Observations of Umbral Flashes and Running Sunspot Waves With the Interface Region Imaging Spectrograph

Abstract

We present simultaneous, high-resolution, multi-temperature observations of running waves and umbral flashes in a sunspot with the Interface Region Imaging Spectrograph. We analyze intensity variations in slit-jaw images to investigate the relationship between running waves in the 1400 Å (Si iv/transition region) passband and umbral flashes in the 2796 Å (Mg ii/chromosphere) passband. Using global wavelet analysis, we find that the dominant wave periods increase from approximately 150 s near the sunspot center to about 300 s in the penumbra in both passbands. This coincides extremely well with the radially increasing inclination of magnetic field lines observed with Solar Dynamics Observatory/Helioseismic and Magnetic Imager, suggesting that the waves are propagating vertically along the inclined field lines. Furthermore, apparent horizontal velocities for running waves decrease from about 12 km s −1 in the umbra to about 4 km s −1 in the penumbra which is predicted by the same inclined field geometry. Finally, we find that umbral flashes lead running waves in both the spatial and temporal domains. The former result is attributable to the inclined field geometry; however, the geometry does not predict the radially increasing time lag which is likely due to the opacity difference between the emission lines that dominate the two passbands. These results suggest that running waves and umbral flashes both originate from photospheric p-mode oscillations and are manifestations of upward-propagating slow magnetoacoustic modes at different altitudes and temperatures. The apparent trans-sunspot propagation of running waves is not real but rather an image of these upward-propagating modes traveling along field lines of radially increasing inclination.

The transition region above sunspots

“…Tian et al (2014a) performed detailed analysis of the TR spectra and identified strong signals of shock waves in the TR above sunspots. IRIS observations of these shocks at different layers appear to be consistent with the scenario of upward propagating magnetoacoustic shock waves along inclined magnetic field lines (Madsen et al 2015).…”

The Frequency-dependent Damping of Slow Magnetoacoustic Waves in a Sunspot Umbral Atmosphere

“…Although the change in magnetic field inclination angles across the umbra has been shown to produce similar effects (Reznikova et al 2012;Madsen et al 2015), the restriction of this behavior in our current observations to chromospheric channels (i.e., excluding the transition region observations) is puzzling and demands further exploration.…”

Signatures of running penumbral waves in sunspot photospheres

“…Vice versa, the larger apparent velocities of around 30-70 km s −1 at photospheric layers fit well into the trend of decreasing horizontal velocities at the inner penumbra toward higher layers. The topological model of the sunspot's magnetic field (e.g., Westendorp Plaza et al 1997), the field-guided propagation of running penumbral waves, and their visual appearance at a certain layer can explain this behavior (Bloomfield et al 2007;Madsen et al 2015). In the penumbra, the magnetic field inclination increases radially from the vertical umbral field.…”