6 July 2011 — Saturn’s storms can produce lightning 10 000 times more intense than Earth’s ones. Convective storms larger than 2000 km have been observed during the past few years, and may last for months. A paper making the cover of Nature this week, co-authored by an astronomer from Paris Observatory, presents observations started in December 2010, of a giant storm that emits in radio waves an energy comparable to that emitted by the entire planet.
The paper published this week presents joint observations in RADIO (by Cassini) and VISIBLE light (by Cassini and from the ground), and gathers thus Co-Is of the Cassini RADIO instrument (the first 4 authors) and of the IMAGING subsystem (the 4 next), as well as 3 amateur astronomers. We know since the start of Cassini’s orbital mission (2004) that storms on Saturn occur very irregularly on the long term, with alternance of activity and inactivity periods lasting several weeks to several months. While all storms have been observed at a latitude of 35° South from 2004 to 2010 (until 1 year after the equinox that occured in August 2009), the new storm here appeared suddenly in December 2010 at a latitude of 35° North, suggesting a seasonal cycle of Saturn’s storm activity, shifted by about one year relative to the equinox for a reason still not understood. This storm, which lasts now for more than 6 months (it is still very active today), is spectacular in radio (with lightning intensity 10 times stronger than during its most intense predecessors, occurrence rate up to 10 flashes / second) as well as in visible light (extension of several tens of thousands of km in longitude, 10000 km in latitude; the main vortex could almost contain the whole Earth !). The energy involved contributes significantly to the energy budget of the atmosphere.
More generally, Saturn’s storm activity shows a tendency to intensify around the vernal equinox (actually since November 2007). The polarisation measured in radio shows that the lightning-associated radio emission is produced on the so-called "Ordinary" mode of propagation. This information is important for modelling the radio propagation of lightning through Saturn’s ionosphere and eventually to derive the ionospheric density profile at the latitude of the storm (this work is in progress).
Finally, amateurs have significantly contributed to the optical observations
Figure 1: Dynamic spectrum of lightning detected in radio by Cassini on 12 December 2010. The intensity of the emissions is plotted as a function of spacecraft receiving time (SCET) and frequency (from 500 kHz to 16 MHz on a logarithmic scale). Cassini coordinates (distance to Saturn’s centre in planetary radii .RS- and west longitude in °) are indicated on the abscissa. Cassini was in the equatorial plane at a local time of 18.6 h. Cassini’s RPWS (Radio and Plasma Wave Science) is a swept-frequency spectrometer in the frequency range considered here. It detects the lightning, intrinsically broadband, at whatever frequency it happens to be tuned to at the time of the flash. The low frequency limit is fixed by Saturn’s ionbosphere, which blocks the atmospheric radio emissions at frequencies lower than its local maximum plasma frequency. The flashes were so numerous (5 to 10 per second) that they are superposed and canot be resolved near the center of the displayed interval, while individual events remain detectable on the edges. The continuous emission below 800 kHz is Saturn’s magnetospheric auroral radio emission.
Figure 2: Images of Saturn with the storm. a, Image taken on 13 December 2010 with an 11-inch Schmidt-Cassegrain telescope in the Philippines. The centre of the storm was at longitude 262° W and latitude 34° N. The latitudinal size of the storm reaches 7° (6,800 km). b, Image taken on 22 December 2010 with a 16- inch Newtonian telescope in Australia. The centre of the storm was at longitude 283° W, and its latitudinal and longitudinal extent reached 9,000 km and 15,000 km, respectively. c, Image taken on 24 December 2010 with the wide-angle camera of Cassini. Here the storm’s latitudinal and longitudinal extent is 10,000 km and 17,000 km, respectively, with an eastward tail that extends much further. The storm centre was at longitude 288° W.
Crédits images : C.G., A.W., NASA/JPL/SSI.
Figure 3: False-colour views showing the height of the storm clouds. Images with three filters sensitive to different amounts of absorption by methane gas were superposed. The filtered image at 889 nm is projected as blue, and sees only the highest clouds. The filtered image at 727 nm is projected as green, and sees high and intermediate clouds but not the lowest clouds. The filtered image at 750 nm is projected as red, and sees clouds at all levels. a, b, Magnified views at full resolution of the areas indicated by white brackets in c. d, Image showing the storm 11 h after image c was taken, and reavling a morphological evolution. The images were taken by Cassini’s narrow angle camera on 26 February 2011 from a distance of 2.4 millions of km.
