I. Ballai | The University of Sheffield (original) (raw)

Papers by I. Ballai

Frontiers in Astronomy and Space Sciences, Oct 12, 2023

The Astrophysical Journal, Aug 20, 2023

Bulletin of Taras Shevchenko National University of Kyiv. Astronomy

Methods of wave analysis of time series are analyzed in detail, namely: two-spacecraft phase diff... more Methods of wave analysis of time series are analyzed in detail, namely: two-spacecraft phase difference, beamforming, k-filtering, MSR (Multi-point Signal Resonator) technique. These multispacecraft methods allow to obtain a set of wave power peaks with different spatial scales at the same frequency, which is a significant advantage over the single-spacecraft approach due to existence a significant number of wave modes at the same frequency in space plasmas. The software implementation of these methods with their successful testing on artificial data is executed. The effectiveness of each method for detecting maxima in the power spectra of the magnetic field is demonstrated.

PADEU, Jun 1, 2004

Observations with the Extreme Ultraviolet Imaging Telescope (EIT) onboard SOHO have revealed the ... more Observations with the Extreme Ultraviolet Imaging Telescope (EIT) onboard SOHO have revealed the existence of transient coronal wave-like phenomena which are generated by an impulsive event and propagate across the visible solar disk for very long distances. Since these waves are able to carry information about the plasma in which they propagate, they are a perfect tool for coronal plasma diagnostics. We propose to study these waves to obtain information about the average values of the magnetic field and mean transport coefficients in the quiet Sun. The present contribution answers some of the 'hottest' questions related to the nature and proprieties of EIT waves.

The Astrophysical Journal

In this study, we explore the possibility of simplifying the modeling of magnetohydrodynamic slow... more In this study, we explore the possibility of simplifying the modeling of magnetohydrodynamic slow body modes observed in photospheric magnetic structures such as the umbrae of sunspots and pores. The simplifying approach assumes that the variation of the eigenvalues of slow body waves can be derived by imposing that the longitudinal component of velocity with respect to the tube axis is zero at the boundary of the magnetic flux tube, which is in good agreement with observations. To justify our approach, we compare the results of our simplified model for slow body modes in cylindrical flux tubes with the model prediction obtained by imposing the continuity of the radial component of the velocity and total pressure at the boundary of the flux tube. Our results show that, to a high accuracy (less than 1% for the considered model), the conditions of continuity of the component of transversal velocity and pressure at the boundary can be neglected when modeling slow body modes under photo...

In this paper we provide clear direct evidence of multiple concurrent higher order magnetohydrody... more In this paper we provide clear direct evidence of multiple concurrent higher order magnetohydrodynamic (MHD) modes in circular and elliptical sunspots by applying both Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) techniques on solar observational data. These techniques are well documented and validated in the areas of fluid mechanics, hydraulics, and granular flows, yet are relatively new to the field of solar physics. While POD identifies modes based on orthogonality in space and it provides a clear ranking of modes in terms of their contribution to the variance of the signal, DMD resolves modes that are orthogonal in time. The clear presence of the fundamental slow sausage and kink body modes, as well as higher order slow sausage and kink body modes have been identified using POD and DMD analysis of the chromospheric H$α$ line at 6562.808~Å for both the circular and elliptical sunspots. Additionally, to the various slow body modes, evidence for the pr...

The International Networking for Young Scientists (INYS), initiated and funded by the British Cou... more The International Networking for Young Scientists (INYS), initiated and funded by the British Council, models itself on the N+N concept, where N+N refers to a workshop involving a number of researchers from the UK and an equal number of local researchers; ideally eight or more from each country. INYS is an initiative that supports the British Council&amp;amp;amp;amp;amp;#39;s purpose of &amp;amp;amp;amp;amp;quot;nurturing

Astronomy & Astrophysics, 2021

Aims. In this paper, we aim to study the time dependence of sunspot group areas in a large sample... more Aims. In this paper, we aim to study the time dependence of sunspot group areas in a large sample composed of various databases spanning over 130 years, used state-of-the-art statistical methods. Methods. For a carefully selected but unbiased sample, we use Bayesian modelling to fit the temporal evolution of the combined umbral and penumbral area of spot groups with a skew-normal function to determine the existence of any asymmetry in spot growth or decay. Our primary selection criteria guaranteed that only spot groups with a well-defined maximum area were taken into account. We also analysed the covariance of the resulting model parameters and their correlations with the physical parameters of the sunspots and the ongoing solar cycle. Results. Our results show that the temporal evolution of well-observed sunspot groups that reach at least 50 millionths of a solar hemisphere at their maximum can be fitted surprisingly well with our model. Furthermore, we show significant asymmetry –...

