78 research outputs found
Forced magnetic reconnection and plasmoid coalescence: I - MHD Simulations
Forced magnetic reconnection, a reconnection event triggered by external
perturbation, should be ubiquitous in the solar corona. Energy released during
such cases can be much greater than that which was introduced by the
perturbation. It is unclear how the properties of the external perturbation and
the initial current sheet affect the reconnection region properties, and
thereby the reconnection dynamics and energy release profile. We investigate
the effect of the form of the external perturbation and initial current sheet
on the evolution of the reconnection region and the energy release process.
Chiefly we explore the non-linear interactions between multiple, simultaneous
perturbations, which represent more realistic scenarios. Future work will use
these results in test particle simulations to investigate particle acceleration
over multiple reconnection events. Simulations are performed using Lare2d, a
2.5D Lagrangian-remap solver for the visco-resistive MHD equations. The model
of forced reconnection is extended to include superpositions of sinusoidal
driving disturbances, including localised Gaussian perturbations. A transient
perturbation is applied to the boundaries of a region containing a force-free
current sheet. The simulation domain is sufficiently wide to allow multiple
magnetic islands to form and coalesce. Island coalescence contributes
significantly to energy release and involves rapid reconnection. Long
wavelength modes in perturbations dominate the evolution, without the presence
of which reconnection is either slow, as in the case of short wavelength modes,
or the initial current sheet remains stable, as in the case of noise
perturbations. Multiple perturbations combine in a highly non-linear manner:
reconnection is typically faster than when either disturbance is applied
individually, with multiple low-energy events contributing to the same total
energy release.Comment: 13 pages (including appendix
Particle Acceleration and Their Escape into the Heliosphere in Solar Flares with Open Magnetic Field
Particle acceleration and transport in reconnecting twisted loops in a stratified atmosphere
Twisted coronal loops should be ubiquitous in the solar corona. Twisted
magnetic fields contain excess magnetic energy, which can be released during
magnetic reconnection, causing solar flares. The aim of this work is to
investigate magnetic reconnection, and particle acceleration and transport in
kink-unstable twisted coronal loops, with a focus on the effects of
resistivity, loop geometry and atmospheric stratification. Another aim is to
perform forward-modelling of bremsstrahlung emission and determine the
structure of hard X-ray sources. We use a combination of magnetohydrodynamic
(MHD) and test-particle methods. First, the evolution of the kinking coronal
loop is considered using resistive MHD model, incorporating atmospheric
stratification and loop curvature. Then, the obtained electric and magnetic
fields and density distributions are used to calculate electron and proton
trajectories using a guiding-centre approximation, taking into account Coulomb
collisions. It is shown that electric fields in twisted coronal loops can
effectively accelerate protons and electrons to energies up to 10 MeV.
High-energy particles have hard, nearly power-law energy spectra. The volume
occupied by high-energy particles demonstrates radial expansion, which results
in the expansion of the visible hard X-ray loop and a gradual increase in hard
X-ray footpoint area. Synthesised hard X-ray emission reveals strong footpoint
sources and the extended coronal source, whose intensity strongly depends on
the coronal loop density
Helicity-conserving relaxation in unstable and merging magnetic flux ropes
Twisted magnetic flux ropes are reservoirs of free magnetic energy. In a
highly-conducting plasma such as the solar corona, energy release through
multiple magnetic reconnections can be modelled as a helicity-conserving
relaxation to a minimum energy state. One possible trigger for this relaxation
is the ideal kink instability in a twisted flux rope. We show that this
provides a good description for confined solar flares, and develop from
idealised cylindrical models to realistic models of coronal loops. Using 3D
magnetohydrodynamic simulations combined with test-particle simulations of
non-thermal electrons and ions, we predict multiple observational signatures of
such flares. We then show how interactions and mergers of flux ropes can
release free magnetic energy, using relaxation theory to complement simulations
of merging-compression formation in spherical tokamaks and heating avalanches
in the solar corona
Analysis of unresolved photospheric magnetic field structure using Fe I 6301 and 6302 lines
Early magnetographic observations indicated that magnetic field in the solar
photosphere has unresolved small-scale structure. Near-infrared and optical
data with extremely high spatial resolution show that these structures have
scales of few tens of kilometres, which are not resolved in the majority of
solar observations. The goal of this study is to establish the effect of
unresolved photospheric magnetic field structure on Stokes profiles observed
with relatively low spatial resolution. Ultimately, we aim to develop methods
for fast estimation of the photospheric magnetic filling factor and
line-of-sight gradient of the photospheric magnetic field, which can be applied
to large observational data sets. We exploit 3D MHD models of
magneto-convection developed using MURAM code. Corresponding profiles of Fe I
6301.5 and 6302.5 spectral lines are calculated using NICOLE
radiative transfer code. The resulting I and V Stokes [x,y,] cubes
with reduced spatial resolution of 150 km are used to calculate magnetic field
values as they would be obtained in observations with Hinode/SOT or SDO/HMI.
