Planetary Lightning: A Review

1. Planetary Lightning: A Review Gus Alaka and Doug Stolz 17/22 April 2014 ATS 780 Atmospheric electricity

2. Lightning on other planets • Lightning is possible on other planets. Theoretical studies predict it but observations were inititally difficult to come by. • We think of charge separations in clouds as requiring rimer/ice crystals in the presence of supercooled liquid. Do these hydrometeor species exist in extraterrestrial atmospheres to facilitate charge separation? • What are the major phenomenological differences between terrestrial and extraterrestrial lightning (e.g., from an energetics perspective)? Are breakdown electric fields on other planets larger? Is lightning on other planets more intense? How does the pressure increase on other planets impact the process? SaturnianSuperBolts!

3. Two METHODS TO ACHIEVE CHARGE Separation • Convection cannot alone explain terrestrial lightning • Need particle collisions • On other planets, strong updrafts can vertically displace distinct charge layers

4. Today’s topics • Jovian Lightning • High Latitude Convection • Saturnian Lightning • Superbolts & SEDs • Lightning on other planets • Venus, Mars, Uranus, Titan (Saturn moon)

5. Jovian CloudsPlan View • Several circulation features, with transient eddies capable of supporting deep convection • Jupiter radiates twice as much as it absorbs from the sun, meaning lapse rates are unstable • Yellow/Brown • Photolytically-destroyed acetylene (Bar-Nun 1975) • Blue/White • Ammonia ice clouds (Rossow 1978) • Great Red Spot • Related to sulfur compounds?? • Not ammonia or acetylene

6. Jovian CloudsPlan View • Several circulation features, with transient eddies capable of supporting deep convection • Jupiter radiates twice as much as it absorbs from the sun, meaning lapse rates are unstable • Yellow/Brown • Photolytically-destroyed acetylene (Bar-Nun 1975) • Blue/White • Ammonia ice clouds (Rossow 1978) • Great Red Spot • Related to sulfur compounds?? • Not ammonia or acetylene

7. Jovian CloudsVertical Structure • UV-absorbing haze in stratosphere • Models predict 3 cloud layers above 60km, or approximately 10bar • Ammonia-Water clouds (60-100km) • Rapid Precip in < 103 s • Ammonia Hydrosulfide clouds (100-110km) • Precip ??? • Ammonia ice clouds (130-135km) • Snow time scale ~ 104 s • High pressure and strong gravity lead to rapid coalescence • Bergeron process, too Rossow 1978

8. Jovian CloudsVertical Structure • Galileo probe dropped in 1995 • 2 cloud layers detected • No low-altitude water layer detected! • Several studies prior to Galileo accept this H2O cloud as “truth” • Clouds are thinner than models predicted • Absence of water and other heavy elements? Desch et al. 2002

9. Jovian CloudsVertical Structure • Galileo probe dropped in 1995 • 2 cloud layers detected • No low-altitude water layer detected! • Several studies prior to Galileo accept this H2O cloud as “truth” • Clouds are thinner than models predicted • Absence of water and other heavy elements? • Authors claim there is a “distinctive feature” between 1.9 and 4.5 bars • What type of condensate? Desch et al. 2002

10. Earliest reference to Jovian LightningBar-Nun (1975) • Some highlights: • Rate of Earthlike lightning strikes is 104 larger than on Earth • Photolytically destroyed acetylene is yellow-brown… color of Jupiter • Calculated the rate of acetylene photolytic destruction • Tstorms restore lost acetylene • Great Red Spot • Tstorms might be an order of magnitude larger than elsewhere on Jupiter • Photolytically destroyed acetylene cannot account for red color

11. First Lightning DetectedVoyager I & II (1979) White lines are outer edge of Jupiter White dots are lightning flashes captured with multiple exposures Borucki et al. 1982

12. Detecting Whistlers • What are whistlers? • Electromagnetic signatures of lightning strokes • Low frequencies  Radio waves • Related to lower frequency plasma events in the ionosphere and/or magnetosphere • Detected by a plasma wave detector around Jupiter • Whistler data confirms lightning observed flashes • Several flashes too weak to detect by Voyager camera Gurnett et al. 1979 Example of a Whistler from Stanford’s wave receiver

