Title: UPDATED REVIEW OF PLANETARY ATMOSPHERIC ELECTRICITY Y. YAIR1, G. FISCHER2, F. SIMES3, N. RENNO4 and
1UPDATED 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
2This 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
3Lightning 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
4Necessary 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
5Operational 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.
6Operational 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).
7VENUS 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)
8Telescopic 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!
9Galileo 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
10Cassini 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)
11New 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
12New 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!
13Sprites 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.
14Summary 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.
15Mars 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
16Electrified 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.
17Evidence 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
18The news from Mars - Pheonix
19Titan 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.
20Convective storms on Titan (Hueso and
Sanchez-Lavega, Nature, 442, 428-431, 2006)
21No 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.
22Jupiter - 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
23Relating cloud features to optical flashes in
Galileo images (Little et al., Icarus 142, 1999)
24New-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
25Summary 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?)
26SED -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
27Statistics of SED storms (Fischer et al., Icarus,
2007)
28The 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
29SED 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
30Clouds 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
31Whistlers 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
32Are 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
33New 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.
34Summary 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
35Summary - 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
36Gaps 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)