Plasma Astrophysics, Part II: Reconnection and Flares: 341 (Astrophysics and Space Science Library)

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The reconnection is seen to occur between a set of ambient chromospheric fibrils and the filament itself. This allows for the relaxation of magnetic tension in the filament by an untwisting motion, demonstrating a flux rope structure. The topology change and untwisting are also found through nonlinear force-free field modelling of the active region in combination with magnetohydrodynamic simulation. These results demonstrate a new role for reconnection in solar eruptions: the release of magnetic twist.

It is widely accepted that magnetic reconnection plays an important role in plasmas, particularly in solar eruptive events, such as flares 1 , 2 , 3 , 4 , 5 , filament eruptions 6 , coronal mass ejections 7 , 8 and jets 9. In the models of magnetic reconnection, magnetic field lines with an oppositely directed component approach each other in a current sheet or at a magnetic null point, break up and reconnect to form new magnetic lines.

Magnetic energy is thereby released into thermal and kinetic energy of the plasma, potentially leading to large-scale phenomena. Evidence of reconnection on the Sun has mostly been limited to single aspects and has been indirect, showing changes of magnetic connections 10 , reconnection inflows 11 , 12 , 13 and outflows 14 , 15 , 16 , 17 , hot cusp-shaped structures at their interface 11 , 13 , 14 , supra-arcade downflows 15 , 18 , loop shrinkage 19 , 20 , sudden brightenings 21 , 22 , current sheets 15 , 20 , 23 , plasmoid ejections 24 , 25 , loop-top hard X-ray sources 20 , 26 , pulsating radio emissions 27 , and coronal heating in the interface between emerging and ambient magnetic flux In recent years, direct and more comprehensive evidence of reconnection has been discovered in a couple of energetic events on the Sun.

Inflows, outflows and the formation of new loops could also be studied in a well-observed case of reconnection between two sets of small-scale chromospheric loops, imaged at high resolution in H-alpha This study showed a transition from slow to fast reconnection.

The three-dimensional 3D topology of reconnecting loops and their heating has been inferred using the combined perspectives and multiple EUV channels of two spacecraft Here we present comprehensive observational evidence of reconnection between a set of chromospheric fibrils and the threads of an erupting filament, which gave rise to a small flare.

We observe the in- and outflows and hot cusp-shaped structures at the ends of a small-scale reconnecting current sheet, as well as newly formed loops that demonstrate the change of magnetic connectivity.

Observing the release of twist by magnetic reconnection in a solar filament eruption

We estimate that the reconnection is fast. The intriguing rotational motion of the erupted filament is found to show the untwisting of a flux rope, enabled by the reconnection with the chromospheric fibrils, which extend to essentially current-free magnetic flux in the corona. Photospheric vector magnetograms taken by the Helioseismic and Magnetic Imager HMI 34 on board SDO allow us to obtain the 3D field structure of the reconnection region, independently demonstrating the change of topology. The dynamics of this source region model are studied in a data-constrained magnetohydrodynamic MHD simulation, which reproduces the observed features of the eruption and confirms that the untwisting of the erupted flux is enabled by reconnection with ambient current-free flux.

Many thin threads extending along the filament spine make up its fine structure see the red arrows in Fig. The threads in the western section show indications of twist of approximately one turn, consistent with the widely but not universally adopted assumption that the magnetic structure of filaments is that of a weakly twisted magnetic flux rope 35 , 36 , Figure 1b shows the position of the filament in the magnetogram. Positive photospheric polarity is given at the western footpoint and southern side of the filament, and negative polarity is given at the eastern footpoint and northern side, so the filament has sinistral chirality This corresponds to positive right handed magnetic helicity, because the axial current of the filament must also point eastward for a force-free equilibrium to exist in the given ambient flux distribution.

The original position is marked by the red-dotted line. The erupting part is indicated by the blue-dotted lines and blue arrows. Cyan-dotted lines in f — h indicate some of the filament threads that rise with a delay. The stable eastern filament section is marked by the red arrows in f — h.

The motion of these threads indicates the untwisting of the filament during its eruption. Associated brightenings, indicating heating due to reconnection, appear soon but only after the onset of the motion.

Plasma Astrophysics, Part II: Reconnection and Flares - Boris V. Somov - Google Books

The eruption comprises the whole western filament section and part of the eastern section. Some of the threads in the western section follow the main eruption with a small delay. Subsequently, other threads become visible in their place; these, as well as all other threads in the eastern filament section, remain stable see the red arrows in Fig. We conclude that part of the magnetic flux in the filament experiences an instability, which causes the motion and subsequently triggers reconnection.

The unstable flux comprises the whole western section and part of the eastern section, where unstable flux lies on top of the stable flux. Because the indicated twist of approximately one turn lies below the threshold of the helical kink instability 39 , 40 and the erupted filament does not build up a clear helical shape as a whole see the blue-dotted line in Fig.

