Author name code: vigeesh ADS astronomy entries on 2022-09-14 author:Vigeesh, Gangadharan ------------------------------------------------------------------------ Title: Acoustic-gravity wave propagation characteristics in 3D radiation hydrodynamic simulations of the solar atmosphere Authors: Fleck, Bernhard; Khomenko, Elena; Carlsson, Mats; Rempel, Matthias; Steiner, Oskar; Riva, Fabio; Vigeesh, Gangadharan Bibcode: 2022cosp...44.2503F Altcode: There has been tremendous progress in the degree of realism of three-dimensional radiation magneto-hydrodynamic simulations of the solar atmosphere in the past decades. Four of the most frequently used numerical codes are Bifrost, CO5BOLD, MANCHA3D, and MURaM. Here we test and compare the wave propagation characteristics in model runs from these four codes by measuring the dispersion relation of acoustic-gravity waves at various heights. We find considerable differences between the various models. Title: Absorption of High-frequency Oscillations and Its Relation to Emissivity Reduction Authors: Waidele, Matthias; Roth, Markus; Vigeesh, Gangadharan; Glogowski, Kolja Bibcode: 2021ApJ...913..108W Altcode: 2021arXiv210601745W Sunspots are known to be strong absorbers of solar oscillation modal power. The most convincing way to demonstrate this is done via Fourier-Hankel decomposition (FHD), where the local oscillation field is separated into in- and outgoing waves, showing the reduction in power. Due to the Helioseismic and Magnetic Imager's high-cadence Doppler measurements, power absorption can be investigated at frequencies beyond the acoustic cutoff frequency. We perform an FHD on five sunspot regions and two quiet-Sun control regions and study the resulting absorption spectra αℓ(ν), specifically at frequencies ν > 5.3 mHz. We observe an unreported high-frequency absorption feature, which only appears in the presence of a sunspot. This feature is confined to phase speeds of one-skip waves whose origins coincide with the sunspot's center, with vph = 85.7 km s-1 in this case. By employing a fit to the absorption spectra at a constant phase speed, we find that the peak absorption strength ${\alpha }_{\max }$ lies between 0.166 and 0.222 at a noise level of about 0.009 (5%). The well-known absorption along ridges at lower frequencies can reach up to ${\alpha }_{\max }\approx 0.5$ . Thus our finding in the absorption spectrum is weaker, but nevertheless significant. From first considerations regarding the energy budget of high-frequency waves, this observation can likely be explained by the reduction of emissivity within the sunspot. We derive a simple relation between emissivity and absorption. We conclude that sunspots yield a wave power absorption signature (for certain phase speeds only), which may help in understanding the effect of strong magnetic fields on convection and source excitation and potentially in understanding the general sunspot subsurface structure. Title: On the effect of oscillatory phenomena on Stokes inversion results Authors: Keys, P. H.; Steiner, O.; Vigeesh, G. Bibcode: 2021RSPTA.37900182K Altcode: 2020arXiv200805539K Stokes inversion codes are crucial in returning properties of the solar atmosphere, such as temperature and magnetic field strength. However, the success of such algorithms to return reliable values can be hindered by the presence of oscillatory phenomena within magnetic wave guides. Returning accurate parameters is crucial to both magnetohydrodynamics (MHD) studies and solar physics in general. Here, we employ a simulation featuring propagating MHD waves within a flux tube with a known driver and atmospheric parameters. We invert the Stokes profiles for the 6301 Å and 6302 Å line pair emergent from the simulations using the well-known Stokes Inversions from Response functions code to see if the atmospheric parameters can be returned for typical spatial resolutions at ground-based observatories. The inversions return synthetic spectra comparable to the original input spectra, even with asymmetries introduced in the spectra from wave propagation in the atmosphere. The output models from the inversions match closely to the simulations in temperature, line-of-sight magnetic field and line-of-sight velocity within typical formation heights of the inverted lines. Deviations from the simulations are seen away from these height regions. The inversions results are less accurate during passage of the waves within the line formation region. The original wave period could be recovered from the atmosphere output by the inversions, with empirical mode decomposition performing better than the wavelet approach in this task.

