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Filament Shape and Coronal Magnetic Field Structure  

Boris Filippov   Submitted: 2015-10-16 02:30

Solar filament shape in projection on disc depends on the structure of the coronal magnetic field. We calculate the position of polarity inversion lines (PILs) of coronal potential magnetic field at different heights above the photosphere, which compose the magnetic neutral surface, and compare with them the distribution of the filament material in Hα chromospheric images. We found that the most of the filament material is enclosed between two polarity inversion lines (PILs), one at a lower height close to the chromosphere and one at a higher level, which can be considered as a height of the filament spine. Observations of the same filament on the limb by the STEREO spacecraft confirm that the height of the spine is really very close to the value obtained from the PIL and filament border matching. Such matching can be used for filament height estimations in on-disk observations. Filament barbs are housed within protruding sections of the low-level PIL. On the base of simple model, we show that the similarity of the neutral surfaces in potential and non-potential fields with the same sub-photospheric sources is the reason for the found tendency for the filament material to gather near the potential-field neutral surface.

Authors: B. Filippov
Projects: SDO-HMI,STEREO

Publication Status: Accepted for publication in MNRAS
Last Modified: 2015-10-17 09:57
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Covert connection of filaments  

Boris Filippov   Submitted: 2015-07-30 00:26

We analyse the relationship between two near filaments, which do not show any connection in Hα images but reveal close magnetic connectivity during filament activations in Extreme Ultraviolet (EUV) observations. A twisted flux rope, which connects a half of one filament with another filament, becomes visible during several activations but seems to exist all the time of the filaments presence on the disc. Solar Dynamic Observatory} (SDO) and Solar Terrestrial Relations Observatory (STEREO) observed the region with the filaments from two points of view separated by the angle of about 120 deg. On 2012 July 27, SDO observed the filament activation on disc, while for the STEREO B position the filaments were visible at the limb. Nearly identical interaction episode was observed on 2012 August 04 by STEREO A on disc and by SDO at the limb. This good opportunity allows us to disentangle the 3-D shape of the connecting flux rope and in particular to determine with high reliability the helicity sign of the flux rope, which looks ambiguous in preliminary inspections of on-disc EUV images only.} The complex magnetic structure of the region consists of three braided flux ropes in the vicinity of the coronal null point. Using observations of the flux rope fine structure and plasma motions within it from two points of view, we determine the negative sign of helicity of the flux rope, which corresponds to dextral chirality of the filaments. The observations, despite the tangled fine structure in some EUV images, support flux rope filament models. They give more evidence for the one-to-one relationship between the filament chirality and the flux rope helicity.

Authors: Boris Filippov
Projects: SDO-AIA,STEREO

Publication Status: MNRAS in press
Last Modified: 2015-08-03 11:57
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Formation of a rotating jet during the filament eruption on 10-11 April 2013  

Boris Filippov   Submitted: 2015-05-08 00:45

We analyze multi-wavelength and multi-viewpoint observations of a helically twisted plasma jet formed during a confined filament eruption on 10-11 April 2013. Given a rather large scale event with its high spatial and temporal resolution observations, it allows us to clearly understand some new physical details about the formation and triggering mechanism of twisting jet. We identify a pre-existing flux rope associated with a sinistral filament, which was observed several days before the event. The confined eruption of the filament within a null point topology, also known as an Eiffel tower (or inverted-Y) magnetic field configuration results in the formation of a twisted jet after the magnetic reconnection near a null point. The sign of helicity in the jet is found to be the same as that of the sign of helicity in the filament. Untwisting motion of the reconnected magnetic field lines gives rise to the accelerating plasma along the jet axis. The event clearly shows the twist injection from the pre-eruptive magnetic field to the jet.

Authors: B. Filippov, A. K. Srivastava, B. N. Dwivedi, S. Masson, G. Aulanier, N. C. Joshi, and W. Uddin
Projects: SDO-AIA,STEREO

Publication Status: Accepted for publication in MNRAS
Last Modified: 2015-05-12 14:02
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Solar Magnetic Flux Ropes  

