Formation of Large-scale Coronal Loops Interconnecting Two Active Regions through Gradual Magnetic Reconnection and an Associated Heating Process

An analysis of the importance of shock heating within coronal magnetic fields has hitherto been a neglected area of study. We present new results obtained from nonlinear magnetohydrodynamic simulations of straight coronal loops. This work shows how the energy released from the magnetic field, following an ideal instability, can be converted into thermal energy, thereby heating the solar corona. Fast dissipation of magnetic energy is necessary for coronal heating and this requirement is compatible with the time scales associated with ideal instabilities. Therefore, we choose an initial loop configuration that is susceptible to the fast-growing kink, an instability that is likely to be created by convectively driven vortices, occurring where the loop field intersects the photosphere (i.e. the loop footpoints). The large-scale deformation of the field caused by the kinking creates the conditions for the formation of strong current sheets and magnetic reconnection, which have previously been considered as sites of heating, under the assumption of an enhanced resistivity. However, our simulations indicate that slow mode shocks are the primary heating mechanism, since, as well as creating current sheets, magnetic reconnection also generates plasma flows that are faster than the slow magnetoacoustic wave speed.

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Formation of solar coronal loops through magnetic reconnection in an emerging active region

10.5194/egusphere-egu21-1013 ◽

2021 ◽

Author(s):

Zhenyong Hou ◽

Hui Tian ◽

Hechao Chen ◽

Xiaoshuai Zhu ◽

Jiansen He ◽

...

Keyword(s):

Magnetic Reconnection ◽

Current Sheet ◽

Emission Measure ◽

Three Dimensional ◽

Transverse Field ◽

Power Spectral Analysis ◽

Active Regions ◽

Building Blocks ◽

Coronal Loops ◽

Field Lines

Coronal loops are building blocks of solar active regions (ARs). However, their formation is not well understood. Here we present direct observational evidence for the formation of coronal loops through magnetic reconnection as new magnetic fluxes emerge to the solar atmosphere. Observations in the EUV passbands of SDO/AIA clearly show the newly formed loops following magnetic reconnection within a vertical current sheet. Formation of the loops is also seen in the H&#945; images taken by NVST. The SDO/HMI observations show that a positive-polarity flux concentration moves toward a negative-polarity one with a speed of ~0.5 km s-1&#160;before the apparent formation of coronal loops. During the formation of coronal loops, we found signatures of flux cancellation and subsequent enhancement of the transverse field between the two polarities. We have reconstructed the three-dimensional magnetic field structure through a magnetohydrostatic model, which shows field lines consistent with the loops in AIA images. Numerous bright blobs with a width of ~1.5 Mm appear intermittently in the current sheet and move upward with apparent velocities of ~80 km s-1. We have also identified plasma blobs moving to the footpoints of the newly formed large loops, with apparent velocities ranging from 30 to 50 km s-1. A differential emission measure analysis shows that the temperature, emission measure and density of the bright blobs are 2.5-3.5 MK, 1.1-2.3&#215;1028&#160;cm-5&#160;and 8.9-12.9&#215;109&#160;cm-3, respectively. Power spectral analysis of these blobs indicates that the magnetic reconnection is inconsistent with the turbulent reconnection scenario.

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Slipping magnetic reconnection and complex evolution of a flux rope and flare ribbons

Proceedings of the International Astronomical Union ◽

10.1017/s1743921316002246 ◽

2015 ◽

Vol 11 (S320) ◽

pp. 179-184

Author(s):

Ting Li ◽

Jun Zhang

Keyword(s):

Magnetic Reconnection ◽

Apparent Motion ◽

Large Scale ◽

Neutral Line ◽

Flux Rope ◽

Heating Process ◽

Flare Event ◽

Flare Loops ◽

Fine Structures ◽

Flare Model

AbstractAbundant observations in recent years show that the flares are more complex than the 2D standard flare model presents. This proposes a challenge to the 2D flare model and 3D flare model has been developed. We report the complex evolution of flare ribbons and a flux rope in a C8.9 flare event. The two ribbons slipped in opposite directions along the neutral line and the eastern ribbon seemed a hook-like structure. The flare loops were crossed each other, composing a “bi-fan” system. The slipping magnetic reconnection is involved in the flare and leads to slipping motion of flare ribbons and complex evolution of flare loops. Overlying the flare loops, a large-scale flux rope was erupted and meanwhile the eastern end of the flux rope changed with time and slipped along the hook-like ribbon. The fine structures of the flux rope delineated a “triangle-flag” surface, which may imply one-half of the coronal quasi-separatrix layers that surrounds a flux rope. We suggest that the heating process of slipping magnetic reconnection during the flare caused the apparent motion of the flux rope ends.

