UPDATED REVIEW OF PLANETARY ATMOSPHERIC ELECTRICITY Y. YAIR1, G. FISCHER2, F. SIMES3, N. RENNO4 and

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UPDATED REVIEW OF PLANETARY ATMOSPHERIC
ELECTRICITYY. YAIR1, G. FISCHER2, F. SIMÕES3,
N. RENNO4 and P. ZARKA51 Department of Life and
Natural Sciences, Open University of Israel,
Ra'anana 43107,Israel2Department of Physics and
Astronomy, University of Iowa, Iowa City, IA
52242, USA3Centre d'Etude des Environnements
Terrestre et Planétaires, 4, Avenue de
Neptune,Saint Maur, France4Department of
Atmospheric, Oceanic and Space Sciences,
University of Michigan, AnnArbor, MI 48109,
USA5 Observatoire de Paris, Meudon, France
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This talk is based on the ISSI-planetary
lightning workshop in Bern, July 2007, to be
published in Space Science Reviews and as a book
by Springer (2008)

• General considerations
• Operational Spacecraft
• Ground-Based observations
• Solid-surface bodies
• Gas giants
• Summary
• Gaps in knowledge

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Lightning fingerprints in planetary atmospheres

• Optical emission, through the intense heating of
  the lightning channel. Continuum and line
  emissions.
• Direct scattered lightning light by clouds
• Indirect transient luminous events (?)
• Electromagnetic
• Whistlers directed by magnetic field-lines
  penetrate the ionosphere
• Low-frequency radio emission by the current
  channel acting as an antenna
• RF emission (VHF) in a broad spectrum peaking (at
  Earth) at 1-10 kHz, and decreasing as f-1 to f-2
  at higher frequencies
• SR Very low-frequency waves (lt a few tens of
  Hz), which are trapped in the surface-ionosphere
  cavity
• Chemical
• Non-equilibrium concentrations of compounds
• Exotic species

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Necessary Conditions for Lightning

• Condensable materials at high enough mixing
  ratios to form clouds with different phases of
  particles
• Polarizable cloud constituents (high e)
• Water (80), Ammonia (25), Sulfuric acid (110)
• SO2 (17), methane (1.7), carbon dioxide (1.6),
  H2S (9)
• Instability is driving cloud dynamics
• Charge is separated in micro and macro scales.
  The resultant field gtgt local breakdown field
• Cloud lifetimes gt charge buildup time

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Operational spacecraft

• Cassini/Huygens to Saturn Launched October 1997
  arrived July 2004.
• ISS (Imaging Science Subsystem) - 2 cameras
  (wide and narrow-angle), equipped with a CCD
  sensor of 1024 pixels squared (Porco et al.,
  2004). RPWS (Radio and Plasma Wave Science) - 3
  electric and magnetic antennas and various
  receivers in the frequency range from a few Hz up
  to 16 MHz (Gurnett et al., 2004). It is capable
  of detecting either lightning whistlers or HF
  radio emissions.
• New Horizons to Pluto Launched January 2006 /
  Jupiter fly-by February 2007.
• LORRI (Long Range Reconnaissance Imager) - an
  8.2-inch (20.8-centimeter) telescope with a CCD
  that provides images of high angular resolution,
  5 µrad. LEISA (Linear Etalon Infrared Spectral
  Imager) and ALICE (ultraviolet imaging
  spectrometer)

Venus Express (VEX) Launched on November 9th
2005, entered a Venusian orbit on April 11th
2006. It is an upgraded version of the
MEX mission with similar instruments. MAG, which
is considered to be optimal for detection of
lightning-associated electromagnetic bursts. The
main camera on board is the Venus Monitoring
Camera (VMC) that takes images of Venus in 4
narrow band filters from UV to near-IR all
sharing one CCD. VIRTIS (Visible and Infrared
Thermal Imaging Spectrometer) which operates at
wavelengths between 0.3 and 5 mic.
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Operational Telescopes

• NASA/IRTF This is the largest infrared telescope
  available for planetary science, allocates 50 of
  its observation time to solar-system bodies
• Has a 3-m diameter mirror and 5 instruments
• (a) SpeX, a 1-5 µm cross-dispersed
  medium-resolution spectrograph
• (b) CSHELL, a 1-5 µm high-resolution
  spectrograph
• (c) MIRSI, a 5-25 µm camera and low-resolution
  spectrometer
• (d) NSFCAM2, a 2048x2048 pixel, 1-5 µm camera
  with a 0.04 arcsec/pixel scale
• (e) Low-resolution 3-14 µm spectrograph and
  high-resolution spectrographs for 8-25 µm.

