Atmospheric Electricity of Planetary Environments Schumann Resonances F. Simões, J.-J. Berthelier, M. Hamelin CETP-IPSL, Saint Maur, France TLE Workshop, Corte, 27 June 2008 1/18

Outline Introduction Schumann resonances on Earth (what are Schumann resonances?) Schumann resonances on Earth (what can we learn from such phenomenon?) Schumann resonances on planetary environments (what are the expected similarities/differences on other planets and moons?) Schumann resonances and TLEs (which modelling improvements are required?) Summary TLE Workshop, Corte, 27 June 2008 2/18 TLE Workshop, Corte, 27 June 2008

Schumann Resonance Theory Earth surface and ionosphere form a cavity where resonant electromagnetic waves can develop (2R) Lightning is a good candidate to excite these waves The frequencies fall in the ELF range and are given by Schumann, Z. Naturforsch. (1952) Where R is Earth radius, c the velocity of light in vacuum and n=1,2,… … Balser and Wagner, Nature (1960) detected such waves. TLE Workshop, Corte, 27 June 2008 3/18

Schumann Resonance Theory The initial assumptions were: Surface is a Perfect Electric Conductor (PEC) Ionosphere is a PEC boundary Cavity thichness is much smaller than radius Lossless cavity with =1 k  E H The objective is solving Maxwell equations in a specific configuration: TLE Workshop, Corte, 27 June 2008 4/18

Schumann Resonances on Earth Schumann spectrum measured on the surface – 5 peaks Schumann spectrum at altitude of ~20 km (balloon) – 7 peaks 8 14 20 26 32 38 44 Hz The ‘World Record’ of Schumann peaks is: 13 ! Füllekrug, GRL (2005) TLE Workshop, Corte, 27 June 2008 5/18

Schumann Resonances on Earth Daily variation of Schumann peaks -Balser and Wagner, Nature (1960) Schumann resonance spectrum is modulated over the solar cycle and responds to solar flares -Reid, in ‘Study in Geophysics: the earth's electrical environment’ (1986) Seasonal variation - ‘global tropical thermometer’ -Williams, Nature (1992) Connection between Schumann resonance and lightning variability, sprites -Boccippio et al., Science (1995) Used to monitoring tropospheric water vapour -Price, Nature (2000) Many other contributions can be find in the book ‘Resonances in the Earth-ionosphere cavity’ of Nickolaenko and Hayakawa (2002) Remark: Schumann resonances respond to sources distribution and ionospheric variability TLE Workshop, Corte, 27 June 2008 6/18

Generalization to Other Planets Environmental parameters: Geometric parameters: R Atmosphere: atm atm Outer boundary: iono, h Inner boundary: in, d R Interior: s s d Transverse mode Simões et al., PSS (2007) TLE Workshop, Corte, 27 June 2008 7/18

Venus “Similar” to the Stark and Zeeman effects in Quantum Mechanics 10.61+0.19i 9.28+0.34i f > 1Hz 9.13+0.62i 17.93+0.52i 17.28+0.23i 15.53+0.62i 16.22+1.01i 25.07+0.61i 24.93+0.87i Simões et al., JGR (2008) Highly asymmetric cavity: Day lasts longer No intrinsic magnetic field Specific atmospheric chemistry 24.71+0.64i Eigenfrequency Splitting 23.22+1.32i 21.48+0.95i TLE Workshop, Corte, 27 June 2008 8/18

Venus h31.5 km Fermat principle – ray tracing  h31.9 km High atmospheric density is responsible for refractivity phenomena  N = (n-1)x10-6. Analytical approximation using Maxwell equations and an exponential permittivity profile  h29.6 km Numerical model  h31.5 km  dense atmosphere h31.5 km  vacuum Simões et al., JGR (2008) Fermat principle – ray tracing  h31.9 km (for high frequencies and light) E shows a maximum at specific altitude TLE Workshop, Corte, 27 June 2008 9/18

Venus Venera 11 and 12 data Ksanfomaliti et al., Kosmich.Issled. (1979) TLE Workshop, Corte, 27 June 2008 10/18

Mars r=2.2-12.5 Christensen and Moore, in ‘Mars’ (1993) Cavity Conditions: Models predict significant atmospheric conductivity down to the surface Surface highly heterogeneous Unknown surface permittivity and conductivity Electric discharging: Lightning – unlikely Triboelectricity from dust storms – likely Surface properties: r=2.2-12.5 Christensen and Moore, in ‘Mars’ (1993) =10-10-10-12 Sm-1 Berthelier et al., PSS (2000) Schumann resonances can be used to: Investigating the sporadic meteor layer – Molina-Cuberos et al., RadSci (2006) Assessing atmospheric propagation conditions - Soriano et al., JGR (2007) Studying triboelectricity But Strong attenuation is expected if the atmospheric conductivity models are confirmed; evanescent modes may occur. TLE Workshop, Corte, 27 June 2008 11/18

