Authors: Richard C. Canfield
Affiliation: Physics Department, Montana State University, Bozeman
A distinctive characteristic of interplanetary magnetic clouds is their rope-like
magnetic structure, i.e. their smoothly-varying helical field lines whose pitch
increases from their core to their boundary. Because this regular structure
helps to make MCs particularly geo-effective, it is important to understand
how it arises. Many MCs are associated with solar filament eruptions, and their
magnetic field properties follow rather predictably from those of the associated
filament and the large-scale solar dipole. However, a comparable number of MCs
are associated with the eruption of solar active regions, particularly sigmoids.
These do not show the same solar--terrestrial correlations as those associated
with filaments. For example, I am unaware of any model of the magnetic fields
of sigmoids and their eruption that gives a demonstrably reliable prediction
of the leading field orientation of their associated MC.
I will discuss recent work which relates the magnetic and topological parameters
of interplanetary MCs to associated solar active regions. This work strongly
supports the notion that MCs associated with active region eruptions are formed
by magnetic reconnection between these regions and their larger-scale surroundings,
rather than simple eruption of pre-existing structures in the corona or chromosphere.
This opens up the possibility that the helicity of the MCs can be understood
in terms of the self (twist) helicity of the active region and the mutual (linking)
helicity of the active region with its surroundings.