Boris Filippov   Submitted: 2015-01-13 02:22

At the beginning of 1990s, it was found that the strongest disturbances of the space-weather were associated with huge ejections of plasma from the solar corona, which took the form of magnetic clouds when moved from the Sun. It is the collisions of the magnetic clouds with the Earth's magnetosphere that lead to strong, sometimes catastrophic changes in space-weather. The onset of a coronal mass ejection (CME) is sudden and no reliable forerunners of СМЕs have been found till date. The CME prediction methodologies are less developed compared to the methods developed for the prediction of solar flares. The most probable initial magnetic configuration of a CME is a flux rope consisting of twisted field lines which fill the whole volume of a dark coronal cavity. The flux ropes can be in stable equilibrium in the coronal magnetic field for weeks and even months, but suddenly they loose their stability and erupt with high speed. Their transition to the unstable phase depends on the parameters of the flux rope (i.e., total electric current, twist, mass loading etc.), as well as on the properties of the ambient coronal magnetic field. One of the major governing factors is the vertical gradient of the coronal magnetic field which is estimated as decay index (n). Cold dense prominence material can be collected in the lower parts of the helical flux tubes. Filaments are therefore good tracers of the flux ropes in the corona, which become visible long before the beginning of the eruption. The perspectives of the filament eruptions and following CMEs can be estimated by the comparison of observed filament heights with calculated decay index distributions. The present paper reviews the formation of magnetic flux ropes, their stable and unstable phases, eruption conditions, and also discusses their physical implications in the solar corona.

Authors: Boris Filippov, Olesya Martsenyuk, Abhishek K. Srivastava, and Wahab Uddin
Projects: None

Publication Status: Accepted for publication in Journal of Astrophysics & Astronomy (Special Issue; Eds: V. Fedun, A.K. Srivastava, R. Erdelyi, J.C. Pandey)
Last Modified: 2015-01-13 14:06
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Filament eruption with apparent reshuffle of endpoints  

Boris Filippov   Submitted: 2014-05-22 23:56

Filament eruption on 30 April - 1 May 2010, which shows the reconnection of one filament leg with a region far away from its initial position, is analyzed. Observations from three viewpoints are used for as precise as possible measurements of endpoint coordinates. The northern leg of the erupting prominence loop 'jumps' laterally to the latitude lower than the latitude of the originally southern endpoint. Thus, the endpoints reshuffled their positions in the limb view. Although this behaviour could be interpreted as the asymmetric zipping-like eruption, it does not look very likely. It seems more likely to be reconnection of the flux-rope field lines in its northern leg with ambient coronal magnetic field lines rooted in a quiet region far from the filament. From calculations of coronal potential magnetic field, we found that the filament before the eruption was stable for vertical displacements, but was liable to violation of the horizontal equilibrium. This is unusual initiation of an eruption with combination of initial horizontal and vertical flux-rope displacements showing a new unexpected possibility for the start of an eruptive event.

Authors: Boris Filippov
Projects: PROBA2/SWAP,SoHO-MDI,STEREO

Publication Status: Accepted in MNRAS
Last Modified: 2014-05-26 13:02
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Filament eruption on 2010 October 21 from three viewpoints  

Boris Filippov   Submitted: 2013-06-05 00:24

A filament eruption on 2010 October 21 observed from three different viewpoints by the Solar Terrestrial Relations Observatory (STEREO) and the Solar Dynamic Observatory (SDO) is analyzed with invoking also data from the Solar and Heliospheric Observatory (SOHO) and the Kanzelhoehe Solar Observatory. The position of the filament just before the eruption at the central meridian not far from the center of the solar disk was favorable for photospheric magnetic field measurements in the area below the filament. Because of this, we were able to calculate with high precision the distribution of the coronal potential magnetic field near the filament. We found that the filament began to erupt when it approached the height in the corona where the magnetic field decay index was greater than one. We determined also that during the initial stage of the eruption the filament moved along the magnetic neutral surface

Authors: Boris Filippov
Projects: STEREO

Publication Status: Accepted, ApJ
Last Modified: 2013-06-05 11:05
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Electric Current Equilibrium in the Corona  

Boris Filippov   Submitted: 2013-02-18 23:33

A hyperbolic flux-tube configuration containing a null point below the flux rope is considered as a pre-eruptive state of coronal mass ejections that start simultaneously with flares. We demonstrate that this configuration is unstable and cannot exist for a long time in the solar corona. The inference follows from general equilibrium conditions and from analyzing simple models of the flux-rope equilibrium. A direct consequence of the stable flux-rope equilibrium in the corona are separatrices in the horizontal-field distribution in the chromosphere. They can be recognized as specific ''herring-bone structures'' in a chromospheric fibril pattern.

