The Experts below are selected from a list of 8814 Experts worldwide ranked by ideXlab platform
V M Nakariakov - One of the best experts on this subject based on the ideXlab platform.
-
standing kink waves in sigmoid solar Coronal Loops implications for Coronal seismology
The Astrophysical Journal, 2020Co-Authors: N Magyar, V M NakariakovAbstract:Using full three-dimensional magnetohydrodynamic numerical simulations, we study the effects of magnetic field sigmoidity or helicity on the properties of the fundamental kink oscillation of solar Coronal Loops. Our model consists of a single denser Coronal loop, embedded in a plasma with dipolar force-free magnetic field with a constant alpha-parameter. For the loop with no sigmoidity, we find that the numerically determined oscillation period of the fundamental kink mode matches the theoretical period calculated using WKB theory. In contrast, with increasing sigmoidity of the loop, the actual period is increasingly smaller than the one estimated by WKB theory. Translated through Coronal seismology, increasing sigmoidity results in magnetic field estimates which are increasingly shifting towards higher values, and even surpassing the average value for the highest alpha value considered. Nevertheless, the estimated range of the Coronal magnetic field value lies within the mimimal/maximal limits, proving the robustness Coronal seismology. We propose that the discrepancy in the estimations of the absolute value of the force-free magnetic field could be exploited seismologically to determine the free energy of Coronal Loops, if averages of the internal magnetic field and density can be reliably estimated by other methods.
-
excitation of kink oscillations of Coronal Loops statistical study
Astronomy and Astrophysics, 2015Co-Authors: V M Nakariakov, I V ZimovetsAbstract:Context. Solar flares are often accompanied by kink (transverse) oscillations of Coronal Loops. Despite intensive study of these oscillations in recent years, the mechanisms that excite them are still not known. Aims. We aim to clarify the excitation mechanisms for these kink oscillations of Coronal Loops. Methods. Weanalysed 58 kink-oscillation events observed by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) during its first four years (2010‐2014) with the use of the JHelioviewer. Association of these oscillation events with flares, lower Coronal (r 1.4R� ) eruptions and plasma ejections, Coronal mass ejections (CMEs), and Coronal Type-II radio bursts is studied. Results. We find that 44 of these 58 oscillation events (76%) were associated with CMEs observed in the white light emission. Moreover, 57 events (98%) were accompanied by lower Coronal eruptions/ejections (LCEs) observed in the extreme-ultraviolet band in the parental active regions. In the remaining event an LCE was not clearly seen, but it was definitely associated with a CME too. The main observational finding is that the kink oscillations were excited by the deviation of Loops from their equilibria by a nearby LCE in 55 events (95%). In three remaining events, it was difficult to reliably determine the cause of the oscillations because of limitations in the observational data. We also found that 53 events (91%) were associated with flares. In five remaining events, the parental active regions were behind the limb and we could not directly see flare sites. It indicates that there is a close relationship between these two kinds of solar activity. However, the estimated speeds of a hypothetical driver of kink oscillations by flares were found to be lower than 500 kms −1 in 80% of the cases. Such low speeds do not favour the association of the oscillation excitation with a shock wave, as usually assumed. That only 23 (40%) of the oscillation events were found to be associated with Coronal Type-II radio bursts also goes against the shock wave mechanism for the excitation of kink oscillations. Conclusions. The statistical analysis shows that the most probable mechanism for exciting the kink oscillations of Coronal Loops is the deviation of Loops from their equilibrium by nearby eruptions or plasma ejections rather than a blast shock wave ignited by a flare.
-
decay less kink oscillations in Coronal Loops
Astronomy and Astrophysics, 2013Co-Authors: Sergey Anfinogentov, Giuseppe Nisticò, V M NakariakovAbstract:Context: Kink oscillations of Coronal Loops in an off-limb active region are detected with the Imaging Assembly Array (AIA) instruments of the Solar Dynamics Observatory (SDO) at 171 A. Aims: We aim to measure periods and amplitudes of kink oscillations of different Loops and to determinate the evolution of the oscillation phase along the oscillating loop. Methods: Oscillating Coronal Loops were visually identified in the field of view of SDO/AIA and STEREO/EUVI-A: the loop length was derived by three-dimensional analysis. Several slits were taken along the Loops to assemble time-distance maps. We identified oscillatory patterns and retrieved periods and amplitudes of the oscillations. We applied the cross-correlation technique to estimate the phase shift between oscillations at different segments of oscillating Loops. Results: We found that all analysed Loops show low-amplitude undamped transverse oscillations. Oscillation periods of Loops in the same active region range from 2.5 to 11 min, and are different for different Loops. The displacement amplitude is lower than 1 Mm. The oscillation phase is constant along each analysed loop. The spatial structure of the phase of the oscillations corresponds to the fundamental standing kink mode. We conclude that the observed behaviour is consistent with the empirical model in terms of a damped harmonic resonator affected by a non-resonant continuously operating external force.
