Foreshock Radio Emissions

The continuous interaction of the solar wind with solar system bodies produces a standoff shock known as a bow shock at which incoming particles from the solar wind are promptly accelerated (see Fig. 1).

Consequently, the region immediately upstream of a bow shock, known as a foreshock, is a propitious site for energetic particle and plasma wave phenomena. Earth's foreshock is naturally the most accessible in the solar system and spacecraft observations over many decades have confirmed that it contains a rich variety of energetic, unstable electron distributions and plasma waves, including Langmuir waves and electromagnetic (radio) waves (see Fig. 2).

We have developed a quantitative model for the terrestrial foreshock radio emissions that takes into account electron acceleration at the bow shock, stochastic growth of Langmuir waves from unstable electron beam distributions, and nonlinear interactions between the Langmuir waves with other waves to produce the electromagnetic waves. Our results have been directly compared with data from the Geotail satellite currently orbiting Earth (see Fig. 3).

We have also generalised our foreshock radio emissions model for applications to other planets and also to planetary satellites such as our own Moon. We have made specific theoretical predictions for the radio emissions from other planetary foreshocks which will be testable with forthcoming space missions.

Key References:

Kuncic, Z. & Cairns, I. H., 2005, Planetary Foreshock Radio Emissions, J. Geophys. Res., in press.

Kuncic, Z. & Cairns, I. H., 2004, Radio Emission from Mini-Magnetospheres on the Moon, Geophys. Res. Lett., 31(11), 809.

Kuncic, Z., Cairns, I. H. & Knock, S. A., 2004, A Quantitative Model for Terrestrial Foreshock Radio Emissions I: Predicted Properties, J. Geophys. Res., 109(A2), 1.

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Figure 1. Schematic illustration depicting the terrestrial foreshock region, immediately upstream of Earth's bow shock and downstream of the connecting interplanetary field line.

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Figure 2. A dynamic spectrum obtained from a crossing of Earth's bow shock by the Wind spacecraft showing first and second harmonic radio emission in the foreshock region.

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Figure 3. Theory vs. data comparison for terrestrial foreshock radio emission: the levels of second harmonic radio emission predicted by our model generally underpredict the observed levels by a factor of a few. Here, we show the predicted radio emission increased by 5dB (left panel) to better match the levels detected by the Geotail satellite (right panel).

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