Coronal heating by magnetic reconnection in loops  with zero net current

Hood, A. W.; Browning, P. K.; Linden, R. A. M. Van der

doi:10.1051/0004-6361/200912285

Cited by 101 publications

(167 citation statements)

References 30 publications

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“…When the two speeds are equal, the ions and electrons are about to separate, and this will lead to the higher (anomalous) resistivities associated with collisionless reconnection. For an ionized hydrogen plasma, Z = 1 and m i = m p , and, since the plasma is still (although only just) a single fluid, T e = T. Equation (3.11) therefore simplifies to 12) which can expressed in code normalized units, 13) through the use of the appropriate normalizing constants,…”

Section: (B) Critical Current Densitymentioning

confidence: 99%

Shock heating in numerical simulations of kink-unstable coronal loops

Bareford

Hood

2015

Phil. Trans. R. Soc. A.

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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 largescale 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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Section: (B) Critical Current Densitymentioning

confidence: 99%

Shock heating in numerical simulations of kink-unstable coronal loops

Bareford

Hood

2015

Phil. Trans. R. Soc. A.

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“…The helical deformation of the kink instability generates current sheets in the nonlinear regime, in which fast magnetic reconnection rapidly dissipates magnetic energy. Recently, it has been demonstrated using 3D MHD simulations that this process causes the field to relax towards a state which is closely-approximated as a constant-α state, and the energy release is in good agreement with relaxation theory (Browning et al 2008;Hood et al 2009). These papers have investigated field profiles, based on the model of Browning & Van der Linden (2003), in the unstable region of parameter space and have shown that fast reconnection develops in a current sheet, with dissipation of magnetic energy and relaxation to a new, lower-energy state.…”

Section: Introductionmentioning

confidence: 52%

“…Note that the magnetic field is continuous everywhere (though the current has discontinuities). Recent work indicates that these α discontinuities have little discernable effect when compared to similar but continuous α-profiles (Hood et al 2009). …”

Section: Equilibrium Fieldsmentioning

confidence: 99%

“…Essentially, helical current sheets become the site of Ohmic dissipation, resulting in a heating event. Further simulations have revealed the appropriate correlation between magnetic energy and Ohmic heating (Browning & Van der Linden 2003;Browning et al 2008;Hood et al 2009). The energy released by an instability depends on the complex dynamics of the magnetic reconnection events that occur inside the loop.…”

Section: Introductionmentioning

confidence: 98%

“…These papers have investigated field profiles, based on the model of Browning & Van der Linden (2003), in the unstable region of parameter space and have shown that fast reconnection develops in a current sheet, with dissipation of magnetic energy and relaxation to a new, lower-energy state. Efficient "mixing" of the α profile is facilitated in the later phases of the evolution, during which the current sheet fragments (Hood et al 2009). …”

Section: Introductionmentioning

confidence: 99%

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A nanoflare distribution generated by repeated relaxations triggered by kink instability

Bareford¹,

Browning²,

Linden

2010

A&A

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Context. It is thought likely that vast numbers of nanoflares are responsible for the corona having a temperature of millions of degrees. Current observational technologies lack the resolving power to confirm the nanoflare hypothesis. An alternative approach is to construct a magnetohydrodynamic coronal loop model that has the ability to predict nanoflare energy distributions. Aims. This paper presents the initial results generated by a coronal loop model that flares whenever it becomes unstable to an ideal MHD kink mode. A feature of the model is that it predicts heating events with a range of sizes, depending on where the instability threshold for linear kink modes is encountered. The aims are to calculate the distribution of event energies and to investigate whether kink instability can be predicted from a single parameter. Methods. The loop is represented as a straight line-tied cylinder. The twisting caused by random photospheric motions is captured by two parameters, representing the ratio of current density to field strength for specific regions of the loop. Instability onset is mapped as a closed boundary in the 2D parameter space. Dissipation of the loop's magnetic energy begins during the nonlinear stage of the instability, which develops as a consequence of current sheet reconnection. After flaring, the loop evolves to the state of lowest energy where, in accordance with relaxation theory, the ratio of current to field is constant throughout the loop and helicity is conserved. Results. There exists substantial variation in the radial magnetic twist profiles for the loop states along the instability threshold. These results suggest that instability cannot be predicted by any simple twist-derived property reaching a critical value. The model is applied such that the loop undergoes repeated episodes of instability followed by energy-releasing relaxation. Hence, an energy distribution of the nanoflares produced is collated. This paper also presents the calculated relaxation states and energy releases for all instability threshold points. Conclusions. The final energy distribution features two nanoflare populations that follow different power laws. The power law index for the higher energy population is more than sufficient for coronal heating.

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Spontaneous reconnection at a separator current layer: 1. Nature of the reconnection

Stevenson

Parnell

2015

JGR Space Physics

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Magnetic separators, which lie on the boundary between four topologically distinct flux domains, are prime locations in three‐dimensional magnetic fields for reconnection, especially in the magnetosphere between the planetary and interplanetary magnetic fields and also in the solar atmosphere. Little is known about the details of separator reconnection, and so the aim of this paper, which is the first of two, is to study the properties of magnetic reconnection at a single separator. Three‐dimensional, resistive magnetohydrodynamic numerical experiments are run to study separator reconnection starting from a magnetohydrostatic equilibrium which contains a twisted current layer along a single separator linking a pair of opposite‐polarity null points. The resulting reconnection occurs in two phases. The first is short involving rapid reconnection in which the current at the separator is reduced by a factor of around 2.3. Most (75%) of the magnetic energy is converted during this phase, via Ohmic dissipation, directly into internal energy, with just 0.1% going into kinetic energy. During this phase the reconnection occurs along most of the separator away from its ends (the nulls) but in an asymmetric manner which changes both spatially and temporally over time. The second phase is much longer and involves slow impulsive bursty reconnection. Again, Ohmic heating dominates over viscous damping. Here the reconnection occurs in small localized bursts at random anywhere along the separator.

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Coronal heating by magnetic reconnection in loops with zero net current

Cited by 101 publications

References 30 publications

Shock heating in numerical simulations of kink-unstable coronal loops

Shock heating in numerical simulations of kink-unstable coronal loops

A nanoflare distribution generated by repeated relaxations triggered by kink instability

Spontaneous reconnection at a separator current layer: 1. Nature of the reconnection

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