-
global sausage modes of Coronal Loops
Astronomy and Astrophysics, 2003Co-Authors: V M Nakariakov, V F Melnikov, Veronika ReznikovaAbstract:Sufficiently thick and dense Coronal Loops can support global sausage magnetoacoustic modes. We demonstrate that the oscillation period of this mode, calculated in the straight cylinder approximation, is determined by the length of the loop, not by its diameter, as it was previously assumed. The existence condition for this mode is the ratio of the loop length to its diameter to be less than about a half of the square root of the density contrast ratio. This mode has a maximum of the magnetic field perturbation at the loop apex and nodes at the footpoints. We demonstrate that the 14−17 s quasi-periodic pulsations, oscillating in phase at a loop apex and at its legs, observed with the Nobeyama Radioheliograph, are interpreted in terms of the global sausage mode.
K Karampelas - One of the best experts on this subject based on the ideXlab platform.
-
Wave Heating in Simulated Multistranded Coronal Loops
The Astrophysical Journal, 2019Co-Authors: Ming-zhe Guo, T Van Doorsselaere, K KarampelasAbstract:It has been found that the Kelvin-Helmholtz instability (KHI) induced by both transverse and torsional oscillations in Coronal Loops can reinforce the effects of wave heating. In this study, we model a Coronal loop as a system of individual strands, and we study wave heating effects by considering a combined transverse and torsional driver at the loop footpoint. We deposit the same energy into the multi-stranded loop and an equivalent monolithic loop, and then observe a faster increase in the internal energy and temperature in the multi-stranded model. Therefore, the multi-stranded model is more efficient in starting the heating process. Moreover, higher temperature is observed near the footpoint in the multi-stranded loop and near the apex in the monolithic loop. The apparent heating location in the multi-stranded loop agrees with the previous predictions and observations. Given the differences in the results from our multi-stranded loop and monolithic loop simulations, and given that Coronal Loops are suggested to be multi-stranded on both theoretical and observational grounds, our results suggest that the multi-strandedness of Coronal Loops needs to be incorporated in future wave-based heating mechanisms.
-
wave heating in gravitationally stratified Coronal Loops in the presence of resistivity and viscosity
arXiv: Solar and Stellar Astrophysics, 2019Co-Authors: K Karampelas, T. Van Doorsselaere, Ming-zhe GuoAbstract: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.
-
heating effects from driven transverse and alfven waves in Coronal Loops
The Astrophysical Journal, 2019Co-Authors: T Van Doorsselaere, Patrick Antoli, K Karampelas, Ming-zhe Guo, Ineke De MoortelAbstract:Recent numerical studies revealed that transverse motions of Coronal Loops can induce the Kelvin–Helmholtz instability (KHI). This process could be important in Coronal heating because it leads to dissipation of energy at small spatial scale plasma interactions. Meanwhile, small-amplitude decayless oscillations in Coronal Loops have been discovered recently in observations of SDO/AIA. We model such oscillations in Coronal Loops and study wave heating effects, considering a kink and Alfven driver separately and a mixed driver at the bottom of flux tubes. Both the transverse and Alfven oscillations can lead to the KHI. Meanwhile, the Alfven oscillations established in Loops will experience phase mixing. Both processes will generate small spatial scale structures, which can help the dissipation of wave energy. Indeed, we observe the increase of internal energy and temperature in loop regions. The heating is more pronounced for the simulation containing the mixed kink and Alfven driver. This means that the mixed wave modes can lead to a more efficient energy dissipation in the turbulent state of the plasma and that the KHI eddies act as an agent to dissipate energy in other wave modes. Furthermore, we also obtained forward-modeling results using the FoMo code. We obtained forward models that are very similar to the observations of decayless oscillations. Due to the limited resolution of instruments, neither Alfven modes nor the fine structures are observable. Therefore, this numerical study shows that Alfven modes probably can coexist with kink modes, leading to enhanced heating.
-
heating by transverse waves in simulated Coronal Loops
Astronomy and Astrophysics, 2017Co-Authors: K Karampelas, T Van Doorsselaere, Patrick AntoliAbstract: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.
-
heating by transverse waves in simulated Coronal Loops
arXiv: Solar and Stellar Astrophysics, 2017Co-Authors: K Karampelas, T. Van Doorsselaere, Patrick AntolinAbstract: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. We aim to study the effects of wave heating from driven and standing kink waves in a Coronal loop. 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. 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.
Tongjiang Wang - One of the best experts on this subject based on the ideXlab platform.
