Johannes Löhner-Böttcher and Nazaret Bello González
Kiepenheuer-Institut für Sonnenphysik, Freiburg, Germany
Running penumbral waves and umbral flashes are the most spectacular and prominent wave phenomena in the chromosphere above sunspots. The nature of both events and their linkage have been under vivid discussion in the last decades. Umbral flashes are described as slow-mode magnetoacoustic waves propagating upward along the vertical magnetic field of the sunspot umbra. In the case of running penumbral waves the interpretation was ambiguous. The temporal imaging of the sunspot chromosphere suggested that running penumbral waves are purely chromospheric horizontal waves excited by the umbral flashes. However, recent studies have confirmed the contrary scenario that the penumbral waves have the same nature as umbral flashes1. They are magneto-acoustic waves which propagate upward and guided by the inclined magnetic field of the penumbra2. This magnetic field inclination increases from the inner to the outer penumbra. A coherent wave front which travels along this umbral and penumbral field to a certain height would have an increasing path length from the central umbra to the outer edge of the sunspot3. The result is a visual pattern of a radial outward propagation which decreases in apparent velocity. While the characteristics of running penumbral waves in the chromosphere are well determined, a detailed observation of their photospheric origin was still lacking until now1,2.
Figure 1 | NOAA11823 in continuum intensity (left panels) and line-of-sight magnetic field strength (right panels) on August 21st 2013 at 15:00:45 UTC as observed by HMI. The insets show the analyzed sunspot region (b) observed with the Dunn Solar Telescope. The contours indicate the sunspot boundaries retrieved from HMI continuum intensity. The HMI magnetogram is scaled from −1kG (black) to 2kG (white).
By means of a multiwavelength observations of an isolated circular sunspot (NOAA11823) shown in Fig. 1, we have found first signatures of running penumbral waves in the middle photosphere4. Simultaneous observations were performed for one hour with HMI and the Interferometric BIdimensional Spectro-polarimeter (IBIS) at the Dunn Solar Telescope in high spatial resolution. The sunspot was observed using the photospheric Fe I lines at 617.33 nm and 630.15 nm, as well as using the low-chromospheric Na I 589.6 nm line. The oscillations in Doppler velocity were studies with a wavelet power analysis. The peak periods of the sunspot waves were retrieved from the global power spectra. The results are shown in Fig. 2.
Figure 2 | Spatial distribution of the peak periods (in min) across the sunspot. The dominating wave periods TPEAK in Doppler velocity power are shown for a) Na I 589.6 nm, b) Fe I 630.15 nm (both IBIS) and c) Fe I 617.33 nm (HMI). The period scale ranges from 2.5 min (dark blue) to 8 min (dark red). The black contours mark the umbral and penumbral boundaries. The black arrow is pointing to the disk center.
In the high photospheric to low chromospheric layer (Figure 2a), the umbra is dominated by waves with periods below 3 min. The penumbra shows a filamentary pattern of increasing wave periods toward the outer penumbra (to 8 min). This phenomena is explained by the increasing cut-off period for larger magnetic field inclinations. Waves with longer periods can propagate to higher atmospheric layers and overpower the shorter waves. In the middle photosphere (Figure 2b), the umbra exhibits a mixture of wave periods which are dominated by the 3 min range. The penumbra in the photosphere also exhibits the filamentary structure which characterizes the chromospheric penumbra. It is obvious, that the peak periods from the HMI Doppler velocities (Figure 2c) resemble the latter. It is confirmed5 that the resolution of HMI is good enough to reveal running penumbral waves in the sunspot photosphere.
Figure 3 | Temporal evolution of the Doppler velocities along three sunspot sectors in the photosphere (Fe I 630.15 nm, lower panels) and lower chromosphere (Na I 589.6 nm, upper panels). For each sector, the azimuthal average at a radial distance X (in arcsec) from the spot center was calculated. The dashed lines mark the umbral boundary. The velocity is scaled between ±0.6 km/s (a)–c)) and ±0.3 km/s (d)–f)). The black tracers of some wave trains indicate the apparent horizontal velocities.
To investigate the apparent horizontal velocities of the propagating waves, a temporal analysis of Doppler velocities has been performed along different circular sectors of the sunspot. In Fig. 3, the running wave fronts are marked by black tracers. The chromospheric waves (upper panels) all have a photospheric counterpart (lower panels). The running waves can be traced back from the penumbra (PU) to the umbral (U) area. The average apparent horizontal velocity of inner penumbral waves amounts to 37 km s−1 in the chromosphere and even 51 km s-1 in the photosphere. These large velocities exceeds the sound speed by far and can only be explained by the visual pattern of coherent propagation along a large gradient of increasing field inclination1,3.
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