Joint CLUSTER and ACE observations of the delayed entry of solar protons in the inner magnetosphere

Using simultaneous observations from the RAPID and EPAM experiments onboard the CLUSTER and ACE spacecraft we study the entry of solar energetic protons (70 < E < 170 keV) into the Earth's magnetotail and their subsequent propagation in the inner magnetopshere. We present a number of interplanetary ion events observed by the ACE/EPAM experiment, which exhibit sharply defined intensity discontinuities associated with magnetic field discontinuities. We find that such discontinuities are clearly identifiable in proton intensity versus time data from the CLUSTER spacecraft inside the Earth's magnetotail. The relative time delay among the spacecraft outside and at various positions inside the magnetotail is determined. Using the observed time delays between ACE and CLUSTER we estimate the effective downstream distances at which low-energy protons gain access into the magnetotail. Furthermore the westward drifts of the solar protons after their injection in the inner magnetosphere are clearly determined between the 4 CLUSTER S/C. The impulsive onset and following propagation of the solar protons in the inner magnetosphere are discussed in the framework of computer simulation models.