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Flux-Rope Twist in Eruptive Flares and CMEs: due to Zipper and Main-Phase Reconnection  

Eric R Priest   Submitted: 2017-01-03 00:54

The nature of three-dimensional reconnection when a twisted flux tube erupts during an eruptive flare or coronal mass ejection is considered. The reconnection has two phases: first of all, 3D ``zipper reconnection" propagates along the initial coronal arcade, parallel to the polarity inversion line (PIL); then subsequent quasi-2D ``main phase reconnection" in the low corona around a flux rope during its eruption produces coronal loops and chromospheric ribbons that propagate away from the PIL in a direction normal to it. One scenario starts with a sheared arcade: the zipper reconnection creates a twisted flux rope of roughly one turn (2π radians of twist), and then main phase reconnection builds up the bulk of the erupting flux rope with a relatively uniform twist of a few turns. A second scenario starts with a pre-existing flux rope under the arcade. Here the zipper phase can create a core with many turns that depend on the ratio of the magnetic fluxes in the newly formed flare ribbons and the new flux rope. Main phase reconnection then adds a layer of roughly uniform twist to the twisted central core. Both phases and scenarios are modeled in a simple way that assumes the initial magnetic flux is fragmented along the PIL. The model uses conservation of magnetic helicity and flux, together with equipartition of magnetic helicity, to deduce the twist of the erupting flux rope in terms the geometry of the initial configuration. Interplanetary observations show some flux ropes have a fairly uniform twist, which could be produced when the zipper phase and any pre-existing flux rope possess small or moderate twist (up to one or two turns). Other interplanetary flux ropes have highly twisted cores (up to five turns), which could be produced when there is a pre-existing flux rope and an active zipper phase that creates substantial extra twist.

Authors: Eric Priest and Dana Longcope
Projects: None

Publication Status: Solar Physics, in press
Last Modified: 2017-01-04 12:18
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Evolution of Magnetic Helicity During Eruptive Flares and Coronal Mass Ejections  

Eric R Priest   Submitted: 2016-08-16 09:00

During eruptive solar flares and coronal mass ejections, a non-pot{\-}ential magnetic arcade with much excess magnetic energy goes unstable and reconnects. It produces a twisted erupting flux rope and leaves behind a sheared arcade of hot coronal loops. We suggest that: the twist of the erupting flux rope can be determined from conservation of magnetic flux and magnetic helicity and equipartition of magnetic helicity. It depends on the geometry of the initial pre-eruptive structure. Two cases are considered, in the first of which a flux rope is not present initially but is created during the eruption by the reconnection. In the second case, a flux rope is present under the arcade in the pre-eruptive state, and the effect of the eruption and reconnection is to add an amount of magnetic helicity that depends on the fluxes of the rope and arcade and the geometry.

Authors: Eric Priest, Dana Longcope and Miho Janvier
Projects: None

Publication Status: in press
Last Modified: 2016-08-16 10:26
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Hinode 7: Conference Summary and Future Suggestions  

Eric R Priest   Submitted: 2014-05-15 03:31

This conclusion to the meeting attempts to summarise what we have learnt during the conference (mainly from the review talks) about new observations from Hinode and about theories stimulated by them. Suggestions for future study are also offered.

Authors: Priest, ER
Projects: None

Publication Status: In press
Last Modified: 2014-05-15 15:19
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A Life of Fun Playing With Solar Magnetic Fields (Special Historical Review)  

Eric R Priest   Submitted: 2014-05-15 01:16

This invited memoire describes my fortunate life, which has been enriched by meeting many wonderful people. The story starts at home and university, and continues with accounts of St Andrews and trips to the USA, together with musings on the book "Solar MHD". The nature and results of collaborations with key people from abroad and with students is mentioned at length. Finally, other important aspects of my life are mentioned briefly before wrapping up.

