Earliest recorded ground-based decameter wavelength observations of Saturn’s lightning during the giant E-storm detected by Cassini spacecraft in early 2006

Abstract : We report the history of the first recorded ground-based radio detection of Saturn’s lightning using the Ukrainian UTR-2 radiotelescope at frequencies from 20 to 25 MHz. The observations were performed between 29 January and 3 February 2006, during which lighting activity (E-storm) on Saturn was detected by the radio experiment onboard Cassini spacecraft. The minimum detectable flux density (1σ1σ-level) at UTR-2 reached 40 Jy (View the MathML source1Jy=10-26Wm-2Hz-1) for narrowband observations (View the MathML sourceΔf=10kHz) and 4 Jy for broadband observations (View the MathML sourceΔf=1MHz), for an effective telescope area of View the MathML source≈100,000m2 and integration time of 20 ms. Selection criteria including comparison of simultaneous ON/OFF-source observations were applied to distinguish detection of lightning-associated radio pulses from interference. This allowed us to identify about 70 events with signal-to-noise ratio more than 5. Measured flux densities (between 50 and 700 Jy) and burst durations (between 60 and 220 ms) are in good agreement with extrapolation of previous Cassini measurements to a ground-based observer. This first detection demonstrates the possibility of Solar System planetary lightning studies using large, present and future ground-based radio instruments. The developed methods of observations and identification criteria are also implemented on the UTR-2 radio telescope for the investigation of the next Saturn’s storms. Together with recently published UTR-2 measurements of activity measured after the 2006 storm reported here, the results have significant implications for detectable planetary radio emission in our Solar System and beyond.