The EUV impact on ionosphere:

J.-E. Wahlund and M. Yamauchi

Swedish Institute of Space Physics (IRF)

ON3 Response of atmospheres and magnetospheres of terrestrial planets to extreme solar/stellar conditions

What do observations indicate for atmospheric evolution of early

Earth and Exo-Earths?

Various escape processes

process Mechanism ExplanationJeans escape thermal,

light neutral/ionThermal tail exceeds escape velocity

Hydrodynamic blow-off thermal, neutral/ion

Extreme EUV condition at early Sun/Star.

Photochemical heating chemical, light neutral

Release of energy in the excited atomic state.

Ion pickup & sub-sequent sputtering

non-thermal, light ion

Newly exposed ions to SW is subject to SW DC field.

Non-thermal ion energization by E// & EM wave

non-thermal, light/heavy ion

Local deposit of SW energy to ionosphere causes EM field that energizes ions.

Large-scale momentum transfer

non-thermal, light/heavy ion

SWDP & EM forces push bulk plasma anti-sunward at the boundary region.

Today's keyword : Ionosphere

1. As source of non-thermally escaping ions.

2. As protector to keep "neutrals to be escape" inside ionosphere (Jeans escape + ion pick-up).

3. As a modifier of large-scale momentum transfer.

(a) The evolution of the planetary atmosphere might be dependent on the ionospheric condition and its activity.

(b) Consider dependence of escape on solar EUV/FUV & solar wind (SW).

hints for extreme conditions at early Sun/star

EUV & SW dependence of ionospheric contribution: 1. as source

As: Magnetized Unmagnetized Key word

Source of

wave-related heating

? localized energy deposit to

ionosphere

Protect from

(Jeans) (Jeans)

ion pick-up

relative height of ionopause & exosphere

Amplify by

O+-related instability

(interaction area increase)

bulk momentum transfer

Fact 1: high rate of non-thermal ion escape

Lundin et al., 2004

(Nilsson et al., 2004)

H+

O+

Cluster/CIS

Escape at solar maximumMars: 0.5 kg/s (O+, O2

+)Venus: 2kg/s (O+)Earth: 1 kg/s (O+)

Fact 2a: Ion escape increases with F10.7 flux

Venus: a factor of 20 change in ionotail density.

Mars: a factor of 102 difference between MEX and Phobos-2 (but need revision).

(Cully et al., 2003)

Between solar max & min (factor 3 difference in F10.7 flux):

Earth: a factor of 102 (or 3) change for O+ (or H+) outflow.

largest contribution & high O/H ratio at early Earth ?

Fact 2b: Non-thermal ion escape increases with geomagnetic activity

(Broad-Band Electrostatic Low Frequency wave)

(Lower Hybrid or Electro-Magnetic Ion Cyclotron wave)

(2) depend strongly on Kp, SWDP, and IMF

Freja@h=1700km (Norqvist et al., 1998)Akebono/DE/Polar (Cully et al., 2003)

(1) in various forms

O+H+

EUV & SW dependence of ionospheric contribution: 2. as protector

As: Magnetized Unmagnetized Key word

Source of

wave-related heating

? localized energy deposit to

ionosphere

Protect from

(Jeans) (Jeans)

ion pick-up

relative height of ionopause & exosphere

Amplify by

O+-related instability

(interaction area increase)

bulk momentum transfer

SW wind interaction with atmosphere

present/ancient Earth? ancient Mars/Venus?

present Mars/Venus?ancient Earth?

For reference

SW is stopped by the magnetic pressure of the dipole field

Interplanetary magnetic field (IMF) is enhanced around the ionosphere due to induction current

Protection by ionosphere

In both magnetized/unmagnetized planets, strong B-field lies between the ionosphere and (shocked) SW.

1. Thick ionosphere means higher ionization rate by the electron impact ionization.

Extra ionization of neutrals with escape velocity, while these ions cannot escape beyond the magnetized ionopause/magnetopause.