Authors: Priest, ER
Projects: None

Publication Status: In press
Last Modified: 2014-05-15 15:19
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Exact Solutions for Reconnective Magnetic Annihilation Annihilation  

Eric R Priest   Submitted: 2000-01-11 11:04

A family of exact solutions of the steady, resistive nonlinear magnetohydrodyna- magnetohydrodynamic equations in two dimensions (x, y) is presented for reconnective annihilation, in which the magnetic field is advected across one pair of separatrices and diffuses across the other pair. They represent a two-fold generalization of the previous Craig-Henton solution, since a dimensio- dimensionless free parameter (gamma) in the new solutions equals unity in the previous solutions and the components (vxe, vye) and (Bxe, Bye) of plasma velocity and magnetic field at a fixed external point (x, y) = (1, 0), say, may all be imposed, whereas only three of these four components are free in the previous solutions. The solutions have the exact forms eq A=A0 (x) + A1 (x), y , quad psi = psi0 (x) + psi1 (x) , y onumber eeq for the magnetic flux function (A) and stream function (psi), so that the electric current is no longer purely a function of x as it was previously. The origin (0, 0) represents both a stagnation point and a magnetic null point, where the plasma velocity ({f v = abla imes psi hat{f z}}) and magnetic field ({f B = abla} imes A hat{f z}) both vanish. A current sheet extends along the y-axis. The non-linear fourth-order equations for A1 and psi1 are solved in the limit of small dimensionless resistivity (large magnetic Reynolds number) using the method of matched asymptotic expansions. Although the solution has a weak boundary layer near x=0, we show that a composite asymptotic representation on 0 leq x leq 1 is given by the leading order outer solution, which has a simple closed-form structure. This enables the equations for A0 and psi0 to be solved explicitly from which their representation for small resistivity is obtained. The effect of the five parameters (vxe, vye, Bxe, Bye, gamma) on the solutions is determined, including their influence on the width of the diffusion region and the inclinations of the streamlines and magnetic field lines at the origin. Several possibilities for generalizing these solutions for asymmetric reconnective annihilation in two and three dimensions are also presented.

Authors: E.R. Priest, V.S. Titov, R.E. Grundy and A.W. Hood
Projects:

Publication Status: Proc. Roy. Soc (in press)
Last Modified: 2000-01-11 11:04
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The Heating of the Solar Corona  

Eric R Priest   Submitted: 2000-01-11 11:03

One of the paradigms about coronal heating has been the belief that the mean or summit temperature of a coronal loop is completely insensitive to the nature of the heating mechanisms. However, we point out that the temperature profile along a coronal loop is highly sensitive to the form of the heating. For example, when a steady-state heating is balanced by thermal conduction, a uniform heating function makes the heat flux a linear function of distance along the loop, while T7/2 increases quadratically from the coronal footpoints; when the heating is concentrated near the coronal base, the heat flux is small and the T7/2 profile is flat above the base; when the heat is focussed near the summit of a loop, the heat flux is constant and T7/2 is a linear function of distance below the summit. This realisation may act as an incentive to proponents of particular heating mechanisms to determine how the heat deposition varies spatially within coronal structures such as loops or arcades and to observers to deduce the temperature profiles with as low an error as possible. We therefore propose a new two-part approach to try and solve the coronal heating problem, namely first of all to use observed temperature profiles to deduce the form of the heating, and secondly to use that heating form to deduce the likely heating mechanism. In particular, we apply this philosophy to a preliminary analysis of Yohkoh observations of the large-scale solar corona. This gives strong evidence against heating concentrated near the loop base for such loops and suggests that heating uniformly distributed along the loop is slightly more likely than heating concentrated at the summit. The implication is that large-scale loops are heated in situ throughout their length, rather than being a steady response to low-lying heating near their feet or at their summits. Unless waves can be shown to produce a heating close enough to uniform, the evidence is therefore at present for these large loops more in favour of turbulent reconnection at many small randomly-distributed current sheets, which is likely to be able to do so. In addition, we address briefly the questions: why does the coronal intensity reduce by a factor of 100 from solar maximum to solar minimum; why is the temperature maximum in large-scale closed regions about 2.3 MK at an altitude 1.5 Rodot; and why are the corresponding values in coronal holes about 1.5 MK and 1.5 Rodot?

