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Imaging Earth’s plasma ducts

The clever use of a radio telescope reveals a long-assumed feature of our near-space environment.

For many decades, researchers have known that the Sun’s light ionizes the upper reaches of Earth’s atmosphere. The resulting charged particles in the ionosphere—and the even more rarefied plasmasphere above it—have a noticeable effect on impinging electromagnetic signals. Additionally, waves of plasma known as “whistler modes” have been observed for the past 50 years or so as they bounce back-and-forth between Earth’s hemispheres; thus have researchers recognized that the plasma supporting those waves is in cylindrical structures aligned with Earth’s magnetic field lines. And now the inferred “plasma ducts” have been imaged in real time, in three dimensions, and over a large swath of sky. University of Sydney undergraduate Cleo Loi, her adviser Tara Murphy, and a host of colleagues used the Murchison Widefield Array (MWA) radio telescope in Western Australia to monitor cosmically distant radio sources that backlight the fluctuating plasma. As myriad sources shifted their apparent positions due to refraction by free electrons, a series of wide-angle snapshots was taken and the sources’ vector displacements—as well as the divergence of those vectors—were plotted. A typical plot is shown here; the red and blue regions correspond to positive and negative divergences and also represent relative overdensities and underdensities of free electrons. The alternating structures are persistent and align perfectly with Earth’s magnetic field lines, superimposed in black. In a further step, Loi and company split the MWA into halves to perceive depth just as our eyes would. The resulting 3D view confirms that the plasma structures are indeed ducts rather than sheets and that they bridge the ionosphere and plasmasphere. (S. T. Loi et al., Geophys. Res. Lett., in press, doi:10.1002/2015GL063699.)

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