Astronomy & Geophysics

Observations of resonant phenomena on the Sun are now being made that have their origins not only... more Observations of resonant phenomena on the Sun are now being made that have their origins not only in the interior, but also in structures in the tenuous solar atmosphere. The challenge for solar researchers is to reconcile the flood of observations and theory from these varied scales and locations to further improve our understanding of the Sun. Here, we look at some of the exciting challenges facing researchers delving into the seismology of the Sun.

This paper considers driven resonant nonlinear slow magnetohydrodynamic (MHD) waves in dissipativ... more This paper considers driven resonant nonlinear slow magnetohydrodynamic (MHD) waves in dissipative steady plasmas. A theory developed by Ruderman, Hollweg, and Goossens (1997) is used and extended to study the effect of steady flows on the nonlinear resonant behaviour of slow MHD waves in slow dissipative layers. The method of matched asymptotic expansions is used to describe the behaviour of the wave variables in the slow dissipative layer. The nonlinear analogue of the connection formulae for slow MHD waves obtained previously by Goossens, Hollweg, and Sakurai (1992) and Erdélyi (1997) in linear MHD, are derived. The effect of an equilibrium flow results partly in a Doppler shift of the available frequency for slow resonance and partly in the modification of the width of the dissipative layer.

Astronomy and Astrophysics, 2008

Aims. In the approximation of linear dissipative magnetohydrodynamics (MHD), it can be shown that... more Aims. In the approximation of linear dissipative magnetohydrodynamics (MHD), it can be shown that driven MHD waves in magnetic plasmas with high Reynolds number exhibit a near resonant behaviour if the frequency of the wave becomes equal to the local Alfvén (or slow) frequency of a magnetic surface. This behaviour is confined to a thin region, known as the dissipative layer, which embraces the resonant magnetic surface. Although driven MHD waves have small dimensionless amplitude far away from the resonant surface, this near-resonant behaviour in the dissipative layer may cause a breakdown of linear theory. Our aim is to study the nonlinear effects in Alfvén dissipative layer Methods. In the present paper, the method of simplified matched asymptotic expansions developed for nonlinear slow resonant waves is used to describe nonlinear effects inside the Alfvén dissipative layer. Results. The nonlinear corrections to resonant waves in the Alfvén dissipative layer are derived, and it is proved that at the Alfvén resonance (with isotropic/anisotropic dissipation) wave dynamics can be described by the linear theory with great accuracy.

Astronomy & Astrophysics, 2006

The spatial structuring of solar and space plasmas is known to have a dispersive effect on waves.... more The spatial structuring of solar and space plasmas is known to have a dispersive effect on waves. Many solar features possess a periodic structure with structures having alternating properties. Here the effect of periodic alternation of magnetic slabs on wave propagation is studied when the equilibrium has a steady motion (in line with observations). The dispersion relation for linear compressional waves is derived and analysed. The propagation of waves is studied, in particular modelling the cases of propagation in penumbral filamentary structures in the photosphere, the plume/interplume region and spaghetti structures in the solar wind. Depending on the width of structures (or the wavelength of oscillations) waves have different behaviour. It is shown that the strength of the equilibrium flow has a strong influence on the propagation speed and character of the wave.

Annales Geophysicae, 2002

Small-amplitude compressional magnetohydrodynamic-type waves are studied in the magnetosphere. Th... more Small-amplitude compressional magnetohydrodynamic-type waves are studied in the magnetosphere. The magnetosphere is treated as a rarefied plasma with anisotropy in the kinetic pressure distribution. The parallel and perpendicular pressures are defined by general polytropic pressure laws. This double-polytropic model can be considered as a natural extension of the magnetohydrodynamic (MHD) model when the plasma is collisionless. Generalized dispersion relations for surface and body waves are derived and analyzed for an isolated magnetic slab. The waves are confined to the slab. For specific polytropic indices, the results obtained in the (i) Chew-Goldberger-Low (CGL) double-adiabatic and (ii) double-isothermal approximations are recovered.