Three different methods of the magnetic filling factor estimation are
considered: the magnetic line ratio method, Stokes V width method and a simple
statistical method. We find that the statistical method and the Stokes V width
method are sufficiently reliable for fast filling factor estimations.
Furthermore, we find that Stokes bisector splitting gradient can be
used for fast estimation of line-of-sight gradient of the photospheric magnetic
field.Comment: Submitted to Astronomy & Astrophysics on 14/05/201
The heating of the solar corona by kink instabilities
The million-degree temperature of the solar corona might be due to the combined effect of barely distinguishable energy releases, called nanoflares, that occur throughout the solar atmosphere. Unfortunately, the high density of nanoflares, implied by this hypothesis, means that conclusive verification is beyond present observational capabilities. Nevertheless, it might be possible to investigate the plausibility of nanoflare heating by constructing a magnetohydrodynamic (MHD) model; one that can derive the energy of nanoflares, based on the assumption that the ideal kink instability of a twisted coronal loop triggers a relaxation to a minimum energy state. The energy release depends on the current profile at the time when the ideal kink instability threshold is crossed. Subsequent to instability onset, fast magnetic reconnection ensues in the non-linear phase. As the flare erupts and declines, the field transitions to a lower energy level, which can be modelled as a helicity-conserving relaxation to a linear force-free state. The aim of this thesis is to determine the implications of such a scheme with respect to coronal heating. Initially, the results of a linear stability analysis for loops that have net current are presented. 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. Photospheric driving is simulated as an entirely random process. Hence, an energy distribution of the nanoflares produced is collated. These results are discussed and unrealistic features of the model are highlighted.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Radial transport and detachment in the University of Manchester linear system
The role of cross field transport and volume recombination are of vital importance for a satisfactory understanding of the plasma edge in magnetically confined devices such as a Tokamak. Plasma fluctuations may travel cross field with significant velocities and play a central role in plasma transport. Cross field transport has been seen to be anomalous in most devices under a very broad range of experimental conditions. In recent years a clear indication of the relation between fluctuation, cross field particle transport and recombination has been reported.The University of Manchester Linear System (the ULS) has been used to observe the Balmer emission of the recombining plasma interacting with a dense neutral Hydrogen gas. The ULS is a device made of a cylindrical vacuum vessel 1.5 m long and 15 cm in radius. The plasma is formed in a separate chamber by a duoplasmatron source in the Demirkhanov configuration; the arc current was limited to 15 A and the potential drop was 100 V. The device is surrounded by a linear solenoid which was used to magnetize the plasma. The highest magnetic field was .1 T. Typical electron temperature in the device spans .1 to 10 eV, and the density 1. E+16 to 5. E+19.Diagnostic includes Langmuir probe and visible spectrometers. In addition, the DivCam imaging system originally designed and built to obtain 2D images of the MAST spherical Tokamak Scrape Off Layer, was used. The DivCam imaging system has enabled to obtain high resolution images of the plasma emission when interacting with the neutral gas. It appears evident that the Electron-Ion Recombination is strongly dependent upon radial transport of plasma particles: light emission attributed to EIR is only observed at a large cross field distance from the plasma source. Moreover, fast imaging of the plasma has also shown the presence of a plasma filament forming and propagating crossfield at the same region of the plasma where the EIR light is observed.To interpret the experimental observations obtained with DivCam, the OSM 1D fluid plasma solver and the EIRENE neutral Monte Carlo solver have been implemented in the linear geometry of the ULS linear system. Both the OSM and the EIRENE solvers were originally intended for tokamak and large