Internal gravity waves in the solar atmosphere

The solar atmosphere shows a wide variety of wave phenomena. Among them are the bouyancy-driven internal gravity waves (IGWs) - a phenomenon common in the terrestrial atmosphere and oceans. We use realistic numerical simulations to study how these waves emanate from the solar surface and propagate through an unmagnetized or a magnetized atmosphere.

Velocity–velocity phase spectra estimated between z=140 km and z=600 km, for a magnetic field-free model (left panel) and a magnetic model with an initial, vertical, homogeneous magnetic field of 50 G flux density (right panel). The propagation of gravity waves can be identified as the blue area below the lower dashed curve that shows a phase difference of around −90° over a height difference of 460 km. In the magnetic model phase differences are significantly reduced.

In a recent paper (Vigeesh et al., 2017), we have shown that a considerable amount of internal waves are produced in the near surface layers, but as they propagate into higher layers these waves get modified if there are strong vertical magnetic fields present. A clear signature of the effect of magnetic fields can be seen in the velocity–velocity (v–v) phase lag between two heights in the atmosphere when examining the spectral region encompassing IGWs (see Fig. 1). The absence of IGWs when magnetic fields are present suggest that they are either mode-converted to other magneto-atmospheric waves or dissipate through interaction with vortical flows higher up in the atmosphere. This study has shown that IGWs are strongly affected by the magnetic fields present on the Sun, hinting at a possible indirect role of these waves in the energy budget of the upper atmosphere.

 

Ref: G. Vigeesh, J. Jackiewicz, & O. Steiner: 2017, ApJ, 835, 148