This article is part of the Theo Murphy meeting issue `High-resolution wave dynamics in the lower solar atmosphere'. Title: Acoustic-gravity wave propagation characteristics in three-dimensional radiation hydrodynamic simulations of the solar atmosphere Authors: Fleck, B.; Carlsson, M.; Khomenko, E.; Rempel, M.; Steiner, O.; Vigeesh, G. Bibcode: 2021RSPTA.37900170F Altcode: 2020arXiv200705847F There has been tremendous progress in the degree of realism of three-dimensional radiation magneto-hydrodynamic simulations of the solar atmosphere in the past decades. Four of the most frequently used numerical codes are Bifrost, CO5BOLD, MANCHA3D and MURaM. Here we test and compare the wave propagation characteristics in model runs from these four codes by measuring the dispersion relation of acoustic-gravity waves at various heights. We find considerable differences between the various models. The height dependence of wave power, in particular of high-frequency waves, varies by up to two orders of magnitude between the models, and the phase difference spectra of several models show unexpected features, including ±180° phase jumps.

This article is part of the Theo Murphy meeting issue `High-resolution wave dynamics in the lower solar atmosphere'. Title: On the influence of magnetic topology on the propagation of internal gravity waves in the solar atmosphere Authors: Vigeesh, G.; Roth, M.; Steiner, O.; Fleck, B. Bibcode: 2021RSPTA.37900177V Altcode: 2020arXiv201006926V The solar surface is a continuous source of internal gravity waves (IGWs). IGWs are believed to supply the bulk of the wave energy for the lower solar atmosphere, but their existence and role for the energy balance of the upper layers is still unclear, largely due to the lack of knowledge about the influence of the Sun's magnetic fields on their propagation. In this work, we look at naturally excited IGWs in realistic models of the solar atmosphere and study the effect of different magnetic field topographies on their propagation. We carry out radiation-magnetohydrodynamic simulations of a magnetic field free and two magnetic models-one with an initial, homogeneous, vertical field of 100 G magnetic flux density and one with an initial horizontal field of 100 G flux density. The propagation properties of IGWs are studied by examining the phase-difference and coherence spectra in the kh - ω diagnostic diagram. We find that IGWs in the upper solar atmosphere show upward propagation in the model with predominantly horizontal field similar to the model without magnetic field. In contrast to that the model with predominantly vertical fields show downward propagation. This crucial difference in the propagation direction is also revealed in the difference in energy transported by waves for heights below 0.8 Mm. Higher up, the propagation properties show a peculiar behaviour, which require further study. Our analysis suggests that IGWs may play a significant role in the heating of the chromospheric layers of the internetwork region where horizontal fields are thought to be prevalent.

This article is part of the Theo Murphy meeting issue `High-resolution wave dynamics in the lower solar atmosphere'. Title: Interaction of Magnetic Fields with a Vortex Tube at Solar Subgranular Scale Authors: Fischer, C. E.; Vigeesh, G.; Lindner, P.; Borrero, J. M.; Calvo, F.; Steiner, O. Bibcode: 2020ApJ...903L..10F Altcode: 2020arXiv201005577F Using high-resolution spectropolarimetric data recorded with the Swedish 1 m Solar Telescope, we have identified several instances of granular lanes traveling into granules. These are believed to be the observational signature of underlying tubes of vortical flow with their axis oriented parallel to the solar surface. Associated with these horizontal vortex tubes, we detect in some cases a significant signal in linear polarization, located at the trailing dark edge of the granular lane. The linear polarization appears at a later stage of the granular lane development, and is flanked by patches of circular polarization. Stokes inversions show that the elongated patch of linear polarization signal arises from the horizontal magnetic field aligned with the granular lane. We analyze snapshots of a magnetohydrodynamic numerical simulation and find cases in which the horizontal vortex tube of the granular lane redistributes and transports the magnetic field to the solar surface causing a polarimetric signature similar to what is observed. We thus witness a mechanism capable of transporting magnetic flux to the solar surface within granules. This mechanism is probably an important component of the small-scale dynamo supposedly acting at the solar surface and generating the quiet-Sun magnetic field. Title: Synthetic observations of internal gravity waves in the solar atmosphere Authors: Vigeesh, G.; Roth, M. Bibcode: 2020A&A...633A.140V Altcode: 2019arXiv191206435V
Aims: We study the properties of internal gravity waves (IGWs) detected in synthetic observations that are obtained from realistic numerical simulation of the solar atmosphere.
