226 research outputs found
Generalised models for torsional spine and fan magnetic reconnection
Three-dimensional null points are present in abundance in the solar corona,
and the same is likely to be true in other astrophysical environments. Recent
studies suggest that reconnection at such 3D nulls may play an important role
in the coronal dynamics. In this paper the properties of the torsional spine
and torsional fan modes of magnetic reconnection at 3D nulls are investigated.
New analytical models are developed, which for the first time include a current
layer that is spatially localised around the null, extending along either the
spine or the fan of the null. These are complemented with numerical
simulations. The principal aim is to investigate the effect of varying the
degree of asymmetry of the null point magnetic field on the resulting
reconnection process - where previous studies always considered a non-generic
radially symmetric null. The geometry of the current layers within which
torsional spine and torsional fan reconnection occur is found to be strongly
dependent on the symmetry of the magnetic field. Torsional spine reconnection
still occurs in a narrow tube around the spine, but with elliptical
cross-section when the fan eigenvalues are different, and with the short axis
of the ellipse being along the strong field direction. The spatiotemporal peak
current, and the peak reconnection rate attained, are found not to depend
strongly on the degree of asymmetry. For torsional fan reconnection, the
reconnection occurs in a planar disk in the fan surface, which is again
elliptical when the symmetry of the magnetic field is broken. The short axis of
the ellipse is along the weak field direction, with the current being peaked in
these weak field regions. The peak current and peak reconnection rate in this
case are clearly dependent on the asymmetry, with the peak current increasing
but the reconnection rate decreasing as the degree of asymmetry is increased
On the nature of reconnection at a solar coronal null point above a separatrix dome
Three-dimensional magnetic null points are ubiquitous in the solar corona,
and in any generic mixed-polarity magnetic field. We consider magnetic
reconnection at an isolated coronal null point, whose fan field lines form a
dome structure. We demonstrate using analytical and computational models
several features of spine-fan reconnection at such a null, including the fact
that substantial magnetic flux transfer from one region of field line
connectivity to another can occur. The flux transfer occurs across the current
sheet that forms around the null point during spine-fan reconnection, and there
is no separator present. Also, flipping of magnetic field lines takes place in
a manner similar to that observed in quasi-separatrix layer or slip-running
reconnection.Comment: Accepted for publication in the Astrophysical Journa
Magnetic reconnection at 3D null points: effect of magnetic field asymmetry
The aim of this paper is to investigate the properties of magnetic
reconnection at a 3D null point, with respect to their dependence on the
symmetry of the magnetic field around the null. In particular we examine the
rate of flux transport across the null point with symmetric/asymmetric
diffusion regions, as well as how the current sheet forms in time, and its
properties. Mathematical modelling and finite difference resistive MHD
simulations are used. It is found that the basic structure of the mode of
magnetic reconnection considered is unaffected by varying the magnetic field
symmetry, that is, the plasma flow is found cross both the spine and fan of the
null. However, the peak intensity and dimensions of the current sheet are
dependent on the symmetry/ asymmetry of the field lines. As a result, the
reconnection rate is also found to be strongly dependent on the field
asymmetry.Comment: 23 pages, 9 figure
Current sheets at three-dimensional magnetic nulls:effect of compressibility
The nature of current sheet formation in the vicinity of three-dimensional
(3D) magnetic null points is investigated. The particular focus is upon the
effect of the compressibility of the plasma on the qualitative and quantitative
properties of the current sheet. An initially potential 3D null is subjected to
shearing perturbations, as in a previous paper [Pontin et al., Phys. Plasmas,
in press (2007)]. It is found that as the incompressible limit is approached,
the collapse of the null point is suppressed, and an approximately planar
current sheet aligned to the fan plane is present instead. This is the case
regardless of whether the spine or fan of the null is sheared. Both the peak
current and peak reconnection rate are reduced. The results have a bearing on
