Synthetic observations of waves in the solar atmosphere

Numerical simulations provide great insight into the various wave phenomena that occur in the solar atmosphere. However, a proper comparison with the observations of the real Sun require us to understand how these phenomena affect spectral lines. Synthetic observables derived from numerical simulation are becoming necessary as they help us to properly interpret observational data.

(a) Power in the k-⍵ dispersion relation diagram of the line core intensity of the Fe I λ 5434 Å line. The phase (b) and coherence (c) spectra between the line-core intensity of the Fe I λ 5576 Å and Fe I λ 5434 Å lines are also shown. The white line marks the dispersion relation of the surface gravity waves (f-mode). The location of the IGWs can be seen by the coloured contour lines in (a)–(c) that mark the coherence levels with the reference colours shown in the colourmap of (c). (d) Power as a function of the horizontal wavenumber (k_h) integrated over the IGW region for the continuum intensity and line-core intensities of the Fe I λλ 5576 Å and 5434 Å lines. Also shown in black is the mean coherence in the IGW region with a distribution quite different from the power spectra.

We study the properties of internal gravity waves (IGWs) detected in synthetic observations that are obtained from realistic numerical simulation of the solar atmosphere. Simultaneous or quasi-simultaneous 2D narrowband imaging by scanning over a single line or multiple lines reveal the presence of IGWs in the solar atmosphere in the phase difference and coherence spectra. The coherent internal waves with their characteristic downward phases cannot be otherwise discerned in the power spectra (see Figure). 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. We find that the energy flux of IGWs determined from the phase difference analysis of observations may be overestimated by an order of magnitude. Our study shows that 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.

Reference: Vigeesh, G.; Roth, M.; 2020, A&A, 633, A140