Identificación de peligros, evaluación de riesgos y sus controles:

It has been suggested that Saturn has similar cloud structure to Jupiter’s, with a top deck com- posed of NH3ice particles, followed by an NH4SH cloud layer, and below that a water ice cloud

and liquid water-ammonia (Atreya & Wong,2005;Yair et al.,2008, Fig. 4.1). Saturn has been ob- served to host large, energetic thunderstorms, which appear infrequently (e.g.Fischer et al.,2008;

Dyudina et al., 2013; Sánchez-Lavega et al., 2016). Li & Ingersoll(2015) modelled moist con- vection in the atmosphere of Saturn, with the aim of explaining the episodic appearance of giant thunderstorms on the planet. They proposed that moist convection is suppressed for decades in Saturn’s atmosphere due to the large molecular weight of water in such H2-He rich atmospheres.

Figure 4.4: Dynamic spectrum (frequency vs time) of Saturn Electrostatic Discharges (SED; dark black spikes) recorded by the RPWS instrument of theCassinispacecraft in 2004. Spacecraft event time (SCET), distance in Saturn radii, western longitude and latitude, and local time of the spacecraft are given along the time axis. Figure reproduced with permission fromFischer et al.(2006a).

They suggested that the quasi-periodic thunderstorm activity can be explained by an oscillation produced by the interaction between moist convection and radiative cooling in the troposphere. Such oscillations produce giant storms with a period of_∼60 years (Li & Ingersoll,2015).

Lightning-induced sferics named Saturn Electrostatic Discharges (SEDs) were first observed by Voyager 1 during its close approach in 1980 (Warwick et al.,1981). The short, strong radio bursts from Saturnian thunderstorms were detected again by the RPWS (Radio and Plasma Wave Science) instrument of theCassinispacecraft in 2004 (Fischer et al., 2006a, Fig. 4.4). Fischer et al.(2006a) andFischer et al.(2007b) analysed the occurrence rate of SEDs during the 2004- 2006 storms and obtained SED rates that generally vary between 30−87 h−1, with two storms with SED rates much higher, 367 h−1. From Cassini data Fischer et al.(2006a) estimated the total energy of 1012−1013J of Saturnian lightning flashes (Fig. 4.6), based on the assumption that SED energy output is proportional to the total energy the same way as is for Earth lightning.

Farrell et al.(2007) suggested that SEDs are much shorter compared to terrestrial discharges and hence the flash should be less energetic, of the order of 107 J. Fischer et al.(2011b) reported

4.1. Extraterrestrial lightning in the Solar System

Figure 4.5: Spectrogram of a whistler event detected byCassini/RPWS. Frequency: 100−600 kHz. The time axis covers 15 s. The intensity is given in V2_m−2_Hz−1_{, with red indicating the largest values. Arrow} labels the curved feature of the whistler. For comparison to Earth whistlers see Fig. 3.6 in Chapter3. Figure reproduced with permission fromAkalin et al.(2006).

the detection of a giant storm that erupted in December 2010, and examined its SED occurrence. They identified the largest SED rates ever detected on Saturn, to be 36000 SED h−1,_∼98 times larger than the SED rate of the largest episode in 2006.

SEDs were confirmed to be a signature of lightning activity by the Cassinispacecraft, when, based on its data, Dyudina et al. (2007) associated the radio emission with clouds visible on the images. Baines et al. (2009) detected these clouds with Cassini/VIMS, and realized that they appear dark in the NIR. They suggested that lightning at 10 bar level, the base of water clouds (Atreya & Wong, 2005, Fig. 4.1, right panel) produces material, mostly carbon, that is transported to the 1 bar level regime where they obscure the clouds in the spectral range 0.8 to 4.1µm. Bjoraker et al.(2011) and Hesman et al.(2011, 2012) found enhanced amounts of C2H2, C2H4 and C2H6 in the atmosphere of Saturn during the 2010/2011 thunderstorm, which

species could have been produced by lightning in the deeper cloud regions and transported to lower pressures by updraft (Yair,2012).

