ALMA Discovery of Solar Umbral Brightness Enhancement at λ=3 mm

Abstract

We report the discovery of a brightness enhancement in the center of a large sunspot umbra at a wavelength of 3 mm using the Atacama Large Millimeter/sub-millimeter Array (ALMA). Sunspots are amongst the most prominent features on the solar surface, but many of their aspects are surprisingly poorly understood. We analyzed a λ=3 mm (100 GHz) mosaic image obtained by ALMA, which includes a large sunspot within the active region AR12470 on December 16, 2015. The 3 mm map has a field-of-view and spatial resolution, which is the highest spatial-resolution map of an entire sunspot in this frequency range. We find a gradient of 3 mm brightness from a high value in the outer penumbra to a low value in the inner penumbra/outer umbra. Within the inner umbra, there is a marked increase in 3mm brightness temperature, which we call an umbral brightness enhancement. This enhanced emission corresponds to a temperature excess of 800 K relative to the surrounding inner penumbral region and coincides with excess brightness in the 1330 and 1400 Å slitjaw images of the Interface Region Imaging Spectrograph (IRIS), adjacent to a partial lightbridge. This λ=3 mm brightness enhancement may be an intrinsic feature of the sunspot umbra at chromospheric heights, such as a manifestation of umbral flashes, or it could be related to a coronal plume since the brightness enhancement was coincident with the footpoint of a coronal loop observed at 171 Å.

arXiv comment16 pages, 5 figures, accepted for publication in ApJ Letter

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• First Spectral Analysis of a Solar Plasma Eruption Using ALMA
• Shock Heating Energy of Umbral Flashes Measured with Integral Field Unit Spectroscopy
• First solar observations with ALMA
• A study of sunspot 3 minute oscillations using ALMA and GST
• The Submillimeter Active Region Excess Brightness Temperature during Solar Cycles 23 and 24
• Flares detected in ALMA single-dish images of the Sun
• Thyr: A Volumetric Ray-Marching Tool for Simulating Microwave Emission