H3+ and the Planets
Steve Miller
+
H3
- the
driver of
planetary
atmospheres
H3+ and the Planets
Steve Miller
H3+ in planetary atmospheres
Formed above the homopause - thermosphere/ionosphere
Na(h) =
Na0 exp[-h/Ha]
Ha =
[kT/mag]
Pressure < 1mbar
Number density < 1018m-3
Temperatures:
Jupiter 900-1100K
Saturn 400K
Uranus 500-750K
H3+ and the Planets
Steve Miller
H3+ chemistry
Formation:
H2 + hn / e- 
H2+ + H2 
H2+ + e- [+e-]
H3+ + H
Charge exchange:
H2(v>4) + H+ 
H2+ + H
Destruction:
H3+ + e-

H2 + H / 3H
Protonation:
H3+ + X

XH+ + H2
In planetary atmospheres, X = CH4, C2H2, C2H6
H3+ and the Planets
Steve Miller
What we observe
High-resolution spectroscopy
- dynamics, energy production
IRTF
- characterising magnetospheres
Medium-resolution spectroscopy
UKIRT
- T, rion mapping, energy flow
- variability
H3+ and the Planets
Steve Miller
Medium resolution spectroscopy
Q(1,0-)
3.953mm
Uranus 1999
L window spectrum
H3+ n2 fundamental
Temperature
~600K
H3+ column density
~ 5x1015 m-2
Total emission
~10-5 W m-2
H3+ and the Planets
Steve Miller
High resolution spectroscopy
Jupiter 1998
Doppler shifting of Q(1,0-)
vion = 1 - 3km s-1
H3+ and the Planets
Steve Miller
A Big Question
Exospheric temperatures:
Jupiter
Saturn
Uranus
Neptune
Calculated
203K
177K
138K
132K
Why are they so hot?
Measured
940K
420K
800K
600K
H3+ and the Planets
Steve Miller
H3+ as a tracer of energy inputs
Particles (keV
electrons) are
accelerated along
magnetic field lines
H3+ formation occurs
Thermalisation then
radiation
What are the
mechanisms that
cause this?
Connerney et al.
H3+ and the Planets
Steve Miller
Planetary aurorae
Earth- solar
wind control
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Jupiter internal,
rotation
Saturn - solar wind, rotation
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
H3+ and the Planets
Steve Miller
The H3+ thermostat
In some spectral regions,
H3+ spectrum dominates
- 3-4mm
Particle inputs to upper
atmosphere ~ n mW m-2
But increased particle flux
creates more H3+
- H3+ emission balances
this for Jupiter and
(probably) Uranus
- but not for Saturn
This does NOT help
the high temperature
problem
BUT…
H3+ and the Planets
Steve Miller
H3+ in exoplanets
Many large exoplanets found
close to central star < 0.5a.u.
Detection?
QuickTime™ and a
FF (Uncompressed) decompressor
are needed to see this picture.
Solar radiation >100 x jovian
Will atmosphere heat up
uncontrollably and boil off?
Becomes less
effective at d<0.4a.u.
due to H2  H + H
More hn creates more H3+
More H3+ more cooling
H3+ and the Planets
Steve Miller
H3+ heating - Joule heating
Downward field-aligned current
Upward field-aligned current
Equatorward electric field
HJ = Eeq2 SP
SP  N(H3+)
Typical values:
Eeq = 1-3 Vm-1 ; SP = 1-10mho
BUT …
H3+ and the Planets
Steve Miller
H3+ heating - ion winds
Magnetic field
Equatorward electric field
Ion drift
vion = -Eeq x BJ / |BJ|2
Typical values:
BJ = 10-3 Tesla ; vion = 1-2 km s-1
BUT …
H3+ and the Planets
Steve Miller
H3+ heating - ion winds and ion drag
Ion drift
Neutral wind
vneut = k vion
k ~ 0.5
HJ =
Hdrag =
Helec =
[(1-k)Eeq]2 SP
k(1-k)Eeq2 SP
HJ + Hdrag
Typical values:
Helec > 1014W planetwide
H3+ and the Planets
Steve Miller
Heating/cooling in an auroral event
Sept. 8, 1998
Sept. 11, 1998
T(H3+)
940K
1065K
N(H3+)
1.55x 1016 m-3
1.80x1016 m-3
vion
0.5 km s-1
1.0 km s-1
Helec
67.0 mW m-2
277.0 mW m-2
Precipitation
10.8 mW m-2
12.0 mW m-2
Conduction
-0.3 mW m-2
-0.4 mW m-2
E(H3+)
-5.1 mW m-2
-10.0 mW m-2
E(CH4)
-65.0 mW m-2
-103.3 mW m-2
Net heating
7.4 mW m-2
175.3 mW m-2
Henrik Melin et al., Icarus Articles in press, 2006.
H3+ and the Planets
H3+ heating - Saturn I
Cassini:
solar wind
control of
Saturn’s polar
dynamics
Steve Miller
H3+ and the Planets
Steve Miller
H3+ heating - Saturn II
Wion = 0.34 WSat
E(r ) = [WSat- Wion]r x BSat
Typical values: Helec = n x 1012W planetwide
H3+ and the Planets
Steve Miller
Uranus
Solar cycle control of
total H3+ emission
QuickTime™ and a
Sorenson Video 3 decompressor
are needed to see this picture.
Auroral emission ~20% of
total emission
Effect of Sun-Magnetic
Pole angle?
H3+ and the Planets
Steve Miller
Jupiter
Saturn
Uranus
Energy Tracer
√
√
√
Thermostat
√
Conductivity
√
√
√
Heating
√
√
?
√
H3+ and the Planets
George Millward
Tom Stallard
Henrik Melin
Bob Joseph
Tom Geballe
Steve Miller
Oka
+
Alan Aylward
Makenzie Lystrup
Chris Smith
H3
- the Jonathan Tennyson
Larry Trafton
driver of
planetary
atmospheres