intensity of protons was measured simultaneously with similar detectors on Explorer 33 and 35 in two nearly orthogonal planes and in 8 different directions on the unit sphere (some overlap). High values of the N/S ratio early in the event corresponded to an extraordinarily strong anisotropy of intensity in interplanetary space with the anisotropy vector pointing dominantly southward. The N/S ratio dropped toward 1 as the interplanetary beam relaxed toward isotropy. Similar, although less well determined, findings applied to protons E p >3.4 Mev and α particles E α >1.18 Mev. The directions of interplanetary field lines that connect with the respective polar caps are uniquely identified by intensity considerations. The interplanetary magnetic vector was measured on Explorer 35 also. Early in the event, the earth was in a solar positive ( B outward from the sun) magnetic sector (ϕ SE ≈140°), with a dominantly southward direction (θ SE ≈−40° to −70°). The anisotropy vector was approximately parallel to the magnetic vector and in the same sense. The composite evidence favors the direct access of energetic particles to the earth's polar caps via magnetic field lines interconnected between the terrestrial field and the interplanetary medium; it strongly contradicts models that contemplate diffusion across the magnetospheric tail as an important feature of particle access.