104 research outputs found
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
Wave heating in gravitationally stratified coronal loops in the presence of resistivity and viscosity
In recent years, coronal loops have been the focus of studies related to the
damping of different magnetohydrodynamic (MHD) surface waves and their
connection with coronal seismology and wave heating. For a better understanding
of wave heating, we need to take into account the effects of different
dissipation coefficients such as resistivity and viscosity, the importance of
the loop physical characteristics, and the ways gravity can factor into the
evolution of these phenomena. We aim to map the sites of energy dissipation
from transverse waves in coronal loops in the presence and absence of
gravitational stratification and to compare ideal, resistive, and viscous MHD.
Using the PLUTO code, we performed 3D MHD simulations of kink waves in single,
straight, density-enhanced coronal flux tubes of multiple temperatures. We see
the creation of spatially expanded Kelvin-Helmholtz eddies along the loop,
which deform the initial monolithic loop profile. For the case of driven
oscillations, the Kelvin-Helmholtz instability develops despite physical
dissipation, unless very high values of shear viscosity are used. Energy
dissipation gets its highest values near the apex, but is present all along the
loop. We observe an increased efficiency of wave heating once the kinetic
energy saturates at the later stages of the simulation and a turbulent density
profile has developed. The inclusion of gravity greatly alters the dynamic
evolution of our systems and should not be ignored in future studies. Stronger
physical dissipation leads to stronger wave heating in our set-ups. Finally,
once the kinetic energy of the oscillating loop starts saturating, all the
excess input energy turns into internal energy, resulting in more efficient
wave heating.Comment: 15 pages, 13 figures. Accepted for publication in Astronomy and
Astrophysics (A&A
Estimating the energy dissipation from Kelvin-Helmholtz instability induced turbulence in oscillating coronal loops
This is the final version. Available from IOP Publishing via the DOI in this recordKelvin-Helmholtz instability induced turbulence is one promising mechanism by which loops in the
solar corona can be heated by MHD waves. In this paper we present an analytical model of the
dissipation rate of Kelvin-Helmholtz instability induced turbulence εD, finding it scales as the wave
amplitude (d) to the third power (εD ∝ d
3
). Based on the concept of steady-state turbulence, we
expect the turbulence heating throughout the volume of the loop to match the total energy injected
through its footpoints. In situations where this holds, the wave amplitude has to vary as the cube-root
of the injected energy. Comparing the analytic results with those of simulations shows that our analytic
formulation captures the key aspects of the turbulent dissipation from the numerical work. Applying
this model to the observed characteristics of decayless kink waves we predict that the amplitudes of
these observed waves is insufficient to turbulently heat the solar corona.Science and Technology Facilities Council (STFC
Deep XMM-Newton observations of the northern disc of M31. I. Source catalogue
We carried out new observations of two fields in the northern ring of M31
with XMM-Newton with two exposures of 100 ks each and obtained a complete list
of X-ray sources down to a sensitivity limit of ~7 x 10^34 erg s^-1 (0.5 - 2.0
keV). The major objective of the observing programme was the study of the hot
phase of the ISM in M31. The analysis of the diffuse emission and the study of
the ISM is presented in a separate paper. We analysed the spectral properties
of all detected sources using hardness ratios and spectra if the statistics
were high enough. We also checked for variability. We cross-correlated the
source list with the source catalogue of a new survey of the northern disc of
M31 carried out with Chandra and Hubble (Panchromatic Hubble Andromeda
Treasury, PHAT) as well as with other existing catalogues. We detected a total
of 389 sources, including 43 foreground stars and candidates and 50 background
sources. Based on the comparison to the Chandra/PHAT survey, we classify 24
hard X-ray sources as new candidates for X-ray binaries (XRBs). In total, we
identified 34 XRBs and candidates and 18 supernova remnants (SNRs) and
candidates. Three of the four brightest SNRs show emission mainly below 2 keV,
consistent with shocked ISM. The spectra of two of them also require an
additional component with a higher temperature. The SNR [SPH11] 1535 has a
harder spectrum and might suggest that there is a pulsar-wind nebula inside the
SNR. We find five new sources showing clear time variability. We also studied
the spectral properties of the transient source SWIFT J004420.1+413702, which
shows significant variation in flux over a period of seven months (June 2015 to
January 2016) and associated change in absorption. Based on the likely optical
counterpart detected in the Chandra/PHAT survey, the source is classified as a
low-mass X-ray binary.Comment: Accepted for publication in A&
Comparative performance of selected variability detection techniques in photometric time series
Photometric measurements are prone to systematic errors presenting a
challenge to low-amplitude variability detection. In search for a
general-purpose variability detection technique able to recover a broad range
of variability types including currently unknown ones, we test 18 statistical
characteristics quantifying scatter and/or correlation between brightness
measurements. We compare their performance in identifying variable objects in
seven time series data sets obtained with telescopes ranging in size from a
telephoto lens to 1m-class and probing variability on time-scales from minutes
to decades. The test data sets together include lightcurves of 127539 objects,
among them 1251 variable stars of various types and represent a range of
observing conditions often found in ground-based variability surveys. The real
data are complemented by simulations. We propose a combination of two indices
that together recover a broad range of variability types from photometric data
characterized by a wide variety of sampling patterns, photometric accuracies,
and percentages of outlier measurements. The first index is the interquartile
range (IQR) of magnitude measurements, sensitive to variability irrespective of
a time-scale and resistant to outliers. It can be complemented by the ratio of
the lightcurve variance to the mean square successive difference, 1/h, which is
efficient in detecting variability on time-scales longer than the typical time
interval between observations. Variable objects have larger 1/h and/or IQR
values than non-variable objects of similar brightness. Another approach to
variability detection is to combine many variability indices using principal
component analysis. We present 124 previously unknown variable stars found in
the test data.Comment: 29 pages, 8 figures, 7 tables; accepted to MNRAS; for additional
plots, see http://scan.sai.msu.ru/~kirx/var_idx_paper
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