13. How many Flashes on Jupiter? • Depends on if you believe the detected whistlers capture flashes too weak to be observed by Voyager • Uncertainty about whether or not to count these “unseen” flashes • Based purely on optically observed flashes: • 3-4 x 10-3 flashes km-2 yr-1(Matthews et al. 1983, Desch et al. 2002) • However, if you believe weaker flashes were missed: • 3-40 flashes km-2 yr-1 (Borucki et al. 1982, Matthews et al. 1983) • We don’t have a ton of data… • Voyager • 20 observable events were detected, between latitudes 30˚N and 55˚N (Cook et al. 1979) • 167 whistlers (Gurnett et al. 1979, Scarf et al. 1979) • Galileo • > 26 storms • 53 lightning spots • Cassini • 4 storms

14. Jovian lightningGalileo (1995-2003) Based on on Galileo and Cassini: “Lightning seems correlated with dark, elongated patches next to isolated, bright, white clouds that are indicative of rising gas.” Lightning! 3m38s lag… it’s likely a transient event Desch et al. 2002

15. Jovian Lightning Galileo (1995-2003) • Lightning captured by the clear filter on Galileo • Typical power of a terrestrial lighting strike is > 1 x109 W • Galileo most powerful: 0.9 x109 W • Cassini most powerful: 0.8 x109 W

16. Jovian LightningCassini (2000-01) Match clouds observed in daylight with lightning strikes observed at night! Storms exist in 500km-2000km clusters The large-scale uplifts are favorable for a “forest” of convective plumes, each creating a fast 100-km-scale updraft which produces repeated lightning, similar to the mesoscale convective systems on the Earth. -- Dyudina et al. 2004 Turbulent wake of large eddies (e.g., GRS) features a lot of lightning -- stronger vertical motions?

17. Jovian LightningCassini (2000-01) • Same images as before, in false-color • WHITE: Optically thick vertically-extended convective towers • Anvils, too • Most lightning occurs here • RED: Very deep clouds from 5-10bar • They still exhibit lightning! • High tops are not a necessary condition • May develop high tops at other times in its lifetime • GREEN: Mid-level clouds from 2-5bar • BLUE: Cirrus/haze from 0.1-0.5bar • No cloud structure beneath • BLACK: Deeper troposphere

18. Flashes In Polar Jupiter From Baines et al. (2007): • Polar lightning observed by New Horizons spacecraft • Up to 80N (3 flashes) and 74S • Similar characteristics to nonpolar lightning • Consistent with internal heat being main driver of convection • If the polar regions are a little colder, this could increase instability • Observations suggest a very weak equator-to-pole temperature gradient • Decadal variability in clouds affects lightning rate • Thin clouds in a wake of eddies (e.g., GRS) From Yair et al. (1998): • Absence near equator • Low water at the equator? (Yair et al. 1998) • More stable temperature profile

19. SaturnIAN Lightning • Saturn Electrostatic Discharges (SED’s), impulsive, short-duration radio pulses (1-40 MHz, lower bound?) detected by Voyager satellites studied by Warwick et al. (1981,1982) and by Cassini (Gurnett et al. 2005; Farrell et al. 2007). • SED’s are episodic with events happening a few times each minute in groups (Fisher et al. 2007) but with periodicity (~10.75 hours) related to the advection of electrically active storms around the planet (Gurnett et al. 2005). • It appears that there is a time resolution limitation with the instrumentation aboard Voyager/Cassini such that individual SED event wave forms cannot be mapped adequately. Jury is still out. • Dyudina et al. (2010, GRL) show the first optical detections of lightning on Saturn using Cassini ISS. • SaturnianSuperbolts (energy dissipation ~ 1013 J) Gurnett et al. (2005) – 1000 times more powerful than terrestrial superbolts. What is a superbolt? • Farrell et al. 2007 (JGR, L06202,34) discuss that SED’s might be shorter than we think in which case the power of the discharges might be more comparable to what happens on Earth – i.e. no such thing as a superbolt. Evans et al. (1983) show distinct “slow” time variability with higher frequency variability superimposed in RF field – we don’t have precise observation of the spectral peak of the SED source since Saturn’s Ionosphere attenuates any signals reaching the space craft.

20. Hypothesized Atmosphere of Saturn Thick ammonia ice clouds aloft prevent much sunlight from reaching…internal energy source is necessary to promote convection in water clouds (which have their bases near 10-15 bars). Cloud bases are around 275-255 K and therefore one explanation for charge separation implied is non-inductive charging during collisions of hydrometeors. 150 m/s updrafts?! With such intense updrafts (and the resulting generation of supersaturation), one expects large LWC. Does the Takahashi ‘78 diagram go out the window? Fischer et al. (2008)

