Update on Venus Lightning from Observations on PVO and Galileo

R. J. Strangeway (IGPP/UCLA) Acknowledgement: C. T. Russell Outline • Overview – Venus Atmosphere. • Lightning pro and con. • Optical and electromagnetic wave evidence for lightning. • Cassini and non-evidence for lightning. • Pioneer Venus Orbiter – whistler-mode emissions. • Predictions for Venus Express. • Summary and Conclusions. Venus 2006Lightning on Venus R. J. Strangeway – 1 The Venus Atmosphere

• Venus’ atmosphere differs greatly from that Venus of the Earth. Principally CO2 (95%) some CO 95% N2(3.5%), with a surface pressure of close to 100 2 N 100 bars and a temperature of close to 730K 2 3.5%

• The lower atmosphere is clear. The 80 temperature and pressure in the middle cloud Upper Cloud are almost Earth like, about 315K and 0.5 bar Middle Cloud 60 Lower Cloud

• The cloud particles are thought to be H2SO4 Lower Thin droplets that like water can be charged. Thus Altitude (km) Aerosols, Dust Haze lightning could occur if convection sustained 40 large potential differences within the clouds TP

20 Clear Atmosphere

0 Temperature 0 200 400 600 800 K

0.01 0.1 1 10 100 bar Pressure

Venus 2006Lightning on Venus R. J. Strangeway – 2 Lightning – Issues

Pro: Venera observations in atmosphere [Krasnopol’sky, Venus, 1983; Ksanfomality et al., Venus, 1983]. Pioneer Venus observations of whistler-mode and broadband emissions [see, e.g., Russell, Space Sci. Rev., 1991]. Venera 9 and Univ. Arizona observations of optical emissions [Grebowsky et al., Venus II, 1997]. Con: Lack of other confirming visible observations. Cassini Venus fly-by observations [Gurnett et al., Nature, 2001] (but c.f. Galileo [Gurnett et al., Science, 1991]). Joule dissipation of whistler-mode waves in lower ionosphere [Cole and Hoegy, JGR, 1996].

Venus 2006Lightning on Venus R. J. Strangeway – 3 Successful Searches for Lightning at Venus

• Atmospheric electricity studies have frequently been the focus of controversy and Venus lightning is no exception. Lightning processes are poorly understood, especially in exotic settings.

• Lightning is most often studied by detecting electromagnetic waves produced by the current surge associated with sudden charge transfer. Such surges also produce heat, light and sound.

• Electromagnetic waves have been detected both above and below the ionosphere. Lightning has been reported from Venus orbit and from Earth.

Venus 2006Lightning on Venus R. J. Strangeway – 4 Venera 11-14 Landers Detected E-M Waves

During Descent Venera 11-14 landers were • 1/2

equipped with search coil Hz -1 magnetometers that are 80 Field Intensity

40V m μ

usually quite unaffected by E, 0 spacecraft noise. 0542 0545 0550Time 0554 48 46 44 42 40 38 36 34 32 30 28 Height of Vehicle above Landing Site, km • The Venera landers saw On the Ground noise bursts consistent with 20

lightning discharges both 10 18 kHz -1/2

during descent and on the Hz 0 ground. -1 40 Vm μ 30 10 kHz 20

10

0 Field Intensity, E, 8 sec8 sec 8 sec 8 sec 8 sec 0654 0658 0704 0707 0714 0717

Venus 2006Lightning on Venus R. J. Strangeway – 5 Optical and Radio Measurements

Optical measurements from Venera 9 Radio Wave Measurement from Galileo Seconds 0 2 4 6 8 10

Peak Intensities Corrected for Instrument Response Instrument Response to 1 W/A/km2 Radiant Interference 10-15 Lightning or, -Hz) 2 Possibly Langmuir

Correction Applied /m Wave Harmonics 2 -18

3 10 Poynting Flux(W/m Lightning ) 2 o

-16 2 10 2 -19 -Hz) Intensity (Watts/A/km 10 Receiver Noise Level 1 Electric Field Spectral Density (V 10-17 105 106 107 Frequency (Hz)

0 4000 6000 8000 o A • From orbit the Venera 9 saw a potential • On its flyby Galileo detected radio sferics lightning storm. Flash intensity varied as symptomatic of lightning. instrument scanned over visible range. • Cassini also saw bursts but Gurnett chose not to interpret the bursts as lightning. Venus 2006Lightning on Venus R. J. Strangeway – 6 Cassini Observations

• From Gurnett et al., Nature, 2001.

• Gurnett argued that the bursts detected near Venus were not due to terrestrial-like cloud to ground lightning – too weak and too infrequent.

• Allowed for possible cloud to cloud or cloud to ionosphere lightning, albeit reluctantly.

Venus 2006Lightning on Venus R. J. Strangeway – 7 Lightning Hypothesis

Venus 2006Lightning on Venus R. J. Strangeway – 8 Pioneer Venus Orbiter Measured the Electric Component of Lighting

• Three types of signals were observed in Typical Nightside Signals night ionosphere:

a. Broadband signals rapidly attenuating with altitude. b. Interference, easily distinguishable from natural signal. c. Whistler mode signals below the electron gyro frequency with little attenuation with altitude.