UTR-2 This is the world largest radio-telescope
in the decametric frequency range (operated from
about 8-32 MHz) located in Kharkov, Ukraine. It
consists of 2040 dipoles and has an effective
area of 150,000 m2 (Konovalenko et al., 2001).
Its sensitivity of a few Jansky (Jy) enables it
to detect lightning from Saturn that is expected
to produce a flux of the order of 100 Jy at Earth
(Zarka et al., 2004).
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VENUS Sister Planet

• Permanent global coverage
• Lower cloud 47.5 - 50.5 km
• mode1 - 1200 cm-3 r0.2 microns
• mode 2 - 50 cm-3 r1.0
• mode 3 - 20 cm-3 r4.0
• Middle cloud 50.5 - 56.5 km
• mode 1 - 300 cm-3 r0.15
• mode 2 - 50 cm-3 r1.3
• mode 3 - 10 cm-3 r3.5
• Upper cloud 56.5 - 70 km
• mode 1 - 1500 cm-3 r0.18
• mode 2 - 50 cm-3 r1.0
• Knollenberg and Hunten, 1980

• Similar to stratiform clouds on Earth, with lower
  mass loading and drop concentrations. The upper
  cloud layer is uniform, middle and lower clouds
  are variable (due to dynamics and microphysics)
• High wind shear, zonal wind speed in the middle
  cloud is 60 m s-1 and 100 m s-1 in the upper
  cloud. Slow vertical motions, but local strong
  motions may exist (VEGA balloons, Sagdeev et al.,
  1986).
• Local convection in clouds may occur at the
  sub-solar point or downwind (day-side)

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Telescopic Lightning Observations

• Venus at inferior conjunction 28 February,
  5,6,7,15 March 1993
• Choronographic optics 153 cm telescope CCD at
  Mt. Bigelow, Arizona. Hansell et al., Icarus,
  1995
• Detection of 7 lightning flashes based on
  emissions in 777.7 nm and 656.4 nm.

Calibrated with known star Detection efficiency
5x106J - 95 1.5x106 J- 50. Global flash rate
1x10-3 relative to Earth Missing the faint
flashes? Measurement not repeated since!
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Galileo Flyby (9.2.90)

• Venus closest approach was 3.67 Venus radii
  (16,000 km above cloud tops)
• Imaging - UV, IR, Vis
• Plasma wave detector 5.6 MHz, Magnotemeter

No lightning images! VLF Activity in the
nightside, active interval defined gt 1 burst
noise in 30 s. 9 distinct VLF events interpreted
as whistlers related to lightning Gurnett et
al., 1992
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Cassini Flyby (26.4.98, 24.6.99)

• Closest approach 600 km (Dayside)
• 20 channels of Radio PW detector search for
  impulsive high-frequency signals (0.125-16 MHz).

No sign of lightning - No detection of VLF
signals (Sferics) above 1 MHz Gurnett et al.,
2001 A similar measurement detected 70 pulses
per second during Earth flyby (18.9.99)
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New evidence based on chemistry
High-resolution spectra of Venus in the NO
fundamental band at 5.3 µm were acquired using
the TEXES spectrograph at NASA IRTF on Mauna Kea,
Hawaii. (Krasnopolski, Icarus, 182, 80-91,
2006) The photochemical impact of the measured NO
abundance is significant and Lightning is the
only known source of NO in the lower atmosphere
of Venus. The required flux of NO corresponds to
the lightning energy deposition of 0.190.06 erg
cm-2 s-1. For a flash energy 109 J, the global
flashing rate is 90 s-1 and 6 km-2 y-1 which is
very high
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New data from VEX (Russel et al., 2007)

• 37 measurements made from range 300 km of the
  Venusian surface as VEX was travelling at 24,000
  km/h
• The fluxgate magnetometer detected bursts of
  clear signals, with rapidly varying amplitudes
  and variable durations and intermissions between
  successive bursts.
• Deduced planetary flash rate based on detector
  footprint of 0.06 of the planet's surface is
  50 s-1.

The three components of the magnetic field as
recorded by the fluxgate magnetometer (MAG)
Signals exhibit repeated bursts with alerting
amplitudes and duration, hinting at lightning as
the probable source. The x-component is towards
the sun, z along the orbit plane and y in
opposite planetary motion.
Still no Optical detection!
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Sprites on Venus (?)

• Huge surface atmospheric pressure (9 MPa) means
  no cloud-to-ground discharge from the high-level
  sulfuric acid clouds (Zs 50 to 70 km).
• Sprites can be produced by IC discharges (van der
  Velde et al., 2006) - so even if all lightning
  activity in Venus is intra-cloud discharges
  sprites are still possible.
• Venusian IC discharges would be slow to build-up
  and would exhibit different characteristics
  compared to their terrestrial analogues (Gurnett
  et al., 2001).
• Potential emission bands from CO2 between 290 and
  500 nm, with prominent lines near 288, 410 and
  427 nm (Goto et al., 2007). Also OI (777.7 nm).

