Parker Lecture: Waves in the Magnetized Solar Atmosphere

Over the last few decades there has been tremendous progress in determining the detailed structure of solar and stellar interiors and envelopes through the observation and interpretation of the properties of p-modes (helioseismology) and g-modes (asteroseismology). These low-frequency modes derive from the broadband `noise' emitted by the star's turbulent convection zone. The high-frequency tail of the convective acoustic emission is not trapped within the stellar envelope, but is instead able to venture out into the optically thin atmosphere. There these waves encounter, amongst other things, the ambient magnetic field (also a by product of the turbulent convection). Close to the stellar surface, where the magnetic field is weak in the sense that the magnetic pressure is small compared to the thermal pressure, or equivalently, the Alfvén speed is much less than the sound speed, the high-frequency acoustic waves propagate freely with little regard for the magnetic field. However, at sufficiently high altitudes these waves will encounter surfaces where the two pressures and characteristic propagation speeds become comparable. In passing through these canopy or equipartition surfaces the incident acoustic waves are transformed into roughly equal amounts of the three magneto-acoustic gravity (MAG) waves. The transmitted MAG waves propagate at different phase speeds and along distinct trajectories through the overlying magneto-atmosphere. They leave distinct imprints on absorption line profiles and the continuum emission, and pave the way for seismology of the solar, and perhaps even stellar, atmospheres.