|ACE News Archives||
ACE News #63 - June 10, 2002
|ACE News Archives|
|ACE Science Nuggets||
|ACE Science Nuggets|
Magnetic clouds are the well-ordered manifestation of coronal mass ejections in the interplanetary medium. Magnetic cloud events are identified in spacecraft data with the following criteria, 1) increase in total field magnitude 2) a smooth rotation of the magnetic field direction through a large angle, and 3) low plasma beta (ratio of particle pressure to magnetic pressure) or low proton temperature.
There have been many successful models for the field structure of these clouds, the simplest being the linear force-free model of Burlaga. In the context of this static cylindrical model, any measured plasma or composition quantity can be mapped to a position within the cloud. The left figure panel shows the spacecraft trajectory projected onto the plane normal to the axis of symmetry of the cylindrical cloud model. Assigning the incoming distances to be negative, we have a one-to-one mapping for time to spatial position. The University of Michigan group has identified and modeled over 50 cloud events in ACE data from Feb 1998 to July 2001. Using this spatial mapping, a number of composite diameter cuts were constructed on the basis of quantities measured by the SWEPAM and SWICS instruments, averaging over all events.
The right figure panel shows two such average diameter cuts. The top panel shows the alpha particle density, while the bottom panel shows the fraction of Fe ions with high charge states (>= +16). The magnetic cloud events were divided into two groups, Fast (>= 500 km/s, solid-triangle line) and Slow (< 500 km/s, dotted line), that have distinct compositional differences. The error bars indicate the bin width and the standard deviation of the mean. The density profiles are very symmetric. The fast clouds have much higher iron charge states, indicating a heated electron temperature environment in the corona, although both cloud types are enhanced over slow solar-wind values (Fe+16 fraction <0.1). Clearly, there are two distinct types of magnetic clouds measured at 1 AU despite identical magnetic field signatures. The compositional structure indicated here will help define not only the solar origins of these clouds but also the cause(s) of their differences.
Contributed by Benjamin Lynch and Thomas Zurbuchen of the University of Michigan.
Last modified 10 June 2002, by