Transient Coronal Sigmoids and Rotating Erupting Flux Ropes |
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| Authors: | L M Green B Kliem T Török L van Driel-Gesztelyi G D R Attrill |
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| Institution: | 1.Mullard Space Science Laboratory,University College London,Surrey,UK;2.Astrophysikalisches Institut Potsdam,Potsdam,Germany;3.Kiepenheuer-Institut für Sonnenphysik,Freiburg,Germany;4.Observatoire de Paris, LESIA, FRE 2461 (CNRS),Meudon Principal Cedex,France;5.Konkoly Observatory of the Hungarian Academy of Sciences,Budapest,Hungary |
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| Abstract: | To determine the relationship between transient coronal (soft X-ray or EUV) sigmoids and erupting flux ropes, we analyse four
events in which a transient sigmoid could be associated with a filament whose apex rotates upon eruption and two further events
in which the two phenomena were spatially but not temporally coincident. We find the helicity sign of the erupting field and
the direction of filament rotation to be consistent with the conversion of twist into writhe under the ideal MHD constraint
of helicity conservation, thus supporting our assumption of flux rope topology for the rising filament. For positive (negative)
helicity the filament apex rotates clockwise (counterclockwise), consistent with the flux rope taking on a reverse (forward)
S shape, which is opposite to that observed for the sigmoid. This result is incompatible with two models for sigmoid formation:
one identifying sigmoids with upward arching kink-unstable flux ropes and one identifying sigmoids with a current layer between
two oppositely sheared arcades. We find instead that the observations agree well with the model by Titov and Démoulin (Astron. Astrophys.
351, 707, 1999), which identifies transient sigmoids with steepened current layers below rising flux ropes. |
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| Keywords: | Sun: sigmoids Sun: flux rope Sun: magnetic fields Sun: eruptions |
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