Methods: We used four different simulations of the solar magneto-convection performed using the CO5BOLD code. A magnetic-field-free model and three magnetic models were simulated. The latter three models start with an initial vertical, homogeneous field of 10, 50, and 100 G magnetic flux density, representing different regions of the quiet solar surface. We used the NICOLE code to compute synthetic spectral maps from all the simulated models for the two magnetically insensitive neutral iron lines Fe I λλ 5434 Å and 5576 Å. We carried out Fourier analyses of the intensity and Doppler velocities to derive the power, phase, and coherence in the kh - ω diagnostic diagram to study the properties of internal gravity waves.
Results: We find the signatures of the internal gravity waves in the synthetic spectra to be consistent with observations of the real Sun. The effect of magnetic field on the wave spectra is not as clearly discernible in synthetic observations as in the case of numerical simulations. The phase differences obtained using the spectral lines are significantly different from the phase differences in the simulation. The phase coherency between two atmospheric layers in the gravity wave regime is height dependent and is seen to decrease with the travel distance between the observed layers. In the studied models, the lower atmosphere shows a phase coherency above the significance level for a height separation of ∼400 km, while in the chromospheric layers it reduces to ∼100-200 km depending on the average magnetic flux density. Conclusion. We conclude that the energy flux of IGWs determined from the phase difference analysis may be overestimated by an order of magnitude. Spectral lines that are weak and less temperature sensitive may be better suited to detecting internal waves and accurately determining their energy flux in the solar atmosphere. Title: Internal Gravity Waves in the Magnetized Solar Atmosphere. II. Energy Transport Authors: Vigeesh, G.; Roth, M.; Steiner, O.; Jackiewicz, J. Bibcode: 2019ApJ...872..166V Altcode: 2019arXiv190108871V In this second paper of the series on internal gravity waves (IGWs), we present a study of the generation and propagation of IGWs in a model solar atmosphere with diverse magnetic conditions. A magnetic field-free and three magnetic models that start with an initial, vertical, homogeneous field of 10, 50, and 100 G magnetic flux density, are simulated using the CO5BOLD code. We find that the IGWs are generated in similar manner in all four models in spite of the differences in the magnetic environment. The mechanical energy carried by IGWs is significantly larger than that of the acoustic waves in the lower part of the atmosphere, making them an important component of the total wave energy budget. The mechanical energy flux (106-103 W m-2) is a few orders of magnitude larger than the Poynting flux (103-101 W m-2). The Poynting fluxes show a downward component in the frequency range corresponding to the IGWs, which confirm that these waves do not propagate upward in the atmosphere when the fields are predominantly vertical and strong. We conclude that, in the upper photosphere, the propagation properties of IGWs depend on the average magnetic field strength and therefore these waves can be potential candidates for magnetic field diagnostics of these layers. However, their subsequent coupling to Alfvénic waves is unlikely in a magnetic environment permeated with predominantly vertical fields, and therefore they may not directly or indirectly contribute to the heating of layers above plasma-β less than 1. Title: On the effect of vorticity on the propagation of internal gravity waves. Authors: Vigeesh, G.; Steiner, O.; Calvo, F.; Roth, M. Bibcode: 2017MmSAI..88...54V Altcode: We compare different models of solar surface convection to study vorticity and how it can influence the propagation of internal gravity waves. We conclude that simulations performed with higher grid resolution may have a reduced gravity wave flux in the lower part of the atmosphere due to strong vorticity. We also show that the vertical extent of the allowed region of propagation depends on the magnetic field inclination. Title: Internal Gravity Waves in the Magnetized Solar Atmosphere. I. Magnetic Field Effects Authors: Vigeesh, G.; Jackiewicz, J.; Steiner, O. Bibcode: 2017ApJ...835..148V Altcode: 2016arXiv161204729V Observations of the solar atmosphere show that internal gravity waves are generated by overshooting convection, but are suppressed at locations of magnetic flux, which is thought to be the result of mode conversion into magnetoacoustic waves. Here, we