The energy of waves in the photosphere and lower chromosphere

Abstract

Context. The energy source powering the solar chromosphere is still undetermined, but leaves its traces in observed intensities. Aims. We investigate the statistics of the intensity distributions as a function of the wavelength for Ca ii H and the Ca ii IR line at 854.2 nm to estimate the energy content in the observed intensity fluctuations. Methods. We derived the intensity variations at different heights of the solar atmosphere, as traced by the line wings and line cores of the two spectral lines. We converted the observed intensities to absolute energy units employing reference profiles calculated in non-local thermal equilibrium (NLTE). We also converted the intensity fluctuations to corresponding brightness temperatures assuming LTE. Results. The root-mean-square (rms) fluctuations of the emitted intensity are about 0.6 (1.2) W m −2 ster −1 pm −1 near the core of the Ca ii IR line at 854.2 nm (Ca ii H), corresponding to relative intensity fluctuations of about 20% (30%). For the line wing, we find rms values of about 0.3 W m −2 ster −1 pm −1 for both lines, corresponding to relative fluctuations below 5%. The relative rms values show a local minimum for wavelengths forming at a height of about 130 km, but otherwise increase smoothly from the wing to the core, i.e., from photosphere to chromosphere. The corresponding rms brightness temperature fluctuations are below 100 K for the photosphere and up to 500 K in the chromosphere. The skewness of the intensity distributions is close to zero in the outer line wing and positive throughout the rest of the line spectrum, owing to the frequent occurrence of high-intensity events. The skewness shows a pronounced local maximum at locations with photospheric magnetic fields for wavelengths in-between those of the line wing and the line core (z ≈ 150−300 km), and a global maximum at the very core (z ≈ 1000 km) for both magnetic and field-free locations. Conclusions. The energy content of the intensity fluctuations is insufficient to create a chromospheric temperature rise that would be similar to the one in most reference models of the solar atmosphere. The increase in the rms fluctuations with height indicates the presence of upwardly propagating acoustic waves of increasing oscillation amplitude. The intensity and temperature variations indicate that there is a clear increase in dynamical activity from photosphere towards the chromosphere, but the variations fall short of the magnitude predicted by fully dynamical chromospheric models by a factor of about five. The enhanced skewness between the photosphere and lower solar chromosphere at magnetic locations is indicative of a mechanism that acts solely on magnetized plasma.

Multiwavelength Studies of MHD Waves in the Solar Chromosphere

“…), researchers attempted to probe the energetics of magnetoacoustic waves further still by harnessing the improved spatial and/or temporal resolutions on offer. However, each time (Fossum & Carlsson 2005a, 2006Beck et al 2009;Bello González et al 2009Beck et al 2012Beck et al , 2013a the authors were unable to conclusively verify that these wave modes carry sufficient energy to play a dominant role in atmospheric heating. Perhaps, as highlighted by the work of Jefferies et al (2006), Wedemeyer-Böhm et al (2007), Kalkofen (2007Kalkofen ( , 2008, to name but a few, we still require yet higher spatial and temporal resolutions to be able to accurately constrain the rapid fluctuating dynamics synonymous with propagating magnetoacoustic wave modes in small-scale magnetic elements.…”

Solar magnetic flux tube simulations with time-dependent ionization

“…Fossum & Carlsson (2005), Cuntz, Rammacher & Musielak (2007), Kalkofen (2007) and Bello González et al (2009). Recent joint observational and theoretical studies aimed at elucidating the significance of wave heating in the solar photosphere and lower chromosphere were given by Beck & Rammacher (2010), E-mail: diaa.gadelmavla@izmirekonomi.edu.tr (DEF); cuntz@uta.edu (MC); wrammacher@online.de (WR) Beck, Rezaei & Puschmann (2012) and Wedemeyer-Böhm et al (2012). The latter study investigates the channelling of magnetic energy through the solar chromosphere into the corona, thus resembling super-tornadoes under solar conditions.…”

The energy of waves in the photosphere and lower chromosphere

“…a time series of about one-hour duration and a large-area map (cf. Beck et al 2009Beck et al , 2012BE09 and BE12 in the following). The other three data sets are large-area maps of the active region NOAA 10978 (cf.…”

“…2) reach about 1000 K at log τ ∼ −4 (z ∼ 700 km). Beck et al (2012;BE12) found a pronounced difference in the skewness of intensity distributions between field-free and magnetic locations, i.e. an increased skewness on magnetic locations at intermediate wavelengths between the line wing and the core, corresponding to formation heights of about z = 135−150 km.…”