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A Discussion on the physics of the solar atmosphere - The x-ray corona from Skylab

Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences ◽

10.1098/rsta.1976.0034 ◽

1976 ◽

Vol 281 (1304) ◽

pp. 365-374 ◽

Cited By ~ 15

Keyword(s):

Large Scale ◽

Dense Plasma ◽

Coronal Holes ◽

Active Regions ◽

Coronal Loops ◽

Closed Loops ◽

X Ray ◽

Physical Conditions ◽

Wide Range ◽

The Magnetic Field

An overview of the images obtained with the A.S. & E. X-ray telescope on Skylab shows the low corona to be highly structured. The plasma is distributed in closed loops shaped by the magnetic field with sizes ranging from the smallest resolvable structures of a few thousand kilometres to loops that reach halfway across the solar disk. Relatively high-temperature and dense plasma loops overlay active regions; large-scale interconnections link active regions to their surrounding fields and in some cases to other active regions. The large-scale loops, which cover most of the Sun outside of active regions, appear to be related to old active regions whose magnetic fields have spread out over the course of several solar rotations. Often at the poles and occasionally on the disk, large regions display radial field configurations (coronal holes) from which the plasma preferentially escapes into high-velocity solar wind streams. A comprehensive view of the structure and evolution of the X-ray corona is given in terms of the physical conditions existing in the various coronal loops, and the importance of active regions is emphasized by examining their structure and time development over a wide range of scales.

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Very Large Array Observations of Large-Scale Coronal Structures

International Astronomical Union Colloquium ◽

10.1017/s0252921100025306 ◽

1994 ◽

Vol 144 ◽

pp. 195-199

Author(s):

K. R. Lang

Keyword(s):

High Resolution ◽

Active Region ◽

Magnetic Fields ◽

Large Scale ◽

Active Regions ◽

Coronal Loops ◽

Large Array ◽

Very Large Array ◽

Magnetically Trapped ◽

High Magnetic Field Strength

AbstractVery Large Array (VLA) observations indicate that electrons accelerated in one active region can travel along otherwise-invisible, large-scale coronal loops to trigger flares in another widely-separated active region, as well as from the magnetic loops connecting them. The VLA provides high-resolution, full-disk images of quiescent, or non-flaring, coronal loops within individual active regions (at 20 cm) and between or beyond them (at 90 cm). Both ground-based radio telescopes and spaceborne X-ray telescopes provide high-resolution images of the ubiquitous coronal loops whose hot, dense magnetically-trapped plasma emits thermal bremsstrahlung. Radio observations can be used to specify the strength and structure of the magnetic fields in the low solar corona. We find a high magnetic field strength in the million-degree plasma above large sunspots – 75 to 80 percent of the value in the underlying photospheric sunspots; as well as coronal regions of non-potential, current-amplified magnetic fields. Some long-lasting (hours) coronal radio sources found on the Sun and other active stars require nonthermal radiation and nearly continuous acceleration of energetic electrons.

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Large-scale Compression Acceleration during Magnetic Reconnection in a Low-β Plasma

The Astrophysical Journal ◽

10.3847/1538-4357/aae07b ◽

2018 ◽

Vol 866 (1) ◽

pp. 4 ◽

Cited By ~ 15

Author(s):

Xiaocan Li ◽

Fan Guo ◽

Hui Li ◽

Shengtai Li

Keyword(s):

Magnetic Reconnection ◽

Large Scale

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Numerical study of the reconnection process between magnetic cloud and heliospheric current sheet

Astronomy and Astrophysics ◽

10.1051/0004-6361/201832951 ◽

2018 ◽

Vol 619 ◽

pp. A82

Author(s):

Man Zhang ◽

Yu Fen Zhou ◽

Xue Shang Feng ◽

Bo Li ◽

Ming Xiong

Keyword(s):

Magnetic Reconnection ◽

Current Sheet ◽

Dynamic Process ◽

Large Scale ◽

Magnetic Cloud ◽

Numerical Study ◽

Three Dimensional ◽

Heliospheric Current Sheet ◽

Reconnection Process ◽

Magnetic Filed

In this paper, we have used a three-dimensional numerical magnetohydrodynamics model to study the reconnection process between magnetic cloud and heliospheric current sheet. Within a steady-state heliospheric model that gives a reasonable large-scale structure of the solar wind near solar minimum, we injected a spherical plasmoid to mimic a magnetic cloud. When the magnetic cloud moves to the heliospheric current sheet, the dynamic process causes the current sheet to become gradually thinner and the magnetic reconnection begin. The numerical simulation can reproduce the basic characteristics of the magnetic reconnection, such as the correlated/anticorrelated signatures in V and B passing a reconnection exhaust. Depending on the initial magnetic helicity of the cloud, magnetic reconnection occurs at points along the boundary of the two systems where antiparallel field lines are forced together. We find the magnetic filed and velocity in the MC have a effect on the reconnection rate, and the magnitude of velocity can also effect the beginning time of reconnection. These results are helpful in understanding and identifying the dynamic process occurring between the magnetic cloud and the heliospheric current sheet.

