457 research outputs found
First comparison of wave observations from CoMP and AIA/SDO
Waves have long been thought to contribute to the heating of the solar corona
and the generation of the solar wind. Recent observations have demonstrated
evidence of quasi-periodic longitudinal disturbances and ubiquitous transverse
wave propagation in many different coronal environments. This paper
investigates signatures of different types of oscillatory behaviour, both above
the solar limb and on-disk, by comparing findings from the Coronal
Multi-channel Polarimeter (CoMP) and the Atmospheric Imaging Assembly (AIA) on
board the Solar Dynamics Observatory (SDO) for the same active region. We study
both transverse and longitudinal motion by comparing and contrasting
time-distance images of parallel and perpendicular cuts along/across active
region fan loops. Comparisons between parallel space-time features in CoMP
Doppler velocity and transverse oscillations in AIA images are made, together
with space-time analysis of propagating quasi-periodic intensity features seen
near the base of loops in AIA. Signatures of transverse motions are observed
along the same magnetic structure using CoMP Doppler velocity
(Vphase=600-750km/s, P=3-6mins) and in AIA/SDO above the limb (P=3-8mins).
Quasi-periodic intensity features (Vphase=100-200km/s, P=6-11mins) also travel
along the base of the same structure. On the disk, signatures of both
transverse and longitudinal intensity features were observed by AIA; both show
similar properties to signatures found along structures anchored in the same
active region three days earlier above the limb. Correlated features are
recovered by space-time analysis of neighbouring tracks over perpendicular
distances of <2.6Mm.Comment: 14 pages, 14 figures, 1 tabl
Nonlinear wave propagation and reconnection at magnetic X-points in the Hall MHD regime
The highly dynamical, complex nature of the solar atmosphere naturally
implies the presence of waves in a topologically varied magnetic environment.
Here, the interaction of waves with topological features such as null points is
inevitable and potentially important for energetics. The low resistivity of the
solar coronal plasma implies that non-MHD effects should be considered in
studies of magnetic energy release in this environment. This paper investigates
the role of the Hall term in the propagation and dissipation of waves, their
interaction with 2D magnetic X-points and the nature of the resulting
reconnection. A Lagrangian remap shock-capturing code (Lare2d) is used to study
the evolution of an initial fast magnetoacoustic wave annulus for a range of
values of the ion skin depth in resistive Hall MHD. A magnetic null-point
finding algorithm is also used to locate and track the evolution of the
multiple null-points that are formed in the system. Depending on the ratio of
ion skin depth to system size, our model demonstrates that Hall effects can
play a key role in the wave-null interaction. In particular, the initial
fast-wave pulse now consists of whistler and ion-cyclotron components; the
dispersive nature of the whistler wave leads to (i) earlier interaction with
the null, (ii) the creation of multiple additional, transient nulls and, hence,
an increased number of energy release sites. In the Hall regime, the relevant
timescales (such as the onset of reconnection and the period of the oscillatory
relaxation) of the system are reduced significantly, and the reconnection rate
is enhanced.Comment: 13 pages, 10 figure
Self-consistent nanoflare heating in model active regions:MHD avalanches
Straightened cylindrical models of coronal loops have been standard for decades, and shown to support nanoflare-like heating, but the influence of geometric curvature in models upon the heating produced has not been discussed in depth. Heating, its spatiotemporal distributions, and the associated mechanisms responsible are discussed, and compared with those from straightened models of a coronal loop. Previously, magnetohydrodynamic avalanches have been generalized to curved loops, and shown to be viable. From that study, the associated heating is analysed and discussed in depth. Heating is seen to arise from processes originally instigated, yet not dominated, by magnetic reconnection, producing bursty, aperiodic nanoflares, dispersed evenly throughout the corona, but with a modest bias away from footpoints. One novelty arising is the simultaneous yet independent occurrence of nanoflare-like events at disjoint sites along individual strands, anticipating some features recently seen in ‘campfires’ by Solar Orbiter. With a view to future refinements in the model and to the inclusion of additional physical effects, the implications of this analysis are discussed
Can multi-threaded flux tubes in coronal arcades support a magnetohydrodynamic avalanche?
Magnetohydrodynamic (MHD) instabilities allow energy to be released from stressed magnetic fields, commonly modelled in cylindrical flux tubes linking parallel planes, but, more recently, also in curved arcades containing flux tubes with both footpoints in the same photospheric plane. Uncurved cylindrical flux tubes containing multiple individual threads have been shown to be capable of sustaining an MHD avalanche, whereby a single unstable thread can destabilise many. We examine the properties of multi-threaded coronal loops, wherein each thread is created by photospheric driving in a realistic, curved coronal arcade structure (with both footpoints of each thread in the same plane). We use three-dimensional MHD simulations to study the evolution of singleand multi-threaded coronal loops, which become unstable and reconnect, while varying the driving velocity of individual threads. Experiments containing a single thread destabilise in a manner indicative of an ideal MHD instability and consistent with previous examples in the literature. The introduction of additional threads modifies this picture, with aspects of the model geometry and relative driving speeds of individual threads affecting the ability of any thread to destabilise others. In both single- and multi-threaded cases, continuous driving of the remnants of disrupted threads produces secondary, aperiodic bursts of energetic release
How is helicity (and twist) partitioned in magnetohydrodynamic simulations of reconnecting magnetic flux tubes?
