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Coronal Oscillations above Sunspots: Why?

Data from various satellite missions concerning emission lines formed at temperatures of 7 x 10⁴K to 1.3 x 10⁵K show oscillations in sunspots with frequencies in the range of 5.8-7.8mHz. With the launch of SoHO there has been renewed interest in the study of umbral oscillations. All the observational data clearly indicate the presence of 3-minute oscillations in sunspots in spectral lines covering a vast temperature range from the low chromosphere to those lines normally associated with coronal temperatures. The question we address here is whether these `coronal lines' are actually formed around the temperature indicated by ionization equilibrium calculations. We show that after folding in the sunspot plume emission measure distribution and a non-Maxwellian electron distribution, the contribution functions for lines normally formed just below 1 x 10⁶K are shifted to lower temperatures. For example, the FeIX 171Å line is shifted to less than 5 x 10⁵K. Other lines such as MgIX 368Å will also be affected. These results question some previous work regarding the suggested detection of 3-minute oscillations in the corona above sunspots.