Observations of Sunspot Oscillations in G Band and Ca II H Line with Solar Optical Telescope on Hinode

Abstract

Exploiting high-resolution observations made by the Solar Optical Telescope onboard Hinode, we investigate the spatial distribution of power spectral density of oscillatory signal in and around active region NOAA 10935. The G-band data show that in the umbra the oscillatory power is suppressed in all frequency ranges. On the other hand, in Ca II H intensity maps oscillations in the umbra, so-called umbral flashes, are clearly seen with the power peaking around 5.5 mHz. The Ca II H power distribution shows the enhanced elements with the spatial scale of the umbral flashes over most of the umbra but there is a region with suppressed power at the center of the umbra. The origin and property of this node-like feature remain unexplained.

Oscillations and Waves in Sunspots

“…Balthasar and Schleicher (2008) tried to detect oscillations observing an Fe i line formed in the lower to middle photosphere and did not find any evidence for penumbral waves. Bellot Rubio et al (2000) found that the amplitude of the velocity oscillations increases toward the umbra/penumbra boundary, in agreement with the results by Nagashima et al (2007) and Lites (1988).…”

“…Using Hinode G-band temporal series of almost five hours of duration and one minute cadence, Nagashima et al (2007) calculated the photospheric power distribution of the intensity fluctuations in the G-band in a region containing a sunspot. Figure 7 shows the power distribution from 0.5 mHz to 5.5 mHz, at intervals of 1 mHz, revealing several important results.…”

“…The group of the panels on the right of Figure 7 shows the chromospheric Ca ii H intensity power maps measured by Nagashima et al (2007) simultaneously to the photospheric G-band intensity maps, discussed above. This figure shows that the higher the frequency of chromospheric umbral oscillations, the higher is the relative power these oscillations have, when compared to the surrounding penumbra and even to the quiet Sun.…”

“…Sigwarth and Mattig (1997) detected 5-min velocity oscillations from the inner to the outer penumbra up to the low chromosphere. According to Figure 7, taken from Nagashima et al (2007), intensity oscillations also exist at frequencies at least up to 5.5 mHz, all around the penumbra. The power of photospheric intensity and velocity oscillations in the penumbra is reduced compared to the surrounding quiet Sun.…”

Sunspot seismic halos generated by fast MHD wave refraction

“…(i) the power enhancement is observed at high frequencies, between 5.5 and 7.5 mHz for waves that are usually nontrapped in the non-magnetic quiet Sun; (ii) the acoustic power measured in halos is higher than in the nearby quite Sun by about 40-60% (Hindman & Brown 1998;Braun & Lindsey 1999;Donea et al 2000;Jain & Haber 2002;Nagashima et al 2007); (iii) the halos are observed at intermediate longitudinal magnetic fluxes B = 50−300 G, while the acoustic power is usually reduced at all frequencies at larger fluxes (Hindman & Brown 1998;Thomas & Stanchfield 2000;Jain & Haber 2002); (iv) the radius of the halo increases with height. In the photosphere the halos are located at the edges of active regions, while in the chromosphere they extend to a large portion of the nearby quiet Sun (Brown et al 1992;Braun et al 1992;Thomas & Stanchfield 2000); (v) the power increase in the halo is qualitatively similar in sunspots, pores and plages; (vi) significant reflection of the upcoming acoustic radiation at 5-6 mHz is detected in active regions, unlike the behavior of such high-frequency waves in the quiet Sun .…”