Internal gravity waves in the presence of magnetic fields

Internal gravity waves (IGWs) are buoyancy-driven waves common in the Earth’s atmosphere and oceans. IGWs have also been observed in the Sun’s atmosphere and are thought to play an important role in the overall dynamics of the solar atmosphere.

Fig. 1: Top panel show snapshots of the bolometric intensity (in grayscale) and absolute magnetic field strength (colored) from four different magnetohydrodynamic simulation of solar near-surface representing different regions on the solar surface. The leftmost plot shows a magnetic field-free model, the next three plots show magnetic models with an initial, vertical, homogeneous magnetic field of 10, 50, and 100 G flux density, respectively. The bottom panel shows the velocity-velocity phase spectra between z=100 km and 140 km corresponding to the models shown above. The characteristic feature of upward propagating gravity waves can be identified from the downward (negative) phase difference seen as green-blue area below the propagation boundary of IGWs (lower dashed curve). We clearly see that the phase spectra of IGWs in the lower layers are the same independent of the average magnetic flux density of the region.

Using state-of-the-art computer simulations, we study the generation and propagation of IGWs in model solar atmosphere representing different regions, like internetwork, network, and plage. We find that, in the near surface layers, the emergent IGW spectra is unaffected by the presence as well as by the strength of the magnetic field (see Fig. 1). IGWs are generated with considerable amount of wave flux independent of the average value of the magnetic flux density, in other words their generation is independent of whether it is completely field-free, internetwork, network, or plage region. However, as they propagate into higher layers in the atmosphere, they are strongly affected by the presence of magnetic field and show differences in behaviour – making them a potential diagnostic tool for determining the average magnetic field properties of upper photospheric layers.

Ref: Vigeesh, G.; Roth, M.; Steiner, O.; Jackiewicz, J. 2019, ApJ, 872, 166   doi: 10.3847/1538-4357/ab020c