Akalin et al.(2006) reported the first detection of a whistler event in Saturn’s magnetosphere. Fig. 4.5illustrates the radio signal, which shows the same pattern as Earth whistlers do (Fig.3.6). Based onCassini/RPWS data,Akalin et al.(2006) proposed that the radio signal originated from

Figure 4.6: Cumulative distribution of optical lightning energies (bottom x-axis; open plane symbols) and spectral powers of 35-ms long SEDs (top x-axis; dotted, dashed, and dot-dashed lines). The figure indi- cates thatCassiniobserved the high-energy tale of lightning flashes, assuming the distribution of lightning energies is similar to Earth lightning (see Fig.5.7in Chapter5). Figure reproduced with permission from Dyudina et al.(2013).

lightning on the northern hemisphere. Hassanzadeh Moghimi(2012) suggested the detection of a whistler event at 430 to 200 Hz (higher frequency detected first) inCassini data from 2004. They concluded that the source of the emission was lightning that occurred at 66.85◦ N, and suggested an internal energy source for lightning activity.

The first Saturnian lightning detection in the visible range was reported by Dyudina et al.

(2010). They detected optical lightning flashes on the night side of the planet in the Cassini

wide- and narrow-angle camera data taken in 2009. Dyudina et al.(2010) treated the lightning spots as light sources on top of the cloud emitting isotropically up and down, and estimated the optical energy of a single lightning flash to be 109 J (Fig. 4.6). This way they confirmed the high total energy output of lightning on Saturn as was originally inferred from SED data (Fig.

4.6;Fischer et al., 2006a). Dyudina et al. (2010) also mentioned that these flashes maybe the most energetic ones occurring on Saturn, as the detection limit of the cameras is 108 J in terms

4.1. Extraterrestrial lightning in the Solar System

Figure 4.7: Optical lightning flashes (bright spots) on the night side of Saturn taken by Cassinion 30 November 2009. The images are shown in chronological order (following the numbers) as they were taken during a 16-min observation time. The light-grey cloud with diameter of∼3000 km, was illuminated by Saturn’s rings and did not change during the observations. Figure reproduced with permission from Dyudina et al.(2013).

of lightning optical energy. Based on the observed diameter of the lightning spots (200 km),

Dyudina et al.(2010) inferred a source altitude of 125-250 km below cloud tops, which is above the liquid water-ammonia cloud base, probably in the NH₄SH cloud or in the H₂O ice cloud (Fig.

4.1, right panel). Dyudina et al.(2013) reported further lightning detections (Fig. 4.7) on the dayside by Cassini at latitude 35◦ N, from a new, much stronger storm than previous storms, observed in February, 2011 (also reported inFischer et al.,2011b).

Zarka et al.(2004) estimated the detectability of planetary lightning with the state-of-the-art radio array, LOFAR (Low Frequency ARray), and suggested that SED activity could be monitored by the instrument, and due to the sporadic occurrence of Saturnian thunderstorms, preferably on a regular basis. Griessmeier et al.(2010) andGrießmeier et al.(2011) presented the results of ground-based search for lightning on Saturn using three arrays, LOFAR, UTR-2 (Ukrainian T-shaped Radio telescope), and the Dutch WSRT (Westerbork Synthesis Radio Telescope). Za- kharenko et al. (2012) described the simultaneous observations of Saturnian lightning activity with UTR-2 at frequencies 12 to 33 MHz, and theCassinispacecraft at 1.8 to 16 MHz. They noted

detection of SEDs. Mylostna et al.(2013) conducted further observations of SEDs with UTR-2. They obtained high time-resolution data (µs resolution), and found that SEDs are composed of 100µs bursts with varying intensity, and no finer structure was observed, which is consistent with a high energy release of 1012−1013J, since a discharge with such duration and observed radio power release (_∼50 W Hz−1,Fischer et al.,2006a) has to be very energetic (Farrell et al.,2007). They also reported the low frequency (<200 kHz) power spectrum of SEDs, which showed an intensity peak around 17 kHz, and a spectral variation of f−2 between 20 and 200 kHz. Kono- valenko et al. (2013) summarized the earliest ground-based detections of Saturnian lightning with the UTR-2.

Dubrovin et al.(2014) modelled the effects of lightning activity on the bottom of the iono- sphere (1000 km) of Saturn, and showed that a conservative estimate of charge moment (104₋ 105C km) produced during a lightning flash could result in the production of transient luminous events in the form of halos and sprites. However, if the ionosphere is lower (600 km), a large (106 C km) moment is needed to produce such events. They also suggested that the blue/UV emission from such TLEs would be very faint and not detectable byCassini. Luque et al.(2014) found that lightning induced electromagnetic pulses could carry energies of the order of 107₋1010J to the ionosphere and produce ELVE-like TLEs with energies of 108J.