Radio broadcasts from the sun have been studied since the beginning of the second half of the 20th century. Most recently, radio astronomers from the Center for Solar-Terrestrial Research (NJIT-CSTR) at the New Jersey Institute of Technology captured the image of a solar eclipse in an unprecedented way, most notably by capturing the first radio images of the famous “Ring of Fire” solar eclipse “Effect of the solar eclipse of October 14, 2023.
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Solar eclipses have been known to humanity for thousands of years, but until the 20th century they could only be observed in the visible. This was also generally true for the study of our parent star, but everything changed with the beginnings of radio astronomy and the accidental discovery of radio signals of cosmic origin by Karl Jansky in 1933. In fact, since the late 19th century, when Hertz’s experiments confirmed James Maxwell’s prediction of the existence of electromagnetic waves, several people believed that the sun could emit electromagnetic waves and searched for them – without success.
Instruments were needed with a sensitivity that these researchers did not have, and besides, in 1902, when the ionosphere was discovered and it was recognized that this layer of ionized gas in the upper atmosphere behaved like a mirror for known radio waves, it was uniform thought that nothing could be discovered in the future because the layer had to reflect all astronomical radio emissions from space, making them undetectable to observers on the ground.
The sun, a radio source that has been increasingly studied since the 1950s
Fortunately, we were wrong and the work on radar devices during the Second World War accelerated the development of technologies that will quickly lead to modern radio telescopes and the study of the Sun in the radio range, as has been the case since the 1950s, for example, in France with the inauguration of the Nançay radio astronomy station (today it is used in particular to hunt for gravitational waves). In fact, it was English radars during the war that detected the Sun’s emissions, and before Nançay, a solar eclipse had even been observed in centimeter and metric waves at Marcoussis using antennas purchased from American Army surplus in Africa in 1951.
Today, an annular solar eclipse was observed in the radio range for the first time, on October 14, 2023. The instruments used are those of the Owens Valley Radio Observatory – Long Wavelength Array (OVRO-LWA). in the United States, in the state of California, about 400 kilometers north of Los Angeles in the Owens Valley.
Radio observations with the Owens Valley Radio Observatory – Long Wavelength Array enabled the production of the short video below, courtesy of Sijie Yu, showing the annular solar eclipse at multiple radio wavelengths. In these images reconstructed in false colors we see the visible solar disk and the lunar occultation, each delimited by the filled and dotted circles. The radio image of the Sun is sometimes distorted due to the refraction of radio waves by Earth’s fluctuating ionosphere, an effect comparable to what one would observe if observing the Sun beneath a rippling water surface.
The October 14 annular solar eclipse was observed for the first time in radio wavelengths. © Sijie Yu
An hour-long solar eclipse on the radio
While the October 14 annular solar eclipse lasted a maximum of five minutes in the United States, the radio phenomenon lasted nearly an hour in the visible due to the existence of the solar corona, which is made of plasma and extends well beyond the surface of the sun and from which photons are emitted Photosphere. Remember that the solar corona is the outermost layer of the sun’s atmosphere, stretching almost ten million kilometers, or about 14 times the sun’s radius.
“It was spectacular to finally see a ‘ring of fire’ solar eclipse in this way… We have never seen this quality of radio images of the Sun before,” said Dale Gary, distinguished professor of physics at the New Jersey Institute of Technology, in a statement NJIT-CSTR and co-investigator of the OVRO-LWA project. “Normally we cannot see the corona from the ground except during a total solar eclipse, but thanks to OVRO-LWA we can now see it at any time. This solar eclipse makes the situation even more dramatic,” he added.