Particle measurements from the low‐altitude polar‐orbiting satellite GRS‐A/Azur and from the Imp 5 satellite inside the magnetosphere near the magnetopause and particle and magnetic‐field measurements in interplanetary space from the Heos A1 satellite are presented for the solar‐proton event of March 1970. Comparison of interplanetary magnetic‐field and particle data indicate that a flux increase lasting 20 min is associated with an interplanetary magnetic filament. A following sharp flux increase is attributed to an interplanetary magnetic discontinuity. As the discontinuity with different proton fluxes on both sides is swept over the earth, the particles immediately enter the magnetosphere near the neutral sheet, and the higher flux is observed without time delay in the quasi‐trapping region over the polar cap. The central polar cap is still dominated by particles entering via the distant magnetospheric tail. The discontinuity is convected along the tail and ultimately reaches the region where field lines originating in the central polar cap are connected to or intermingled with the interplanetary field. Filling up of the polar cap in less than 2 hours to the higher flux level results in a maximum ‘tail length’ of about 550 R_E.