Paradoxically, it may be easier to detect optical
emissions from sprites in Venus than from the
lightning discharges hidden below the clouds.
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Summary Venus Lightning

• Lightning occur in Venus, probably intra-cloud
  flashes in the middle cloud layer.
• Long duration pulses (70 ms).
• Total energy (?) 108 J - 1010 J
• Best observed at Oxygen 777.4 nm, 656.4 nm.
• Global rate - debatable, conflicting evidence
  maybe low lt 40 km-2 year-1 maybe high gt 50 s-1
  or is it just obscured by clouds?
• Sprites are possible and may be observed by
  looking toward the limb.

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Mars Dust storms dust devils

• Dust storms are a global phenomena that can
  obscure the entire planet for months
• Occur mainly by aeolian processes, stronger in
  regions of slopes and near the polar caps.
• Dust Devils are turbulent vertical small-scale
  storms caused by sharp local instabilities (like
  funnel clouds in tornado).
• Dust particles suspended in air acquire charge by
  tribolelectric interaction with other particles
  (friction charging).
• Triboelectrric charging depends on
• Size and composition of interacting particles
• Frequency of collisions

Dust devils are bigger and stronger than on
earth reach up to 7 km and have diameters 100m
-1 km. Martian dust devils are 700 times more
dense in dust particles than the ambient
atmosphere
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Electrified dust devils on Mars (Farrell et al.,
GRL, 2004)

• Two distinct particle populations (Lgtgts),
  gravitational settling will lead to large-scale
  charge separation and the build-up of electric
  fields within the dust cloud.
• In mild winds, there will be size differentiation
  and large grains will congregate at the bottom of
  the devil, smaller ones will be carried aloft.
• Model results exponential increase of E field,
  reaches 20 kV/m (breakdown field) within 20
  seconds.
• Assumed to be similar to terrestrial dust-devils,
  which attain a larger electric field due to the
  lower conductivity of the lower atmosphere on
  Earth.

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Evidence for Martian Dust Electrification (Farrel
et al., JGR, 2004)

• To date, there are no direct measurements of
  electrical activity on Mars. Laboratory
  experiments show that dust grains in a
  Martian-like chamber acquire significant charge

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The news from Mars - Pheonix
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Titan methane clouds

• 3D dynamical calculations of convective cloud
  evolution show severe flash floods can occur in
  methane clouds, provided that RH gt 80.
• Cloud can reach 30 km altitude in 5-8 hours with
  max updrafts 20 m s-1. Raindrops grow to r 1-5
  mm before precipitating, with total rainfall of
  110 kg m-2, similar to flash-floods on Earth.
• These clouds are similar to super Cbs and if
  charge generation is active, are potentially
  lightning producing.

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Convective storms on Titan (Hueso and
Sanchez-Lavega, Nature, 442, 428-431, 2006)
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No Titan lightning, yet (Fischer et al., GRL
2007)

• The Cassini/RPWS instrument searched for radio
  emissions from Titan lightning, but nothing was
  found during any of the first 35 Titan close
  flybys.
• In case of a Titan lightning storm RPWS should
  easily detect burst signals above Titans
  ionospheric cutoff frequency of about 500 kHz
  (Bird et al., 1997) that also show a quadratic
  fall-off of signal intensity with spacecraft
  distance.
• The non-detection of RPWS tells us that Titan
  lightning is an extremely rare event if it exists
  at all. The RPWS result does not rule out the
  existence of other forms of atmospheric
  electricity like corona discharges.
• The search for Titan lightning with RPWS will
  continue at least until the end of Cassinis
  extended mission in mid-2010, which will increase
  the total number of close Titan flybys to 70.

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Jupiter - Optical Detection Voyager, Galileo,
Cassini
Storms are long lasting and the flash rate is
high, very bright and energetic, factor 1000
compared to earth. Occur at mid- and high
latitudes, less near the equator
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Relating cloud features to optical flashes in
Galileo images (Little et al., Icarus 142, 1999)
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New-Horizons discovery of polar lightning
activity in Jupiter (Baines et al., Science, 2007)

• LORRI (Long Range Reconnaissance Imager) camera
  identified Jovian lightning at high latitudes up
  to 80N and 74S
• Flashes also at the anticyclonic sides of 16
  eastward jets (60S and 66S), and the polar
  strikes (80N, 74S) are located in regions of
  relatively weak winds

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Summary Jovian Lightning

• Location Mid and high-latitudes, Polar(!)
• Flash rate lt 40 km-2 year-1
• Optical energy 2.5109 J
• RF energy - 2.0107 J
• Total Energy gt 1012 J
• Active Clouds deep water clouds (?) with
  ammonia anvils, embedded in larger storms
• Charging is probably ice-ice non-inductive
  other processes (contamination of NH4?)

