685 research outputs found
Fine strand-like structure in the solar corona from MHD transverse oscillations
Current analytical and numerical modelling suggest the existence of
ubiquitous thin current sheets in the corona that could explain the observed
heating requirements. On the other hand, new high resolution observations of
the corona indicate that its magnetic field may tend to organise itself in fine
strand-like structures of few hundred kilometres widths. The link between small
structure in models and the observed widths of strand-like structure several
orders of magnitude larger is still not clear. A popular theoretical scenario
is the nanoflare model, in which each strand is the product of an ensemble of
heating events. Here, we suggest an alternative mechanism for strand
generation. Through forward modelling of 3D MHD simulations we show that small
amplitude transverse MHD waves can lead in a few periods time to strand-like
structure in loops in EUV intensity images. Our model is based on previous
numerical work showing that transverse MHD oscillations can lead to
Kelvin-Helmholtz instabilities that deform the cross-sectional area of loops.
While previous work has focused on large amplitude oscillations, here we show
that the instability can occur even for low wave amplitudes for long and thin
loops, matching those presently observed in the corona. We show that the
vortices generated from the instability are velocity sheared regions with
enhanced emissivity hosting current sheets. Strands result as a complex
combination of the vortices and the line-of-sight angle, last for timescales of
a period and can be observed for spatial resolutions of a tenth of loop radius.Comment: Accepted for publication in ApJ
Forward Modelling of Standing Slow Modes in Flaring Coronal Loops
Standing slow mode waves in hot flaring loops are exclusively observed in
spectrometers and are used to diagnose the magnetic field strength and
temperature of the loop structure. Due to the lack of spatial information, the
longitudinal mode cannot be effectively identified. In this study, we simulate
standing slow mode waves in flaring loops and compare the synthesized line
emission properties with SUMER spectrographic and SDO/AIA imaging observations.
We find that the emission intensity and line width oscillations are a quarter
period out of phase with Doppler shift velocity both in time and spatial
domain, which can be used to identify a standing slow mode wave from
spectroscopic observations. However, the longitudinal overtones could be only
measured with the assistance of imagers. We find emission intensity asymmetry
in the positive and negative modulations, this is because the contribution
function pertaining to the atomic emission process responds differently to
positive and negative temperature variations. One may detect \textbf{half}
periodicity close to the loop apex, where emission intensity modulation is
relatively small. The line-of-sight projection affects the observation of
Doppler shift significantly. A more accurate estimate of the amplitude of
velocity perturbation is obtained by de-projecting the Doppler shift by a
factor of rather than the traditionally used .
\textbf{If a loop is heated to the hotter wing, the intensity modulation could
be overwhelmed by background emission, while the Doppler shift velocity could
still be detected to a certain extent.Comment: 18 pages, 10 figures, Astrophysics Journa
Kelvin–Helmholtz Instability and Alfvénic Vortex Shedding in Solar Eruptions
We report on a three-dimensional MHD numerical experiment of a small-scale coronal mass ejection (CME)-like eruption propagating though a nonmagnetized solar atmosphere. We find that the Kelvin–Helmholtz instability (KHI) develops at various but specific locations at the boundary layer between the erupting field and the background atmosphere, depending on the relative angle between the velocity and magnetic field. KHI develops at the front and at two of the four sides of the eruption. KHI is suppressed at the other two sides of the eruption. We also find the development of Alfvénic vortex shedding flows at the wake of the developing CME due to the 3D geometry of the field. Forward modeling reveals that the observational detectability of the KHI in solar eruptions is confined to a narrow ≈10° range when observing off-limb, and therefore its occurrence could be underestimated due to projection effects. The new findings can have significant implications for observations, for heating, and for particle acceleration by turbulence from flow-driven instabilities associated with solar eruptions of all scales
The role of torsional Alfven waves in coronal heating
In the context of coronal heating, among the zoo of MHD waves that exist in
the solar atmosphere, Alfven waves receive special attention. Indeed, these
waves constitute an attractive heating agent due to their ability to carry over
the many different layers of the solar atmosphere sufficient energy to heat and
maintain a corona. However, due to their incompressible nature these waves need
a mechanism such as mode conversion (leading to shock heating), phase mixing,
resonant absorption or turbulent cascade in order to heat the plasma. New
observations with polarimetric, spectroscopic and imaging instruments such as
those on board of the japanese satellite Hinode, or the SST or CoMP, are
bringing strong evidence for the existence of energetic Alfven waves in the
solar corona. In order to assess the role of Alfven waves in coronal heating,
in this work we model a magnetic flux tube being subject to Alfven wave heating
through the mode conversion mechanism. Using a 1.5-dimensional MHD code we
carry out a parameter survey varying the magnetic flux tube geometry (length
and expansion), the photospheric magnetic field, the photospheric velocity
amplitudes and the nature of the waves (monochromatic or white noise spectrum).
It is found that independently of the photospheric wave amplitude and magnetic
field a corona can be produced and maintained only for long (> 80 Mm) and thick
(area ratio between photosphere and corona > 500) loops. Above a critical value
of the photospheric velocity amplitude (generally a few km/s) the corona can no
longer be maintained over extended periods of time and collapses due to the
large momentum of the waves. These results establish several constraints on
Alfven wave heating as a coronal heating mechanism, especially for active
region loops.Comment: 39 pages, 8 figures; http://stacks.iop.org/0004-637X/712/49
The multi-thermal and multi-stranded nature of coronal rain
In this work, we analyse coordinated observations spanning chromospheric, TR
and coronal temperatures at very high resolution which reveal essential
characteristics of thermally unstable plasmas. Coronal rain is found to be a
highly multi-thermal phenomenon with a high degree of co-spatiality in the
multi-wavelength emission. EUV darkening and quasi-periodic intensity
variations are found to be strongly correlated to coronal rain showers.
