A new hypothesis explains the radar signature of plasma waves in Earth’s ionosphere
Puerto Rico’s massive Arecibo Observatory, 305 meters across, can be used to observe radio waves coming from distant objects—and also as a radar facility to observe the Earth’s ionosphere. Credit: NAIC–Arecibo Observatory
Source: Radio Science

For decades, space physicists have used radio telescopes like Arecibo in Puerto Rico and Jicamarca in Peru as enormous, unconventional radar facilities: Instead of detecting individual targets like aircraft, the beam scatters off of the ions and electrons in the ionosphere, giving a faint return signal that indicates their density and temperature.

Although this signal is mostly smooth, its strength spikes at certain frequencies where various kinds of plasma waves in the ionosphere resonate and give the signal a boost. The plasma in motion can be either ions or electrons, each of which modulates the signal in different ways.

One of the most curious of these spectral spikes is the so-called gyro line, which appears as a signal shifted a fraction of a megahertz off the main radar frequency, depending on the plasma conditions.

In 1978 a seminal paper laid out a theoretical foundation for gyro lines, arguing they are caused by electrostatic whistler waves, in which electrons move back and forth. Using this foundation, an equation to predict the frequency of the gyro line for a given set of a conditions was produced. But many have noted over the years that it has limited success in predicting observations.

Now, in a new paper, Hysell et al. upend the conventional wisdom by correcting some significant errors in the original 1978 paper. They point out that in deriving the equations that define the wave properties, some mathematical simplifications were made that don’t fit the conditions. By undoing them, they arrive at a new framework that much better matches observations.

The key equation is the dispersion relation, which defines how the waveform behaves as it propagates. The calculations in the original paper assumed that the wave’s frequency is much lower than that of the gyrations of the free electrons surrounding it, which simplifies the math. However, this condition can be met only under certain conditions involving the angle of the magnetic field. Because of the geometry of Earth’s magnetic field, which “dips” toward the poles, this condition cannot be met at middle to high latitudes, where most incoherent scatter radar facilities are located.

Furthermore, for such waves, electrons aren’t the only plasma particles involved: At lower frequencies, the much heavier ions can also move and resonate with the wave. The current authors note that this means that a different kind of wave can emerge in which ions and electrons move together, called a lower hybrid wave.

So although the original paper presented a neat, clean, analytic expression, the current authors find that no such expression exists for the variety of conditions that typically occur at incoherent scatter radar facilities.

The authors propose a new method: Instead of using fixed parameters in a single equation, the parameters are allowed to vary and are fit according to observations. The authors tested this approach using observations at the Arecibo Observatory in Puerto Rico in October 2016 and were able to reproduce the gyro lines they observed much more accurately. (Radio Sciencehttps://doi.org/10.1002/2017RS006283, 2017)

—Mark Zastrow, Freelance Writer


Zastrow, M. (2017), Mystery of the ionosphere’s “gyro line” solved, Eos, 98, https://doi.org/10.1029/2017EO081279. Published on 06 September 2017.

Text © 2017. The authors. CC BY-NC-ND 3.0
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