magnetic confinement devices. Modelling of the EIR emissivity in the ULS device has demonstrated the importance of the inclusion of turbulent and blob transport in the model to obtain reasonable agreement between the observations and the theoretical predictions. The central density of the plasma filament has been estimated to be approximately .7 E+19 m-3 using EIRENE results.The emission attributed to hydrogenic ions (negative atomic H- and positive molecular ions H2+) and related to Molecular Assisted Recombinations can be estimated within EIRENE using the AMJUEL database. The database provides ion population estimations for three different collisional regimes: in the first regime a large population of vibrational excited hydrogen molecules are assumed to exist within the plasma volume; the second assumes strong Charge Exchange reactions and not vibrational excited molecule; the third assumes electron impact collisions with ground states molecule to be the only ion source. A reasonable agreement between the observations and the EIRENE prediction is only found when using the third estimation suggesting that molecular excitation and charge exchange processes are relatively unimportant under the experimental conditions considered.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Energy release in driven twisted coronal loops
This work is funded by Science and Technology Facilities Council (UK). This equipment was funded by a BIS National E-Infrastructure capital grant ST/K00042X/1, DiRAC Operations grant ST/K003267/1 and Durham University.Magnetic reconnection in twisted magnetic flux tubes, representing coronal loops, is investigated. The main goal is to establish the influence of the field geometry and various thermodynamic effects on the stability of twisted flux tubes and on the size and distribution of heated regions. In particular, we aim to investigate to what extent the earlier idealised models, based on the initially cylindrically symmetric flux tubes, are different from more realistic models, including the large-scale curvature, atmospheric stratification, thermal conduction and other effects. In addition, we compare the roles of Ohmic heating and shock heating in energy conversion during magnetic reconnection in twisted loops. The models with straight flux tubes show similar distribution of heated plasma during the reconnection: it initially forms a helical shape, which subsequently becomes very fragmented. The heating in these models is rather uniformly distributed along flux tubes. At the same time, the hot plasma regions in curved loops are asymmetric, and concentrate close to the loop tops. Large-scale curvature has a destabilising in influence: lower twist is needed for instability. Footpoint convergence normally delays instability slightly, although, in some cases converging flux tubes can be less stable. Finally, introducing a stratified atmosphere gives rise to decaying wave propagation, which has destabilising effect.PostprintPeer reviewe
Particle Acceleration and Their Escape into the Heliosphere in Solar Flares with Open Magnetic Field
© 2023, The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/Energetic particle populations in the solar corona and in the heliosphere appear to have different characteristics even when produced in the same solar flare. It is not clear what causes this difference: properties of the acceleration region, the large-scale magnetic field configuration in the flare, or particle transport effects, such as scattering. In this study, we use a combination of magnetohydrodynamic and test-particle approaches to investigate magnetic reconnection, particle acceleration, and transport in two solar flares: an M-class flare on 2013 June 19, and an X-class flare on 2011 September 6. We show that in both events, the same regions are responsible for the acceleration of particles remaining in the coronal and being ejected toward the heliosphere. However, the magnetic field structure around the acceleration region acts as a filter, resulting in different characteristics (such as energy spectra) acquired by these two populations. We argue that this effect is an intrinsic property of particle acceleration in the current layers created by the interchange reconnection, and therefore, may be ubiquitous, particularly, in noneruptive solar flares with substantial particle emission into the heliosphere.Peer reviewe
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