Abstract
Coronal fan loops rooted in sunspot umbra show outward propagating waves with subsonic phase speed and a period around 3 min. However, their source region in the lower atmosphere is still ambiguous. We performed multi-wavelength observations of a clean fan loop system rooted in a sunspot utilizing data from the Interface Region Imaging Spectrograph (IRIS) and the Solar Dynamics Observatory (SDO). We explored the less studied property of frequency modulation of these 3-min waves from the photosphere to the corona, and identified periodicities in the ranges of 14-20 min, and 24-35 min. Based on our findings, we interpret that the 3-min slow waves observed in the coronal fan loops are driven by 3-min oscillations observed at the photospheric footpoints of these fan loops in the umbral region. We also investigated possible connections between the 3-min and 5-min oscillations observed at the photosphere, which are still poorly understood. Our results provide clear evidence of magnetic coupling of the solar umbral atmosphere through the propagation of 3-min waves along the fan loops at different atmospheric heights.
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