New Data and Models of Running Penumbral Waves in Sunspots

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Abstract
We present new data on running penumbral waves (RPWs) obtained by appliction of special filtering techniques to high-resolution CCD images. We find for the first time that the waves decelerate from an initial velocity of 25 to around 10 km s-1 at the outer edge and are azimuthally symmetric. They are more or less circular and are only seen in symmetric, radial sectors of sunspot penumbrae. We did not see any outward decrease in amplitude. If present, it is not large. There appears to be a connection between umbral flashes and RPWs, but we cannot define it with the present data. We present a geometric model for the waves, which fits the observed deceleration. In this model, RPWs are spherical waves expanding from a subsurface region on the axis of the spot. A good fit is obtained with a depth of 15,000 km and a vertical velocity of 4.5 km s-1. This model predicts a lag in the wave arrival at successive heights. For the single spot for which we had good data in three wavelengths, we found an appropriate lag between the waves at Hα ± 0.6 Å and Hα. Therefore, we propose a spherical wave geometric model for RPWs, although we have no physical model.

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