J. Kraskiewicz

Context. This study was performed in the context of the chromosphere and low corona heating. Aims. We considered the evolution of driven Alfvén waves in the solar atmosphere, whose initial state was modeled with a realistic ionization profile. We discuss their potential role in plasma heating. Methods. Two- and half-dimensional (2.5 D) numerical simulations of the solar atmosphere were performed with the use of the code JOANNA. The dynamics of the atmosphere was described by the two-fluid equations (and ionization and recombination terms were taken into account) for ions (protons) combined with electrons and neutrals (hydrogen atoms). The initial atmosphere was described by a hydrostatic background supplemented by the Saha equation and embedded in a magnetic flux tube. This background was perturbed by a monochromatic driver that operated in the transversal components of the ion and neutral velocities. Results. We demonstrated that as a result of ion-neutral collisions, Alfvén waves are damped. The damping length grows with the wave period. This is anticipated based on the linear theory for a homogeneous medium. The developed flux-tube model results at higher temperatures rise for shorter periods of the driving wave, and the effect is stronger than for the pure hydrostatic state. Conclusions. We highlight the importance of taking the realistic background plasma into account in the evolution of two-fluid Alfvén waves that participate in the thermal energy release and in the heating in the solar chromosphere and low corona.

We present new results towards the explanation of the chromospheric-heating problem and the solar-wind origin, using a two-fluid model that takes into account the collisional interaction between ions (protons) and neutrals (hydrogen atoms). Our aim is to further reveal the mechanism behind chromospheric heating and plasma outflows. We simulate and analyse the propagation and evolution of Alfvén waves in the partially ionised solar chromosphere, consisting of ions + electrons and neutral fluids. The simplified model chromosphere is permeated by a vertical, uniform magnetic field. We perform numerical simulations in the framework of a quasi-1.5-dimensional (1.5D), two-fluid model in which Alfvén waves are excited by a harmonic driver in the transverse component of the ion and neutral velocities, operating in the chromosphere. In the case of a small-amplitude driver, Alfvén waves are weakly damped, and for the chosen wave periods of a few seconds, Alfvén waves manage to propagate through the chromosphere and enter the solar corona. Non-linear Alfvén waves excited by a large-amplitude driver cause significant chromospheric heating and plasma outflows. We thus conclude that two-fluid Alfvén waves with larger amplitudes can contribute to chromospheric heating and plasma outflows, which may result higher up in the solar-wind origin.

We study vertical variations of wave periods of magnetoacoustic two-fluid waves in the partially ionized lower solar atmosphere, consisting of ion (proton) + electron and neutral (atomic hydrogen) fluids, which are coupled by ion–neutral collisions. The study allows finding the wave period cut-offs and their variations in the solar atmosphere, as well as establishing the role of these cut-offs in determining the wave propagation conditions. The atmosphere is permitted by a uniform vertical magnetic field. We perform numerical simulations in the framework of a one-dimensional (1D), two-fluid model in which plane waves are exited by a harmonic driver in the vertical ion and neutral velocities, operating at the bottom of the solar photosphere. We observe excitation of waves with cut-off wave periods in addition to waves set directly by the driver. We also see that some waves exited by that driver can reach the solar corona. Despite of its limitations such as the lack of non-adiabatic and non-ideal terms and a simple 1D structure, the developed two-fluid model of the solar atmosphere sheds a new light on the role of cut-offs in setting up the wave propagation conditions in the solar atmosphere and finding periods of waves that may carry their energy from the solar surface to the corona.

We study numerically the propagation of small-amplitude acoustic-gravity waves in a quiet region of the gravitationally stratified solar atmosphere. These waves are excited by a localized in-space periodic driver which acts in the vertical component of velocity at the bottom of the photosphere. Our aim is to study these waves and their period variations in space in 2D medium approximation. We collect the wave signals along few spatial directions to mimic line-of-sight (LOS) effects. The results of numerical simulations we performed reveal that the detected wave periods depend strongly on the LOS directions; above the driver, along the vertical direction, only acoustic waves with their periods smaller than about 300 s are free to propagate, while obliquely to the gravity action longer-period gravity waves can penetrate a quiet region of the solar atmosphere.