21. Saturn Electrostatic Discharges (SED’s) • The earliest evidence for lightning on Saturn came from Voyager satellites – Saturn Electrostatic Discharges (Warwick et al. 1981). Impulsive radio signals were again measured by CASSINI’s Radio and Plasma Wave Science instrument (Evans et al. 1983; Fischer et al. 2006 and many others). EPISODIC SED RADIO SIGNALS Quiet period Fischer et al. (2011)

22. Cloud darkening • Some researchers report that clouds appear brighter during times of high lightning rates (Dyudina et al. 2007); meanwhile others find that lightning producing clouds are 20% darker/less reflective in the near-infrared spectrum relative to surrounding clouds – a product of lightning generated chemical species that are transported vertically and darken the cloud (Baines et al. 2009). ELECTRICALLY ACTIVE REGION <<< DOWNSTREAM DARKENING Baines et al. (2009) Clouds appear to darken in time as ammonia species become coated with hydrocarbon material.

23. Saturn’s lightning Chemistry • The outer ammonia cloud deck may be thicker on Saturn (Russel) which could complicate observation of optical lightning emissions. • Given what researchers knew about the chemical composition of Saturn early on, researchers conducted spark experiments in controlled “Saturnian atmospheres” to study products that might be generated during the discharge process. Then the spectral absorption of these substance was investigated to verify observed darkening in near-infrared imagery. • “…We have shown that the dark clouds on Saturn associated with lightning strikes are unusually dark throughout the visual and near-infrared. This spectral behavior is consistent with a combination of materials expected to be produced by lightning chemistry in the deep atmosphere of Saturn…A variety of hydrocarbon, sulfur, phosphorus, and water-based materials absorb in the near-infrared…” (Baines et al. 2009)

24. More Saturn’s Clouds • In the right panel, bright clouds at the leading edge were electrically active (SED producers) but gradually darken. SED PRODUCER Fischer et al. (2011)

25. Optical Lightning On Saturn Confirmed! • Numerous difficulties associated with the direct observation of optical lightning emissions on Saturn.Enough evidence has been gathered now to confidently say that lightning occurs on Saturn (Dyudina et al. 2010; 2013). First optical images came from August 17, 2009 from CASSINI’s Imaging Science Subsystem or ISS (Dyudina et al. 2010, Fischer et al. 2011). ***Some of the first visible images of optical lightning emissions on Saturn’s night side lightning taken during equinox by Cassini. Complications: 1. Reflected light from Saturn’s rings (ring shine) drowns the optical lightning signal. 2. Lightning occurring deep in the Saturnian atmosphere is obscured by intervening cloud and haze. 3. Infrequent storms which produce lightning. Flash areas of a few hundred km2!!! Dyudina et al. (2013)

26. Comparisonlightning – Saturn/Jupiter/Earth • There is some uncertainty about where the SED sources originate but it is believed that the origins lie some 125-250 km below cloud tops in a possible water layer (Dyudina et al. 2010; Fischer et al. 2011) consistent with the larger illuminated cloud region at cloud top. Fischer et al. (2011)

27. SuperBolts? • How powerful are Saturnian lightning discharges? • An average CG stroke generates roughly 4 x 108 J of energy with the strong strokes generating up to 2 x 1010 J – a small fraction is diverted into optical radiation while the bulk goes into heating and ionizing air (Russel). Do “superbolts” or discharges with energy dissipation that greatly exceeds terrestrial events exist? • Desch et al. (1992) estimated the energy dissipated during SED’s to be on the order of 1012-1013 J…orders of magnitude larger than any terrestrial “superbolts”. • Gurnett et al. (2001,2005) inferred that SED’s that were 106 times more powerful than electrostatic discharges observed as the Cassini satellite was leaving Earth. • According to Farrell et al. (2007), the Cassini RPWS instrument measured discharge peak power density of 50 W Hz-1 during an orbital pass of Saturn. • Either SED’s are analogous to terrestrial discharge in that they last on the order of 70 µs or their duration is much shorter (~1 µs) and they radiate at much higher frequencies relative to terrestrial discharges. If “superbolts” of Saturn are shorter in duration then their peak energy dissipation would not have to be as large to generate the same power. Farrell et al. (2007) and Evans et al. (1983) are at odds.

