The Atacama Large Millimeter/submillimeter Array (ALMA) telescope has been used to study the Sun for the first time. The new ALMA images reveal otherwise invisible details of our Sun, including a new view of the dark, contorted center of an evolving sunspot that is nearly twice the diameter of the Earth.
A map of the whole disc of the Sun was also made with a single ALMA antenna, using a technique called fast-scanning, at a wavelength of 1.25 mm. Image credit: ALMA / ESO / NAOJ / NRAO.
During a 30-month period beginning in 2014, an international team of astronomers from Europe, North America and East Asia harnessed ALMA’s single-antenna and array capabilities to detect and image the millimeter-wavelength light emitted by the Sun’s chromosphere — the region that lies just above the photosphere, the visible surface of the Sun.
“We’re accustomed to seeing how our Sun appears in visible light, but that can only tell us so much about the dynamic surface and energetic atmosphere of our nearest star,” explained team member Dr. Tim Bastian, an astronomer with the National Radio Astronomy Observatory.
“To fully understand the Sun, we need to study it across the entire electromagnetic spectrum, including the millimeter and submillimeter 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 the technique of radio interferometry — and avoid damage from the intense heat of the focused sunlight.
The result of this work is a series of images that demonstrate ALMA’s unique vision and ability to study the Sun.
The astronomers also observed an enormous sunspot at wavelengths of 1.25 mm and 3 mm using two of ALMA’s receiver bands.
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 mm. The full-disc solar image was taken with the Helioseismic and Magnetic Imager (HMI) on board NASA’s Solar Dynamics Observatory. Image credit: ALMA / ESO / NAOJ / NRAO / NASA.
“Sunspots are transient features that occur in regions where the Sun’s magnetic field is extremely concentrated and powerful,” the scientists explained.
“They are lower in temperature than the surrounding regions, which is why they appear relatively dark.”
“Observations at shorter wavelengths are able to probe deeper into the Sun, meaning the 1.25 mm images show a layer of the chromosphere that is deeper, and therefore closer to the photosphere, than those made at a wavelength of 3 mm.”
