Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, Maryland 20771

We present simultaneous observations of magnetic fields and flow anisotropies of solar energetic (several MeV) protons and electrons made by the ISEE 3 and IMP 8 spacecraft during the passage of an interplanetary magnetic cloud on February 16-17, 1980. ISEE 3 was situated ~ 250 R_E upstream of Earth at the L₁ libration point, and IMP 8 was crossing the Earth's northern tail lobe and magnetosheath in a dusk-dawn direction at a mean downtail distance of ~ 27 R_E. Several solar particle onsets were seen at ISEE 3 during cloud passage, with corresponding energetic particle signatures being observed in the geomagnetic tail by IMP 8. Various differences in the particle signatures at the two spacecraft occur, however, and these may be interpreted in terms of the magnetic field line topology of the cloud, the connectivity of the cloud field lines to the solar surface, and the interconnection between the magnetic fields of the magnetic cloud and of the Earth. The observations are consistent with a model of magnetic clouds according to which these mesoscale configurations are curved magnetic flux ropes attached at both "ends" to the Sun's surface and extending to 1 AU. The energetic particles stream out of the Sun along the cloud's helical field lines, arrive with appropriate time delays at an observation site inside the cloud, and become counterstreaming after mirroring within the cloud. When the field in the magnetic cloud had a persistent southward component, we can directly confirm the interconnection between the terrestrial and cloud magnetic fields at the location of IMP 8 in the magnetotail. While at ISEE 3 inside the magnetic cloud the energetic particles have a strong east-west bidirectional flow, IMP 8 observes a tail-aligned bidirectional flow (i.e., roughly orthogonal to the flow direction at ISEE 3) as the energetic particles travel earthward along lobe field lines closer to Earth. Thus we have a case of solar particles gaining access into the magnetosphere at low altitudes first through magnetic connection of the magnetic cloud to the Sun and, subsequently, via the interconnection of the cloud's field lines to those of the Earth.

J. Geophys. Res., 98, No. A9, 15497-15507, Sept. 1993