Solar atmospheric oscillations and the chromospheric magnetic topology

Abstract

Aims. We investigate the oscillatory properties of the quiet solar chromosphere in relation to the underlying photosphere, with particular regard to the effects of the magnetic topology. Methods. For the first time we perform a Fourier analysis on a sequence of line-of-sight velocities measured simultaneously in a photospheric (Fe I 709.0 nm) and a chromospheric line (Ca II 854.2 nm). The velocities were obtained from full spectroscopic data acquired at high spatial resolution with the Interferometric BIdimensional Spectrometer (IBIS). The field of view encompasses a full supergranular cell, allowing us to discriminate between areas with different magnetic characteristics. Results. We show that waves with frequencies above the acoustic cut-off propagate from the photosphere to upper layers only in restricted areas of the quiet Sun. A large fraction of the quiet chromosphere is in fact occupied by "magnetic shadows", surrounding network regions, that we identify as originating from fibril-like structures observed in the core intensity of the Ca II line. We show that a large fraction of the chromospheric acoustic power at frequencies below the acoustic cut-off, residing in the proximity of the magnetic network elements, directly propagates from the underlying photosphere. This supports recent results arguing that network magnetic elements can channel low-frequency photospheric oscillations into the chromosphere, thus providing a way to input mechanical energy in the upper layers.

The magnetic field in the solar atmosphere

“…The fibril pattern around sunspots is often observed to be oriented radially outwards and forming whirls which exhibit rotation patterns specific to the hemisphere where they are observed (Hale 1908b;Richardson 1941;Peter 1996). Vecchio et al (2007) underlined the likeliness of fibril-like structures seen in Ca ii 8542 Å images to outline the canopy at chromospheric levels. The fibrils are thought to follow the canopy magnetic field of sunspot super-penumbrae, whose base rises slowly from the edge of the spots as one moves radially outward alongside a decreasing magnetic field strength (Giovanelli 1980;Giovanelli and Jones 1982;Solanki et al 1994, and see also Sect.…”

The solar chromosphere at high resolution with IBIS

“…Their presence indicates that even at the relatively low chromospheric heights sampled by the Ca II 854.2 line, the atmosphere is already highly structured by the pervasive magnetic fields, entirely consistent with the picture provided by Hα images. The fibrils in turn play a crucial role in shaping the chromospheric dynamics, in particular producing a strong reduction in the oscillatory power at periods around three minutes, correlated with the absence of chromospheric acoustic shocks (Vecchio et al 2007; Paper I; Paper II).…”

Space – Time Distribution of G-band and Ca ii H-line Intensity Oscillations in Hinode/SOT – FG Observations

“…In Figure 4 we present xy images of time-integrated G-band wavelet power. To make contact with the work of Vecchio et al (2007) and their analysis of Doppler oscillations in the Interferometric…”

“…This last band was named the "high frequency" (HF) band by Vecchio et al (2007). The 21 s cadence of our Hinode data set allows us also to define a "very high frequency" (VHF) band including frequencies 10 < f < 24 mHz (42 < T < 100 s).…”