previous analytical solutions for steady-state reconnection in incompressible
plasmas, implying that fan current sheet solutions are dynamically accessible,
while spine current sheet solutions are not.Comment: to appear in Physics of Plasmas. This version contains updated
figures and references, additional discussion, and typos are fixed. This is
the second in a series of papers - the first of which (by the same authors)
is located at astro-ph/0701462. A version with higher quality figures can be
found at http://www.maths.dundee.ac.uk/~dpontin
Enhanced relapse prevention for bipolar disorder: A qualitative investigation of value perceived for service users and care coordinators
Background: Enhanced relapse prevention (ERP) is a psychological intervention delivered by mental health professionals to help individuals with bipolar disorder (BD) recognise and manage early warning signs for mania and depression. ERP has an emerging evidence base and is recommended as good practice for mental health professionals. However, without highly perceived value to both those receiving (services users) or delivering it (health professionals), implementation will not occur. The aim of this study is to determine what values of ERP are perceived by service users (SUs) and mental health professionals (care coordinators, CCs) providing community case management. Methods: A nested qualitative study design was employed as part of a randomised controlled trial of ERP. Semi-structured interviews were conducted with a purposive sub-sample of 21 CCs and 21 SUs, and an iterative approach used to develop a framework of conceptual categories that was applied systematically to the data. Results: The process of implementing and receiving ERP was valued by both SUs and CCs for three similar sets of reasons: improved understanding of BD (where a knowledge deficit of BD was perceived), enhanced working relationships, and improved ways of managing the condition. There were some differences in the implications these had for both CCs and SUs who also held some reservations. Conclusion: CCs and SUs perceive similar value in early warning signs interventions to prevent relapse, and these have particular benefits to them. If this perceived value is maintained, CCs and SUs in routine practice may use ERP long-term
Why are flare ribbons associated with the spines of magnetic null points generically elongated?
Coronal magnetic null points exist in abundance as demonstrated by
extrapolations of the coronal field, and have been inferred to be important for
a broad range of energetic events. These null points and their associated
separatrix and spine field lines represent discontinuities of the field line
mapping, making them preferential locations for reconnection. This field line
mapping also exhibits strong gradients adjacent to the separatrix (fan) and
spine field lines, that can be analysed using the `squashing factor', . In
this paper we make a detailed analysis of the distribution of in the
presence of magnetic nulls. While is formally infinite on both the spine
and fan of the null, the decay of away from these structures is shown in
general to depend strongly on the null-point structure. For the generic case of
a non-radially-symmetric null, decays most slowly away from the spine/fan
in the direction in which increases most slowly. In particular,
this demonstrates that the extended, elliptical high- halo around the spine
footpoints observed by Masson et al. (Astrophys. J., 700, 559, 2009) is a
generic feature. This extension of the halos around the spine/fan
footpoints is important for diagnosing the regions of the photosphere that are
magnetically connected to any current layer that forms at the null. In light of
this, we discuss how our results can be used to interpret the geometry of
observed flare ribbons in `circular ribbon flares', in which typically a
coronal null is implicated. We conclude that both the physics in the vicinity
of the null and how this is related to the extension of away from the
spine/fan can be used in tandem to understand observational signatures of
reconnection at coronal null points.Comment: Pre-print version of article accepted for publication in Solar
Physic
2.5D magnetohydrodynamic simulation of the formation and evolution of plasmoids in coronal current sheets
