Signatures of running penumbral waves in sunspot photospheres

Abstract

Context. The highly dynamic atmosphere above sunspots exhibits a wealth of magnetohydrodynamic waves. Recent studies suggest a coupled nature of the most prominent phenomena: umbral flashes and running penumbral waves (RPWs). Aims. From an observational point of view, we perform a height-dependent study of RPWs, compare their wave characteristics, and aim to track down these so far only chromospherically observed phenomena to photospheric layers to prove the upward propagating field-guided nature of RPWs. Methods. We analyze a time series (58 min) of multiwavelength observations of an isolated circular sunspot (NOAA11823) taken at high spatial and temporal resolution in spectroscopic mode with the Interferometric BIdimensional Spectro-polarimeter (IBIS/DST). By means of a multilayer intensity sampling, velocity comparisons, wavelet power analysis, and sectorial studies of time slices, we retrieve the power distribution, characteristic periodicities, and propagation characteristics of sunspot waves at photospheric and chromospheric levels. Results. Signatures of RPWs are found at photospheric layers. Those continuous oscillations occur preferably at periods between 4-6 min starting at the inner penumbral boundary. The photospheric oscillations all have a slightly delayed, more defined chromospheric counterpart with larger relative velocities, which are linked to preceding umbral flash events. In all of the layers, the power of RPWs follows a filamentary fine-structure and shows a typical ring-shaped power distribution increasing in radius for larger wave periods. The analysis of time slices reveals apparent horizontal velocities for RPWs at photospheric layers of ≈51 km s −1 , which decrease to ≈37 km s −1 at chromospheric heights. Conclusions. The observations strongly support the scenario of RPWs being upward propagating slow-mode waves guided by the magnetic field lines. Clear evidence for RPWs at photospheric layers is given. The inverse proportionality of the peak period and cutoff period on the field inclination is supported by the observations. The larger apparent horizontal velocities at photospheric heights hint at the more horizontal penumbral field inclination.

Dependence of sunspot photospheric waves on the depth of the source of solarp-modes

“…From a comparison between the observed photospheric RPWs' velocities and the sourcedepth dependence of their apparent wave velocities in the simulations, we can estimate the location of the sources in sunspots. The velocity of the fast-moving wave obtained from the cross-correlation of the simulations with stochastic sources is within the limits of the speed of the photospheric RPWs measured by Löhner-Böttcher & Bello González (2015) for source depths between z s = −1.3 Mm and z s = −4.3 Mm. As reported by Zhao et al (2015), the fast-moving wave only appears in sunspots.…”

“…For each map at log τ, we crosscorrelated the vertical velocity signal at x ref = 7.5 Mm with the vertical velocity at all other locations inside the area of interest. The location x ref corresponds to the penumbra at approximately the same radial distance where the RPWs were reported by Löhner-Böttcher & Bello González (2015). The crosscorrelation at time lag t indicates the position of a wave packet at time t after (before) it has left (arrived) from (at) x ref .…”

“…The cross-correlation of an independent simulation with no sources located beneath the sunspot within radial distances of 10 Mm shows a radiallyinward-propagating wave, excited by the sources located outside of the spot. Löhner-Böttcher & Bello González (2015) recently reported the observation of RPWs at photospheric layers. They present apparent horizontal velocities of 51 ± 13 km s −1 , which decrease to 37 ± 10 km s −1 at the chromosphere.…”

Hinode and Iris Observations of the Magnetohydrodynamic Waves Propagating From the Photosphere to the Chromosphere in a Sunspot

“…Tracing waves from the photosphere to the chromosphere also helps us understand their true connection. Löhner-Böttcher & Bello González (2015) found photospheric oscillations in sunspot penumbrae that have a slightly delayed counterpart of more defined chromospheric running penumbral waves with larger relative velocities, suggesting that the running penumbral waves propagate upward along inclined magnetic field lines. Inside sunspot umbrae, since waveforms in the photosphere and the chromosphere are not similar to each other because of acoustic cutoff and nonlinear interaction, it is not easy to trace waves like Löhner-Böttcher & Bello González (2015).…”

Magnetic field reconstruction based on sunspot oscillations

“…The three-dimensional evolution of wave periods is illustrated in Fig. 2 of Löhner-Böttcher & Bello González (2015).…”