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Exoplanets turn the knobs on radio star transmissions

posted Oct 20, 2019, 3:37 PM by Jeremy Drake
Many postings on this site deal in one way or another with the energetic radiation from the hot, multi-million degree outer atmospheres - the "corona" - of stars like the Sun. Heated by the dissipation of magnetic energy generated by an interior dynamo, the most conspicuous manifestation of stellar coronae is their X-ray emission. The X-ray emission is a natural consequence of the interaction of fast-moving electrons and ions in an ionized gas at such high temperatures. Electron impacts with ions "excite" electrons still bound to ions into higher energy stats which then spontaneously decay, emitting the energy in the form of energetics photons - X-rays - at discrete energies. But an electron simply moving in the electric field of an ion, or in the magnetic field of the star, can also lead to photon emission over a very large range of energies, including the radio. Stars like the Sun are, in fact, routinely detected at radio wavelengths.

Light travelling through plasma is subject to refraction and bending - just like through other substances such as glass or water. The effects are generally negligible in stellar coronae but become significant at radio frequencies. Exoplanets close in to their parent stars have the potential to affect the radio
propagation because they act as a barrier to the stellar wind plasma and change its flow and density structure as the plasma streams past. University of Massachusetts Lowell professor, Ofer Cohen, lead a study published in the Astrophysical Journal to examine this effect using 3D numerical simulations and determine whether it has any observable signature.  We found quite large modulations of up to a factor of 2 in the synthetic stellar radio emission in the 10-100 MHz range could be induced by a planet as it moved on its orbit. The intensity modulations were sensitive to both the strength and polarity of the planetary magnetic field.

Several studies in the past have examined the possibility of detecting radio emission from exoplanets themselves, but the predicted signal tends to lie mostly at very low frequencies less than a few MHz that do not pass through the Earth's ionosphere. Our study indicates that radio observations from the ground at higher frequencies could instead be used to investigate the magnetic properties of exoplanets. The magnetic field of a planet is particularly interesting as a probe of interior planetary structure and also for the protection it affords from stellar winds and coronal mass ejections that can erode planetary atmospheres.