Funding: S.M. would like to acknowledge the financial support provided by the Prime Ministerʼs Research Fellowship of India. A.K.S. acknowledges the ISRO grant DS 2B-13012(2)/26/2022-Sec.2 for the support of his scientific research. D.I.P. gratefully acknowledges support through an Australian Research Council Discovery Project (DP210100709). D.Y. is supported by the National Natural Science Foundation of China (NSFC; grant Nos. 12173012, 12111530078, and 11803005), the Guangdong Natural Science Funds for Distinguished Young Scholar (grant No. 2023B1515020049), the Shenzhen Technology Project (grant No. GXWD20201230155427003-20200804151658001) and the Shenzhen Key Laboratory Launching Project (grant No. ZDSYS20210702140800001).In the present paper, using MPI-AMRVAC, we perform a 2.5D numerical magnetohydrodynamic simulation of the dynamics and associated thermodynamical evolution of an initially force-free Harris current sheet subjected to an external velocity perturbation under the condition of uniform resistivity. The amplitude of the magnetic field is taken to be 10 G, typical of the solar corona. We impose a Gaussian velocity pulse across this current sheet that mimics the interaction of fast magnetoacoustic waves with a current sheet in the corona. This leads to a variety of dynamics and plasma processes in the current sheet, which is initially quasi-static. The initial pulse interacts with the current sheet and splits into a pair of counterpropagating wavefronts, which form a rarefied region that leads to an inflow and a thinning of the current sheet. The thinning results in Petschek-type magnetic reconnection followed by a tearing instability and plasmoid formation. The reconnection outflows containing outward-moving plasmoids have accelerated motions with velocities ranging from 105 to 303 km s−1. The average temperature and density of the plasmoids are found to be 8 MK and twice the background density of the solar corona, respectively. These estimates of the velocity, temperature, and density of the plasmoids are similar to values reported from various solar coronal observations. Therefore, we infer that the external triggering of a quasi-static current sheet by a single-velocity pulse is capable of initiating magnetic reconnection and plasmoid formation in the absence of a localized enhancement of resistivity in the solar corona.Peer reviewe
2.5-D MHD Simulation of the Formation and Evolution of Plasmoids in Coronal Current Sheets
In the present paper, using MPI-AMRVAC, we perform a 2.5-D numerical MHD
simulation of the dynamics and associated thermodynamical evolution of an
initially force-free Harris current sheet subjected to an external velocity
perturbation under the condition of uniform resistivity. The amplitude of the
magnetic field is taken to be 10 Gauss, typical of the solar corona. We impose
a Gaussian velocity pulse across this current sheet mimicking the interaction
of fast magnetoacoustic waves with a current sheet in corona. This leads to a
variety of dynamics and plasma processes in the current sheet, which is
initially quasi-static. The initial pulse interacts with the current sheet and
splits into a pair of counter-propagating wavefronts, which forms a rarefied
region and leads to inflow and a thinning of the current sheet. The thinning
results in Petschek-type magnetic reconnection followed by tearing instability
and plasmoid formation. The reconnection outflows containing outward-moving
plasmoids have accelerated motions with velocities ranging from 105-303 km/s.
The average temperature and density of the plasmoids are found to be 8 MK and
twice the background density of the solar corona, respectively. These estimates
of velocity, temperature and density of plasmoids are similar to values
reported from various solar coronal observations. Therefore, we infer that the
external triggering of a quasi-static current sheet by a single velocity pulse
is capable of initiating magnetic reconnection and plasmoid formation in the
absence of a localized enhancement of resistivity in the solar corona.Comment: 20 pages, 10 figures, Accepted for publication in The Astrophysical
Journa
Phase Mixing of Alfvén Waves Near a 2D Magnetic Null Point
The propagation of linear Alfvén wave pulses in an inhomogeneous plasma near a 2D coronal null point is investigated. When a uniform plasma density is considered, it is seen that an initially planar Alfvén wavefront remains planar, despite the varying equilibrium Alfvén speed, and that all the wave collects at the separatrices. Thus, in the non-ideal case, these Alfvénic disturbances preferentially dissipate their energy at these locations. For a non-uniform equilibrium density, it is found that the Alfvén wavefront is significantly distorted away from the initially planar geometry, inviting the possibility of dissipation due to phase mixing. Despite this however, we conclude that for the Alfvén wave, current density accumulation and preferential heating still primarily occur at the separatrices, even when an extremely non-uniform density profile is considered
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