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Cross-body comparison:Gravitational effect on photo-ionisation rates
Marina Galand, Erik Vigren (Imperial College London, UK)
Michael Mendillo (Boston University, USA)
Gravitational effect on photo-ionisation rates
OUTLINE
–Context–Photo-ionisation rates at different bodies–Hydrostatic equilibrium versus
outgassing/expansion– Impact on electron density
Gravitational effect on photo-ionisation rates
OUTLINE
–Context–Photo-ionisation rates at different bodies–Hydrostatic equilibrium versus
outgassing/expansion– Impact on electron density
ion, e- Photoelectron
XUV solar photons(0.1-100 nm)
+
*
Solar deposition
Photo-ionisationExcitation
Dissociation
Atmospheric species ion produced
Ionisation potential
N2 N2+ 15.58 eV 79.6 nm
H2O H2O+ 12.61 eV 98.3 nm
Motivation
WHY high rate & high column density for CG?
67P/CG1.3 AUPerihelion*
Earth1 AU*
Surface
- Low latitudes- Sun at zenith- F10.7 = 100
- Neutral atmosphere:* NRLMSISE-00(Picone et al. 2002)* Kinetic model(Tenishev et al. 2008)
Solar deposition at 67P/CG
Gravitational effect on photo-ionisation rates
OUTLINE
–Motivation–Photo-ionisation rates at different bodies–Hydrostatic equilibrium versus
outgassing/expansion– Impact on electron density
Comparative study
Body Neutral species
considered
Distance from the Sun (AU)
Equatorial surface gravity
(m/s2)Earth N2 1 9.78Titan N2 9.6 1.35Ganymede H2O 5.2 1.4367P/CG H2O
(Q=5.5x1027 s-1)1.3
(perihelion)0
1P/Halley H2O(Q=5.5x1029 s-1)
0.9(Giotto)
0
Conditions: Low latitude, 0° SZA, F10.7 = 100 (TIMED/SEE)
Calculation of the photo-ionisation rates
In the XUV (0.1-100 nm), primarily extinction in the beam
apply Beer-Lambert Law:
Attenuated solar flux at wavelength l and at altitude z:
Photoelectron production rate:
Ionization
photoelectron
` ` ` ` ` `
Optical depth t
Gravitational effect on photo-ionisation rates
OUTLINE
–Motivation–Photo-ionisation rates at different bodies–Hydrostatic equilibrium versus
outgassing/expansion– Impact on electron density
Neutral density
- Planet/Moon with significant gravity and dense atmosphere:Hydrostatic equilibrium prevails
with
Assuming isothermal region (Hn=H):
- Comets with negligible gravity:Sublimation followed by expansion
Assuming that un is independent of r:
t=1
t=2
Photo-ionisation rates (1 AU)
67PEarth
1P
Titan
67P
Earth
1PTitan
27-28 nm
[Galand et al., to be submitted, 2014]
Gravitational effect on photo-ionisation rates
OUTLINE
–Motivation–Photo-ionisation rates at different bodies–Hydrostatic equilibrium versus
outgassing/expansion– Impact on electron density
Electron density
Body Electron dissociative recombination rate (cm-3 s-1)
Earth 1.0 x 10-7 (1000 K) [O2+]
Titan 3.2 x 10-6 * (Vigren et al. 2013)Comet 1.9 x 10-6 (100 K) [H3O+] (Neau et al. 2000)
* Empirically derived from estimation of Pe and observed Ne
Assuming photo-chemical equilibrium near the peak(E-region peak at Earth):
Electron density (1 AU)
Transport included
Transport at comets
further reduction in
Ne near surface
67P Earth
1P
Titan
Gravitational effect on photo-ionisation rates
- At a given heliocentric distance, for low outgassing comets, peak photo-ionisation rate has:
- high magnitude and low altitudecompared with the hydrostatic case, valid at planets & moons.
- For hot ionospheres, such as Earth:- Small recombination coeff. large Ne (for given Pe)
- For cold ionospheres, such as comets:- High recombination coeff. + transport low Ne (close to surface)
- Negligible gravity at comets means that photo-ionisation rates at comets need to be treated differently from planetary atmospheres which are controlled by gravity. Cannot just extrapolate from planetary atmospheres to comets!