present a study of the acoustic-gravity wave spectrum emerging from a realistic, self-consistent simulation of solar (magneto)convection. A magnetic field free, hydrodynamic simulation and a magnetohydrodynamic (MHD) simulation with an initial, vertical, homogeneous field of 50 G flux density were carried out and compared with each other to highlight the effect of magnetic fields on the internal gravity wave propagation in the Sun’s atmosphere. We find that the internal gravity waves are absent or partially reflected back into the lower layers in the presence of magnetic fields and argue that the suppression is due to the coupling of internal gravity waves to slow magnetoacoustic waves still within the high-β region of the upper photosphere. The conversion to Alfvén waves is highly unlikely in our model because there is no strongly inclined magnetic field present. We argue that the suppression of internal waves observed within magnetic flux concentrations may also be due to nonlinear breaking of internal waves due to vortex flows that are ubiquitously present in the upper photosphere and the chromosphere. Title: CO5BOLD for MHD: progresses and deficiencies . Authors: Steiner, O.; Calvo, F.; Salhab, R.; Vigeesh, G. Bibcode: 2017MmSAI..88...37S Altcode: The magnetohydrodynamics module of CO5BOLD has been steadily improved over the past decade and has been used for various solar and stellar physical applications. We give an overview of current work with it and of remaining and newly emerged shortcomings. Title: Gravity waves in magnetized solar atmospheres from MHD simulations. Authors: Jackiewicz, Jason; Vigeesh, Gangadharan Bibcode: 2014AAS...22412350J Altcode: The solar atmosphere is believed to be a region where gravity waves are generated and propagate, but a variety of effects makes observations of them rather difficult. Measurements of gravity wave properties could, however, show how they play an important role in the upper photosphere and chromosphere and even deposit energy there. Here we show how analysis of gravity waves from detailed numerical simulations can be used to study magnetic fields and energy deposition in the atmosphere, and how mode conversion to slow magneto-acoustic waves changes their observable properties. Title: Seismology of Small-Scale Magnetic Features using Numerical Simulation Authors: Vigeesh, G.; Jackiewicz, J. Bibcode: 2013ASPC..478..259V Altcode: We present results of 3D magnetohydrodynamic simulations as part of a preliminary study aimed at understanding the interaction of seismic waves with small-scale magnetic flux concentrations on the Sun. A model solar atmosphere without magnetic fields (“Quiet Sun” model) and a model with magnetic fields are constructed. We consider the solar surface-gravity waves (f-mode) that are naturally excited in both the simulated models and use them to measure travel times between different locations on the surface. When compared to the field-free simulation, we observe that a strong f-mode scattering occurs in the presence of magnetic fields and results in travel-time shifts. With the help of realistic numerical simulations, we show the seismic influence of small-scale magnetic features in travel-time differences and its possible effects on the helioseismic measurements of the structure and dynamics of the solar interior. Title: Acoustic emission from magnetic flux tubes in the solar network Authors: Vigeesh, G.; Hasan, S. S. Bibcode: 2013JPhCS.440a2045V Altcode: 2013arXiv1304.5193V We present the results of three-dimensional numerical simulations to investigate the excitation of waves in the magnetic network of the Sun due to footpoint motions of a magnetic flux tube. We consider motions that typically mimic granular buffeting and vortex flows and implement them as driving motions at the base of the flux tube. The driving motions generates various MHD modes within the flux tube and acoustic waves in the ambient medium. The response of the upper atmosphere to the underlying photospheric motion and the role of the flux tube in channeling the waves is investigated. We compute the acoustic energy flux in the various wave modes across different boundary layers defined by the plasma and magnetic field parameters and examine the observational implications for chromospheric and coronal heating. Title: First steps with CO5BOLD using HLLMHD and PP reconstruction . Authors: Steiner, O.; Rajaguru, S. P.; Vigeesh, G.; Steffen, M.; Schaffenberger, W.; Freytag, B. Bibcode: 2013MSAIS..24..100S Altcode: We report on first experiences with real-life applications using the MHD-module of CO5BOLD together with the piecewise parabolic