Groundbreaking method pierces the ionosphere to reveal the radio universe in stunning clarity: ScienceAlert

The very lowest frequencies of the radio universe have just been revealed in spectacular clarity.

A team of astronomers has used a new calibration technique to give us the first sharp images of the radio universe in the frequency range of 16-30 megahertz – a feat previously thought impossible due to the turbulent interference created by Earth’s ionosphere is generated.

“It’s as if you put on glasses for the first time and your vision is no longer blurred,” says astronomer Christian Groeneveld of Leiden University, who led the study.

Seeing the universe with radio eyes presents some interesting challenges at the best of times.

At the lower end of the electromagnetic spectrum, the radio range consists of the longest waves, meaning they can penetrate the Earth’s atmosphere. However, because the signals are often quite weak and because the wavelengths are quite long, the antennas with which we detect them must be quite large.

So putting a radio telescope in space is simply not a cost-effective way to study the radio sky, and most radio telescopes are deployed and rotating here on Earth. But for the decameter frequency range, below 30 megahertz, this means we haven’t been able to see in detail what’s happening beyond that.

This is due to the ionosphere, which scatters low-frequency radio waves so effectively that they arrive very damaged. The variable number of electrons in the ionosphere causes variable phase delays in the low-frequency wavefront; and interactions between electrons and magnetic fields in the ionosphere can cause the radio waves to rotate. The result is very blurry, out-of-focus images.

It’s been a problem for as long as we’ve had radio astronomy. But as early as 2004, astronomers predicted that we might be able to achieve much better resolution with projects like LOFAR, a radio telescope array that had yet to be built.

LOFAR is now the largest radio telescope in the world and sees the universe in the lowest frequencies we can see from Earth. But the ionosphere is still the same old problem, so Groeneveld and his colleagues looked for a way to correct its interference.

Their calibration strategy works similarly to adaptive optics, which uses a guide star to help optical telescopes correct for the effects of atmospheric distortion. The researchers used the radio sources themselves as calibration targets, with sensitivity and resolution being an order of magnitude higher than previous decameter observations.

The technique is not perfect: lines radiate around radio sources in the new image; this is because the ionosphere makes the source appear to be moving. The calibration has been able to locate the source with greater precision, but some artifacts of the ionospheric influence remain. That is something that can be refined in further work.

But for now, the team’s efforts show a level of precision that reveals details we’d never seen before. High-frequency and low-frequency radio emissions are created by different processes and objects; Studying clusters of galaxies that we had previously only seen in high-frequency radio waves showed that the emission is not evenly distributed, but shows a kind of spotty pattern.

Bursts from very distant black holes also produce low-frequency radio waves, so the new technique means astronomers have a much better tool for understanding the accretion of black holes in the early universe.

But for now we know that the technology works. The researchers are hard at work processing more data, hoping to eventually map the entire northern sky from ten meters. And, Groeneveld notes: “There is of course a chance that we will eventually discover something unexpected.”

Yes please.

The research was published in Nature Astronomy.