Comparison of solar photospheric bright points between Sunrise observations and MHD simulations

Abstract

Bright points (BPs) in the solar photosphere are thought to be the radiative signatures (small-scale brightness enhancements) of magnetic elements described by slender flux tubes or sheets located in the darker intergranular lanes in the solar photosphere. They contribute to the ultraviolet (UV) flux variations over the solar cycle and hence may play a role in influencing the Earth's climate. Here we aim to obtain a better insight into their properties by combining high-resolution UV and spectro-polarimetric observations of BPs by the Sunrise Observatory with 3D compressible radiation magnetohydrodynamical (MHD) simulations. To this end, full spectral line syntheses are performed with the MHD data and a careful degradation is applied to take into account all relevant instrumental effects of the observations. In a first step it is demonstrated that the selected MHD simulations reproduce the measured distributions of intensity at multiple wavelengths, line-of-sight velocity, spectral line width, and polarization degree rather well. The simulated line width also displays the correct mean, but a scatter that is too small. In the second step, the properties of observed BPs are compared with synthetic ones. Again, these are found to match relatively well, except that the observations display a tail of large BPs with strong polarization signals (most likely network elements) not found in the simulations, possibly due to the small size of the simulation box. The higher spatial resolution of the simulations has a significant effect, leading to smaller and more numerous BPs. The observation that most BPs are weakly polarized is explained mainly by the spatial degradation, the stray light contamination, and the temperature sensitivity of the Fe i line at 5250.2 Å. Finally, given that the MHD simulations are highly consistent with the observations, we used the simulations to explore the properties of BPs further. The Stokes V asymmetries increase with the distance to the center of the mean BP in both observations and simulations, consistent with the classical picture of a production of the asymmetry in the canopy. This is the first time that this has been found also in the internetwork. More or less vertical kilogauss magnetic fields are found for 98% of the synthetic BPs underlining that basically every BP is associated with kilogauss fields. At the continuum formation height, the simulated BPs are on average 190 K hotter than the mean quiet Sun, the mean BP field strength is found to be 1750 G, and the mean inclination is 17 • , supporting the physical flux-tube paradigm to describe BPs. On average, the synthetic BPs harbor downflows increasing with depth. The origin of these downflows is not yet understood very well and needs further investigation.

The Small-scale Structure of Photospheric Convection Retrieved by a Deconvolution Technique Applied to Hinode/SP Data

“…Moreover, these values are consistent with the previous work of Danilovic et al (2008), who found RMS contrasts of 14.4%, 7.5%, and 7.0% in the original pixel sampling (≈10 km), the summed sampling (≈100 km) with spatial convolution, and the Hinode/SP observation used in their work, respectively. Their contrast values are slightly lower than ours because their simulation included magnetic flux, which hinders the granulation and, hence, reduces the intensity contrast (Biermann 1941;Riethmüller et al 2014). The contrast of 13.0% found in our work is also consistent with the value of 12.8% estimated by the observational approach of Mathew et al (2009), who deconvolved Hinode images using a PSF of the Hinode/SOT derived using images during the Mercury transit.…”

“…The chosen setup was the same as that used in Riethmüller et al (2014). The calculation ran for 1 hour of solar time and we stored a snapshot containing all the physical parameters (needed for the line computation) every 30 seconds, thus producing 120 snapshots in all.…”

Variation of the temperature gradient in the solar photosphere with magnetic activity

“…This is consistent with the results of MHD simulations (Spruit 1976;Deinzer et al 1984;Riethmüller et al 2014), it is mainly an effect of the reduced opacity in the magnetic region.…”

Magnetic fields of opposite polarity in sunspot penumbrae

“…Modeling the fringes by a superposition of sine curves of various amplitudes and frequencies requires little computing time when compared to more involved approaches using wavelet analysis (Rojo & Harrington 2006) or 2D pattern recognition (Casini et al 2012). Drawbacks are the danger of removing or modifying an actual signal, that is a spectral line.…”