A new look at the Earth’s sky through the eyes of a radio telescope

By Cleo Loi, guest contributor

Cleo Loi is credited with proving the existence of plasma tubes inside the Earth’s magnetosphere and extending into the plasmasphere. Her study was recently published in the journal Geophysical Research Letters. She wrote a blog post for Aurorasaurus describing how this affects the aurora. Thanks, Cleo! 

Radio astronomers are gearing up for a new generation of radio telescopes that will be based on radically new design concepts: a wide field of view and a high-fidelity snapshot capability. Testbed instruments are springing up around the world to explore the technical challenges and scientific potential that these next-generation instruments will bring.

(Photo credit: “MWA 32T Tile” by Natasha Hurley-Walker)

One such instrument is the Murchison Widefield Array (MWA), a radio telescope in the Australian outback. The MWA consists of 128 phased tiles. The picture to the left shows an individual tile.

It has a superb ability to image spatial variations in the electron content of the atmosphere. Electrons get freed from gas molecules by a variety of natural processes, including ionization by sunlight and impacts by energetic particles coming from outer space. This ionized gas is called a plasma, and the ionized part of the atmosphere is called the ionosphere. The MWA can “image” features in the ionosphere through how it warps images of distant stars and galaxies, just as imperfections in a glass window pane warp a background image.

Many things generate irregularities in the sea of electrons floating around the Earth, such as thunderstorms, earthquakes and meteors. Bubbles, ripples and plumes are examples of structures that have been discovered by scientists over the years. At high latitudes near the auroral regions and poles, the Earth’s magnetic field lines join with those of the Sun, which are swept outwards by a supersonic wind and stream past the Earth. In these regions, charged particles from the Sun can enter the Earth’s magnetosphere, the region of influence of the Earth’s magnetic field. Energetic processes funnel these particles down onto the atmosphere, producing auroral displays as they collide with oxygen and nitrogen gas. The dragging around of magnetic field lines also stirs up the plasma, forming strange features such as blobs, holes and tongues of ionization. At these latitudes, the ionosphere is active indeed.

At low and middle latitudes, the Earth’s magnetic field lines form closed loops within the magnetosphere, shielding the atmosphere from energetic particles. The ionosphere here is much quieter, although it still has its fair share of irregularities. Among these are the plasma tubes discovered in MWA data, cylindrical volumes several tens of kilometres in diameter in which there are more electrons than their surroundings. Like aurorae and many other plasma structures, these tubes align with the Earth’s magnetic field. However unlike aurora, they are invisible to the naked eye because they do not glow. They are subtle “warps in the glass” formed by an unknown process, and can only be seen with the help of backlighting: the stars and galaxies in the sky.

An animated visualization showing the plasma tubes (in red and orange) aligning with the Earth’s magnetic field. Credit: Loi et al./University of Sydney via The Age.

Scientists have long taken for granted the existence of these tubes, observing them through their properties as optic-fiber-like guides for electromagnetic waves called “whistlers”, and naming them “whistler ducts”. They live at high altitudes, much higher than do aurora. However, they have been notoriously difficult to directly direct because their sizes and altitudes are not easily probed by most existing techniques. The exquisite sensitivity of the MWA has produced spectacular visualizations of a phenomenon that scientists have been struggling to glimpse all these years. This demonstrates the tremendous potential for an MWA-like radio telescope as a geospace monitoring tool, and opens up exciting new possibilities.

If such an instrument were to be built at high latitudes, it could provide a radio telescope’s view of auroral activity that would complement visible-light sightings. This could be used to forge a better understanding of what happens to plasma near the Earth during an auroral display, since radio telescopes can see much higher up. Some advantages of a radio telescope over visible-light observations are that it can observe through clouds, when the moon is up, and even in the daytime. It would have a unique perspective to offer on auroral activity, just as it has for the sky over the Australian outback, and it is likely to reveal more things we didn’t think we’d ever see.

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Cleo Loi is a guest contributor to the Aurorasaurus blog. She is an Australian astrophysicist and undergraduate at the University of Sydney School of Physics.

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