K. Murawski

Numerical simulations of solar macrospicules

Context. We consider a localized pulse in the component of velocity, parallel to the ambient magnetic field lines, that is initially launched in the solar chromosphere. Aims. We aim to generalize our recent numerical model of spicule formation by implementing a VAL-C model of solar temperature. Methods. With the use of the code FLASH we solve two-dimensional ideal magnetohydrodynamic equations numerically to simulate the solar macrospicules. Results. Our numerical results reveal that the pulse located below the transition region triggers plasma perturbations, which exhibit many features of macrospicules. We also present an observational (SDO/AIA 304 A) case study of the macrospicule that approximately mimics the numerical simulations. Conclusions. In the frame of the model we devised, the solar macrospicules can be triggered by velocity pulses launched from the chromosphere.

The Kinematics and Plasma Properties of a Solar Surge Triggered by Chromospheric Activity in AR11271

We observe a solar surge in NOAA AR11271 using the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly 304 A image data on 2011 August 25. The surge rises vertically from its origin up to a height of ≈65 Mm with a terminal velocity of ≈100 km s–1, and thereafter falls and fades gradually. The total lifetime of the surge was ≈20 minutes. We also measure the temperature and density distribution of the observed surge during its maximum rise and find an average temperature and a density of 2.0 MK and 4.1 × 109 cm–3, respectively. The temperature map shows the expansion and mixing of cool plasma lagging behind the hot coronal plasma along the surge. Because SDO/HMI temporal image data do not show any detectable evidence of significant photospheric magnetic field cancellation for the formation of the observed surge, we infer that it is probably driven by magnetic-reconnection-generated thermal energy in the lower chromosphere. The radiance (and thus the mass density) oscillations near the base of the surge are also evident, which may be the most likely signature of its formation by a reconnection-generated pulse. In support of the present observational baseline of the triggering of the surge due to chromospheric heating, we devise a numerical model with conceivable implementation of the VAL-C atmosphere and a thermal pulse as an initial trigger. We find that the pulse steepens into a slow shock at higher altitudes which triggers plasma perturbations exhibiting the observed features of the surge, e.g., terminal velocity, height, width, lifetime, and heated fine structures near its base.

Observations of a pulse-driven cool polar jet by SDO/AIA

Context. We observe a solar jet at north polar coronal hole (NPCH) using SDO AIA 304 A image data on 3 August 2010. The jet rises obliquely above the solar limb and then retraces its propagation path to fall back. Aims. We numerically model this solar jet by implementing a realistic (VAL-C) model of solar temperature. Methods. We solve two-dimensional ideal magnetohydrodynamic equations numerically to simulate the solar jet. We consider a localized velocity pulse that is essentially parallel to the background magnetic field lines and is initially launched at the top of the solar photosphere. The pulse steepens into a shock at higher altitudes, which triggers plasma perturbations that exhibit the observed features of the jet. The typical direction of the pulse also clearly exhibits the leading front of the moving jet. Results. Our numerical simulations reveal that a large amplitude initial velocity pulse launched at the top of the solar photosphere in general produces the observed properties of the jet, e.g., upward and backward average velocities, height, width, life-time, and ballistic nature. Conclusions. The close match between the jet observations and numerical simulations provides a first strong evidence that this jet is formed by a single velocity pulse. The strong velocity pulse is most likely generated by the low-atmospheric reconnection in the polar region, which triggers the jet. The downflowing material of the jet most likely is absorbed in the next upcoming velocity pulses from the lower solar atmosphere, and because of that we only see a single jet moving upward in the solar atmosphere.

Pulse-driven non-linear Alfvén waves and their role in the spectral line broadening

We study the impulsively generated non-linear Alfv´ en waves in the solar atmosphere and describe their most likely role in the observed non-thermal broadening of some spectral lines in solar coronal holes. We solve numerically the time-dependent magnetohydrodynamic equations to find temporal signatures of large-amplitude Alfv´ en waves in the solar atmosphere model of open and expanding magnetic field configuration, with a realistic temperature distribution. We calculate the temporally and spatially averaged, instantaneous transversal velocity of non-linear Alfv´ en waves at different heights of the model atmosphere and estimate its contribution to the unresolved non-thermal motions caused by the waves. We find that the

Torsional Alfvén waves in solar magnetic flux tubes of axial symmetry

Aims. Propagation and energy transfer of torsional Alfven 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 Alfven 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 Alfven waves, as well as energy transfer to the magnetoacoustic waves that are triggered by the Alfven waves and are akin to the vertical jet flows. Alfven 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 Alfven 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.

