Association of chromospheric sunspot umbral oscillations and running penumbral waves I. Morphological study

Observations of a sunspot region located near the center of the solar disk were obtained on October 3, 1994, with the Multichannel Subtractive Double Pass Spectrograph (MSDP). This instrument, operating in Hα, was installed at the focus of the VTT at Tenerife (Canary Islands) and provided Hα intensity profiles at every pixel of the field of view. Reconstruction of the Hα profile allowed the computation of two dimensional intensity and Doppler velocity images at different wavelengths within the line. We analyse a time series of 1 hour and 8 min, obtained with a cadence of 36 sec and investigate the relation between umbral oscillations and running penumbral waves. The Doppler velocity as a function of time, along radial cuts through the center of the spot, shows several clear cases where waves that originate inside the umbra continue to propagate in the penumbra. In one case we were able to follow the evolution of an oscillating element for 216 sec, from the inner part of the umbra to the penumbra and we describe the propagation characteristics. We confirm the close association between sunspot oscillations and running penumbral waves and suggest that they are probably due to the same resonator.