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Radio bursts: from Jupiter to the stars

3 October 2023

The combined results of two recent papers illustrate how studies of radio emissions from Jupiter can be transposed to star systems. Mauduit et al. [1] have developed a new method for detecting radio bursts of Jupiter drifting in the time-frequency plane. Applied to high-resolution data from the Nançay Decameter Array, it was possible to discover bursts linked to the Jovian auroras and others induced by the interaction of Ganymede with Jupiter. These bursts led to study the acceleration of electrons near Jupiter. In parallel, Zhang et al. [2] discovered, using the FAST radio telescope, similar bursts emitted by the active star AD Leo, which can be interpreted by analogy with the results obtained on Jupiter.

Detecting radio bursts drifting in the time-frequency plane is a powerful way to remotely study electron acceleration processes in astrophysical plasmas. It has been known for 50 years that such bursts are produced by the interaction between Jupiter and its satellite Io. A new detection algorithm has also made it possible to discover, via the analysis of massive time-frequency data recorded by the Nançay Decameter Array, similar bursts from the Ganymede-Jupiter interaction and the Jovian polar auroras [1]. The acceleration of electrons involves magnetic oscillations called “Alfvén waves”. This potentially universal mechanism is powered by the motion of Io, Ganymede, or magnetospheric plasma through the rapidly rotating magnetic field of Jupiter. In parallel, very sensitive observations with the giant Chinese radio telescope FAST have led to the discovery of radio bursts from the active star AD Leonis [2]. If the frequencies, intervals between bursts, intensities, direction and speed of drift in the time-frequency plane are different, the morphology, the discrete, drifting, quasi-periodic character (and even the polarization, strongly circular) of the stellar bursts are remarkably similar to those of Jupiter. The experience acquired on Jupiter suggests in the case of AD Leo the same mechanism of radio emission, fed by electrons of modest energy (≤ 20 keV), accelerated by Alfvén waves resulting from the interaction of the star with its massive plasma wind or with a planetary companion.

The figure illustrates and compares the radio bursts of Jupiter (b) recorded between 11 and 13.5 MHz by (a) the Nançay Decameter Array (NDA) with its Juno-N high resolutio receiver, and the radio bursts of AD Leonis (e ) observed between 1290 and 1470 MHz by the giant Chinese radio telescope FAST (d). Both types of bursts reveal a strong circular polarization, suggesting the same emission mechanism linked to the cyclotron (helical) motion of electrons in the magnetic field of the object. The difference in frequencies is explained by the amplitude of the magnetic field of AD Leo 100 times greater than that of Jupiter, which also explains the much faster time-frequency drifts for AD Leo ( 500 to 900 MHz/s) than for Jupiter (-10 to -20 MHz/s), even if the electrons at the origin of the emissions have similar energies (a few keV). The positive drifts observed for AD Leo imply that these electrons are moving towards the star. The negative drifts observed for Jupiter indicate electrons magnetically reflecting and “going up” Jupiter’s magnetic field lines (yellow arrows in panels (c) & (f)). The radio fluxes detected for Jupiter (millions of Jansky) are much higher than those detected for AD Leo (<0.1 Jansky), due to the huge difference in distance between these objects and the Earth ( 5 Astronomical Units for Jupiter, 5 parsecs for AD Leo, with 1 parsec = 200000 AU), knowing that the flux decreases as the inverse of the square of the distance. But the intrinsic flux of the AD Leo bursts is actually 100 to 1000 times higher than that of the Jupiter bursts. The discrete and quasi-periodic character of the bursts suggests an acceleration of the electrons by Alfvén waves (magnetic oscillations). These two articles are a first example of how the mechanisms and physical scenarios developed for a solar system object can be extrapolated and transposed to more distant and inaccessible astrophysical sources. Sources and copyrights (a) Nançay Decameter Array (NDA, © ORN). (b) NDA/Juno-N observation provided by L. Lamy. (c) Diagram adapted from Fig. 3 from Szalay et al., Geophys. Res. Lett. 49, e2022GL098111, 2022, https://doi.org/10.1029/2022GL098111. (d) https://apod.nasa.gov/apod/image/1609/DaiFAST_1500.jpg. (e) Figure adapted from Fig. 1 of [2]. (f) Artist’s impression of a dwarf star (© ASTRON/Danielle Futselaar).
References:

[1] Emilie Mauduit, Philippe Zarka, Laurent Lamy, & Sébastien Hess, Drifting discrete Jovian radio bursts reveal acceleration processes related to Ganymede and the main aurora, Nature Communications, 14, 5981 (2023)

[2] Jiale Zhang, Hui Tian, Philippe Zarka, Corentin Louis, Hongpeng Lu, Dongyang Gao, Xiaohui Sun, Sijie Yu, Bin Chen, Xin Cheng, & Ke Wang, Fine structures of radio bursts from flare star AD Leo with FAST observations, The Astrophysical Journal, 2023, 953, 65

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