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Lucie Green   Submitted: 2007-09-05 13:38

In order 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-S (forward-S) shape, which is opposite to that observed for the sigmoid. This result is incompatible with two models for sigmoid formation: one which identifies sigmoids with upward arching kink-unstable flux ropes and one which identifies sigmoids with a current layer between two oppositely sheared arcades. We find instead that the observations agree well with the model by Titov and D́emoulin (1999) which identifies transient sigmoids with steepened current layers below rising flux ropes.

Authors: L. M. GREEN, B. KLIEM, T. TOEROEK, L. van DRIEL-GESZTELYI and G. D. R. ATTRILL
Projects: None

Publication Status: Submitted to Solar Physics
Last Modified: 2007-09-05 13:38
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Subject will be restored when possible  

Lucie Green   Submitted: 2007-09-05 13:38

In order 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 (counter-clockwise), consistent with the flux rope taking on a reverse-S (forward-S) shape, which is opposite to that observed for the sigmoid. This result is incompatible with two models for sigmoid formation: one which identifies sigmoids with upward arching kink-unstable flux ropes and one which identifies sigmoids with a current layer between two oppositely sheared arcades. We find instead that the observations agree well with the model by Titov and D́emoulin (1999) which identifies transient sigmoids with steepened current layers below rising flux ropes.

Authors: L. M. GREEN, B. KLIEM, T. TOEROEK, L. van DRIEL-GESZTELYI and G. D. R. ATTRILL
Projects: None

Publication Status: Submitted to Solar Physics
Last Modified: 2007-09-06 08:00
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Lucie Green   Submitted: 2007-09-05 13:38

In order 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 (counter-clockwise), consistent with the flux rope taking on a reverse-S (forward-S) shape, which is opposite to that observed for the sigmoid. This result is incompatible with two models for sigmoid formation: one which identifies sigmoids with upward arching kink-unstable flux ropes and one which identifies sigmoids with a current layer between two oppositely sheared arcades. We find instead that the observations agree well with the model by Titov and D́emoulin (1999) which identifies transient sigmoids with steepened current layers below rising flux ropes.

Authors: L. M. GREEN, B. KLIEM, T. TOEROEK, L. van DRIEL-GESZTELYI and G. D. R. ATTRILL
Projects:

Publication Status: Submitted to Solar Physics
Last Modified: 2007-09-24 05:37
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Subject will be restored when possible  

Lucie Green   Submitted: 2007-09-05 13:38

In order 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 (counter-clockwise), consistent with the flux rope taking on a reverse-S (forward-S) shape, which is opposite to that observed for the sigmoid. This result is incompatible with two models for sigmoid formation: one which identifies sigmoids with upward arching kink-unstable flux ropes and one which identifies sigmoids with a current layer between two oppositely sheared arcades. We find instead that the observations agree well with the model by Titov and D́emoulin (1999) which identifies transient sigmoids with steepened current layers below rising flux ropes.

Authors: L. M. GREEN, B. KLIEM, T. TOEROEK, L. van DRIEL-GESZTELYI and G. D. R. ATTRILL
Projects:

Publication Status: Submitted to Solar Physics
Last Modified: 2007-10-01 05:21
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Subject will be restored when possible  

Lucie Green   Submitted: 2007-09-05 13:38

In order 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 (counter-clockwise), consistent with the flux rope taking on a reverse-S (forward-S) shape, which is opposite to that observed for the sigmoid. This result is incompatible with two models for sigmoid formation: one which identifies sigmoids with upward arching kink-unstable flux ropes and one which identifies sigmoids with a current layer between two oppositely sheared arcades. We find instead that the observations agree well with the model by Titov and D́emoulin (1999) which identifies transient sigmoids with steepened current layers below rising flux ropes.

Authors: L. M. GREEN, B. KLIEM, T. TOEROEK, L. van DRIEL-GESZTELYI and G. D. R. ATTRILL
Projects: None

Publication Status: Published in 2007, Solar Physics, 246, 365.
Last Modified: 2007-11-27 07:32
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Subject will be restored when possible  

Lucie Green   Submitted: 2007-09-05 13:37

In order 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 (coun-terclockwise), consistent with the flux rope taking on a reverse-S (forward-S) shape, which is opposite to that observed for the sigmoid. This result is incompatible with two models for sigmoid formation: one which identifies sigmoids with upward arching kink-unstable flux ropes and one which identifies sigmoids with a current layer between two oppositely sheared arcades. We find instead that the observations agree well with the model by Titov and D́emoulin (1999) which identifies transient sigmoids with steepened current layers below rising flux ropes.