ORIGIN OF MACROSPICULE AND JET IN POLAR CORONA BY A SMALL-SCALE KINKED FLUX TUBE

We report an observation of a small-scale flux tube that undergoes kinking and triggers the macrospicule and a jet on 2010 November 11 in the north polar corona. The small-scale flux tube emerged well before the triggering of the macrospicule and as time progresses the two opposite halves of this omega-shaped flux tube bent transversely and approach each other. After ~2 minutes, the two approaching halves of the kinked flux tube touch each other and an internal reconnection as well as an energy release takes place at the adjoining location and a macrospicule was launched which goes up to a height of 12 Mm. Plasma begins to move horizontally as well as vertically upward along with the onset of the macrospicule and thereafter converts into a large-scale jet in which the core denser plasma reaches up to ~40 Mm in the solar atmosphere with a projected speed of ~95 km s–1. The fainter and decelerating plasma chunks of this jet were also seen up to ~60 Mm. We perform a two-dimensional numerical simulation by considering the VAL-C initial atmospheric conditions to understand the physical scenario of the observed macrospicule and associated jet. The simulation results show that reconnection-generated velocity pulse in the lower solar atmosphere steepens into slow shock and the cool plasma is driven behind it in the form of macrospicule. The horizontal surface waves also appeared with shock fronts at different heights, which most likely drove and spread the large-scale jet associated with the macrospicule.

Fast Magnetic Twister and Plasma Perturbations in a Three-dimensional Coronal Arcade

We present results of three-dimensional (3D) numerical simulations of a fast magnetic twister excited above a foot-point of the potential solar coronal arcade that is embedded in the solar atmosphere with the initial VAL-IIIC temperature profile, which is smoothly extended into the solar corona. With the use of the FLASH code, we solve 3D ideal magnetohydrodynamic equations by specifying a twist in the azimuthal component of magnetic field in the solar chromosphere. The imposed perturbation generates torsional Alfven waves as well as plasma swirls that reach the other foot-point of the arcade and partially reflect back from the transition region. The two vortex channels are evident in the generated twisted flux-tube with a fragmentation near its apex which results from the initial twist as well as from the morphology of the tube. The numerical results are compared to observational data of plasma motions in a solar prominence. The comparison shows that the numerical results and the data qualitatively agree even though the observed plasma motions occur over comparatively large spatio-temporal scales in the prominence.

Numerical simulations of magnetoacoustic oscillations in a gravitationally stratified solar corona

Aims. We consider magnetoacoustic oscillations in a gravitationally stratified solar corona, that are triggered by an initial pulse in the vertical component of velocity launched from various altitudes of the solar atmosphere. Methods. We numerically solve two-dimensional magnetohydrodynamic equations for an ideal plasma to determine the spatial and temporal signatures of excited oscillations. Results. Our numerical results reveal that few-min oscillations are effectively excited by the initial velocity pulses and that their waveperiods depend on the vertical location and amplitude of the pulse. Conclusions. The building block of this scenario consists of a one-dimensional rebound shock model.

Numerical simulations of the attenuation of the fundamental slow magnetoacoustic standing mode in a gravitationally stratified solar coronal arcade

Aims. We aim to explore the influence of thermal conduction on the attenuation of the fundamental standing slow magnetoacoustic mode in a two-dimensional (2D) potential arcade that is embedded in a gravitationally stratified solar corona. Methods. We numerically solve the time-dependent magnetohydrodynamic equations to find the spatial and temporal signatures of the mode. Results. We find that this mode is strongly attenuated on a time-scale of about 6 waveperiods. Conclusions. The effect of non-ideal plasma such as thermal conduction is to enhance the attenuation of slow standing waves. The numerical results are similar to previous observational data and theoretical findings for the one-dimensional plasma.

Three-dimensional numerical simulation of magnetohydrodynamic-gravity waves and vortices in the solar atmosphere

With the adaptation of the FLASH code, we simulate magnetohydrodynamic-gravity waves and vortices as well as their response in the magnetized three-dimensional (3D) solar atmosphere at different heights to understand the localized energy transport processes. In the solar atmosphere, strongly structured by gravitational and magnetic forces, we launch a localized velocity pulse (in horizontal and vertical components) within a bottom layer of 3D solar atmosphere modelled by initial VAL-IIIC conditions, which triggers waves and vortices. The rotation direction of vortices depends on the orientation of an initial perturbation. The vertical driver generates magnetoacoustic-gravity waves which result in oscillations of the transition region, and it leads to the eddies with their symmetry axis oriented vertically. The horizontal pulse excites all magnetohydrodynamic-gravity waves and horizontally oriented eddies. These waves propagate upwards, penetrate the transition region and enter the solar corona. In the high-beta plasma regions, the magnetic field lines move with the plasma and the temporal evolution show that they swirl with eddies. We estimate the energy fluxes carried out by the waves in the magnetized solar atmosphere and conclude that such wave dynamics and vortices may be significant in transporting the energy to sufficiently balance the energy losses in the localized corona. Moreover, the structure of the transition region highly affects such energy transports and causes the channelling of the propagating waves into the inner corona.