Advances in Space Research, 2019

Compressional waves propagating in the partially ionised solar lower atmospheric plasmas can easi... more Compressional waves propagating in the partially ionised solar lower atmospheric plasmas can easily steepen into nonlinear waves, including shocks. Here we investigate the effect of weak dispersion generated by Hall currents perpendicular to the ambient magnetic field on the characteristics of shock waves. Our study will also focus on the interplay between weak dispersion and partial ionisation of the plasma. Using a multiple scale technique we derive the governing equation in the form of a Korteweg-de Vries-Burgers equation. The effect of weak dispersion on shock waves is obtained using a perturbation technique. The secular behaviour of second order terms is addressed with the help of a renormalisation technique. Our results show that dispersion modifies the characteristics of shock waves and this change is dependent also on the ionisation degree of the plasma. Dispersion can create short lived oscillations in the shocked plasma. The shock fronts become wider with the increase in the number of neutrals in the plasma.

Linear and nonlinear resonant interaction of sound waves in dissipative layers

Astronomy & Astrophysics, 2001

Nonlinear resonant absorption of fast magnetoacoustic (FMA) waves in inhomogeneous weakly dissipa... more Nonlinear resonant absorption of fast magnetoacoustic (FMA) waves in inhomogeneous weakly dissipative, isotropic and anisotropic plasmas in static and steady equilibria is studied. Both isotropic and anisotropic plasmas are considered and for the background equilibrium state 1D planar static and steady models are used. The equilibrium configuration consists of three layers, where an inhomogeneous magnetised plasma slab is surrounded by two homogeneous magnetised semi-infinite plasma regions. The propagating FMA waves are partly absorbed due to coupling to local nonlinear slow magnetohydrodynamic (MHD) waves in the inhomogeneous layer, and are partly reflected. The coefficient of wave energy resonant absorption is derived using two simplifying assumptions (i) weak nonlinearity and (ii) the thickness of the inhomogeneous layer is small compared to the wavelength of the waves, i.e. the so-called long-wavelength approximation is used.

Astronomy & Astrophysics

This study deals with the dissipative instability that appears in a compressible partially ionise... more This study deals with the dissipative instability that appears in a compressible partially ionised plasma slab embedded in a uniform magnetic field, modelling the state of the plasma in solar prominences. In the partially ionised plasma, the dominant dissipative effect is the Cowling resistivity. The regions outside the slab (modelling the solar corona) are fully ionised, and the dominant mechanism of dissipation is viscosity. Analytical solutions to the extended magnetohydrodynamic (MHD) equations are found inside and outside of the slab and solutions are matched at the boundaries of the slab. The dispersion relation is derived and solutions are found analytically in the slender slab limit, while the conditions necessary for the appearance of the instability is investigated numerically for the entire parameter space. Our study is focussed on the effect of the compressibility on the generation and evolution of instabilities. We find that compressibility reduces the threshold of the ...

Philosophical Transactions of the Royal Society A, 2020

High-resolution solar observations show the complex structure of the magnetohydrodynamic (MHD) wa... more High-resolution solar observations show the complex structure of the magnetohydrodynamic (MHD) wave motion. We apply the techniques of proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) to identify the dominant MHD wave modes in a sunspot using the intensity time series. The POD technique was used to find modes that are spatially orthogonal, whereas the DMD technique identifies temporal orthogonality. Here, we show that the combined POD and DMD approaches can successfully identify both sausage and kink modes in a sunspot umbra with an approximately circular cross-sectional shape. This article is part of the Theo Murphy meeting issue ‘High-resolution wave dynamics in the lower solar atmosphere’.

arXiv: Solar and Stellar Astrophysics, 2020

The plasma dynamics at frequencies comparable with collisional frequency between various species ... more The plasma dynamics at frequencies comparable with collisional frequency between various species has to be described in multi-fluid framework, where collisional interaction between particles is an important ingredient. In our study we will assume that charged particles are strongly coupled, meaning that they form a single fluid that interacts with neutrals, therefore we will employ a two-fluid model. Here we aim to investigate the evolutionary equation of slow sausage waves propagating in a gravitationally stratified flux tube in the two-fluid solar atmosphere in a strongly ionised limit using an initial value analysis. Due to the collisional interaction between massive particles (ions and neutrals) the governing equations are coupled. Solutions are sought in the strongly ionised limit and the density ratio between neutrals and charged particles is a small parameter. This limit is relevant to the upper part of the chromosphere. Our results show that slow sausage waves associated wit...