reconstruction scheme and present preliminary results of stellar magnetic models with Teff = 4000 K to Teff = 5770 K. Title: Three-dimensional Simulations of Magnetohydrodynamic Waves in Magnetized Solar Atmosphere Authors: Vigeesh, G.; Fedun, V.; Hasan, S. S.; Erdélyi, R. Bibcode: 2012ApJ...755...18V Altcode: 2011arXiv1109.6471V We present results of three-dimensional numerical simulations of magnetohydrodynamic (MHD) wave propagation in a solar magnetic flux tube. Our study aims at understanding the properties of a range of MHD wave modes generated by different photospheric motions. We consider two scenarios observed in the lower solar photosphere, namely, granular buffeting and vortex-like motion, among the simplest mechanism for the generation of waves within a strong, localized magnetic flux concentration. We show that granular buffeting is likely to generate stronger slow and fast magnetoacoustic waves as compared to swirly motions. Correspondingly, the energy flux transported differs as a result of the driving motions. We also demonstrate that the waves generated by granular buffeting are likely to manifest in stronger emission in the chromospheric network. We argue that different mechanisms of wave generation are active during the evolution of a magnetic element in the intergranular lane, resulting in temporally varying emission at chromospheric heights. Title: Stokes Diagnostics of Magneto-Acoustic Wave Propagation in the Magnetic Network on the Sun Authors: Vigeesh, G.; Steiner, O.; Hasan, S. S. Bibcode: 2011SoPh..273...15V Altcode: 2011SoPh..tmp..349V; 2011arXiv1104.4069V The solar atmosphere is magnetically structured and highly dynamic. Owing to the dynamic nature of the regions in which the magnetic structures exist, waves can be excited in them. Numerical investigations of wave propagation in small-scale magnetic flux concentrations in the magnetic network on the Sun have shown that the nature of the excited modes depends on the value of plasma β (the ratio of gas to magnetic pressure) where the driving motion occurs. Considering that these waves should give rise to observable characteristic signatures, we have attempted a study of synthesised emergent spectra from numerical simulations of magneto-acoustic wave propagation. We find that the signatures of wave propagation in a magnetic element can be detected when the spatial resolution is sufficiently high to clearly resolve it, enabling observations in different regions within the flux concentration. The possibility to probe various lines of sight around the flux concentration bears the potential to reveal different modes of the magnetohydrodynamic waves and mode conversion. We highlight the feasibility of using the Stokes-V asymmetries as a diagnostic tool to study the wave propagation within magnetic flux concentrations. These quantities can possibly be compared with existing and new observations in order to place constraints on different wave excitation mechanisms. Title: Wave propagation and energy transport in the magnetic network of the Sun Authors: Vigeesh, G.; Hasan, S. S.; Steiner, O. Bibcode: 2009A&A...508..951V Altcode: 2009arXiv0909.2325V Aims. We investigate wave propagation and energy transport in magnetic elements, which are representatives of small scale magnetic flux concentrations in the magnetic network on the Sun. This is a continuation of earlier work by Hasan et al. (2005, ApJ, 631, 1270). The new features in the present investigation include a quantitative evaluation of the energy transport in the various modes and for different field strengths, as well as the effect of the boundary-layer thickness on wave propagation.
Methods: We carry out 2D MHD numerical simulations of magnetic flux concentrations for strong and moderate magnetic fields for which β (the ratio of gas to magnetic pressure) on the tube axis at the photospheric base is 0.4 and 1.7, respectively. Waves are excited in the tube and ambient medium by a transverse impulsive motion of the lower boundary.
Results: The nature of the modes excited depends on the value of β. Mode conversion occurs in the moderate field case when the fast mode crosses the β = 1 contour. In the strong field case the fast mode undergoes conversion from predominantly magnetic to predominantly acoustic when waves are leaking from the interior of the flux concentration to the ambient medium. We also estimate the energy fluxes in the acoustic and magnetic modes and find that in the strong field case, the vertically directed acoustic wave fluxes reach spatially averaged, temporal maximum values of a few times 106 erg cm-2 s-1 at chromospheric height levels.