We perform numerical simulations of magnetoacoustic waves (MAWs) in the solar atmosphere, which is gravitationally stratified and structured by either vertical or horizontal uniform magnetic fields. These waves are excited by a monochromatic driver that operates in the photosphere. We show that the gradients of the atmospheric parameters lead to filtering of the waves through the solar atmosphere and to variations of the dominant wave period with height. We use these variations to determine a local cutoff period, which shows a good agreement with the previously obtained analytical and numerical results in an isothermal solar atmosphere. In our numerical simulations, the propagation of MAWs in a more realistic model of the solar atmosphere is considered, and the obtained results demonstrate that the waves with periods higher than a local cutoff wave period are strongly reflected and become evanescent with height, while the waves with shorter wave periods are propagating, and may even reach the solar corona. Some of the evanescent waves may also tunnel and reach the atmospheric heights that would not be otherwise accessible to them. An important result of our study is excitation of chromospheric oscillations with periods equal to the period that is comparable to the observed solar chromospheric oscillations. Implications of our theoretical predictions are discussed.

We study numerically the propagation of magnetoacoustic-gravity waves in a quiet region of the solar atmosphere which is either magnetic free or permeated by a weak magnetic field, B. In the latter case we consider the following configurations of B: (a) horizontal; (b) vertical. The waves are excited by a driver which mimics, resulting from granulation, stochastic flow that acts at the bottom of the photosphere. Our aim is to study periods of the waves generated by this driver and their variations with height. We find that in the magnetic-free atmosphere acoustic-gravity waves are generated, while in the magnetic case magnetoacoustic-gravity waves are excited. In all cases our numerical results show that waveperiods follow approximately the trend of analytical findings for linear waves and recent observational data.

Aims. Propagation and energy transfer of torsional Alfvén waves in solar magnetic flux tubes of axial symmetry is studied. Methods. An analytical model of a solar magnetic flux tube of axial symmetry is developed by specifying a magnetic flux and deriving general analytical formulas for the equilibrium mass density and gas pressure. The main advantage of this model is that it can be easily adopted to any axisymmetric magnetic structure. The model is used to numerically simulate the propagation of nonlinear Alfvén waves in such 2D flux tubes of axial symmetry embedded in the solar atmosphere. The waves are excited by a localized pulse in the azimuthal component of velocity and launched at the top of the solar photosphere, and they propagate through the solar chromosphere, the transition region, and into the solar corona.Results. The results of our numerical simulations reveal a complex scenario of twisted magnetic field lines and flows associated with torsional Alfvén waves, as well as energy transfer to the magnetoacoustic waves that are triggered by the Alfvén waves and are akin to the vertical jet flows. Alfvén waves experience about 5% amplitude reflection at the transition region. Magnetic (velocity) field perturbations that experience attenuation (growth) with height agree with analytical findings. The kinetic energy of magnetoacoustic waves consists of 25% of the total energy of Alfvén waves. The energy transfer may lead to localized mass transport in the form of vertical jets, as well as to localized heating because slow magnetoacoustic waves are prone to dissipation in the inner corona.

Aims. We construct a new analytical model of a solar coronal loop that is embedded in a gravitationally stratified and magnetically confined atmosphere. On the basis of this analytical model, we devise a numerical model of solar coronal loops. We adopt this model to perform the numerical simulations of its vertical kink oscillations excited by an external driver. Methods. Our model of the solar atmosphere is constructed by adopting a realistic temperature distribution and specifying the curved magnetic field lines that constitute a coronal loop. This loop is described by 2D, ideal magnetohydrodynamic equations that are numerically solved by the FLASH code. Results. The vertical kink oscillations are excited by a periodic driver in the vertical component of velocity, which is acting at the top of the photosphere. For this forced driver with its amplitude 3 km s-1, the excited oscillations exhibit about 1.2 km s-1 amplitude in their velocity and the loop apex oscillates with an amplitude in displacement of about 100 km. Conclusions. The newly devised analytical model of the coronal loops is utilized for the numerical simulations of the vertical kink oscillations, which match well with the recent observations of decayless kink oscillations excited in solar loops. The model will have further implications on the study of waves and plasma dynamics in coronal loops, revealing physics of energy and mass transport mechanisms in the localized solar atmosphere.