Reduction of Jeans escape (of mainly H, He)

2. Higher ionopause location means less neutrals (corona) beyond the ionopause.

Reduction of ion pick-up (of mainly H, He)

Fact 3: Ionopause is EUV/FUV dependentSolar cycle variation of the ionopause height:

Venus : 1700 km difference between solar maximum (high) and solar minimum (low) (Zhang et al., 2007). The same tendency for Mars (Zhang et al., 1990).

Therma/non-thermal ratio = out-of-phase of solar cycle

cf. SW dependence of ionopause heightWe expect:

(a) strong (stable) IMF no change

(b) variable IMF lower balance altitude (by cancellation of B)

(c) strong SWDP lower balance altitude

Therma/non-thermal ratio = out-of-phase of SW activity

Fact 4a: extra ionization (cold case)

high ionization (by electron impact) & subsequent escape are observed at Titan

(Wahlund et al., 2005)

Fact 4b: extra ionization (hot case)

Possible extra ionization by, e.g., critical ionization velocity mechanism

Critical ionization velocity (CIV)

EUV & SW dependence of ionospheric contribution

As: Magnetized Unmagnetized Key word

Source of

wave-related heating

? localized energy deposit to

ionosphere

Protect from

(Jeans) (Jeans)

ion pick-up

relative height of ionopause & exosphere

Amplify by

O+-related instability

(interaction area increase)

bulk momentum transfer

Magnetized planet

increase in EUV/FUV SWDP IMF (IMF)

Non-thermal heating +++ ++ + +

Jeans + photo-chemical

++ same same same

Ion pick-up same same same same

Large-scale momentum transfer

+ (?) + + (?) + (?)

O/H ratio (#1) ++ + +

(#1) Increase or decrease depending on the relative importance of non-thermal heating

Unmagnetized planet

increase in EUV/FUV SWDP IMF (IMF)

Non-thermal heating ++ (?) + (?) same +

Jeans + photo-chemical

++ same same same

Ion pick-up (#2) ++ same +

Large-scale momentum transfer

++ + same ++

O/H ratio (#1) ++ same +

#1) depending on relative importance of non-thermal heating.#2) depending on relative extent of ionosphere and exosphere

Since ancient Earth's ionosphere is

* Most likely High EUV/FUV* More likely High SWDP* Probably strong/active IMF

much higher O escape & much higher O/H ratio of escape than present.

The ancient atmosphere can be chemically quite reduced

Unclear parameters : Magnetized or non-magnetized, atmospheric composition, internal condition

End

Extra slides for Q & A

Budget above the Earth's ionosphereH+/O+ in major return routeion escape H+ O+

< 10 eV (2~3 Re) 2~5 1~3

> 10 eV (3~4 Re) 2~8 1.5~20

ion precipitation ion electron

> 10 eV (DMSP) 0.2~0.9 9~60in 1025 /s

mass budget H+ O+ meteors

out 0.05~0.2 0.5~5 -

in < 0.02 ? 0.5in kg/s

After Moore et al., 1999

Magnetized planet (Earth, Mercury)

Magnetopause : balance between SW PD Planetary magnetic field

(a) stronger but stable IMF lower altitude of magnetopause but more return flow

(b) more variable IMF more internal process (non-thermal escape)

(c) stronger SW PD lower altitude of magnetopause + escape

How about UV dependence ? (important for ancient condition)

Height and density of the ionosphere

(1) Ionization (source) = Chapman model One-component atmosphere (scale height = H 1/gravity): cross section, F0:incoming solar flux, n0:density at z=0

Peak altitude : zmax(, F0, H) = H ln(n0H/cos()) does not depends on F0 , but on H (i.e., gravity)

Peak production : qmax (, F0 , H) = F0cos()/H exp(1) depends on F0 and H (i.e., gravity)

(2) Transport (recombination loss is ignorable) Moves peak of ne(z) much higher with less sharp ne(z) profile

Transport (convection) is mainly driven by heating ( q) Ionospheric extent depends on both F0 and gravity

Escape from the cusp

Earth ?

Mars ?

Venus ?

Io & other

Satellites?