ACE News Archives Fluctuation Anisotropy within Magnetic Clouds Magnetic clouds are a frequently observed feature of the solar wind plasma seen by the ACE spacecraft at L1.
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.