Two distinct plasma and energetic ion distributions within the June 1998 magnetic cloud

Abstract

On June 24–25, 1998 a magnetic cloud was observed by ACE near the L1 Lagrangian point. The cloud contained what at first appears to be a single interplanetary magnetic field (IMF) flux rope. However, within this flux rope we found two distinct distributions of plasma and energetic particles. The first region of the cloud was populated with anti-sunward streaming energetic ions and exhibited 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 were the two regions within the flux rope occupied by two different plasma regimes, implying different coronal origins, but the ion anisotropies were completely different, (unidirectional vs. bidirectional), implying markedly dif...