-
three dimensional magnetohydrodynamic modeling of propagating disturbances in fan like Coronal Loops
The Astrophysical Journal, 2013Co-Authors: Tongjiang Wang, L Ofman, Joseph M DavilaAbstract:Quasi-periodic propagating intensity disturbances (PDs) have been observed in large Coronal Loops in EUV images over a decade, and are widely accepted to be slow magnetosonic waves. However, spectroscopic observations from Hinode/EIS revealed their association with persistent Coronal upflows, making this interpretation debatable. Motivated by the scenario that the Coronal upflows could be the cumulative result of numerous individual flow pulses generated by sporadic heating events (nanoflares) at the loop base, we construct a velocity driver with repetitive tiny pulses, whose energy frequency distribution follows the flare power-law scaling. We then perform three-dimensional MHD modeling of an idealized bipolar active region by applying this broadband velocity driver at the footpoints of large Coronal Loops which appear open in the computational domain. Our model successfully reproduces the PDs with similar features as the observed, and shows that any upflow pulses inevitably excite slow magnetosonic wave disturbances propagating along the loop. We find that the generated PDs are dominated by the wave signature as their propagation speeds are consistent with the wave speed in the presence of flows, and the injected flows rapidly decelerate with height. Our simulation results suggest that the observed PDs and associated persistent upflows may be produced by small-scale impulsive heating events (nanoflares) at the loop base in the corona, and that the flows and waves may both contribute to the PDs at lower heights.
-
standing slow mode waves in hot Coronal Loops observations modeling and Coronal seismology
Space Science Reviews, 2011Co-Authors: Tongjiang WangAbstract:Strongly damped Doppler shift oscillations are observed frequently associated with flarelike events in hot Coronal Loops. In this paper, a review of the observed properties and the theoretical modeling is presented. Statistical measurements of physical parameters (period, decay time, and amplitude) have been obtained based on a large number of events observed by SOHO/SUMER and Yohkoh/BCS. Several pieces of evidence are found to support their interpretation in terms of the fundamental standing longitudinal slow mode. The high excitation rate of these oscillations in small- or micro-flares suggest that the slow mode waves are a natural response of the Coronal plasma to impulsive heating in closed magnetic structure. The strong damping and the rapid excitation of the observed waves are two major aspects of the waves that are poorly understood, and are the main subject of theoretical modelling. The slow waves are found mainly damped by thermal conduction and viscosity in hot Coronal Loops. The mode coupling seems to play an important role in rapid excitation of the standing slow mode. Several seismology applications such as determination of the magnetic field, temperature, and density in Coronal Loops are demonstrated. Further, some open issues are discussed.
Ming-zhe Guo - One of the best experts on this subject based on the ideXlab platform.
-
kink oscillations in solar Coronal Loops with elliptical cross sections i the linear regime
arXiv: Solar and Stellar Astrophysics, 2020Co-Authors: Ming-zhe Guo, T Van DoorsselaereAbstract:The cross sections of solar Coronal Loops are suggested to be rarely circular. We examine linear kink oscillations in straight, density-enhanced, magnetic cylinders with elliptical cross-sections by solving the three-dimensional magnetohydrodynamic equations from an initial-value-problem perspective. Motivated by relevant eigen-mode analyses, we distinguish between two independent polarizations, one along the major axis (the M-modes) and the other along the minor one (the m-modes). We find that, as happens for Coronal Loops with circular cross-sections, the apparent damping of the transverse displacement of the loop axis is accompanied by the accumulation of transverse Alfv\'enic motions and the consequent development of small-scales therein, suggesting the robustness of the concepts of resonant absorption and phase-mixing. In addition, two stages can in general be told apart in the temporal evolution of the loop displacement; a Gaussian time dependence precedes an exponential one. For the two examined density ratios between Loops and their surroundings, the periods of the M-modes (m-modes) tend to increase (decrease) with the major-to-minor-half-axis ratio, and the damping times in the exponential stage for the M-modes tend to exceed their m-mode counterparts. This is true for the two transverse profiles we examine. However, the relative magnitudes of the damping times in the exponential stage for different polarizations depend on the specification of the transverse profile and/or the density contrast. The applications of our numerical findings are discussed in the context of Coronal seismology.
-
Wave Heating in Simulated Multistranded Coronal Loops
The Astrophysical Journal, 2019Co-Authors: Ming-zhe Guo, T Van Doorsselaere, K KarampelasAbstract:It has been found that the Kelvin-Helmholtz instability (KHI) induced by both transverse and torsional oscillations in Coronal Loops can reinforce the effects of wave heating. In this study, we model a Coronal loop as a system of individual strands, and we study wave heating effects by considering a combined transverse and torsional driver at the loop footpoint. We deposit the same energy into the multi-stranded loop and an equivalent monolithic loop, and then observe a faster increase in the internal energy and temperature in the multi-stranded model. Therefore, the multi-stranded model is more efficient in starting the heating process. Moreover, higher temperature is observed near the footpoint in the multi-stranded loop and near the apex in the monolithic loop. The apparent heating location in the multi-stranded loop agrees with the previous predictions and observations. Given the differences in the results from our multi-stranded loop and monolithic loop simulations, and given that Coronal Loops are suggested to be multi-stranded on both theoretical and observational grounds, our results suggest that the multi-strandedness of Coronal Loops needs to be incorporated in future wave-based heating mechanisms.
