The structure and evolution of a sigmoidal active region
Sarah Gibson Submitted: 2002-10-04 14:43
Solar coronal sigmoidal active regions have been shown to be precursors to some coronal mass ejections (CMEs).
Sigmoids, or ``S''-shaped structures, may be indicators of twisted or
helical magnetic structures, having an increased likelihood of eruption.
We present here an analysis of a sigmoidal region's 3-d structure and how it evolves
in relation to its eruptive dynamics. We use data
taken during a recent study of a sigmoidal active
region passing across the solar disk (an element of the third ``Whole Sun Month''
campaign). While S-shaped structures are generally observed in soft X-ray
emission, the observations we present demonstrate their visibility at a range of
wavelengths including those showing an associated sigmoidal filament. We
examine the relationship between the S-shaped structures seen in SXR and those seen in
cooler lines in order to probe the sigmoidal region's 3-d density and temperature
structure. We also consider magnetic field observations and extrapolations
in relation to these coronal structures.
We present an interpretation of the disk passage of the sigmoidal region,
in terms of a twisted magnetic flux rope that emerges into and equilibrates
with overlying coronal magnetic field structures,
which explains many of the key observed aspects of the region's structure and evolution. In particular, the evolving flux rope interpretation provides insight into
why and how the region moves between active and quiescent phases, how the region's sigmoidicity is maintained during its evolution, and under what circumstances sigmoidal structures are apparent at a range of wavelengths.
Authors: S. E. Gibson, L. Fletcher, G. Del Zanna, C. D. Pike, H. E. Mason,C. H. Mandrini, P. Demoulin, H. Gilbert, J. Burkepile, T. Holzer, D. Alexander, Y. Liu, N. Nitta, J. Qiu, B. Schmieder, and B. J. Thompson
Publication Status: ApJ, 574, 1021, 2002
Last Modified: 2006-01-12 09:43