Aims. We aim to study the formation and evolution of solar spicules using numerical simulations of a vertical velocity pulse that is launched from the upper chromosphere. Methods. With the use of the PLUTO code, we numerically solved adiabatic and non-adiabatic magnetohydrodynamic (MHD) equations in 2D cylindrical geometry. We followed the evolution of spicules triggered by pulses that are launched in a vertical velocity component from the upper chromosphere. Then we compared the results obtained with and without non-adiabatic terms in the MHD equations. Results. Our numerical results reveal that the velocity pulse is steepened into a shock that propagates upward into the corona. The chromospheric cold and dense plasma follows the shock and rises into the corona with the mean speed of 20-25 km s −1 . The nonlinear wake behind the pulse in the stratified atmosphere leads to quasi-periodic rebound shocks, which lead to quasi-periodic rising of chromospheric plasma into the corona with a period close to the acoustic cut-off period of the chromosphere. We found that the effect of non-adiabatic terms on spicule evolution is minor; the general properties of spicules such as their heights and rising-time remain slightly affected by these terms. Conclusions. In the framework of the axisymmetric model we devised, we show that the solar spicules can be triggered by the vertical velocity pulses, and thermal conduction and radiative cooling terms do not exert any significant influence on the dynamics of these spicules.
Recently Wiśniewska et al. demonstrated observationally how the acoustic cutoff frequency varies with height in the solar atmosphere including the upper photosphere and the lower and middle chromosphere, and showed that the observational results cannot be accounted for by the existing theoretical formulas for the acoustic cutoff. In order to reproduce the observed variation of the cutoff with atmospheric height, numerical simulations of impulsively generated acoustic waves in the solar atmosphere are performed, and the spectral analysis of temporal wave profiles is used to compute numerically changes of the acoustic cutoff with height. Comparison of the numerical results with the observational data shows good agreement, which clearly indicates that the obtained results may be used to determine the structure of the background solar atmosphere.
We observe plasma flows in cool loops using the Slit-Jaw Imager on board the Interface Region Imaging Spectrometer (IRIS). Huang et al. observed unusually broadened Si IV 1403 Åline profiles at the footpoints of such loops that were attributed to signatures of explosive events (EEs). We have chosen one such unidirectional flowing cool-loop system observed by IRIS where one of the footpoints is associated with significantly broadened Si IV line profiles. The line-profile broadening indirectly indicates the occurrence of numerous EEs below the transition region (TR), while it directly infers a large velocity enhancement/perturbation, further causing the plasma flows in the observed loop system. The observed features are implemented in a model atmosphere in which a low-lying bipolar magnetic field system is perturbed in the chromosphere by a velocity pulse with a maximum amplitude of 200 km s −1 . The data-driven 2D numerical simulation shows that the plasma motions evolve in a similar manner as observed by IRIS in the form of flowing plasma filling the skeleton of a cool-loop system. We compare the spatio-temporal evolution of the cool-loop system in the framework of our model with the observations, and conclude that their formation is mostly associated with the velocity response of the transient energy release above their footpoints in the chromosphere/TR. Our observations and modeling results suggest that the velocity responses most likely associated to the EEs could be one of the main candidates for the dynamics and energetics of the flowing cool-loop systems in the lower solar atmosphere.
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.
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