Synthetic observations of internal gravity waves in the solar atmosphere

Abstract

Aims. We study the properties of internal gravity waves (IGWs) detected in synthetic observations that are obtained from realistic numerical simulation of the solar atmosphere. Methods. We used four different simulations of the solar magneto-convection performed using the CO 5 BOLD code. A magneticfield-free model and three magnetic models were simulated. The latter three models start with an initial vertical, homogeneous field of 10, 50, and 100 G magnetic flux density, representing different regions of the quiet solar surface. We used the NICOLE code to compute synthetic spectral maps from all the simulated models for the two magnetically insensitive neutral iron lines Fe i λλ 5434 Å and 5576 Å. We carried out Fourier analyses of the intensity and Doppler velocities to derive the power, phase, and coherence in the k h − ω diagnostic diagram to study the properties of internal gravity waves. Results. We find the signatures of the internal gravity waves in the synthetic spectra to be consistent with observations of the real Sun. The effect of magnetic field on the wave spectra is not as clearly discernible in synthetic observations as in the case of numerical simulations. The phase differences obtained using the spectral lines are significantly different from the phase differences in the simulation. The phase coherency between two atmospheric layers in the gravity wave regime is height dependent and is seen to decrease with the travel distance between the observed layers. In the studied models, the lower atmosphere shows a phase coherency above the significance level for a height separation of ∼400 km, while in the chromospheric layers it reduces to ∼100-200 km depending on the average magnetic flux density. Conclusions. We conclude that the energy flux of IGWs determined from the phase difference analysis may be overestimated by an order of magnitude. Spectral lines that are weak and less temperature sensitive may be better suited to detecting internal waves and accurately determining their energy flux in the solar atmosphere.

On the influence of magnetic topology on the propagation of internal gravity waves in the solar atmosphere

“…The phase difference and coherence spectra of the velocities at two separate layers are then computed from the complex cross-spectrum, 2 (k, ω), where v 1 represents the velocity at layer 1 and likewise for v 2 , and the overbar represents the complex conjugate (see eqns (4-6) of Vigeesh et al [5]). The confidence interval for the phase and coherence measurements and the zero coherence threshold are also computed as described in Vigeesh et al [7]. To study the energy transport by these waves, we estimate the mechanical energy flux spectra by computing the perturbed pressure-velocity ( p − v) co-spectrum represented on the k h -ω dispersion relation diagram.…”

“…Low-frequency ultraviolet (UV) brightness fluctuations observed in the internetwork region of the low solar chromosphere are believed to be caused by IGWs dissipating in the higher layers [2]. However, the strong effects of magnetic field orientation (attack angle) on the propagation of IGW have been demonstrated by Newington & Cally [3,4] and realistic numerical simulations have shown that the magnetic field present in the solar atmosphere influence the propagation of these waves and likely hinder them from propagating into the upper atmosphere [5][6][7]. At this point, it is still unclear what role the magnetic field plays in the propagation of IGWs into higher layers and how this fits with the brightness fluctuations seen in chromosphere.…”

High-resolution wave dynamics in the lower solar atmosphere

“…Likewise, the link between brightness fluctuations in internetwork regions and internal gravity waves is not fully resolved. Using three-dimensional simulations of the near-surface obtained with the CO 5 BOLD code [37], Vigeesh et al [38] find that in low layers the magnetic field does not play a significant role in the propagation properties of internal gravity waves. However, the authors find that in higher layers (approx.…”

“…In relatively weak magnetic regions of the lower solar atmosphere, gravity waves with frequencies shorter than ≲2 mHz (periods ≳500 s) have been identified in both observations and simulations [30–33]. Such waves have been found to have larger energy flux, by approximately one order of magnitude, compared to co-spatial acoustic waves [34], and thus can potentially contribute towards heating of the chromosphere and beyond.…”

A new method for detecting solar atmospheric gravity waves

“…IGWs have also been proposed as an agent for the mechanical heating of stellar atmospheres and coronae [10]. More recent studies [11][12][13] show the diagnostic potential of the IGWs for magnetic field in the solar atmosphere at different heights above the solar surface.…”