Jupiter and Saturn Cavity Parameterization: Radius is one order of magnitude larger than Earth Inner boundary is significantly lower than planetary radius Interior conductivity is required - Liu, PhD Thesis, Caltech (2006) Permittivity profile must be used because gas density cannot be neglected – range [1, ~1.25 (liquid hydrogen)] Strong intrinsic magnetic field – effect is neglected though Eigenfrequencies: Simões et al., Icarus (2008) Planet f1 [Hz] Q f2 [Hz] f3 [Hz] Jupiter 0.68 8.5 1.21 8.6 1.74 8.7 Saturn 0.93 7.8 1.63 6.8 2.34 6.5 TLE Workshop, Corte, 27 June 2008 12/18

Uranus and Neptune Cavity Parameterization: And: Planet f1 [Hz] Q F2 [Hz] F3 [Hz] Uranus 2.44 20.3 4.24 19.3 6.00 20.0 1.02 2.0 1.99 3.03 2.3 Neptune 2.33 9.7 4.12 9.4 5.90 9.5 1.10 1.0 2.03 1.1 2.96 0.9 Cavity Parameterization: Radius is smaller than in the Jovian planets Inner boundary is significantly lower than the radius Interior permittivity profile must be used because gas density cannot be neglected – range [1, ~1.25 (liquid hydrogen)] Simões et al., Icarus (2008) And: Conductivity is driven by water content in the gaseous envelope - Liu, PhD Thesis, Caltech (2006) 15% H2O H2O depleted TLE Workshop, Corte, 27 June 2008 13/18

Earth Titan Radius ~ 2575 km Ionospheric Layer height ~ 700 km TLE Workshop, Corte, 27 June 2008 14/18

Huygens Probe - ELF spectra recorded with the PWA analyzer Titan Huygens Probe - ELF spectra recorded with the PWA analyzer 36 Hz spectral line Simões et al., PSS (2007) This signal can not be the lowest eigenmode but is consistent to the second eigenfrequency TLE Workshop, Corte, 27 June 2008 15/18

Titan Cavity Parameterization: Data: Interpretation: Radius 2575 km Height of the ionosphere ~ 1200 km (cavity upper boundary ~700 km) Low surface conductivity (10-10-10-9 Sm-1) – Grard et al., PSS (2006)  skin depth >103 km Theoretical models predict buried ocean - Lunine and Stevenson, Icarus (1987) Data: Huygens Probe provided ELF data – Fulchignoni et al., Nature (2005); Grard et al., PSS (2006) Cassini Orbiter did not detect lightning - Fischer et al., Icarus (2007) Conductivity measurements below 140 km – Hamelin et al., PSS (2006); López-Moreno et al., GRL (submitted) Interpretation: If the signal detected by Huygens is natural, the Schumann resonance is likely excited by an interaction with the magnetosphere of Saturn - Béghin et al., Icarus (2007) Schumann resonances can be used to investigate the interior of Titan and assess the depth of the buried ocean - Simões et al., PSS (2007) TLE Workshop, Corte, 27 June 2008 16/18

And now back to Earth… 2D axisymmetric and 3D models of ELF-VLF electromagnetic wave propagation We can apply the same numerical model to improve studies of the Earth cavity The finite element model includes transient, eigenfrequency, harmonic propagation, and parametric analysis Currently: Modelling eigenfrequency splitting Near future: Model may be used to study TLEs Cavity Parameterization: Inner boundary (Earth radius) Outer boundary (D-layer) 3D Ionospheric data 3D Atmospheric data 3D Geomagnetic field Sources are Hertz dipoles S(t) or S() E(t), E() H(t), H() Sources and medium properties 3D model TLE Workshop, Corte, 27 June 2008 17/18

Summary Schumann resonances respond to sources distribution and ionospheric variability Schumann resonances can be used to investigate atmospheric electricity through the Solar System, including rocky planets, icy moons, and the giant planets The eigenmodes can be used to investigate Venus atmospheric turbulence and refractivity phenomena Venus cavity asymmetry is expected to induce line splitting larger than 1 Hz Schumann resonance monitoring can be used to investigate the sporadic meteor layer of Mars The water content of the gaseous envelope of Uranus and Neptune can be investigated by means of Schumann resonance measurements Schumann resonances can be used to investigate the interior of Titan and estimate the depth o the buried ocean The model can run under specific configurations that might be useful for TLE investigations – current contact: Fernando.Simoes@cetp.ipsl.fr A comparative planetology review of Schumann resonances is expected to be available soon – Simões et al., SSR (in press) TLE Workshop, Corte, 27 June 2008 18/18