273 research outputs found
Alfv\`en wave phase-mixing and damping in the ion cyclotron range of frequencies
Aims. To determine the effect of the Hall term in the generalised Ohm's law
on the damping and phase mixing of Alfven waves in the ion cyclotron range of
frequencies in uniform and non-uniform equilibrium plasmas. Methods. Wave
damping in a uniform plasma is treated analytically, whilst a Lagrangian remap
code (Lare2d) is used to study Hall effects on damping and phase mixing in the
presence of an equilibrium density gradient. Results. The magnetic energy
associated with an initially Gaussian field perturbation in a uniform resistive
plasma is shown to decay algebraically at a rate that is unaffected by the Hall
term to leading order in k^2di^2 where k is wavenumber and di is ion skin
depth. A similar algebraic decay law applies to whistler perturbations in the
limit k^2di^2>>1. In a non-uniform plasma it is found that the
spatially-integrated damping rate due to phase mixing is lower in Hall MHD than
it is in MHD, but the reduction in the damping rate, which can be attributed to
the effects of wave dispersion, tends to zero in both the weak and strong phase
mixing limits
JPEG2000 Image Compression on Solar EUV Images
For future solar missions as well as ground-based telescopes, efficient ways
to return and process data have become increasingly important. Solar Orbiter,
e.g., which is the next ESA/NASA mission to explore the Sun and the
heliosphere, is a deep-space mission, which implies a limited telemetry rate
that makes efficient onboard data compression a necessity to achieve the
mission science goals. Missions like the Solar Dynamics Observatory (SDO) and
future ground-based telescopes such as the Daniel K. Inouye Solar Telescope, on
the other hand, face the challenge of making petabyte-sized solar data archives
accessible to the solar community. New image compression standards address
these challenges by implementing efficient and flexible compression algorithms
that can be tailored to user requirements. We analyse solar images from the
Atmospheric Imaging Assembly (AIA) instrument onboard SDO to study the effect
of lossy JPEG2000 (from the Joint Photographic Experts Group 2000) image
compression at different bit rates. To assess the quality of compressed images,
we use the mean structural similarity (MSSIM) index as well as the widely used
peak signal-to-noise ratio (PSNR) as metrics and compare the two in the context
of solar EUV images. In addition, we perform tests to validate the scientific
use of the lossily compressed images by analysing examples of an on-disk and
off-limb coronal-loop oscillation time-series observed by AIA/SDO.Comment: 25 pages, published in Solar Physic
Recent advances in coronal heating
The solar corona, the tenuous outer atmosphere of the Sun, is orders of magnitude hotter than the solar surface. This 'coronal heating problem' requires the identification of a heat source to balance losses due to thermal conduction, radiation and (in some locations) convection. The review papers in this Theo Murphy meeting issue present an overview of recent observational findings, large- and small-scale numerical modelling of physical processes occurring in the solar atmosphere and other aspects which may affect our understanding of the proposed heating mechanisms. At the same time, they also set out the directions and challenges which must be tackled by future research. In this brief introduction, we summarize some of the issues and themes which reoccur throughout this issue.PostprintPeer reviewe
Coronal Seismology and the Propagation of Acoustic Waves Along Coronal Loops
We use a combination of analytical theory, numerical simulation, and data
analysis to study the propagation of acoustic waves along coronal loops. We
show that the intensity perturbation of a wave depends on a number of factors,
including dissipation of the wave energy, pressure and temperature gradients in
the loop atmosphere, work action between the wave and a flow, and the
sensitivity properties of the observing instrument. In particular, the scale
length of the intensity perturbation varies directly with the dissipation scale
length (i.e., damping length) and the scale lengths of pressure, temperature,
and velocity. We simulate wave propagation in three different equilibrium loop
models and find that dissipation and pressure and temperature stratification
are the most important effects in the low corona where the waves are most
easily detected. Velocity effects are small, and cross-sectional area
variations play no direct role for lines-of-sight that are normal to the loop
axis. The intensity perturbation scale lengths in our simulations agree very
well with the scale lengths we measure in a sample of loops observed by TRACE.
The median observed value is 4.35x10^9 cm. In some cases the intensity
perturbation increases with height, which is likely an indication of a
temperature inversion in the loop (i.e., temperature that decreases with
height). Our most important conclusion is that thermal conduction, the primary
damping mechanism, is accurately described by classical transport theory. There
is no need to invoke anomalous processes to explain the observations.Comment: To appear in the Dec. 1, 2004 issue of the Astrophysical Journa
Contribution of mode-coupling and phase-mixing of Alfvén waves to coronal heating
This research has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 647214) and from the UK Science and Technology Facilities Council. This work used the DiRAC Data Centric system at Durham University, operated by the Institute for Computational Cosmology on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk. This equipment was funded by a BIS National E-infrastructure capital grant ST/K00042X/1, STFC capital grant ST/K00087X/1, DiRAC Operations grant ST/K003267/1 and Durham University.Context. Phase-mixing of Alfvén waves in the solar corona has been identified as one possible candidate to explain coronal heating. While this scenario is supported by observations of ubiquitous oscillations in the corona carrying sufficient wave energy and by theoretical models that have described the concentration of energy in small-scale structures, it is still unclear whether this wave energy can be converted into thermal energy in order to maintain the million-degree hot solar corona. Aims. The aim of this work is to assess how much energy can be converted into thermal energy by a phase-mixing process triggered by the propagation of Alfvénic waves in a cylindric coronal structure, such as a coronal loop, and to estimate the impact of this conversion on the coronal heating and thermal structure of the solar corona. Methods. To this end, we ran 3D MHD simulations of a magnetised cylinder where the Alfvén speed varies through a boundary shell, and a footpoint driver is set to trigger kink modes that mode couple to torsional Alfvén modes in the boundary shell. These Alfvén waves are expected to phase-mix, and the system allows us to study the subsequent thermal energy deposition. We ran a reference simulation to explain the main process and then we varied the simulation parameters, such as the size of the boundary shell, its structure, and the persistence of the driver. Results. When we take high values of magnetic resistivity and strong footpoint drivers into consideration, we find that i) phase-mixing leads to a temperature increase of the order of 105 K or less, depending on the structure of the boundary shell, ii) this energy is able to balance the radiative losses only in the localised region involved in the heating, and iii) we can determine the influence of the boundary layer and the persistence of the driver on the thermal structure of the system. Conclusions. Our conclusion is that as a result of the extreme physical parameters we adopted and the moderate impact on the heating of the system, it is unlikely that phase-mixing can contribute on a global scale to the heating of the solar corona.PostprintPeer reviewe
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
Magnetic reconnection in flux-tubes undergoing spinning footpoint motions
Aims. Photospheric motions acting on the coronal magnetic field have the potential to build up huge amounts of magnetic energy. The energy may be released through magnetic reconnection, and so a detailed understanding of the 3D process is crucial if its implications for coronal heating are to be fully addressed. Methods. A 3D MHD experiment is described in which misaligned magnetic flux tubes are subjected to simple spinning boundary motions. Results. The resulting shear between adjacent flux systems generates a twisted central separator current sheet that extends vertically throughout the domain. Current density is amplified to a sufficient extent that reconnection begins, and occurs everywhere along the separator current sheet, while the separatrix current sheets that exist in the early stages of the experiment are found to be unimportant in the systems dynamical evolution. In 2D cross-sections, the reconnection process exhibits many similarities to the regime of flux pile-up reconnection
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
A New Approach for Modelling Chromospheric Evaporation in Response to Enhanced Coronal Heating: 1 The Method
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