Abstract:

On June 24-25, 1998 a magnetic cloud was observed by ACE near the L1 Lagrangian point. The cloud contains what at first appears to be a single interplanetary magnetic field (IMF) flux rope. However, within this flux rope we find two distinct distributions of plasma and energetic particles. The first region of the cloud is populated with anti-sunward streaming energetic ions and exhibits a low alpha to proton ratio, typical of the solar wind. Halfway into the magnetic cloud a second and distinct spatial region was encountered, demarcated by a significant drop in the proton temperature, increase in the proton density, and increase in the alpha to proton ratio. This second region was also populated with “pancake” energetic ion pitch angle distributions (PADs). Therefore not only are the two regions within the flux rope occupied by two different plasma regimes, implying different coronal origins, but the ion anisotropies are completely different, (unidirectional vs. bidirectional), implying markedly different global topologies for the magnetic field lines.