Tracking Sunspot Waves Across the Solar Disk using IRIS

Waves can be observed in the transition region and upper chromosphere of sunspots. Two particular phenomena, running waves and umbral flashes, can be seen in the 1400 Å and 2796 Å bandpasses, respectively, on slit-jaw images from the Interface Region Imaging Spectrograph (IRIS). How running waves propagate through the solar atmosphere is still a topic of debate revolving around two theories. The trans-sunspot theory explains that these waves propagate radially across the sunspot from its center just as they appear visually. The upward propagating theory illustrates that these waves are tied to the magnetic field lines and are therefore slow magnetoacoustic waves, indicating that the observed apparent motion of the waves is not real. Previous studies focused on individual observations at a fixed viewing angle, leaving the results susceptible to line-of-sight bias. To test for this bias, we observed the leading sunspots of AR 11836 and AR 12546 as they traveled from the disk center to the western limb. To assess the two theories, we applied global wavelet analysis to our high-pass filtered slit-jaw images and were able to find the spatial distribution of dominate wave periods for both phenomena. We then compared the period distribution to magnetic field line inclinations from the Helioseismic and Magnetic Imager (HMI). From this, we find a clear correlation between magnetic field line inclination and wave period as both increase outward from the sunspot center. The period increases from 180 s from the sunspot center to 240 s near the edge of the penumbra for all of our viewing angles, nullifying the existence of a line-of-sight bias and lending credence to the upward propagating theory. The importance of discovering how these waves are moving through the solar atmosphere could provide a mechanism for transporting local seismic energy from the photosphere to the corona.