Properties of oscillatory motions in a facular region

Abstract

Aims. We study the properties of waves in a facular region of moderate strength in the photosphere and chromosphere. Our aim is to statistically analyse the wave periods, power, and phase relations as a function of the magnetic field strength and inclination. Methods. Our work is based on observations obtained at the German Vacuum Tower Telescope (Observatorio del Teide, Tenerife) using two different instruments: the Triple Etalon SOlar Spectrometer (TESOS) in the Ba ii 4554 Å line to measure velocity and intensity variations through the photosphere and, simultaneously, the Tenerife Infrared Polarimeter (TIP-II), in the Fe i 1.56 μm lines to measure the Stokes parameters and magnetic field strength in the lower photosphere. Additionally, we use the simultaneous broadband filtergrams in the Ca ii H line to obtain information about intensity oscillations in the chromosphere.Results. We find several clear trends in the oscillation behaviour: (i) the period of oscillation increases by 15-20% with the magnetic field increasing from 500 to 1500 G. (ii) The temperature-velocity phase shifts show a strikingly different distribution in the facular region compared to the quiet region, a significant number of cases in the range from −180• to 180• is detected in the facula. (iii) The most powerful chromospheric Ca ii H intensity oscillations are observed at locations with strong magnetic fields (1.3-1.5 kG) inclined by 10-12 degrees, as a result of upward propagating waves with rather low phase speeds, and temperature-velocity phase shifts between 0• and 90• . (iv) The power of the photospheric velocity oscillations from the Ba ii line increases linearly with decreasing magnetic field inclination, reaching its maximum at strong field locations.

The possible origin of facular brightness in the solar atmosphere

“…Intensity oscillations affect the facular contrast less, possibly because of the magnetic nature of the observed waves, in which case the principal restoring force is the magnetic field and oscillations of thermodynamic parameters are less important. As was shown in our previous work (Kostik & Khomenko 2013), the 5-min oscillations more effectively reach the temperature minimum from the bottom photosphere when the phase shift between the oscillations of temperature and velocity are in the range of 0 • and 90 • . If these waves were observed in a quiet area, this type of phase shift would suggest clear deviations from adiabatic wave propagation (an adiabatic propagation leads to 180 • phase shift).…”

“…The line becomes deeper and its intensity decreases; i.e., the temperature and intensity oscillations are 180 deg out of phase, see also Shchukina et al (2009). Similar assumption was done in Kostik & Khomenko (2013). We operate in terms of temperature-velocity phase shift for a better comparison with theoretical models, see Noyes & Leighton (1963), Mihalas & Toomre (1981, 1982, Deubner (1990).…”

“…5) are in agreement with an intuitive picture of upward running waves transferring their energy to the chromosphere and producing an increase in its brightness. Indeed, according to analysis of the same dataset in (Kostik & Khomenko 2013), in about 67% of locations in the observed area, the φ(V, V) is negative, i.e., in most of the area the waves are propagating upwards. The greater the power of velocity oscillations is, the larger the facular contrast (Fig.…”

“…5 since these show negative values of the φ(V, V) phase shifts (upward propagation) when φ(T, V) phase shifts lie in the range between 0 • and 90 • . Extra care, however, has to be take when interpreting φ(T, V) phase shifts in our case, since the waves are observed in a strongly magnetized facular area and the dependence between the temperature-velocity phase shift and non-adiabaticity of oscillations in the presence of the magnetic field is not well studied theoretically (see the discussion in Kostik & Khomenko 2013). Determining the phase shift between temperature and velocity for magneto-acoustic waves would need numerical modeling for particular cases of the observed magnetic field geometries.…”

“…We obtained maps of the magnetic field strength and inclination by inverting the Stokes parameters of the Fe i 15 648 and 15 652 Å lines using the SIR inversion code (Ruiz Cobo & del Toro Iniesta 1992), see the details in Kostik & Khomenko (2013). Both the field strength and the inclination were assumed to be constant with height.…”