CAASTRO/Loi et al. Plasma tubes in the Earth's magnetosphere have been observed for the first time

A 60-year-old theory about the structure of the magnetic fields that surround Earth has been confirmed directly for the first time. The lead author of the paper is an undergraduate student who invented a way to view the Earth's magnetosphere in three dimensions.

The sun emits a constant stream of charged particles that's supplemented by cosmic rays from sources such as supernovae. As these particles approach Earth, their path is altered by the Earth's magnetic field, which deflects some and funnels others towards the poles, leading to displays such as aurora.

This region, known as the magnetosphere, includes the ionosphere and plasmasphere as its inner layers. These distinctions aside, we don't know all that much about the structure of these regions.

A better understanding would be useful because the ionosphere interferes with satellite navigation systems and the images received by radio telescopes. During her honors project at the University of Sydney, Cleo Loi realized she could use the Murchison Widefield Array (MWA) radio telescope to probe these regions in a way that had never been done before, leading to a paper in the journal Geophysical Research Letters.

The MWA is a forerunner of the Square Kilometer Array (SKA), soon to transform radio astronomy. It consists of 128 antennae spread over three kilometers (almost two miles). Loi suggested that by splitting the observations between those from the eastern and western ends of the array, she would achieve something equivalent to binocular vision, allowing us to see in three dimensions.

For the MWA's usual astronomical work, a three-kilometer baseline does not give the parallax required to see in-depth, but the situation is very different when we are looking close to the Earth.

Loi detected a series of high and low density plasma tubes connecting the ionosphere and plasmasphere running in parallel to the magnetic field. "We measured their position to be about 600 kilometres [373 miles] above the ground, in the upper ionosphere, and they appear to be continuing upwards into the plasmasphere. This is around where the neutral atmosphere ends, and we are transitioning to the plasma of outer space," Loi said. The tubes move slowly with time, so telescopes experience changing interference effects.

Loi told IFLScience that while some limited probing has been done on the ionosphere with other radio telescopes such as the Very Large Array, no one to her knowledge has applied parallax to the problem previously.

“People theorized something like this from observations of a type of very low frequency electromagnetic wave. We can detect lightning from another hemisphere and people concluded there must be plasma tubes guiding the signal,” says Loi. “It's a very indirect conclusion, and no one had much idea what these tubes were like.”

Loi says she was amazed at how many tubes the MWA's enormous 30° field revealed. “There are no plans to use the SKA for studying the ionosphere, but I am hoping with publicity about this work to change that,” she says.