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The largest terrestrial radio telescope photographed the Sun.

A snapshot of the Sun in the visible range and a radio image of a sunspot on it, obtained by ALMA at a wavelength of 1.25 millimeters
ALMA (ESO / NAOJ / NRAO), NASA

The largest terrestrial array of radio telescopes ALMA for the first time during the work carried out a survey of the surface of the Sun in the millimeter range. These observations sharply differ from the usual objects of ALMA – galaxies of the reionization epoch, protoplanetary discs of stars and gas-dust nebulae. Radio astronomers managed to obtain detailed images of a giant sunspot with a radius twice as large as the earth’s. This was reported by the press release of the Southern European Observatory.

Observations of astronomical objects in different spectral ranges allow one to obtain information about fundamentally different physical processes taking place in them. For example, studies of the Crab Nebula in the visible spectrum show the expansion of gas clouds after a supernova explosion, and X-ray data allow us to see the neutron pulsar star and the consequences of its activity.

Observations of the Sun are also conducted in different bands by several spacecraft and ground-based observatories. In the X-ray, hard and soft ultraviolet, the Sun is photographed by the space observatories SOHO, Hinode and STEREO. Snapshots in the first two ranges are designed to investigate the most high-energy processes occurring in a preheated to a million degrees of the Sun’s crown. In particular, they allow fixing coronal mass ejections.

ALMA is an array of 66 radio telescopes located in the Atacama Desert (Chile). Thanks to the joint processing of data from several telescopes simultaneously, the system can be viewed as a single radio telescope with a base of about 18.5 kilometers. Her observations relate to the millimeter range – this radiation corresponds to much less energetic processes than the ultraviolet and visible (hundreds of nanometers).

With the help of radio scans, in particular, you can “look” into the area under the sun’s crown – the chromosphere (even lower is the “surface” of the luminary – the photosphere). Observations of ALMA allowed radio astronomers to estimate the temperature distribution in it, as well as determine the shape of the central region of a large sunspot.

ALMA conducted two series of observations: at wavelengths of 1.25 millimeters and 3 millimeters. They correspond to two different depths of the chromosphere, with shorter-wavelength radiation corresponding to deeper layers. In particular, scientists received a couple of images of a large sunspot, about twice as large as the diameter of the Earth. The pictures show a difference in the temperature distribution above it.

Interestingly, when designing ALMA engineers took into account in advance that the radio telescope will be used for observations of the Sun. Therefore, the antennas were protected from the focused thermal radiation of the star. In the future, solar observations of ALMA will be used to study the dynamics of the chromosphere and its thermal features. This will help to better understand the physics of the sun.

Radio observations of the Sun began back in the 1940s after the accidentaldiscovery of the radio emission of the solar corona. Among the active observatories can be mentioned the Siberian solar radio telescope  – an array of 256 2.5-meter radio antennas, arranged in the form of two perpendicular lines. He conducts observations at a wavelength of 5.2 centimeters. As notedon the site of the observatory, radio telescopes allow you to observe the Sun’s crown against the backdrop of the luminary disk, and not just on the limb. In addition, this is the only reliable opportunity to study magnetic fields in the corona.

One of the unresolved problems of the physics of the Sun is the anomalous warming up of the crown. The temperature of the outer regions of the photosphere is about four thousand degrees Celsius. The chromosphere from internal to external areas warms up to 20 thousand degrees, and the corona temperature reaches already one million degrees. The mechanism of this warm-up is not fully understood. One of the theories assigns a central role to resonance absorption processes (they were recently observed directly). Also, the heating may be associated with a magnetic reconnection.

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