Detection of Fast-Moving Waves Propagating from Penumbra to Outside of Sunspots in the Photosphere

Abstract:

It is well known that different types of magnetohydrodynamic waves and
oscillations exist in and above sunspots from the photospheric level
through the chromosphere to the corona. The SDO/HMI provides continuous
observation of the Sun with a high spatial resolution and temporal
cadence, and this allows us to monitor the wave activities around sunspot
areas in the photospheric level with an unprecedented quality. Using
time-distance helioseismic analysis technique, we identify fast-moving
waves traveling along the sunspot's radial direction from the sunspot
penumbra to approximately 30 megameters outside of the sunspot. The
apparent speed of the wave is about 45 km/s, far exceeding the typical
acoustic wave speed in the photospheric level, and also faster than
the penumbral waves observed in the chromosphere. The dominant frequency
of this newly detected wave is about 3 mHz, falling into the category of
5-minute oscillation. The observation is compatible with a wave source
at a depth of about 8 megameters beneath the sunspot's surface, and
the above-described waves are expanding wavefronts sweeping across the
photosphere. We will discuss how this wave is linked to other waves
observed in the chromosphere and corona.