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Sun-to-Earth MHD Simulation of the 14 July 2000 "Bastille Day" Eruption  

Tibor Torok   Submitted: 2018-01-19 08:54

Solar eruptions are the main driver of space-weather disturbances at the Earth. Extreme events are of particular interest, not only because of the scientific challenges they pose, but also because of their possible societal consequences. Here we present a magnetohydrodynamic (MHD) simulation of the 14 July 2000 Bastille Day eruption, which produced a very strong geomagnetic storm. After constructing a thermodynamic MHD model of the corona and solar wind, we insert a magnetically stable flux rope along the polarity inversion line of the eruption's source region and initiate the eruption by boundary flows. More than 1033 ergs of magnetic energy are released in the eruption within a few minutes, driving a flare, an EUV wave, and a coronal mass ejection (CME) that travels in the outer corona at about 1500 km s-1, close to the observed speed. We then propagate the CME to Earth, using a heliospheric MHD code. Our simulation thus provides the opportunity to test how well in situ observations of extreme events are matched if the eruption is initiated from a stable magnetic-equilibrium state. We find that the flux-rope center is very similar in character to the observed magnetic cloud, but arrives about 8.5 hours later and about 15 degrees too far to the North, with field strengths that are too weak by a factor of about 1.6. The front of the flux rope is highly distorted, exhibiting localized magnetic-field concentrations as it passes 1 AU. We discuss these properties with regard to the development of space-weather predictions based on MHD simulations of solar eruptions.

Authors: Tibor Török, Cooper Downs, Jon A. Linker, Roberto Lionello, Viacheslav S. Titov, Zoran Mikić, Pete Riley, Ron M. Caplan, Janvier Wijaya
Projects: None

Publication Status: ApJ (under revision)
Last Modified: 2018-01-23 11:53
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The origin, early evolution and predictability of solar eruptions  

Tibor Torok   Submitted: 2018-01-15 18:39

Coronal mass ejections (CMEs) were discovered in the early 1970s when space-borne coronagraphs revealed that eruptions of plasma are ejected from the Sun. Today, it is known that the Sun produces eruptive flares, filament eruptions, coronal mass ejections and failed eruptions; all thought to be due to a release of energy stored in the coronal magnetic field during its drastic reconfiguration. This review discusses the observations and physical mechanisms behind this eruptive activity, with a view to making an assessment of the current capability of forecasting these events for space weather risk and impact mitigation. Whilst a wealth of observations exist, and detailed models have been developed, there still exists a need to draw these approaches together. In particular more realistic models are encouraged in order to asses the full range of complexity of the solar atmosphere and the criteria for which an eruption is formed. From the observational side, a more detailed understanding of the role of photospheric flows and reconnection is needed in order to identify the evolutionary path that ultimately means a magnetic structure will erupt.

Authors: L.M. Green, T. Torok, B. Vrsnak, W. Manchester IV, A. Veronig
Projects: None

Publication Status: Accepted for publication in Space Science Reviews
Last Modified: 2018-01-16 11:11
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Modeling Jets in the Corona and Solar Wind  

Tibor Torok   Submitted: 2015-11-30 23:03

Coronal jets are transient, collimated eruptions that occur in regions of predominantly open magnetic field in the solar corona. Our understanding of these events has greatly evolved in recent years but several open questions, such as the contribution of coronal jets to the solar wind, remain. Here we present an overview of the observations and numerical modeling of coronal jets, followed by a brief description of "next-generation" simulations that include an advanced description of the energy transfer in the corona ("thermodynamic MHD"), large spherical computational domains, and the solar wind. These new models will allow us to address some of the open questions.

Authors: T. Torok, R. Lionello, V.S. Titov, J.E. Leake, Z. Mikic, J.A. Linker, M.G. Linton
Projects: None

Publication Status: Submitted
Last Modified: 2015-12-02 13:53
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The Evolution of Writhe in Kink-Unstable Flux Ropes and Erupting Filaments  

Tibor Torok   Submitted: 2014-03-07 07:38

The helical kink instability of a twisted magnetic flux tube has been suggested as a trigger mechanism for solar filament eruptions and coronal mass ejections (CMEs). In order to investigate if estimations of the pre-eruptive twist can be obtained from observations of writhe in such events, we quantitatively analyze the conversion of twist into writhe in the course of the instability, using numerical simulations. We consider the line tied, cylindrically symmetric Gold-Hoyle flux rope model and measure the writhe using the formulae by Berger and Prior which express the quantity as a single integral in space. We find that the amount of twist converted into writhe does not simply scale with the initial flux rope twist, but depends mainly on the growth rates of the instability eigenmodes of higher longitudinal order than the basic mode. The saturation levels of the writhe, as well as the shapes of the kinked flux ropes, are very similar for considerable ranges of initial flux rope twists, which essentially precludes estimations of pre-eruptive twist from measurements of writhe. However, our simulations suggest an upper twist limit of ∼6π for the majority of filaments prior to their eruption.