Authors: Boris Filippov
Projects: None

Publication Status: Accepted, Solar Phys.
Last Modified: 2013-02-19 10:01
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Formation of a White-Light Jet within a Quadrupolar Magnetic Configuration  

Boris Filippov   Submitted: 2011-11-29 03:52

We analyze multi-wavelength and multi-viewpoint observations of a large-scaleevent viewed on 7 April 2011 originating from an active region complex. Theactivity leads to a white-light jet being formed in the outer corona. Thetopology and evolution of the coronal structures were imaged in high resolutionusing the Atmospheric Imaging Assembly (AIA) onboard the Solar DynamicsObservatory (SDO). In addition, large field-of-view images of the corona wereobtained using the Sun Watcher using Active Pixel System detector and ImageProcessing (SWAP) telescope onboard the PRoject for Onboard Autonomy (PROBA2)microsatellite, providing evidence for the connectivity of the coronalstructures with outer coronal features that were imaged with the Large AngleSpectrometric Coronagraph (LASCO) C2 on Solar and Heliospheric Observatory(SOHO). The data-sets reveal an Eiffel-tower type jet configuration extendinginto a narrow jet in the outer corona. The event starts from the growth of adark area in the central part of the structure. The darkening was also observedin projection on the disk by the Solar TErrestrial RElations Observatory-Ahead(STEREO-A) spacecraft from a different point of view. We assume that the darkvolume in the corona descends from a coronal cavity of a flux rope that movedup higher in the corona but still failed to erupt. The quadrupolar magneticconfiguration corresponds to a saddle-like shape of the dark volume andprovides a possibility for the plasma to escape along the open field lines intothe outer corona, forming the white-light jet.

Authors: Boris Filippov, Serge Koutchmy, and Ehsan Tavabi
Projects: SDO-AIA

Publication Status: Accepted for publication in Solar Physics
Last Modified: 2011-11-30 08:16
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Crossing Filaments  

Boris Filippov   Submitted: 2011-01-28 00:51

Solar filaments show the position of large scale polarity inversion lines and are used for the reconstruction of large-scale solar magnetic field structure on the basis of Hα synoptic charts for the periods when magnetographic measurements were not available. Sometimes crossing filaments are seen in Hα filtergrams. We analyze daily Hα filtergrams from the archive of Big Bear Solar Observatory for the period of 1999-2003 to find crossing and interacting filaments. A number of examples are presented and filament patterns are compared with photospheric magnetic field distributions. We have found that all crossing filaments reveal quadrupolar magnetic configurations of the photospheric field and presume the presence of null points in the corona.

Authors: Boris Filippov and A.K. Srivastava
Projects: None

Publication Status: Submitted to Solar Physics
Last Modified: 2011-01-28 09:15
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Deflection of coronal rays by remote CMEs: shock wave or magnetic pressure?  

Boris Filippov   Submitted: 2010-07-01 00:16

We analyze five events of the interaction of coronal mass ejections (CMEs) with the remote coronal rays located up to 90 degrees away from the CME as observed by the SOHO/LASCO C2 coronagraph. Using sequences of SOHO/LASCO C2 images, we estimate the kink propagation in the coronal rays during their interaction with the corresponding CMEs ranging from 180 to 920 km s-1 within the interval of radial distances form 3 R to 6 R. We conclude that all studied events do not correspond to the expected pattern of shock wave propagation in the corona. Coronal ray deflection can be interpreted as the influence of the magnetic field of a moving flux rope related to a CME. The motion of a large-scale flux rope away from the Sun creates changes in the structure of surrounding field lines, which are similar to the kink propagation along coronal rays. The retardation of the potential should be taken into account since the flux rope moves at high speed comparable with the Alfvén speed.

Authors: Boris Filippov and A.K. Srivastava
Projects: None

Publication Status: Solar Physics (in press)
Last Modified: 2010-07-02 14:55
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Abstracts by Author
Filament Shape and Coronal Magnetic Field Structure
Covert connection of filaments
Formation of a rotating jet during the filament eruption on 10-11 April 2013
Solar Magnetic Flux Ropes
Filament eruption with apparent reshuffle of endpoints
Filament eruption on 2010 October 21 from three viewpoints
Electric Current Equilibrium in the Corona
Formation of a White-Light Jet within a Quadrupolar Magnetic Configuration
Crossing Filaments
Deflection of coronal rays by remote CMEs: shock wave or magnetic pressure?

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