A comparison of G-band brightness as a proxy-magnetometer in various magnetic configurations

TL;DR

The study addresses whether centering G-band filters at 430.4 nm improves detection of small-scale solar magnetic fields compared with the traditional 430.5 nm setting. It combines realistic 3D MHD simulations with RH 1.5D spectral synthesis to generate synthetic G-band images across quiet-Sun and plage environments, applying Gaussian spectral broadening to emulate instrument filters. The results show that 430.4 nm yields higher intensity contrast, particularly for narrowband filters, due to higher formation heights and enhanced CH-opacity sensitivity in magnetized regions, though some strong-field areas can appear darker due to heating balance and Wilson depression effects. The findings suggest 430.4 nm as a promising alternative for high-contrast magnetic diagnostics, with potential applications to stellar contexts, while highlighting the need for further work on instrumental effects and spectropolarimetric capabilities.

Abstract

We investigate the diagnostic potential of the G-band at 430.4 nm for probing small-scale magnetic fields in the solar photosphere. Combining three-dimensional MHD simulations from the MURaM code and spectral synthesis via the RH 1.5D code, we evaluate the intensity contrast in the G-band filtergrams by comparing the filter centered at 430.4 nm in comparison to the conventional 430.5 nm. Our results show that filtergrams centered at 430.4 nm provide higher contrast across varying magnetic environments, particularly at narrow filter widths. This enhancement arises from its slightly higher formation height and greater sensitivity to temperature variations in magnetized regions. These findings indicate that G-band filtergrams centered at 430.4 nm show enhanced diagnostic potential under the assumptions of the present modeling. The obtained results are also relevant and suggest potential applications in stellar contexts, where molecular bands are often used as proxies for magnetic activity.

Figures (9)

• Figure 1: Maps of magnetic field strength ($B_z$), line-of-sight velocity ($v_z$), and temperature (T) at the $\tau = 1$ layer for three simulation setups corresponding to QS, WP, and SP regions..
• Figure 2: Spatially averaged profiles of density ($\rho$), gas pressure ($p$), and temperature ($T$) as functions of log($\tau_{500}$) for the three simulation setups. The averaging is performed over the full horizontal extent (12 Mm $\times$ 12 Mm) of each domain. The plotted range in log($\tau_{500}$) corresponds to the atmospheric layers between approximately -0.5 Mm to +1.0 Mm in geometric height, the region used as input for the RH 1.5D radiative transfer calculations.
• Figure 3: Brightness intensity maps for three different simulation setups (columns). The top row shows the continuum intensity at 430 nm, emphasising the overall photospheric brightness distribution. The middle row displays the intensity map at 430.4 nm, where the intergranular lanes and granules exhibit higher contrast. In comparison, the bottom row illustrates the intensity map at 430.5 nm, showing different intensity distributions between intergranular lanes and granules.
• Figure 4: A plot showing the spatially averaged G-band spectrum (solid black line) for the QS simulation, compared with the reference spectrum from the ATLAS database presented with red line. Overplotted are Gaussian filter profiles representing different spectral resolutions: dash-dotted lines correspond to filters with FWHM = 0.5 nm centered at 430.4 nm and 430.5 nm, while dashed lines correspond to filters with FWHM = 0.01 nm centered at the same wavelengths. These profiles illustrate the effect of spectral broadening on the observed G-band.
• Figure 5: A six-panel scatter plot illustrating the relationship between the G-band brightness at 430.5 nm and the magnetic field strength at the optical depth layer $\tau_{500} = 1$. The top row corresponds to a Gaussian convolution with FWHM = 0.5 nm, while the bottom row corresponds to FWHM = 0.01 nm. Each column represents a different simulation setup with average magnetic field strengths of 10 G, 50 G, and 200 G. Data points are color-coded by temperature at $\tau_{500} = 1$. Distributions are shown separately for regions with $B \geq 1000$ G and $B < 1000$ G.