Magnetic reconnection and the Kelvin-Helmholtz instability in the solar corona

Howson, Thomas; Moortel, I. De; Pontin, D. I.

doi:10.1051/0004-6361/202141620

Cited by 13 publications

(11 citation statements)

References 72 publications

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“…We can clearly see that the loop boundary is fully turbulent and extremely fine structures in vorticity have already been generated. Several authors have studied the evolution of vorticity in numerical simulations where the kink mode is excited (see, e.g., Terradas et al 2008;Antolin et al 2015;Howson et al 2017b;Karampelas et al 2017;Karampelas & Van Doorsselaere 2018;Guo et al 2019;Howson et al 2020Howson et al , 2021 and some of their results are comparable with the present ones. Similar vorticity structures as those displayed in Fig.…”

Section: Revisiting the Straight Magnetic Field Casesupporting

confidence: 78%

Transition to turbulence in nonuniform coronal loops driven by torsional Alfvén waves

Díaz-Suárez

Soler

2022

A&A

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It has been shown in a previous work that torsional Alfvén waves can drive turbulence in nonuniform coronal loops with a purely axial magnetic field. Here we explore the role of the magnetic twist. We model a coronal loop as a transversely nonuniform straight flux tube, anchored in the photosphere, and embedded in a uniform coronal environment. We consider that the magnetic field is twisted and control the strength of magnetic twist by a free parameter of the model. We excite the longitudinally fundamental mode of standing torsional Alfvén waves, whose temporal evolution is obtained by means of high-resolution three-dimensional ideal magnetohydrodynamic numerical simulations. We find that phase mixing of torsional Alfvén waves creates velocity shear in the direction perpendicular to the magnetic field lines. The velocity shear eventually triggers the Kelvin-Helmholtz instability (KHi). In weakly twisted magnetic tubes, the KHi is able to grow nonlinearly and, subsequently, turbulence is driven in the coronal loop in a similar manner as in the untwisted case. Provided that magnetic twist remains weak, the effect of magnetic twist is to delay the onset of the KHi and to slow down the development of turbulence. In contrast, magnetic tension can suppress the nonlinear growth of the KHi when magnetic twist is strong enough, even if the KHi has locally been excited by the phase-mixing shear. Thus, turbulence is not generated in strongly twisted loops.

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Section: Revisiting the Straight Magnetic Field Casesupporting

confidence: 78%

Transition to turbulence in nonuniform coronal loops driven by torsional Alfvén waves

Díaz-Suárez

Soler

2022

A&A

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“…The results from the observations and simulations have suggested that the KHI and RTI vortices can twist and deform the magnetic field as well as change the local magnetic pressure, and a myriad of current sheets and vortices are produced through a turbulent cascade. It is also likely that magnetic reconnection is enhanced in the solar corona due to such wave processes (Howson et al 2021). However, it is unclear whether this sort of reconnection is only gradual and confined to the smallest turbulent scales, hence, not affecting the overall topology of the loop.…”

Section: Introductionmentioning

confidence: 99%

Observations of Instability-driven Nanojets in Coronal Loops

Antolin

McLaughlin

2022

ApJ

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The recent discovery of nanojets by Antolin et al. represents magnetic reconnection in a braided field, thus clearly identifying reconnection-driven nanoflares. Due to their small scale (500 km in width, 1500 km in length) and short timescales (<15 s), it is unclear how pervasive nanojets are in the solar corona. In this paper, we present Interface Region Imaging Spectrograph and Solar Dynamics Observatory observations of nanojets found in multiple coronal structures, namely, in a coronal loop powered by a blowout jet, and in two other coronal loops with coronal rain. In agreement with previous findings, we observe that nanojets are accompanied by small nanoflare-like intensity bursts in the (E)UV, have velocities of 150–250 km s−1 and occur transversely to the field line of origin, which is sometimes observed to split. However, we find a variety of nanojet directions in the plane transverse to the loop axis. These nanojets are found to have kinetic and thermal energies within the nanoflare range, and often occur in clusters. In the blowout jet case study, the Kelvin–Helmholtz instability (KHI) is directly identified as the reconnection driver. For the other two loops, we find that both, KHI and Rayleigh–Taylor instability (RTI) are likely to be the drivers. However, we find that KHI and RTI are each more likely in one of the other two cases. These observations of nanojets in a variety of structures and environments support nanojets being a general result of reconnection that are driven here by dynamic instabilities.

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“…This process has been well studied in both analytical (e.g., Zaqarashvili et al 2015;Barbulescu et al 2019;Van Doorsselaere et al 2021) and numerical settings (e.g., Antolin et al 2014Antolin et al , 2015Antolin et al , 2018Magyar & Van Doorsselaere 2016;Karampelas & Van Doorsselaere 2018;Afanasyev et al 2019;Antolin & Van Doorsselaere 2019). The small spatial scales that form and the misalignment of magnetic field lines at the boundaries of Kelvin-Helmholtz vortices can drive plasma heating (Karampelas et al 2017) and lead to magnetic reconnection (Howson et al 2021). Due to the nature of the fundamental standing waves (perturbed velocities/ magnetic field largest at apex/footpoints), small scales in the velocity field tend to generate viscous heating at the loop apex, while small scales in the magnetic field typically produce ohmic heating at the loop footpoints (e.g., Karampelas et al 2017).…”

Section: Introductionmentioning

confidence: 99%

(When) Can Wave Heating Balance Optically Thin Radiative Losses in the Corona?

Moortel

Howson

2022

ApJ

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Why the atmosphere of the Sun is orders of magnitudes hotter than its surface is a long standing question in solar physics. Over the years, many studies have looked at the potential role of magnetohydrodynamic (MHD) waves in sustaining these high temperatures. In this study, we use 3D MHD simulations to investigate (driven) transverse waves in a coronal loop. As the boundary-driven transverse waves propagate along the flux tube, the radial density profile leads to resonant absorption (or mode coupling) and phase mixing in the boundaries of the flux tube and the large velocity shears are subject to the Kelvin–Helmholtz instability (KHI). The combination of these effects leads to enhanced energy dissipation and wave heating. Considering both resonant and nonresonant boundary driving as well as different densities for the flux tube, we show that only wave heating associated with a resonant driver in a lower-density loop (with a loop core density ∼5 × 10−13 kg m−3) is able to balance radiative losses in the loop shell. Changing the model parameters to consider a denser loop or a driver with a nonresonant frequency, or both, leads to cooling of the coronal loop as the energy losses are greater than the energy injection and dissipation rates.

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Magnetic reconnection and the Kelvin-Helmholtz instability in the solar corona

Cited by 13 publications

References 72 publications

Transition to turbulence in nonuniform coronal loops driven by torsional Alfvén waves

Transition to turbulence in nonuniform coronal loops driven by torsional Alfvén waves

Observations of Instability-driven Nanojets in Coronal Loops

(When) Can Wave Heating Balance Optically Thin Radiative Losses in the Corona?

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