Nonlinear sunspot transition region oscillations in NOAA 8378

Observations obtained with the Solar and Heliospheric Observatory, joint observing program for velocity fields in sunspot regions are used to study the 3 minute transition region oscillations above four sunspots. Here we report mainly on the results obtained for NOAA 8378, based on simultaneous recordings of the transition region lines O v lambda 629 and N v lambda lambda 1238, 1242 and the chromospheric Si II lambda 1260 line with the Solar Ultraviolet Measurements of Emitted Radiation instrument. The 3 minute transition region oscillations in NOAA 8378 occur mainly above the umbra and show (1) larger peak line intensity amplitudes than reported before, (2) clear signs of nonlinearities, (3) significant oscillations in line width, (4) maxima in peak line intensity and maxima in velocity directed toward the observer that are nearly, but not exactly in phase, and (5) a clear connection to the oscillations in the sunspot chromosphere. The suggestion that the waves are upward-propagating acoustic waves is confronted with the observations by a simple test.