Magnetic Reconnection in Strongly Magnetized Regions of the Low Solar Chromosphere

Ni, Lei; Lukin, V. S.; Murphy, N. A.; Lin, Jin‐Ming

doi:10.3847/1538-4357/aa9edb

Cited by 49 publications

(88 citation statements)

References 52 publications

(79 reference statements)

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“…In our previous paper 32 , the numerical results demonstrated that the collisions between electrons and neutrals are not important for magnetic reconnection in low β plasmas. In order to compare with the corresponding cases in Ni et al (2018), we also ignore the collisions between electrons and neutrals in this work. The dimensionless magnetic diffusivity is…”

Section: Normalization and Initial Conditionsmentioning

confidence: 93%

“…Our simulations are performed by using the reactive multi-fluid plasma-neutral module of the HiFi modeling framework 30,32 . Here, we normalize the equations by using the character- be uniform with a neutral particle number density of n n0 = 0.5n = 0.5 × 10 21 m −3 , and the initial ionization degree is f i0 = n i0 /(n i0 + n n0 ) = 0.01%.…”

Section: Normalization and Initial Conditionsmentioning

confidence: 99%

“…Our recent numerical simulations 32 have studied magnetic reconnection in strongly magnetized regions around the solar TMR. In that paper, we have presented the first reactive multi-fluid simulations of magnetic reconnection in low β plasmas with a guide field.…”

Section: Introductionmentioning

confidence: 99%

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Magnetic reconnection in the low solar chromosphere with a more realistic radiative cooling model

Lukin

Murphy

et al. 2018

Physics of Plasmas

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Magnetic reconnection is the most likely mechanism responsible for the high temperature events that are observed in strongly magnetized locations around the temperature minimum in the low solar chromosphere. This work improves upon our previous work ["Magnetic Reconnection in Strongly Magnetized Regions of the Low Solar Chromosphere", The Astrophysical Journal 852, 95 (2018) ] by using a more realistic radiative cooling model computed from the OPACITY project and the CHI-ANTI database. We find that the rate of ionization of the neutral component of the plasma is still faster than recombination within the current sheet region. For low β plasmas, the ionized and neutral fluid flows are well-coupled throughout the reconnection region resembling the single-fluid Sweet-Parker model dynamics. Decoupling of the ion and neutral inflows appears in the higher β case with β 0 = 1.46, which leads to a reconnection rate about three times faster than the rate predicted by the Sweet-Parker model. In all cases, the plasma temperature increases with time inside the current sheet, and the maximum value is above 2 × 10 4 K when the reconnection magnetic field strength is greater than 500 G. While the more realistic radiative cooling model does not result in qualitative changes of the characteristics of magnetic reconnection, it is necessary for studying the variations of the plasma temperature and ionization fraction inside current sheets in strongly magnetized regions of the low solar atmosphere. It is also important for studying energy conversion during the magnetic reconnection process when the hydrogen-dominated plasma approaches full ionization.

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Section: Normalization and Initial Conditionsmentioning

confidence: 93%

Section: Normalization and Initial Conditionsmentioning

confidence: 99%

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Magnetic reconnection in the low solar chromosphere with a more realistic radiative cooling model

Lukin

Murphy

et al. 2018

Physics of Plasmas

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“…EBs result when the reconnection occurs in the upper photosphere with temperatures some 1 000-3 000 K higher than ambient (e.g., Rutten et al 2013), while UV bursts are reproduced when the reconnection is located higher in the chromosphere and entails much higher temperatures than those found in EB models, at least high enough to ionize silicon three times: 20 kK if in dense photospheric material, 80 kK at lower densities, as pointed out by Rutten (2016). It may be possible to generate temperatures as high as 25 kK through reconnection in the upper photosphere or lower chromosphere, as shown by Ni et al (2016Ni et al ( , 2018. Priest et al (2018) and Syntelis et al (2019) pointed out in a theoretical work that flux cancellation can cause simultaneous reconnection at different heights in the solar atmosphere.…”