Authors: L. M. GREEN, B. KLIEM, T. TOEROEK, L. van DRIEL-GESZTELYI and G. D. R. ATTRILL
Projects: None

Publication Status: Submitted to Solar Physics
Last Modified: 2007-09-05 13:37
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Subject will be restored when possible  

Lucie Green   Submitted: 2007-09-05 13:34

In order 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 (coun-terclockwise), consistent with the flux rope taking on a reverse-S (forward-S) shape, which is opposite to that observed for the sigmoid. This result is incompatible with two models for sigmoid formation: one which identifies sigmoids with upward arching kink-unstable flux ropes and one which identifies sigmoids with a current layer between two oppositely sheared arcades. We find instead that the observations agree well with the model by Titov and D́emoulin (1999) which identifies transient sigmoids with steepened current layers below rising flux ropes.

Authors: L. M. GREEN, B. KLIEM, T. TOEROEK, L. van DRIEL-GESZTELYI and G. D. R. ATTRILL
Projects: None

Publication Status: Submitted to Solar Physics
Last Modified: 2007-09-05 13:34
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THE MAGNETIC HELICITY BUDGET OF A CME-PROLIFIC ACTIVE REGION  

Lucie Green   Submitted: 2002-04-30 07:39

Coronal mass ejections (CMEs) are thought to be the way by which the solar corona expels accumulated magnetic helicity which is injected into the corona via several methods. DeVore (2000, ApJ, 539, 944) suggests that a significant quantity is injected by the action of differential rotation, however Demoulin et al. (2002, Astron. Astrophys., 382, 650), based on the study of a simple bipolar active region, show that this may not be the case. This paper studies the magnetic helicity evolution in an active region (NOAA 8100) in which the main photospheric polarities rotate around each other during five Carrington rotations. As a result of this changing orientation of the bipole, the helicity injection by differential rotation is not a monotonic function of time. Instead, it experiences a maximum and even a change of sign. In this particular AR, both differential rotation and localized shearing motions are actually depleting the coronal helicity instead of building it. During this period of five solar rotations, a high number of CMEs (35 observed, 65 estimated) erupted from the active region and the helicity carried away has been calculated, assuming that each can be modeled by a twisted flux rope. It is found that the helicity injected by differential rotation (~ -7 x 1042 Mx2) into the active region cannot provide the amount of helicity ejected via CMEs, which is a factor 5 to 46 larger and of the opposite sign. Instead, it is proposed that the ejected helicity is provided by the twist in the sub-photospheric part of the magnetic flux tube forming the active region.

Authors: L.M.GREEN, M.C. L'OPEZ FUENTES, C.H. MANDRINI, P. D'EMOULIN, L. VAN DRIEL-GESZTELYI, J.L. CULHANE
Projects:

Publication Status: Solar Physics, in press
Last Modified: 2002-04-30 07:39
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Coronal mass ejections and their association to active region flaring.  

Lucie Green   Submitted: 2002-04-30 06:59

Since the discovery of coronal mass ejections (CMEs), flaring has been thought to be associated in some way with the ejection in either cause or effect. When CMEs were first discovered in the 1970s it was suggested that they were powered by solar flares (e.g. Dryer, 1982). Research since then (Harrison, 1986) has indicated that there is an associated flare that occurs shortly after the CME. To investigate this further, and making no assumption that a particular flare is causally connected to the CME, flaring activity in nine active regions that show one or more CME signatures has been studied for several hours before and after CME launch. Although the initiation of the CME may occur on size scales larger than the active region itself, definite changes are seen in the flaring activity which may be related to the ejection. This work indicates that the energy released from the active region magnetic field via flaring is greater prior to the CME launch than after.

Authors: L.M.Green, L.K.Harra, S.A.Matthews, J.L.Culhane
Projects:

Publication Status: Solar Physics, 2001, 200, 189
Last Modified: 2002-04-30 06:59
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THE MAGNETIC HELICITY BUDGET OF A CME-PROLIFIC ACTIVE REGION
Coronal mass ejections and their association to active region flaring.

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