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

Photo-ionisation rates

Photo-ionisation rates

Neutral density unchanged

`

1 AU

N2 H2OConditions: 1 AU, F10.7 = 100 (TIMED/SEE)

Photo-ionisation frequency

Neutral density unchanged

`

Photo-ionisation rates

1 AU

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:

Neutral density

Hydrostaticequilibrium

Comets:Outgassing+Expansion

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)

Photo-chemical

equilibrium

Low a at Earth large Ne

67P Earth

1PTitan

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!