-
wave heating in gravitationally stratified Coronal Loops in the presence of resistivity and viscosity
arXiv: Solar and Stellar Astrophysics, 2019Co-Authors: K Karampelas, T. Van Doorsselaere, Ming-zhe GuoAbstract: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.
-
heating effects from driven transverse and alfven waves in Coronal Loops
The Astrophysical Journal, 2019Co-Authors: T Van Doorsselaere, Patrick Antoli, K Karampelas, Ming-zhe Guo, Ineke De MoortelAbstract:Recent numerical studies revealed that transverse motions of Coronal Loops can induce the Kelvin–Helmholtz instability (KHI). This process could be important in Coronal heating because it leads to dissipation of energy at small spatial scale plasma interactions. Meanwhile, small-amplitude decayless oscillations in Coronal Loops have been discovered recently in observations of SDO/AIA. We model such oscillations in Coronal Loops and study wave heating effects, considering a kink and Alfven driver separately and a mixed driver at the bottom of flux tubes. Both the transverse and Alfven oscillations can lead to the KHI. Meanwhile, the Alfven oscillations established in Loops will experience phase mixing. Both processes will generate small spatial scale structures, which can help the dissipation of wave energy. Indeed, we observe the increase of internal energy and temperature in loop regions. The heating is more pronounced for the simulation containing the mixed kink and Alfven driver. This means that the mixed wave modes can lead to a more efficient energy dissipation in the turbulent state of the plasma and that the KHI eddies act as an agent to dissipate energy in other wave modes. Furthermore, we also obtained forward-modeling results using the FoMo code. We obtained forward models that are very similar to the observations of decayless oscillations. Due to the limited resolution of instruments, neither Alfven modes nor the fine structures are observable. Therefore, this numerical study shows that Alfven modes probably can coexist with kink modes, leading to enhanced heating.
L A Fisk - One of the best experts on this subject based on the ideXlab platform.
-
the open magnetic flux of the sun i transport by reconnections with Coronal Loops
The Astrophysical Journal, 2005Co-Authors: L A FiskAbstract:The magnetic field of the Sun includes a component that opens into and creates the heliospheric magnetic field, the so-called open magnetic flux of the Sun. Open flux is transported along the solar surface by random convective motions in the photosphere and also by reconnection with Coronal Loops and the subsequent displacement of the open field lines. A model is developed to describe the interactions through reconnection of open magnetic field lines and smaller Coronal Loops on the quiet Sun. The displacement of open field lines by reconnection with such Loops can be described as a diffusive process, and the model yields the appropriate diffusion coefficient. The model also relates the total diffusive transport of open flux, as a result of reconnection with Loops and random convective motions, to a basic solar parameter, the rate of emergence of magnetic flux per unit surface area. This in turn provides an explanation for the formation of Coronal holes: accumulations of open flux. The diffusion process will tend to accumulate open flux into regions where the rate of emergence of new magnetic flux is a local minimum. It is also shown that the parameters used in this model are consistent with and can interrelate a variety of different observations. This work serves as the basis for subsequent papers in which the distribution of open magnetic flux on the Sun and its evolution during the solar cycle is determined, and the transport of open magnetic flux is related to the heating and acceleration of the solar wind.
-
acceleration of the solar wind as a result of the reconnection of open magnetic flux with Coronal Loops
Journal of Geophysical Research, 2003Co-Authors: L A FiskAbstract:[1] There are compelling observations of a clear anticorrelation between solar wind flow speed and Coronal electron temperature, as determined from solar wind ionic charge states. A simple theory is presented which can account for these observations, including the functional form of the correlation: Solar wind flow speed squared varies essentially linearly as the inverse of the Coronal electron temperature. In this theory, magnetic field lines in the corona that open into the heliosphere reconnect with Coronal Loops near their base. This process displaces the open field line and disturbs and imparts energy into the overlying corona, thereby determining the Poynting vector into the corona. This process releases mass from the loop into the corona and determines the mass flux of the solar wind. The Poynting vector and mass flux into the corona determine the final speed of the solar wind and yield a relationship that provides an excellent fit to observations. The reconnection of open field lines with Coronal Loops, and their subsequent displacements, also results in a diffusive transport of open field lines, which influences the configuration of the heliospheric magnetic field.