Authors: Tibor Torok, Bernhard Kliem, Mitchell A. Berger, Mark G. Linton, Pascal Demoulin, Lidia van Driel-Gesztelyi
Projects: None

Publication Status: To appear in Plasma Physics and Controlled Fusion
Last Modified: 2014-03-07 07:38
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Initiation of Coronal Mass Ejections by Sunspot Rotation  

Tibor Torok   Submitted: 2014-01-14 05:28

We study a filament eruption, two-ribbon flare, and coronal mass ejection (CME) that occurred in Active Region NOAA 10898 on 6 July 2006. The filament was located South of a strong sunspot that dominated the region. In the evolution leading up to the eruption, and for some time after it, a counter-clockwise rotation of the sunspot of about 30 degrees was observed. We suggest that the rotation triggered the eruption by progressively expanding the magnetic field above the filament. To test this scenario, we study the effect of twisting the initially potential field overlying a pre-existing flux-rope, using three-dimensional zero-β MHD simulations. We first consider a relatively simple and symmetric system, and then study a more complex and asymmetric magnetic configuration, whose photospheric flux distribution and coronal structure are guided by the observations and a potential field extrapolation. In both cases, we find that the twisting leads to the expansion of the overlying field. As a consequence of the progressively reduced magnetic tension, the flux-rope quasi-statically adapts to the changed environmental field, rising slowly. Once the tension is sufficiently reduced, a distinct second phase of evolution occurs where the flux-rope enters an unstable regime characterized by a strong acceleration. Our simulations thus suggest a new mechanism for the triggering of eruptions in the vicinity of rotating sunspots.

Authors: Tibor Török, Manuela Temmer, Gherardo Valori, Astrid Veronig, Lidia van Driel-Gesztelyi, Bojan Vr?nak
Projects: None

Publication Status: Solar Physics Volume 286, Issue 2, pp.453-477 (2013)
Last Modified: 2014-01-15 12:51
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Distribution of Electric Currents in Solar Active Regions  

Tibor Torok   Submitted: 2014-01-14 05:25

There has been a long-lasting debate on the question of whether or not electric currents in solar active regions are neutralized. That is, whether or not the main (or direct) coronal currents connecting the active region polarities are surrounded by shielding (or return) currents of equal total value and opposite direction. Both theory and observations are not yet fully conclusive regarding this question, and numerical simulations have, surprisingly, barely been used to address it. Here we quantify the evolution of electric currents during the formation of a bipolar active region by considering a three-dimensional magnetohydrodynamic simulation of the emergence of a sub-photospheric, current-neutralized magnetic flux rope into the solar atmosphere. We find that a strong deviation from current neutralization develops simultaneously with the onset of significant flux emergence into the corona, accompanied by the development of substantial magnetic shear along the active region's polarity inversion line. After the region has formed and flux emergence has ceased, the strong magnetic fields in the region's center are connected solely by direct currents, and the total direct current is several times larger than the total return current. These results suggest that active regions, the main sources of coronal mass ejections and flares, are born with substantial net currents, in agreement with recent observations. Furthermore, they support eruption models that employ pre-eruption magnetic fields containing such currents.

Authors: Tibor Torok, James E. Leake, Viacheslav S. Titov, Vasilis Archontis, Zoran Mikić, Mark G. Linton, K?vin Dalmasse, Guillaume Aulanier, Bernhard Kliem
Projects: None

Publication Status: ApJ (under revision)
Last Modified: 2014-01-15 12:52
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Initiation of Coronal Mass Ejections by Sunspot Rotation  

Tibor Torok   Submitted: 2013-02-28 12:23

We study a filament eruption, two-ribbon flare, and coronal mass ejection (CME) that occurred in active region NOAA 10898 on 6 July 2006. The filament was located south of a strong sunspot that dominated the region. In the evolution leading up to the eruption, and for some time after it, a counter-clockwise rotation of the sunspot of about 30 degrees was observed. We suggest that the rotation triggered the eruption by progressively expanding the magnetic field above the filament. To test this scenario, we study the effect of twisting the initially potential field overlying a pre-existing flux rope, using three-dimensional zero-beta MHD simulations. We first consider a relatively simple and symmetric system, and then study a more complex and asymmetric magnetic configuration, whose photospheric flux distribution and coronal structure are guided by the observations and a potential field extrapolation. In both cases, we find that the twisting leads to the expansion of the overlying field. As a consequence of the progressively reduced magnetic tension, the flux rope quasi-statically adapts to the changed environmental field, rising slowly. Once the tension is sufficiently reduced, a distinct second phase of evolution occurs where the flux rope enters an unstable regime characterized by a strong acceleration. Our simulations thus suggest a new mechanism for the triggering of eruptions in the vicinity of rotating sunspots.

Authors: Torok, T., Temmer, M., Valori, G., Veronig, A. M., van Driel-Gesztelyi, L., Vrsnak, B.
Projects: SoHO-EIT,SoHO-MDI,SoHO-LASCO,TRACE

Publication Status: Accepted for publication by Solar Physics
Last Modified: 2013-02-28 12:32
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A model for magnetically coupled sympathetic eruptions  

Tibor Torok   Submitted: 2011-08-09 18:49

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: T. Torok, O. Panasenco, V.S. Titov, Z. Mikic, K.K. Reeves, M. Velli, J.A. Linker, G. De Toma
Projects: None

Publication Status: ApJ Letters (accepted)
Last Modified: 2011-08-10 09:32
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Abstracts by Author
Sun-to-Earth MHD Simulation of the 14 July 2000 "Bastille Day" Eruption
The origin, early evolution and predictability of solar eruptions
Modeling Jets in the Corona and Solar Wind
The Evolution of Writhe in Kink-Unstable Flux Ropes and Erupting Filaments
Initiation of Coronal Mass Ejections by Sunspot Rotation
Distribution of Electric Currents in Solar Active Regions
Initiation of Coronal Mass Ejections by Sunspot Rotation
A model for magnetically coupled sympathetic eruptions

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