All southward motions end by UT, and all material is subsequently seen to slide down towards the western end point of the filament in this confined eruption, which does not produce a coronal mass ejection. Obviously, the southward end region of the erupted filament threads is the highest part of the visible erupted structure. The data do not definitely reveal whether the erupted flux is still magnetically rooted in the eastern filament section or if it now connects to other negative flux in the photosphere.

However, the erupted threads point approximately towards the neighbouring AR in the southeast, away from the direction of the eastern filament section, but similar to pre-existing interconnecting loops between the two ARs. This is suggestive of reconnection between the erupting filament and these loops. The eruption gives rise to two signatures of magnetic reconnection. The two prominent brightenings that develop at the sides of the eastern filament section Fig. Ambient flux passing over the filament is lifted; subsequently, its legs approach each other and reconnect in a vertical current sheet that is known to form under the erupting flux, but not imaged in the present event.

The released energy is channelled downward along the field lines, producing the chromospheric brightenings where the reconnecting ambient flux is rooted. This can likewise be seen under the western filament section Fig. Chromospheric fibrils south of the bend point reconnect with the trailing threads of the erupting western filament section on the north side of the bend point.

The observations of the reconnecting current sheet are analysed in detail in the following section, where we find that it also forms under the erupting flux. However, here it is the erupting flux that reconnects with ambient flux, a process not envisioned in the standard flare model.

Plasma Astrophysics, Part II

In addition, the erupted filament displays an intriguing motion that is highly suggestive of a rotation about its main direction Supplementary Movie 1. At least one full turn is indicated.

The motion of the better visible threads on the upper side is also shown in a time-slice plot white-dotted lines in Fig. When looking along the filament toward the western footpoint, the rotation is clockwise. Erupted flux is generally assumed to have the structure of a flux rope 5 , 6 , 7 , 8.

The positive helicity inferred above implies that the twist of the rope is right handed. The clockwise rotation thus represents an untwisting that is equivalent to the relaxation of magnetic tension and supports the conjecture of a flux rope structure for the erupted filament. Erupting flux ropes show an apparent untwisting simply as the result of their expansion.

While the total number of field line turns is preserved in the absence of reconnection , the twist per unit length decreases. The stretching can appear as a propagation of twist if only a part of the rope displays a twist pattern. Although the threads of the erupting filament do not display a pronounced twist pattern in images Fig.

The stretching can also mimic a rotation in a time-slice plot, as in Figure 1i. However, it cannot explain two effects visible in the present event: many threads shift nearly completely to the other side of the rope cyan lines in Fig. True untwisting results from the conversion of twist into writhe of the flux rope axis and from reconnection with less twisted flux.

The first effect is negligible here because a clear helical shape does not develop Fig. Following reconnection with less twisted flux, the twist tends to equilibrate along the new structure. Hence, the observed rotational motion indicates that the erupted filament flux reconnects with ambient flux, which usually has no twist. The H-alpha observations reveal only a small part of that flux the chromospheric fibrils south of the bend point of the filament channel , but a consistent whole picture is provided below by our models for the coronal field of the AR and by the evolution of one of these models in an MHD simulation.

The reconnection process and formation of the associated current sheet are shown in Fig. Their full evolution can be better seen in Supplementary Movie 1. Figure 2a shows the positions of the original structures in an H-alpha image overlaid by a line-of-sight magnetogram. The reconnection occurs between two sets of magnetic loops with opposite directions see the arrows in Fig. One set of magnetic loops, that is, the filament threads whose eruption is delayed, is indicated by the red arrow before the reconnection.

The other set, that is, the chromospheric fibrils, is indicated by the white arrow. The reconnection occurs in a small region marked by the black rectangle in Fig. In Fig.

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During the reconnection process, two cusp-shaped structures are formed blue-dotted lines in Fig. Simultaneously, new loops appear on the other side of the cusp structure northeast of the reconnection region the black arrows in Fig. Subsequently, these loops move away from the cusp. After the reconnection ceases, the newly formed loops can be seen more obviously marked by black arrows in Fig. They have accumulated in the northeast reconnection outflow region. The new loops, which form early in the southwest of the reconnection region at the right cusp , follow the filament eruption Supplementary Movie 1 , while those that form there later do not rise further and can be clearly seen near the end of the event indicated by the yellow arrow in Fig.

The three time-slice plots shown in Fig. During the eruptive process the reconnection region is stretched, apparently to form a current sheet, which is visible in H-alpha as a bright linear structure extending between the tips of the two cusps Fig. The average length and width of the current sheet are estimated to be 4. At the same time, we find that the plasma at the tip of the northeast cusp structure is significantly heated, causing the brightening Fig. The cusp and current sheet can be seen in multiple channels Fig.

The southwestern cusp structure is weaker, as is to be expected from lower plasma densities at the upper end of a current sheet that is formed and stretched out upward by the eruption. This also shows up only intermittently owing to absorption by the moving threads of the erupted filament, which demonstrates that the current sheet forms under the erupting flux.