Most clouds are very large structures that pass over the spacecraft in a day or more. Magnetic clouds evolve in relative isolation from the surrounding plasma, which explains at least in part their large-scale features. The figure shows an example of a magnetic cloud observed by ACE in March 1998. As the plasma temperature within the cloud drops the magnetic fluctuations become increasing perpendicular to the mean magnetic field. This can be seen in the bottom panel which is a plot of the ratio of magnetic fluctuation energies perpendicular and parallel to the mean field.
Preliminary analyses indicate that the wave vectors also rotate to a more perpendicular orientation.
This means that a smaller fraction of the observed magnetic fluctuation energy will be available for scattering the energetic particles so that energetic solar particles should possess longer mean free paths within magnetic clouds. Contributed by Charles Smith , Bartol Research Institute, University of Delaware.