Tikalon Blog by Dev Gualtieri

Jovian Radiation

August 30, 2021

One obvious thing done by early radio astronomers was to point their antennas towards the Sun. Something that big and energetically radiating must be pumping out detectable radio waves. Radar operators had detected solar radio waves during World War II, but this information was kept secret. The first published account was by pioneer radio astronomer, Grote Reber (1911-2002), who conducted observations at 160 MHz.[1] Reber found a solar radio intensity at this frequency of 10^-22 watts per cm² per circular degree per MHz, and he noted that this intensity was much less than that observed for the central Milky Way source in Sagitarius, now known as Sagittarius A.[1] The solar radio emission is still much greater than what would be expected from black body radiation.

The now iconic image of Grote Reber's first radio telescope antenna, built at his home in Wheaton, Illinois, a suburb of Chicago, in 1937.

Reber was an an amateur radio operator (W9GFZ). He built this 31.4 foot diameter parabolic antenna from galvanized iron, which was an inexpensive, conducting metal. Reber stated that he spent $2,000 of his own money, roughly equivalent to his annual salary, in its construction.

Wheaton, Illinois was also the home of astronomer, Edwin Hubble (1889–1953), originator of Hubble's law and for whom the Hubble Space Telescope is named.

(NRAO photograph by Grote Reber, via Wikimedia Commons. Click for larger image.)

Jupiter is often called a failed star. It has an atmosphere that's composed of about 75% hydrogen and 24% helium by mass, which are the same elements in stars. However, Jupiter would need about 20 times its mass to develop enough internal pressure and temperature to initiate hydrogen fusion. Even a failed star emits radio waves, a discovery made in 1955 by Bernard Burke and Kenneth Franklin of the Carnegie Institution of Science in Washington.[2] They detected Jovian radio bursts at 22.2 MHz that matched the period of the planet's rotation.[2] Jovian radio bursts are of two types - long bursts of up to several seconds, and short bursts lasting a few tens of milliseconds.

While other cosmic radio waves are so weak that they can only be detected with the huge radio telescopes used by radio astronomers, such radio signals from Jupiter are so powerful that anyone can detect them with a simple radio receiver. Radio JOVE, created in 1998 by James Thieman, a radio astronomer at NASA's Goddard Space Flight Center (Greenbelt, Maryland), and presently run as a non-profit organization by Thieman and a group of other radio astronomers, sells kits for building such receivers.[3]

Radio JOVE receiver circuit board.

The receiver operates at 20.1 MHz, and the circuit design dates back to 2012. Other options are the presently available software-defined radios (SDRs) that interface to computers by a USB connection. These are inexpensive, but an additional upconverter is needed to allow reception at Jovian radio frequencies.

A simple dipole antenna operable at 20.1 MHz is about twenty feet long, and building it takes a little work. The Radio Jove website gives antenna construction directions, but I would advise checking information from the American Radio Relay League about antenna safety.

The software that I use for my (non-Jovian) SDR radio reception in Linux is gqrx, but other operating systems have similar applications.

(NASA/Radio JOVE Project image from the JOVE Receiver Kit Assembly Manual. Click for larger image.)

One thing that the Earth and Jupiter have in common is lightning. lightning strikes occur on Earth at least a billion times each year, and lightning strikes have been detected on Jupiter.[4] There's radio emission from Jupiter lightning, just as on Earth, but only at low frequencies.[4] Experiments by Joseph R. Dwyer (b. 1963) and his colleagues in 2003 showed that the leaders of Earth lightning emit bright bursts of x-rays thought to be caused by Bremsstrahlung emissions from relativistic electrons accelerated in air by the intense electric fields.[5-6] Jupiter is also a strong emitter of x-rays, not by lightning, but by another process.[7-10]

Recently, a huge international team of planetary scientists have published an open access paper in Science Advances that describes the mechanism for x-ray emission from Jupiter.[7] The team has members from the Chinese Academy of Sciences (Beijing, China), University College London (Dorking, United Kingdom), the Smithsonian Astrophysical Observatory (Cambridge, Massachusetts), the British Antarctic Survey (Cambridge, United Kingdom), Johns Hopkins University (Laurel, Maryland), the Southwest Research Institute (San Antonio, Texas), the University of Texas at San Antonio (San Antonio, Texas), the Université de Liège (Liège, Belgium), Northumbria University (Newcastle upon Tyne, United Kingdom), Wuhan University (Wuhan, China), the European Space Agency (ESA), the NASA Goddard Space Flight Center (Greenbelt, Maryland), the University of Iowa (Iowa City, Iowa), and the Imperial College London (London, United Kingdom).

The origin of Jupiter's x-rays has remained unknown since its discovery forty years ago.[7] This recent research shows that the x-rays are produced by Jovian aurorae that are similar to aurorae on Earth; this, despite the fact that the energies are orders of magnitude greater - hundreds of gigawatts.[7-8] Jupiter's x-ray aurorae are produced by mega-electronvolt (MeV) ions of sulfur and oxygen captured from volcanic activity of its moon, Io.[7]

Overlaid images of Jupiter's north pole (left) and south pole (right) from NASA's JJuno spacecraft and Chandra X-ray Observatory. The x-ray aurorae are shown in purple atop visible images from Juno.

(NASA Chandra/Juno Wolk/Dunn image via University College London. Click for larger image.)

The x-rays often pulse in a period of a few tens of minutes coincident with emissions in the ultraviolet, infrared, and radio spectra.[7] Jupiter's x-ray bursts are triggered by periodic vibrations of Jupiter's magnetic field lines that create plasma waves that cause the sulfur and oxygen ions to travel along them, ultimately impacting Jupiter's atmosphere and generating x-rays in the process.[8,10] Says William Dunn of University College London and co-principal investigator of the study,

"We have seen Jupiter producing X-ray aurora for four decades, but we didn't know how this happened. We only knew they were produced when ions crashed into the planet's atmosphere... Now we know these ions are transported by plasma waves - an explanation that has not been proposed before, even though a similar process produces Earth's own aurora. It could, therefore, be a universal phenomenon, present across many different environments in space."[8]

The x-ray emission from Jupiter's south pole consistently pulse with an 11 minute period, but the north pole emissions are erratic with intensity that's independent of those at the south pole.[10] Earth's north and south aurorae behave similarly to each other.[10] Why Jupiter's magnetic field lines vibrate with this periodicity is unclear, but the vibration might be caused by interaction with the solar wind or some plasma dynamics in Jupiter's magnetosphere.[8] Jupiter's magnetic field is about 20,000 times stronger than Earth's field, so its magnetosphere is extremely large.[8]

This research was funded by the Chinese Academy of Sciences, the Science and Technology Facilities Council (STFC) of the United Kingdom, the Royal Society, the European Space Agency (ESA) and NASA, among other sources.[8]

Jovian Radiation

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