Funding: STFC through the Consolidated grant, ST/N000609/1, to the University of St Andrews.Magnetic helicity conservation provides a convenient way to analyze specific properties (namely, the linkage and twist) of reconnecting flux tubes and yield additional insight into the pre- and post-reconnection states of magnetic structures in the solar atmosphere. A previous study considered two flux tubes with footpoints anchored in two parallel planes. They showed that reconnection would add self-helicity equivalent to a half turn of twist to each flux tube. We address a related and fundamental question here: if two flux tubes anchored in a single plane reconnect, what are the resulting twists imparted to each of the reconnected tubes? Are they equal and do they have a simple exact value independent of footpoint location? To do this, we employ a new (computationally efficient) method which subdivides each flux tube into distinct elements and calculates the mutual helicity of many elemental pairs, the sum of which determines the self-helicity of the overall flux tube. Having tested the method using a simple analytical model, we apply the technique to a magnetohydrodynamic simulation where initially untwisted magnetic flux tubes are sheared and allowed to reconnect (based on a previous reconnection model). We recover values of self-helicity and twist in the final end state of the simulations which show excellent agreement with theoretical predictions.Publisher PDFPeer reviewe
Flare particle acceleration in the interaction of twisted coronal flux ropes
The authors gratefully acknowledge the support of the U.K. Science and Technology Facilities Council. JT and AWH acknowledge the financial support of STFC through the Consolidated grant, ST/N000609/1, to the University of St Andrews. PKB acknowledges STFC support through ST/P000428/1 at the University of Manchester.Aims. The aim of this work is to investigate and characterise non-thermal particle behaviour in a three-dimensional (3D) magnetohydrodynamical (MHD) model of unstable multi-threaded flaring coronal loops. Methods. We have used a numerical scheme which solves the relativistic guiding centre approximation to study the motion of electrons and protons. The scheme uses snapshots from high resolution numerical MHD simulations of coronal loops containing two threads, where a single thread becomes unstable and (in one case) destabilises and merges with an additional thread. Results. The particle responses to the reconnection and fragmentation in MHD simulations of two loop threads are examined in detail. We illustrate the role played by uniform background resistivity and distinguish this from the role of anomalous resistivity using orbits in an MHD simulation where only one thread becomes unstable without destabilising further loop threads. We examine the (scalable) orbit energy gains and final positions recovered at different stages of a second MHD simulation wherein a secondary loop thread is destabilised by (and merges with) the first thread. We compare these results with other theoretical particle acceleration models in the context of observed energetic particle populations during solar flares.PostprintPeer reviewe
Topological Lensing in Spherical Spaces
This article gives the construction and complete classification of all
three-dimensional spherical manifolds, and orders them by decreasing volume, in
the context of multiconnected universe models with positive spatial curvature.
It discusses which spherical topologies are likely to be detectable by
crystallographic methods using three-dimensional catalogs of cosmic objects.
The expected form of the pair separation histogram is predicted (including the
location and height of the spikes) and is compared to computer simulations,
showing that this method is stable with respect to observational uncertainties
and is well suited for detecting spherical topologies.Comment: 32 pages, 26 figure
Flux rope formation due to shearing and zipper reconnection
The authors acknowledge the financial support of STFC through the Consolidated grant, ST/N000609/1, to the University of St Andrews. These systems are operated on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk). The equipment was funded by BIS National E-infrastructure capital grants (ST/K00042X/1 and ST/K001590/1), STFC capital grants (ST/K00087X/1, ST/H008861/1 and ST/H00887X/1) and DiRAC Operations grants (ST/K003267/1 and ST/K00333X/1). The research data supporting this publication can be accessed at http://dx.doi.org/10.17630/dea50f38-7aff-48c3-b090-c689d02dd305.Zipper reconnection has been proposed as a mechanism for creating most of the twist in the flux tubes that are present {prior to} eruptive flares and coronal mass ejections. We have conducted a first numerical experiment on this new regime of reconnection, where two initially untwisted parallel flux tubes are sheared and reconnected to form a large flux rope. We describe the properties of this experiment, including the linkage of magnetic flux between concentrated flux sources at the base of the simulation, the twist of the newly formed flux rope and the conversion of mutual magnetic helicity in the sheared pre-reconnection state into the self-helicity of the newly formed flux rope.Publisher PDFPeer reviewe
Generalized Chaplygin gas model, supernovae and cosmic topology
In this work we study to which extent the knowledge of spatial topology may
place constraints on the parameters of the generalized Chaplygin gas (GCG)
model for unification of dark energy and dark matter. By using both the
Poincar\'e dodecahedral and binary octahedral spaces as the observable spatial
topologies, we examine the current type Ia supernovae (SNe Ia) constraints on
the GCG model parameters. We show that the knowledge of spatial topology does
provide additional constraints on the parameter of the GCG model but does
not lift the degeneracy of the parameter.Comment: Revtex 4, 8 pages, 10 figures, 1 table; version to match the
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