Conclusions: The main conclusions of our work are twofold: firstly, for transverse, impulsive excitation, flux tubes/sheets with strong fields are more efficient than those with weak fields in providing acoustic flux to the chromosphere. However, there is insufficient energy in the acoustic flux to balance the chromospheric radiative losses in the network, even for the strong field case. Secondly, the acoustic emission from the interface between the flux concentration and the ambient medium decreases with the width of the boundary layer. Title: Numerical simulation of wave propagation in magnetic network Authors: Vigeesh, G.; Hasan, S. S.; Steiner, O. Bibcode: 2009IAUS..257..185V Altcode: We present 2-D numerical simulations of wave propagation in the magnetic network. The network is modelled as consisting of individual magnetic flux sheets located in intergranular lanes. They have a typical horizontal size of about 150 km at the base of the photosphere and expand upward and become uniform. We consider flux sheets of different field strengths. Waves are excited by means of transverse motions at the lower boundary, to simulate the effect of granular buffeting. We look at the magneto-acoustic waves generated within the flux sheet and the acoustic waves generated in the ambient medium due to the excitation. We calculate the wave energy fluxes separating them into contributions from the acoustic and the Poynting part and study the effect of the different field strengths. Title: Numerical simulation of wave propagation in the presence of a magnetic flux sheet Authors: Vigeesh, G.; Steiner, O.; Hasan, S. S. Bibcode: 2008ESPM...12.3.24V Altcode: We model network magnetic fields as consisting of individual magnetic flux sheets located in intergranular lanes. With a typical horizontal size of about 150 km at the base of the photosphere, they expand upward and merge with their neighbors at a height of about 600 km. Above a height of approximately 1000 km the magnetic field starts to become uniform. Granular buffeting is thought to excite waves in this medium, which is modeled by means of transversal motions at the lower boundary. The transverse driving, generates both fast and slow waves within the flux sheet and acoustic waves in the ambient medium. We consider flux sheets of different field strengths and different boundary-layer widths. Separating the energy flux of the waves into contributions due to the acoustic flux and the Poynting flux, we show the longitudinal and transversal components of both and study their temporal evolution. Title: First local helioseismic experiments with CO5BOLD Authors: Steiner, O.; Vigeesh, G.; Krieger, L.; Wedemeyer-Böhm, S.; Schaffenberger, W.; Freytag, B. Bibcode: 2007AN....328..323S Altcode: 2007astro.ph..1029S With numerical experiments we explore the feasibility of using high frequency waves for probing the magnetic fields in the photosphere and the chromosphere of the Sun. We track a plane-parallel, monochromatic wave that propagates through a non-stationary, realistic atmosphere, from the convection-zone through the photosphere into the magnetically dominated chromosphere, where it gets refracted and reflected. We compare the wave travel time between two fixed geometrical height levels in the atmosphere (representing the formation height of two spectral lines) with the topography of the surface of equal magnetic and thermal energy density (the magnetic canopy or β=1 contour) and find good correspondence between the two. We conclude that high frequency waves indeed bear information on the topography of the `magnetic canopy'. Title: Wave Propagation in the Magnetic Network on the Sun Authors: Hasan, S. S.; Vigeesh, G.; van Ballegooijen, A. A. Bibcode: 2006IAUS..233..116H Altcode: Hasan et al. (2005) have recently presented 2-D dynamical calculations on wave propagation in in the magnetic network of the Sun. The latter is idealized as consisting of non-potential flux tubes in the quiet solar chromosphere. It is of interest to understand how the nature of wave propagation is influenced by the choice of initial equilibrium configuration of the magnetic field. We examine this by comparing the earlier calculations with those when the network is modelled as a potential structure. Our calculations demonstrate that the nature of the wave propagation is significantly different, particularly the transport of energy which for the potential case, occurs more isotropically than for the non-potential configuration.