Saturn Electrostatic Discharges
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Saturn Electrostatic Discharges (also referred to as SEDs) are atmospheric lightning events in convective weather storms on Saturn that produce high frequency (HF) radio emissions (1–40 MHz). Terrestrial lighting events on Earth, in comparison, occur in the very low frequency (VLF) radio band, between 3 Hz and 30 kHz. This makes SED signals at least 10,000 times stronger.[1] While first discovered by NASA's Voyager 1 mission, the scientific community has gained further understanding through the following Voyager 2 and Cassini missions in conjunction with ground-based observation and data gathering methods.
The charging of particles in thunderstorms on earth is most effective at 248 to 263 K. On Saturn this is at 8–10 bars or 200 km below the cloud tops. Both lighting on Earth and SEDs probably have a similar charging mechanism in water clouds around freezing level. Flashes on Saturn have a total energy of about 1012–13 Joules and typically last for 70 milliseconds. Strokes typically last for 100 microseconds on Saturn. Thunderstorms can last days or even months on Saturn, but storms can also be absent for years. One SED illuminates a cloud region about 200 km in diameter.[2]
Saturn Electrostatic discharges were first recorded by the Voyager 1 spacecraft as it passed near Saturn in November 1980. The term was subsequently coined by Warwick et al. in April 1981 in the Journal Planetary Radio Astronomy Observations from Voyager 1 Near Saturn.[3] It was initially uncertain whether these SEDs were associated with storms in the planet's atmosphere or if they were originating in its rings. Evans et al. hypothesized that they originated from a satellite located within Saturn's B Ring, which was also the proposed reason for a narrow feature also found.[4] This was disputed in 1983 by Kaiser et al., who argued that the occultation caused by the planet lasted too long for SEDs to originate in the rings.[2][5]
