Lightning activity on Titan: can Cassini detect it?

Although no lightning discharges were observed during the Voyager 1 flyby of Titan, this lack of evidence does not rule out the existence of lightning phenomena which could be detected by the Radio and Plasma Wave Science (RPWS) instrument on board of the Cassini spacecraft. The existence of lightning or other electromagnetic discharges has been suggested to explain the formation of hydrocarbons and nitriles in the context of Titanʹs complex organic chemistry. Although thunderclouds may be a rare phenomenon in Titanʹs lower atmosphere, recent investigations show that such clouds may cause temporary maximum electrical fields in the order of 2×106 V m−1 sufficient to initiate 20 km long Earth-like cloud-to-ground type 2 lightning strokes. Since such clouds are likely to be correlated with strong convection near the subsolar point, we expect possible lightning flashes to occur only on the dayside. Recent telescopic infrared observations have detected localized, high, short-lived clouds on Titan. We have calculated the flash characteristics, frequency spectrum, maximum spectral energy and the electromagnetic energy radiated into the troposphere by using a wave guide model of lightning currents for Titanʹs lightning strokes. Our study indicates that cloud-to-ground lightning strokes on Titan would be comparable with so-called type 2 lightning strokes on Earth. Their total radiated energy to the far field could be about 130 kJ and their maximum energy at a frequency of about 4 kHz. In order to estimate the capability of the Cassini/RPWS instrument to detect lightning discharges during several close Titan flybys, we distinguish the atmospheric regions, where the propagation of electromagnetic waves is unperturbed or where it is impossible. We found that the Cassini/RPWS instrument should be able to detect electromagnetic signals generated from a representative cloud-to-ground lightning stroke in Titanʹs lower atmosphere in a frequency range above 500 kHz or 1 MHz up to 200 Titan radii away. We suggest that for the search of lightning signals the RPWS high-frequency receiver HF2 with its H2-1E or H2-1E/F receiver modes and low integration times Δt of 10 or 20 ms should be chosen. Since the lightning flash rate might be low (<1 flash per hour) it is important to have long observation times. An analysis of all Cassini trajectories of Titan close flybys shows that the spacecraft would have the opportunity to observe Titanʹs dayside within 100 Titan radii during nearly all flybys. This time should be long enough, even if the lightning flash rate is low.