Authors: E. R. Priest, C. R. Foley, J. Heyvaerts, T.D. Arber$^{*}$, D. Mackay$^{*}$, J. L. Culhane$^{+}$ and L.W. Acton
Projects:

Publication Status: Ap J (submitted)
Last Modified: 2000-01-11 11:03
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Aspects of 3D Magnetic Reconnection  

Eric R Priest   Submitted: 2000-01-11 11:02

In this review paper we discuss several aspects of magnetic reconnection theory, focussing on the field-line motions that are associated with reconnection. A new exact solution of the nonlinear MHD equations for {it reconnective annihilation annihilation} is presented which represents a two-fold generalisation of the previous solutions. Magnetic reconnection at null points by several mechanisms is summarised, including {it spine reconnection, fan reconnection} and {it separator reconnection}, where it is pointed out that two common features of separator reconnection are the rapid flipping of magnetic field lines and the collapse of the separator to a current sheet. In addition, a formula for the rate of reconnection between two flux tubes is derived. The magnetic field of the corona is highly complex, since the magnetic carpet consists of a multitude of sources in the photosphere. Progress in understanding this compexity may, however, be made by constructing the {it skeleton} of the field and developing a theory for the local and global bifurcations between the different topologies. The eruption of flux from the Sun may even sometimes be due to a change of topology caused by {it emerging flux breakout}. A CD Rom attached to this paper presents the results of a toy model of vacuum reconnection, which suggests that rapid flipping of field lines in fan and separator reconnection is an essential ingredient also in real non-vacuum conditions. In addition, it gives an example of {it binary reconnection} between a pair of unbalanced sources as they move around, which may contribute significantly to coronal heating. Finally, we present examples in TRACE movies of geometrical changes of the coronal magnetic field that are a likely result of large-scale magnetic reconnection.

Authors: Priest, ER and Schrijver, CJ
Projects:

Publication Status: Solar Phys. (in press)
Last Modified: 2000-01-11 11:02
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Solar Flare Theory and the Status of Flare Understanding  

Eric R Priest   Submitted: 2000-01-11 11:01

  A review is given of our current understanding of the MHD of solar flares. The theory of reconnection in 2D is now well understood, but in 3D we are only starting to understand the complexity of magnetic topology and the different ways in which reconnection may occur. Small flares may be driven by footpoint motions in emerging or interacting regions or they may occur at quasi-sepratrix layers in complex regions. For large eruptive two-ribbon flares, there has recently been a major advance in our understanding: first of all, an MHD catastrophe is likely to be responsible for the basic eruption; and secondly, the eruption then drives reconnection in the stretched-out field lines to create a set of rising soft x-ray loops. This model is well supported by numerical experiments and detailed Yohkoh observations. Particle acceleration may occur by turbulent or direct electric fields at the reconnection site or in the different kinds of MHD shock wave that are associated with the flare process.

Authors: Priest, ER
Projects:

Publication Status: ASP Conf Series, in press
Last Modified: 2000-01-11 11:01
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Abstracts by Author
Flux-Rope Twist in Eruptive Flares and CMEs: due to Zipper and Main-Phase Reconnection
Evolution of Magnetic Helicity During Eruptive Flares and Coronal Mass Ejections
Hinode 7: Conference Summary and Future Suggestions
A Life of Fun Playing With Solar Magnetic Fields (Special Historical Review)
Exact Solutions for Reconnective Magnetic Annihilation Annihilation
The Heating of the Solar Corona
Aspects of 3D Magnetic Reconnection
Solar Flare Theory and the Status of Flare Understanding

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