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Formation of coronal rain triggered by impulsive heating associated with magnetic reconnection

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936253 ◽

2019 ◽

Vol 630 ◽

pp. A123 ◽

Cited By ~ 5

Author(s):

P. Kohutova ◽

E. Verwichte ◽

C. Froment

Keyword(s):

Magnetic Reconnection ◽

Thermal Instability ◽

Flux Rope ◽

Coronal Loops ◽

Rain Event ◽

Subsequent Formation ◽

Standard Models ◽

Field Lines ◽

Impulsive Heating ◽

Coronal Rain

Context. Coronal rain consists of cool plasma condensations formed in coronal loops as a result of thermal instability. The standard models of coronal rain formation assume that the heating is quasi-steady and localised at the coronal loop footpoints. Aims. We present an observation of magnetic reconnection in the corona and the associated impulsive heating triggering formation of coronal rain condensations. Methods. We analyse combined SDO/AIA and IRIS observations of a coronal rain event following a reconnection between threads of a low-lying prominence flux rope and surrounding coronal field lines. Results. The reconnection of the twisted flux rope and open field lines leads to a release of magnetic twist. Evolution of the emission of one of the coronal loops involved in the reconnection process in different AIA bandpasses suggests that the loop becomes thermally unstable and is subject to the formation of coronal rain condensations following the reconnection and that the associated heating is localised in the upper part of the loop leg. Conclusions. In addition to the standard models of thermally unstable coronal loops with heating localised exclusively in the footpoints, thermal instability and subsequent formation of condensations can be triggered by the impulsive heating associated with magnetic reconnection occurring anywhere along a magnetic field line.

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Why are CMEs large-scale coronal events: nature or nurture?

Annales Geophysicae ◽

10.5194/angeo-26-3077-2008 ◽

2008 ◽

Vol 26 (10) ◽

pp. 3077-3088 ◽

Cited By ~ 24

Author(s):

L. van Driel-Gesztelyi ◽

G. D. R. Attrill ◽

P. Démoulin ◽

C. H. Mandrini ◽

L. K. Harra

Keyword(s):

Magnetic Reconnection ◽

Large Scale ◽

Observational Evidence ◽

Angular Width ◽

Small Scale ◽

Apparent Contradiction ◽

Lower Corona ◽

Magnetic Structures ◽

Source Regions ◽

Scale Response

Abstract. The apparent contradiction between small-scale source regions of, and large-scale coronal response to, coronal mass ejections (CMEs) has been a long-standing puzzle. For some, CMEs are considered to be inherently large-scale events – eruptions in which a number of flux systems participate in an unspecified manner, while others consider magnetic reconnection in special global topologies to be responsible for the large-scale response of the lower corona to CME events. Some of these ideas may indeed be correct in specific cases. However, what is the key element which makes CMEs large-scale? Observations show that the extent of the coronal disturbance matches the angular width of the CME – an important clue, which does not feature strongly in any of the above suggestions. We review observational evidence for the large-scale nature of CME source regions and find them lacking. Then we compare different ideas regarding how CMEs evolve to become large-scale. The large-scale magnetic topology plays an important role in this process. There is amounting evidence, however, that the key process is magnetic reconnection between the CME and other magnetic structures. We outline a CME evolution model, which is able to account for all the key observational signatures of large-scale CMEs and presents a clear picture how large portions of the Sun become constituents of the CME. In this model reconnection is driven by the expansion of the CME core resulting from an over-pressure relative to the pressure in the CME's surroundings. This implies that the extent of the lower coronal signatures match the final angular width of the CME.

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3-D numerical simulations of coronal loops oscillations

Annales Geophysicae ◽

10.5194/angeo-27-3899-2009 ◽

2009 ◽

Vol 27 (10) ◽

pp. 3899-3908 ◽

Cited By ~ 16

Author(s):

M. Selwa ◽

L. Ofman

Keyword(s):

Active Regions ◽

External Source ◽

Coronal Loops ◽

Velocity Perturbation ◽

Impulsive Excitation ◽

Kink Mode ◽

Parametric Studies ◽

Field Lines ◽

Significant Distance

Abstract. We present numerical results of 3-D MHD model of a dipole active region field containing a loop with a higher density than its surroundings. We study different ways of excitation of vertical kink oscillations by velocity perturbation: as an initial condition, and as an impulsive excitation with a pulse of a given position, duration, and amplitude. These properties are varied in the parametric studies. We find that the amplitude of vertical kink oscillations is significantly amplified in comparison to horizontal kink oscillations for exciters located centrally (symmetrically) below the loop, but not if the exciter is located a significant distance to the side of the loop. This explains why the pure vertical kink mode is so rarely observed in comparison to the horizontally polarized one. We discuss the role of curved magnetic field lines and the pulse overlapping at one of the loop's footpoints in 3-D active regions (AR's) on the excitation and the damping of slow standing waves. We find that footpoint excitation becomes more efficient in 3-D curved loops than in 2-D curved arcades and that slow waves can be excited within an interval of time that is comparable to the observed one wave-period due to the combined effect of the pulse inside and outside the loop. Additionally, we study the effect of AR topology on the excitation and trapping of loop oscillations. We find that a perturbation acting directly on a single loop excites oscillations, but results in an increased leakage compared to excitation of oscillations in an AR field by an external source.

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