Study of solar system planetary lightning with LOFAR

Radio signatures of lightning discharges have been detected by the Voyager spacecraft near Saturn and Uranus up to 40 MHz. Corresponding flux densities at the distance of the Earth are up to 1000 Jansky (Jy) for Saturn (1 event per minute above 50 Jy, with 30–300 ms duration) and up to a few tens of Jansky for Uranus. Low Frequency ARray LOFAR will allow us to detect and monitor the lightning activity at these two planets. Imaging will allow us to locate lightning sources on Saturn's disk (even if with moderate accuracy), which could then be correlated to optical imaging of clouds. Such observations could provide new information on electrification processes, atmospheric dynamics, composition, and geographical and seasonal variations, compared to the Earth. In addition, lightning may play a role in the atmospheric chemistry, through the production of non-equilibrium trace organic constituents potentially important for biological processes. LOFAR observations can also help us to assess the existence of lightning at Neptune (marginally detected by Voyager), at Venus (where their existence is very controversial), and at Mars (possibly resulting from dust cloud charging). At Jupiter, low-altitude ionospheric layers of meteoritic origin and/or intrinsically long discharge duration seem to prevent the emission and escape of high-frequency radio waves associated with lightning. LOFAR thus presents good possibilities for the detection and study of solar system planetary lightning; we also discuss its relevance to bring new information on Terrestrial lightning-related upper atmosphere transient phenomena (sprites, TIPPs…). Instrumental constraints are outlined.