Waves in the Magnetized Solar Atmosphere. I. Basic Processes and Internetwork Oscillations

Abstract

We have modeled numerically the propagation of waves through magnetic structures in a stratiÐed atmosphere. We Ðrst simulate the propagation of waves through a number of simple, exemplary Ðeld geometries in order to obtain a better insight into the e †ect of di †ering Ðeld structures on the wave speeds, amplitudes, polarizations, direction of propagation, etc., with a view to understanding the wide variety of wavelike and oscillatory processes observed in the solar atmosphere. As a particular example, we then apply the method to oscillations in the chromospheric network and internetwork. We Ðnd that in regions where the Ðeld is signiÐcantly inclined to the vertical, refraction by the rapidly increasing phase speed of the fast modes results in total internal reÑection of the waves at a surface whose altitude is highly variable. We conjecture a relationship between this phenomenon and the observed spatiotemporal intermittancy of the oscillations. By contrast, in regions where the Ðeld is close to vertical, the waves continue to propagate upward, channeled along the Ðeld lines but otherwise largely una †ected by the Ðeld.

The magnetic field in the solar atmosphere

“…The height at which the magnetic forces start to dominate over others (i.e., where β 1) is expected to be strongly corrugated relative to the solar surface. In the QS, that height is expected to vary between ≈800 km and 1.6 Mm above the photosphere (Rosenthal et al 2002). In ARs, this height is likely to be lower, as shown by Metcalf et al (1995).…”

“…Quite some time after the first speculations on the height of canopy-type magnetic fields, observational evidence for the merging heights in plage of in the order of several hundreds of km has been delivered (Steiner and Pizzo 1989;Guenther and Mattig 1991;Bruls and Solanki 1995). Rosenthal et al (2002) performed numerical simulations of the propagation of waves through a model atmosphere, resembling properties of the chromospheric network and internetwork, and found the canopy height to vary between ≈800 km and 1.6 Mm above the base of the photosphere. However, a considerable number of findings, especially in the QS, led to serious doubts upon the reality of a large-scale, undisturbed magnetic canopy there (for details see Sect.…”

“…Another is that it is a horizontal boundary between the β > 1 and β < 1 field. Instead of vertically more or less well-divided domains of high or low β, such domains may well be mixed up, even "islands" may exist (Rosenthal et al 2002).…”

Dynamical Behavior of the Upper Solar Photosphere

“…Acoustic waves run upward and steepen into shocks as computed by Carlsson & Stein (1997). At the canopy they tend to be reflected and converted into magnetic modes as observed by McIntosh et al (2001) and computed by Rosenthal et al (2002). Spicules were entered with the separatrix geometry of van Ballegooijen & Nisenson (1999) in mind.…”

“…The spatially averaged temporal power spectrum at right shows a high wide peak representing the three-minute oscillation (often called "chromospheric" but it is mostly an under-the-canopy phenomenon, cf. Rosenthal et al 2002), which is resolved in p-mode ridges and pseudo-ridges at left. At slower periodicities there is a gradual increase toward much higher power.…”