Section: Introductionmentioning

confidence: 91%

Ellerman bombs and UV bursts: transient events in chromospheric current sheets

Hansteen

Ortiz

Archontis

et al. 2019

A&A

119

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Context. Ellerman bombs (EBs), observed in the photospheric wings of the Hα line, and UV bursts, observed in the transition region Si iv line, are both brightenings related to flux emergence regions and specifically to magnetic flux of opposite polarity that meet in the photosphere. These two reconnection-related phenomena, nominally formed far apart, occasionally occur in the same location and at the same time, thus challenging our understanding of reconnection and heating of the lower solar atmosphere. Aims. We consider the formation of an active region, including long fibrils and hot and dense coronal plasma. The emergence of a untwisted magnetic flux sheet, injected 2.5 Mm below the photosphere, is studied as it pierces the photosphere and interacts with the preexisting ambient field. Specifically, we aim to study whether EBs and UV bursts are generated as a result of such flux emergence and examine their physical relationship. Methods. The Bifrost radiative magnetohydrodynamics code was used to model flux emerging into a model atmosphere that contained a fairly strong ambient field, constraining the emerging field to a limited volume wherein multiple reconnection events occur as the field breaks through the photosphere and expands into the outer atmosphere. Synthetic spectra of the different reconnection events were computed using the 1.5D RH code and the fully 3D MULTI3D code. Results. The formation of UV bursts and EBs at intensities and with line profiles that are highly reminiscent of observed spectra are understood to be a result of the reconnection of emerging flux with itself in a long-lasting current sheet that extends over several scale heights through the chromosphere. Synthetic spectra in the Hα and Si iv 139.376 nm lines both show characteristics that are typical of the observations. These synthetic diagnostics suggest that there are no compelling reasons to assume that UV bursts occur in the photosphere. Instead, EBs and UV bursts are occasionally formed at opposite ends of a long current sheet that resides in an extended bubble of cool gas.

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“…If β is lower, that is, higher in the atmosphere, the plasma can be heated to higher temperatures, and in the low chromosphere gravity can hinder the formation of plasmoids (Ni et al 2016). However, the situation changes if one accounts for the presence of neutrals in the chromosphere and the reconnection in the low chromosphere can result in peak temperatures of about 30 kK (Ni et al 2018a;Ni & Lukin 2018). During reconnection, the 2D models can produce turbulent small-scale structures in plasmoids that cover a range of temperatures from 10 4 K to 10 5 K, that is, covering the temperatures expected in Ellerman bombs and UV bursts.…”

Section: Introductionmentioning

confidence: 99%

Plasmoid-mediated reconnection in solar UV bursts

Peter

Huang²,

Chitta

et al. 2019

A&A

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Context. Ultraviolet bursts are transients in the solar atmosphere with an increased impulsive emission in the extreme UV lasting for one to several tens of minutes. They often show spectral profiles indicative of a bi-directional outflow in response to magnetic reconnection. Aims. To understand UV bursts, we study how motions of magnetic elements at the surface can drive the self-consistent formation of a current sheet resulting in plasmoid-mediated reconnection. In particular, we want to study the role of the height of the reconnection in the atmosphere. Methods. We conducted numerical experiments solving the 2D magnetohydrodynamic equations from the solar surface to the upper atmosphere. Motivated by observations, we drove a small magnetic patch embedded in a larger system of magnetic field of opposite polarity. This type of configuration creates an X-type neutral point in the initial potential field. The models are characterized by the (average) plasma-β at the height of this X point. Results. The driving at the surface stretches the X-point into a thin current sheet, where plasmoids appear, accelerating the reconnection, and a bi-directional jet forms. This is consistent with what is expected for UV bursts or explosive events, and we provide a self-consistent model of the formation of the reconnection region in such events. The gravitational stratification gives a natural explanation for why explosive events are restricted to a temperature range around a few 0.1 MK, and the presence of plasmoids in the reconnection process provides an understanding of the observed variability during the transient events on a timescale of minutes. Conclusions. Our numerical experiments provide a comprehensive understanding of UV bursts and explosive events, in particular of how the atmospheric response changes if the reconnection happens at different plasma-β, that is, at different heights in the atmosphere. This analysis also gives new insight into how UV bursts might be related to the photospheric Ellerman bombs.

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Magnetic Reconnection in Strongly Magnetized Regions of the Low Solar Chromosphere

Cited by 49 publications

References 52 publications

Magnetic reconnection in the low solar chromosphere with a more realistic radiative cooling model

Magnetic reconnection in the low solar chromosphere with a more realistic radiative cooling model

Ellerman bombs and UV bursts: transient events in chromospheric current sheets

Plasmoid-mediated reconnection in solar UV bursts

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