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SED -Saturn Electrostatic Discharges

• Impulsive short-duration radio bursts, detected
  by the planetary radio astronomy (PRA) on both
  Voyagers Warwick et al., Science, 212 (1981),
  215, (1982) and by Cassini RPWS Gurnett et al.,
  Science, 307,2005
• Organized in episodes, occurrence rate - few
  events per minute, typical duration 30-300 ms per
  event, broadband from lt20kHz to gt40MHz
• Recorded by the instrument when the source is
  facing the spacecraft

Source - clouds/rings, recurrence period hints
that source rotates with the planet 10 h
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Statistics of SED storms (Fischer et al., Icarus,
2007)
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The SED storm of January 23rd 2006
Asterisks denote sub-spacecraft western longitude
and mean time of the SED bursts
Dynamic spectrum of SED episode E56 in the
frequency range 600 kHz to 4 MHz
First SED source associated with a distinctive
bright atmospheric cloud feature located in
latitude 35 S strong indication for lightning
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SED related cloud feature
No direct flashes of light on Saturn detected
(like for Jupiter) due to optically thick clouds,
depth of lightning, or ring shine
from Dyudina et al., 2007
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Clouds and SEDs as seen from Earth
Konovalenko et al., Ground-based Decameter
Wavelength Observations of Saturn Electrostatic
Discharges.)

• Cloud feature related to SEDs was first spotted
  by amateur astronomers on Earth
• Detection of SED with huge decametric radio
  telescope UTR-2 in Ukraine, which has a
  sensitivity of a few Jansky
• SEDs have source power of 50 W/Hz corresponding
  to flux of 200 Jy at Earth
• Detection criteria ON and OFF beam, right
  duration and intensity, broadband emission, etc.,
  some tens of bursts were detected and two of them
  by UTR-2 as well as RPWS

Feb. 2, 2006, Photos by Ralf Vandebergh
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Whistlers in Saturns magnetosphere
Only 2 whistlers observed DOY 302, 2004 and DOY
115, 2007
2004 whistler at L6.49 maps to 67N (from
dispersion D81 Hz1/2 s) 2007 whistler at L13.84
maps 75N (s/c at 45S and large dispersion)
67N and 75N are night-day boundary latitude at
noon. No corresponding SED observations!
from Akalin et al., 2006
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Are there also less powerful and permanent
lightning on Saturn?

• Cassini spent about 4.5 hours within 2.5 RS and
  no flashes with terrestrial strength were
  detected
• Cassini spent nearly 20 hours within 3 RS in 6
  periapsis passes, but no lightning was detected
• We might miss some weak storms, but all SED
  storms detected thus far had a strength radiating
  out to at least 40 RS
• Permanent lightning activity slightly stronger
  than Earth lightning should have been detected at
  periapsis passes, so probably there is
  considerable time with absolutely no lightning
  activity
• Lightning at Saturn located at higher pressures ?
  higher breakdown field ? more charges can
  accumulate ? more powerful lightning

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New SED storm on Saturn November 07-June 08
(update courtesy G. Fischer)

• The storm is still going on and by tomorrow
  (June 27th) it will be exactly 7 months (!!) old.
  Since about 3 months it is in fact not only one
  storm system, but there are two. Both are located
  at 35 South latitude, and currently they are
  separated by about 30 in longitude. From RPWS
  data I can see that both storm systems radiate
  SEDs and we have optical confirmation from
  ground-based amateur astronomers as well as from
  Cassini/ISS.

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Summary Saturnian Lightning

• Source of SED atmospheric
• Location S. hemisphere 35
• Flash rate lt 210-2 km-2 year-1
• Optical energy (?) 1010 J
• RF energy - 108 J should be re-evaluated!
• Total Energy (superbolts?) 1013 J
• Clouds deep water clouds or ammonia clouds
• Charging still unknown

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Summary - Lightning Activity in our solar system

• Earth, Jupiter, Saturn - definite
• Venus probable, debatable
• Uranus, Neptune probable, partly confirmed
• Mars, Titan - theoretically possible
• Moon, Mercury, Pluto - impossible

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Gaps in knowledge

• Limited knowledge of charge separation mechanisms
  in various compounds, typical of planetary
  clouds need new laboratory studies
• Cloud dynamics and microphysics are not
  well-known need advanced modeling
• Breakdown processes and consequent emission not
  fully understood need theoretical and laboratory
  studies
• Limited sample of storms and flashes need for
  increase of ground-based observations (optical,
  radio LOFAR)