The first images of the Sun generated by observations carried out using the ALMA radio telescope have been published. This is the first time that the largest radio telescope in the world has been used in this way and this is the beginning of an important expansion in ALMA’s use. The first results are details of the Sun’s chromosphere such as a sunspot twice the size of Earth.
Even before its inauguration, which happened in March 2013, the ALMA (Atacama Large Millimeter/submillimeter Array) radio telescope has been generally used to observe very distant objects, even billions of light years away, sometimes very faint, or details of solar systems that are relatively close. However, this great instrument is designed also to observe objects that are closer such as the Sun at wavelengths greater than those generally detected by solar observatories.
The use of instruments designed especially for observation of distant objects for solar astronomy isn’t unprecedented. For example, in 2014 NASA’s NuSTAR Space Telescope was used to study the Sun at X-rays. Using ALMA for solar astronomy marks the first time of a project of which ESO is a partner in this kind of research.
Not all telescopes can be used to observe the Sun, in fact generally they’d be damaged by the intense solar emissions. Even with radio telescopes cautions are needed, in this case to avoid damages to the electronic equipment following the high temperatures caused by sunlight. Years ago, a fire broke out in an equipment of the Swedish-ESO submillimetre telescope Telescope (SEST) when it was pointed at the Sun by mistake.
The ALMA radio telescope was designed for solar astronomy as well so it can be pointed at the Sun even if it’s billions of times brighter than the objects typically observed by this instrument. This allowed to observe the Sun’s chromosphere, the region just above the photosphere, that is the visible surface, at wavelengths of 1.25 and 3 mm thanks to two bands of receivers.
ALMA’s capabilities allowed to obtain various details of the solar chromosphere using the full array of antennas but also a map of the entire solar disk using just one of the 66 radio telescope antennas, a technique called fast scanning, at 1.25 millimeters. The observation accuracy and speed with a single antenna makes it possible to produce that type of map in a few minutes. These are two ways of use that complement each other.
The observations made at two wavelengths reveal among other things the temperature differences between the various parts of the solar chromosphere. It’s exactly the kind of details that can be detected with a radio telescope and is a central element of future research that will be carried out with ALMA. This instrument demonstrates once again how valuable it is for astronomy.