28. SED POWER SPECTRUM SED BROADBAND EMISSIONS • Are there implications for atmospheric chemistry if Saturnian discharges are truly more energetic? More energetic discharges would imply an efficient charge separation process or an ability of the Saturnian atmosphere to sustain large charge separation (i.e., a higher breakdown potential gradient). How does the duration of the discharge affect the frequency distribution of radiated power? Farrell et al. (2007) Evans et al. (1983)

29. Superbolt interpretation Wd = Discharge dissipation energy • Appears clearly that the wave forms do last longer on average but higher frequency variability is smeared out by “coarse” 0.1 ms time resolution of the instrument! Farrell et al. (2007)

30. Gaps in Understanding • Limited understanding of charge separation in different substances that are thought to exist in planetary atmospheres. Charge separation in methane and ammonia based substances appears to be at the heart of the issue. • We have a very small sample of optical/radio observations that document lightning. To characterize extraterrestrial lightning, more observations from radio telescopes and satellites are required – specifically time structure of flashes/strokes leading to understanding of power/energy dissipation >> lightning chemistry.

31. lightning ON OTHER PLANETS

32. General • We have a reasonable knowledge of the composition of planetary atmospheres. Use a laboratory setting to simulate the atmospheres of other planets (different chemistry) and observe electrical discharges to understand the expected optical emissions. Total difference in streamer structure with varying pressure and somewhat due to different chemistry. • Jupiter, Earth, Saturn, Venus lightning confirmed. Uranus, Neptune probable. Moon, Pluto, Mercury impossible. Why? Availability of gaseous/liquid/solid substances for charge separation?

33. Venusian Lightning • Many satellite overpasses have occurred and indirect evidence of lightning has been gathered (magnetic field pulses) but optical signals have proven elusive until only recently.*** • Venus has distinct cloud layers and strong vertical shear but drastically different atmospheric chemistry! • Gurnett et al. (1992) document 9 distinct VLF signals using instrumentation aboard Galileo satellite but no images. • Gurnett et al. (2001) document no lightning signals in Cassini satellite overpass (whereas the same measurement detected 70 pulses/sec as the satellite passed Earth). • But we deduce that lightning must occur because high-resolution observations of NO spectra at 5.3 µm show distinct peaks of absorption and we know lightning is the only source of NO (Nitrogen Oxide) in the lower atmosphere of Venus (Krasnopolski, 2006, Icarus).

34. Venusian Lightning • *** Landmark result: Hansell et al. (1995, Icarus) used telescope on Mt. Bigelow, AZ to observe Venus in the 777.7 micron and 656.4 micron bands. 7 distinct optical emissions which were significant over the background. • Significant amplitude pulses in the magnetic field measured by VEX during a pass of Venus (Russel et al. 2007, 2011). • Garcia Munoz et al. (2011) attempted to repeat the observations of lightning on Venus’s night side using bigger and better optics…unsuccessful documentation of optical pulses. • In summary, lightning does occur on Venus. Global flash rate is debated: low end 40/km^2/yr to high end 50/sec (Yair).

35. Martian Lightning • The search for water on Mars has been ongoing-therefore we look to other means for charge separation • Triboelectric effect (friction charging) – relative size of dust particles and frequency of collisions important. • Dust devils with lofted dust of drastically different size…gravitational settling/centrifuging leads to charge separation (perhaps as strong as 20 kV/m in 20 sec according to laboratory results). Thus modeling and laboratory studies confirm that triboelectric effect can separate enough charge for lightning on Mars, but can we observe it directly? • Farrell et al. (2004, JGR) look at potentially electrified dust devils on Mars. No direct evidence of lightning occurring on Mars to date though. • Ruf et al. (2009, GRL) used a radio-telescope to document Schumann Resonances on Mars and found distinct peaks at the first three predicted SR modes for the Martian resonant cavity. • Anderson et al. (2012) used a radio-telescope array to conduct a similar survey. They found peaks but no large Martian dust storms were observed at the same time…open question. • Gurnett et al. (2010, GRL) studied data from MARSIS – looking for impulsive signals from lightning in Martian dust storms. Two large dust storms and many small storms but no lightning signals!

36. Titan Lightning • Saturn’s largest moon. Average temperature is -150C. Is it too cold to spark? • Dramatically different atmospheric chemistry…the result of lightning? Lammer et al. (2001) suggest that lightning might be possible. Models seem to show the possibility of lightning Borucki et al. (1996). • Hueso and Sanchez Lavega(2006, Nature) investigate convective methane clouds and the possibility of charge separation between methane droplets/drops. • Fisher et al. (2007, GRL) and Fisher and Gurnett (2011, GRL) breakdown 70 passes of Titan by Cassini and find NO radio pulses indicative of lightning on Titan. • Huygens Probe descent through the atmosphere of Titan documenting “significant” peak in SR at 36 Hz.

37. Uranian Lightning • Voyager passes within 600,000 km of Uranus and measures 140 HF radio bursts…but no optical detection. Are these flashes happening too deep in the atmosphere and are obscured by upper haze layers?