• Both broadband noise and whistler mode noise were consistent with a single source, the different bandwidth coming from the propagation to PVO.

Venus 2006Lightning on Venus R. J. Strangeway – 9 PVO Local Time Distribution

100

80 B > 15 nT

60

Ω F < ge 40 Bursts

Percent Occurrence Percent Ω 20 F > ge Bursts

0 06 04 02 00 22 20 18 Dawn Dusk Local Time

• Both types of signals identified as lightning arose more often near dusk than near dawn.

• Local time distribution was not a reflection of plasma properties.

Venus 2006Lightning on Venus R. J. Strangeway – 10 PVO – 100 Hz Inside Ionospheric Hole

Venus 2006Lightning on Venus R. J. Strangeway – 11 Altitude Dependence of PVO Signals

100 Hz inside cone 10-1 ) -1

100 Hz outside cone 10-2 730 Hz Burst Rate (sec 10-3 30 kHz 5.4 kHz

10-4 150 200 250 300 Altitude (km)

• Whistler mode should propagate vertically • Poynting flux can be estimated from above the ionosphere (atmospheric electric field amplitude and plasma refraction). Thus, the propagation cone can conditions. be calculated from the magnetic field direction and strength. • Poynting flux is nearly constant with altitude, decreasing slowly. • Inside propagation cone whistler mode burst rates do not vary with altitude. Non- whistler mode frequencies do vary rapidly.

Venus 2006Lightning on Venus R. J. Strangeway – 12 Nightside Maps – 30 kHz

Based on results of Ho et al., P&SS, 1994.

Venus 2006Lightning on Venus R. J. Strangeway – 13 Nightside Maps – 5.4 kHz

Venus 2006Lightning on Venus R. J. Strangeway – 14 Nightside Maps – 100 Hz Whistler

Venus 2006Lightning on Venus R. J. Strangeway – 15 PVO Entry Phase – Below Ionosphere?

Venus 2006Lightning on Venus R. J. Strangeway – 16 PVO Entry Phase

• On final orbits of PVO when it was entering the atmosphere, measurements were made just below the atmosphere.

• Electric field observations climbed to large values as they should in free spaces as wave phase speed approached speed of light. Median Poynting flux below 129 km was same as above the ionosphere.

• Attenuation scales consistent with propagation through the collisional ionosphere.

Venus 2006Lightning on Venus R. J. Strangeway – 17 Evidence for Whistler Mode

100 Hz wave Poynting flux decreases slowly with altitude [Russell et al., GRL, 1989]. Waves restricted to whistler-mode resonance cone for vertical propagation are polarized perpendicular to the ambient field [Strangeway, JGR, 1991]. Burst rates highest for vertical magnetic fields [Ho et al., JGR, 1992]. Waves occur for low electron beta [Strangeway, JGR, 1992].

Issues with whistler-mode identification:

z Joule dissipation [Cole and Hoegy, JGR, 1996; Strangeway, 1996].

z Non-linear dispersion [Cole and Hoegy, JGR, 1997], but see Strangeway [JGR, 1997] and Strangeway [Adv. Space Res., 2000]. Wave magnetic fields can be comparable with ambient field – BUT this is why VEX can detect EM lightning signals.

Venus 2006Lightning on Venus R. J. Strangeway – 18 Joule Dissipation

From Strangeway, Adv. Space Res., 2000.

Venus 2006Lightning on Venus R. J. Strangeway – 19 What will Venus Express See?

• Venus Express carries two fluxgate sensors each capable of 128 vectors per second (64 Hz Nyquist frequency). • The estimated signal strength is well above the noise threshold of these sensors: For µ = 1000, expect signals of order 1 - 10 nT over the bandwidth of the instrument. • The access of lightning signals to the spacecraft will be modulated by the direction of the magnetic field and the local time of periapsis. Access will also be affected by the presence or absence of ionospheric holes. • Optical instruments should also detect lightning on the night side: Venus Monitoring Camera (365nm, 935nm, 1010nm) and VIRTIS. • Optical flashes should be much more frequent near dusk.

Venus 2006Lightning on Venus R. J. Strangeway – 20 Summary and Conclusions • Atmospheric electricity is a complex phenomenon and at best only poorly understood on Earth.

• Venus lightning contains all the complexity and ambiguity of its terrestrial counterpart and its study was compounded by several intense personal rivalries.

• Evidence for lightning comes from electromagnetic waves above and below the ionosphere; from low frequency and high frequency components, from optical observations in orbit and from Earth.

• The behavior of the signals is consistent with lightning emissions in Venus’ sulfuric acid clouds and propagation (for the EM waves) upward through the ionosphere.

• We expect that Venus Express will detect lightning both optically and electromagnetically.

Venus 2006Lightning on Venus R. J. Strangeway – 21