Progressive cooling of coronal rain is observed, leading to a height dependence
of the emission. A fast-slow two-step catastrophic cooling progression is
found, which may reflect the transition to optically thick plasma states. The
intermittent and clumpy appearance of coronal rain at coronal heights becomes
more continuous and persistent at chromospheric heights just before impact,
mainly due to a funnel effect from the observed expansion of the magnetic
field. Strong density inhomogeneities on spatial scales of 0.2"-0.5" are found,
in which TR to chromospheric temperature transition occurs at the lowest
detectable scales. The shape of the distribution of coronal rain widths is
found to be independent of temperature with peaks close to the resolution limit
of each telescope, ranging from 0.2" to 0.8". However we find a sharp increase
of clump numbers at the coolest wavelengths and especially at higher
resolution, suggesting that the bulk of the rain distribution remains
undetected. Rain clumps appear organised in strands in both chromospheric and
TR temperatures, suggesting an important role of thermal instability in the
shaping of fundamental loop substructure. We further find structure reminiscent
of the MHD thermal mode. Rain core densities are estimated to vary between
2x10^{10} cm^{-3} and 2.5x10^{11} cm^{-3} leading to significant downward mass
fluxes per loop of 1-5x10^{9} g s^{-1}, suggesting a major role in the
chromosphere-corona mass cycle.Comment: Abstract is only short version. See paper for full. Countless pages,
figures (and movies, but not included here). Accepted for publication in the
Astrophysical Journa
Fine strand-like structure in the solar corona from magnetohydrodynamic transverse oscillations
Current analytical and numerical modeling suggest the existence of ubiquitous thin current sheets in the corona that could explain the observed heating requirements. On the other hand, new high resolution observations of the corona indicate that its magnetic field may tend to organize itself in fine strand-like structures of few hundred kilometers widths. The link between small structure in models and the observed widths of strand-like structure several orders of magnitude larger is still not clear. A popular theoretical scenario is the nanoflare model, in which each strand is the product of an ensemble of heating events. Here, we suggest an alternative mechanism for strand generation. Through forward modeling of three-dimensional MHD simulations we show that small amplitude transverse MHD waves can lead in a few periods time to strand-like structure in loops in EUV intensity images. Our model is based on previous numerical work showing that transverse MHD oscillations can lead to Kelvin-Helmholtz instabilities that deform the cross-sectional area of loops. While previous work has focused on large amplitude oscillations, here we show that the instability can occur even for low wave amplitudes for long and thin loops, matching those presently observed in the corona. We show that the vortices generated from the instability are velocity sheared regions with enhanced emissivity hosting current sheets. Strands result as a complex combination of the vortices and the line-of-sight angle, last for timescales of a period, and can be observed for spatial resolutions of a tenth of loop radius.Publisher PDFPeer reviewe
Public resources for chemical probes: the journey so far and the road ahead.
High-quality small molecule chemical probes are extremely valuable for biological research and target validation. However, frequent use of flawed small-molecule inhibitors produces misleading results and diminishes the robustness of biomedical research. Several public resources are available to facilitate assessment and selection of better chemical probes for specific protein targets. Here, we review chemical probe resources, discuss their current strengths and limitations, and make recommendations for further improvements. Expert review resources provide in-depth analysis but currently cover only a limited portion of the liganded proteome. Computational resources encompass more proteins and are regularly updated, but have limitations in data availability and curation. We show how biomedical scientists may use these resources to choose the best available chemical probes for their research
Heating by transverse waves in simulated coronal loops
K.K. was funded by GOA-2015-014 (KU Leuven). T.V.D was supported by the IAP P7/08 CHARM (Belspo) and the GOA-2015-014 (KU Leuven). P.A. acknowledges funding from the UK Science and Technology Facilities Council and the European Union Horizon 2020 research and innovation programme (grant agreement No. 647214).Context. Recent numerical studies of oscillating flux tubes have established the significance of resonant absorption in the damping of propagating transverse oscillations in coronal loops. The nonlinear nature of the mechanism has been examined alongside the Kelvin-Helmholtz instability,which is expected to manifest in the resonant layers at the edges of the flux tubes. While these two processes have been hypothesized to heat coronal loops through the dissipation of wave energy into smaller scales, the occurring mixing with the hotter surroundings can potentially hide this effect. Aims. We aim to study the effects of wave heating from driven and standing kink waves in a coronal loop. Methods. Using the MPI-AMRVAC code, we perform ideal, three dimensional magnetohydrodynamic (MHD) simulations of both (a) footpoint driven and (b) free standing oscillations in a straight coronal flux tube, in the presence of numerical resistivity. Results. We have observed the development of Kelvin-Helmholtz eddies at the loop boundary layer of all three models considered here, as well as an increase of the volume averaged temperature inside the loop. The main heating mechanism in our setups was Ohmic dissipation, as indicated by the higher values for the temperatures and current densities located near the footpoints. The introduction of a temperature gradient between the inner tube and the surrounding plasma, suggests that the mixing of the two regions, in the case of hotter environment, greatly increases the temperature of the tube at the site of the strongest turbulence, beyond the contribution of the aforementioned wave heating mechanism.PostprintPeer reviewe
- …