Solar polar brightening and radius at 100 and 230 GHz observed by ALMA

Abstract

Polar brightening of the Sun at radio frequencies has been studied for almost fifty years and yet a disagreement persists between solar atmospheric models and observations. Some observations reported brightening values much smaller than the expected values obtained from the models, with discrepancies being particularly large at millimeter wavelengths. New clues to calibrate the atmospheric models can be obtained with the advent of the Atacama Large Millimeter/submillimeter Array (ALMA) radio interferometer. In this work, we analyzed the lower limit of the polar brightening observed at 100 and 230 GHz by ALMA, during its Science Verification period, 2015 December 16-20. We find that the average polar intensity is higher than the disk intensity at 100 and 230 GHz, with larger brightness intensities at the South pole in eight of the nine maps analyzed. The observational results were compared with calculations of the millimetric limb brightnening emission for two semi-empirical atmospheric models, FAL- C (Fontenla et al. 1993) and SSC (Selhorst et al. 2005a). Both models presented larger limb intensities than the averaged observed values. The intensities obtained with the SSC model were closer to the observations, with polar brightenings of 10.5% and 17.8% at 100 and 230 GHz, respectively. This discrepancy may be due to the presence of chromospheric features (like spicules) at regions close to the limb.

10 pages, 6 figures, 2 tables, accepted for publication in ApJ

References in corpus (11)

• Solar science with the Atacama Large Millimeter/submillimeter Array - A new view of our Sun
• Observing the Sun with the Atacama Large Millimeter-submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping
• Accurate determination of the free-free Gaunt factor; I - non-relativistic Gaunt factors
• Center-to-limb observations of the Sun with ALMA
• Formation of the thermal infrared continuum in solar flares
• First analysis of solar structures in 1.21 mm full-disc ALMA image of the Sun
• The Brightness Temperature of the Quiet Solar Chromosphere at 2.6 mm
• Observation of Chromospheric Sunspot at Millimeter Range with the Nobeyama 45 m Telescope
• Solar Radius at Subterahertz Frequencies and its Relation to Solar Activity
• Comparison of solar radio and EUV synoptic limb charts during the present solar maximum
• Association of radio polar cap brightening with bright patches and coronal holes

Cited by in corpus (17)

• The Sun at millimeter wavelengths I. Introduction to ALMA Band 3 observations
• Observations of solar chromospheric oscillations at 3 mm with ALMA
• Imaging of the Quiet Sun in the Frequency Range of 20-80 MHz
• Transient brightenings in the quiet Sun detected by ALMA at 3 mm
• A first look at the submillimeter Sun with ALMA
• Evaluating Non-LTE Spectral Inversions with ALMA and IBIS
• The subterahertz solar cycle: Polar and equatorial radii derived from SST and ALMA
• A Genetic Algorithm to model Solar Radio Active Regions from 3D Magnetic Field Extrapolations
• Subterahertz Radius and Limb Brightening of the Sun Derived from SST and ALMA
• Planetary transits at radio wavelengths: secondary eclipses of hot Jupiter extended atmospheres
• Flares detected in ALMA single-dish images of the Sun
• The Solar Radius at 37 GHz through Cycles 22 to 24
• Non-Linear Convergence of Solar-like Stars Chromospheres using Millimeter, Sub-millimeter, and Infrared Observations
• Calculated brightness temperatures of solar structures compared with ALMA and Metsähovi measurements
• Modeling brightness temperature of prominences on the solar disk using ALMA single-dish observations
• The Solar ALMA Science Archive (SALSA)
• Modeling brightness temperature of sunspots using ALMA single-dish observations