No abstract

The existence of the Sun’s hot atmosphere and the solar wind acceleration continues to be an outstanding problem in solar-astrophysics. Although magnetohydrodynamic (MHD) modes and dissipation of magnetic energy contribute to heating and the mass cycle of the solar atmosphere, yet direct evidence of such processes often generates debate. Ground-based 1-m Swedish Solar Telescope (SST)/CRISP, Hα 6562.8 Å observations reveal, for the first time, the ubiquitous presence of high frequency (~12–42 mHz) torsional motions in thin spicular-type structures in the chromosphere. We detect numerous oscillating flux tubes on 10 June 2014 between 07:17 UT to 08:08 UT in a quiet-Sun field-of-view of 60” × 60” (1” = 725 km). Stringent numerical model shows that these observations resemble torsional Alfvén waves associated with high frequency drivers which contain a huge amount of energy (~105 W m−2) in the chromosphere. Even after partial reflection from the transition region, a significant amount of energy (~103 W m−2) is transferred onto the overlying corona. We find that oscillating tubes serve as substantial sources of Alfvén wave generation that provide sufficient Poynting flux not only to heat the corona but also to originate the supersonic solar wind.

Abstract. We consider slow magnetosonic standing waves that are impulsively excited in a solar coronal loop. The onedimensional numerical model we implement includes the effects of nonlinearity, optionally thermal conduction, heating, and cooling of the solar plasma. We numerically evaluate excitation and damping times of a standing wave in hot coronal loops on the basis of a parametric study. Results of the numerical simulations reveal that initially launched impulses mainly trigger the fundamental mode and its first harmonic, depending on the location of these pulses in space. Parametric study shows that these standing waves are excited in a dozen or so wave periods corresponding roughly to 13 min and that they are strongly damped over a similar time-scale.

Context. We study the upward propagation of a localized velocity pulse that is initially launched below the transition region within the solar atmosphere. The pulse quickly steepens into a shock, which may lead to the formation of spicules. Aims. We aim to explore the spicule formation scenario in the framework of the rebound shock model. Methods. We solve two-dimensional time-dependent magnetohydrodynamic equations numerically to find spatial and temporal dynamics of spicules.Results. The numerical simulations show that the strong initial pulse may lead to the quasi periodic rising of chromospheric material into the lower corona in the form of spicules. The periodicity results from the nonlinear wake that is formed behind the pulse in the stratified atmosphere. The superposition of rising and falling off plasma portions resembles the time sequence of single and double (sometimes even triple) spicules, which is consistent with observational findings. Conclusions. The two-dimensional rebound shock model may explain the observed speed, width, and heights of type I spicules, as well as observed multi-structural and bi-directional flows. The model also predicts the appearance of spicules with 3-5 min period due to the consecutive shocks.

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 Å) 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.

Aims. The main aims of the paper are to carry out numerical simulations of the vertical oscillations in a coronal loop in order to determine their dependence on various parameters and to compare them with recent TRACE observations. Methods. We consider impulsively generated oscillations in a solar coronal arcade loop. The two-dimensional numerical model we implement in the ideal MHD regime includes the effects of nonlinearity and line curvature. We perform parametric studies by varying both the position and the width/strength of the pulse. Results. A pulse launched below a loop is in general found to excite multiple wave modes, in particular a vertical oscillation with many properties of a kink mode, fast mode oscillations and a slow mode pulse (or two slow mode pulses, depending on the location of the original pulse). From our parametric studies we deduce that wave periods and attenuation times of the excited waves depend on the position below the loop summit, as well as on the width of the pulse. Wider pulses launched closer to a foot-point and to the loop's apex trigger wave packets of longer period waves which are more strongly attenuated. A perturbed loop does not return to its initial state but is instead stretched, with its apex shifted upwards. As a result the perturbations propagate along the stretched loop and consequently stronger and wider pulses which stretch a loop more lead to longer period oscillations. A pulse located near one of the foot-points is found to excite a distortion mode leading to asymmetric oscillations which are distinct from the vertical or horizontal kink modes that have been identified in TRACE data.

Aims. We aim to analytically and numerically explore small-amplitude Alfvén waves in the solar atmosphere. Methods. We transform the wave equations to obtain the cutoff-frequency and wave travel time for strictly linear Alfvén waves. The wave equations are solved numerically to find out spatial and temporal signatures of the waves. Results. The analytical predictions are verified by numerically solving the wave equations for linear Alfvén waves. The waves are impulsively generated and their characteristics and behavior in the solar atmosphere are investigated by the numerical simulations. The derived cutoff-frequency is used to determine regions in the solar atmosphere where strong reflection occurs for Alfvén waves of different frequencies. Conclusions. The numerical results reveal that impulsively generated small-amplitude waves exhibit characteristic spatial and temporal signatures which agree with the predictions of the analytical theory.

Aims. We study wave leakage as a possible attenuation mechanism of coronal loop oscillations in the ideal MHD regime. Methods. We consider impulsively generated oscillations in solar coronal magnetic wave guides such as a straight slab and a curved arcade loop. The two-dimensional numerical model we implement includes the effects of nonlinearity and line curvature on attenuation of fast magnetosonic kink waves. Results. We show that these waves are more strongly attenuated in the arcade loop than in the slab and provide evidence that the curvature of magnetic field lines results in excess energy leakage. For parameters appropriate for a coronal loop the kink oscillation is too efficiently attenuated by energy leakage, suggesting that in the solar atmosphere wave leakage must be reduced compared to our simulations. We conclude that energy leakage is an efficient source of attenuation of coronal loop oscillations.