This image of the entire Sun was taken in the red visible light emitted by iron atoms in the Sun’s atmosphere. Light at this wavelength originates from the visible solar surface, the photosphere. A cooler, darker sunspot is clearly visible in the disc, and as a visual comparison is shown alongside the image from ALMA at a wavelength of 1.25 millimetres. The full-disc solar image was taken with the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). Credit: ALMA (ESO/NAOJ/NRAO), NASA.
New images taken with the ALMA in Chile have revealed otherwise invisible details of our Sun, including a new view of the dark, contorted centre of a sunspot that is nearly twice the diameter of the Earth. The images are the first ever made of the Sun with a facility where ESO is a partner. The results are an important expansion of the range of observations that can be used to probe the physics of our nearest star. ALMA antennas had been carefully designed so they could image the Sun without being damaged by the intense heat of the focussed light.
Astronomers have harnessed ALMA’s capabilities to image the millimetre-wavelength light emitted by the Sun’s chromosphere – the region that lies just above the photosphere, which forms the visible surface of the Sun. The solar campaign team, an international group of astronomers with members from Europe, North America and East Asia, produced the images as a demonstration of ALMA’s ability to study solar activity at longer wavelengths of light than are typically available to solar observatories on Earth.
To achieve a fuller understanding, astronomers need to study the sun across the entire electromagnetic spectrum, including the millimetre and submillimetre portion that ALMA can observe. Since the Sun is many billions of times brighter than the faint objects ALMA typically observes, the ALMA antennas were specially designed to allow them to image the Sun in exquisite detail using radio interferometry – and avoid damage from the intense heat of the focused sunlight.
The team observed an enormous sunspot at wavelengths of 1.25 millimetres and 3 millimetres using 2 of ALMA’s receiver bands. The images reveal differences in temperature between parts of the Sun’s chromosphere. Understanding the heating and dynamics of the chromosphere are key areas of research that will be addressed in the future using ALMA. Sunspots are transient features that occur in regions where the Sun’s magnetic field is extremely concentrated and powerful. They are lower temp than the surrounding regions, which is why they appear relatively dark.
The difference in appearance between the 2 images is due to the different wavelengths of emitted light being observed. Observations at shorter wavelengths are able to probe deeper into the Sun, ie 1.25 millimetre images show a layer of the chromosphere that is deeper, and therefore closer to the photosphere, than those made at a wavelength of 3 millimetres.
ALMA is the first facility where ESO is a partner that allows astronomers to study the nearest star, our own Sun. All other existing and past ESO facilities need to be protected from the intense solar radiation to avoid damage. The new ALMA capabilities will expand the ESO community to include solar astronomers. http://www.eso.org/public/usa/news/eso1703/
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