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Radio Emission Properties of Jupiter and Saturn

-- Teresa | December 30, 2020

Question:

The NRAO recently shared on their Facebook page a hypothetical picture of what Jupiter’s and Saturn’s conjunction would look like if observed by the VLA at a wavelength of 2 cm (15 GHz). I found it really interesting how both planets look radically different from each other at that wavelength and was wondering if you could tell me why. Both Saturn’s disc and rings were easily visible at 2 cm–do the rings naturally radiate at around 15 GHz due to their low temperature, or are we seeing any sort of interaction with Saturn’s magnetic field? And Jupiter’s strong magnetic field appears to be easily visible at 2 cm, but the disc of the planet itself was missing. What do the shape of Jupiter’s radio emissions at this wavelength tell us about its magnetic field and how it differs from Saturn’s? Was the 2 cm wavelength chosen specifically because it would show interesting contrasts between Jupiter and Saturn in the radio wavelengths?

Thank you!

-- Teresa

Answer:

Both planets have strong radio emission due to several different emission processes. Below a frequency of 40 MHz, cyclotron-maser emission is the dominant mechanism, with much of this being associated with the rotational phase of the nearby volcanic moon Io. Between 40 MHz and a few GHz, the radio emission is dominated by synchrotron emission from magnetically trapped, highly relativistic electrons which spiral around the strong magnetic field lines of Jupiter. This emission occurs mainly within a region that is a few Jovian radii from the planet. At frequencies above 40 GHz, the thermal emission from the heating and re-emission of dust in the planet’s atmosphere dominates. Saturn’s radio emission is also produced by these emission processes, but due to the properties of its rings, it appears bright at frequencies higher than a few GHz. I suspect that the image of Jupiter chosen in the comparison was most sensitive to the intense radio emission produced by interactions with its magnetic fields, with the weaker emission from Jupiter’s disk suppressed. I suspect that the 2cm wavelength for this comparison was chosen by convenience, as it is one of the most sensitive wavelengths for observations with the VLA.

-- Jeff Mangum

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