H 3 + and the Planets Steve Miller H the driver of planetary atmospheres

H 3 + and the Planets Steve Miller H 3 + in planetary atmospheres Formed above the homopause - thermosphere/ionosphere N a (h)=N a0 exp[-h/H a ] H a =[kT/m a g] Pressure < 1  bar Number density < m -3 Temperatures: Jupiter K Saturn400K Uranus K

H 3 + and the Planets Steve Miller H 3 + chemistry Formation:H 2 + h / e -  H e - [+e - ] H H 2  H H Charge exchange:H 2 (v>4) + H +  H H Destruction:H e -  H 2 + H / 3H Protonation:H X  XH + + H 2 In planetary atmospheres, X = CH 4, C 2 H 2, C 2 H 6

H 3 + and the Planets Steve Miller What we observe IRTF High-resolution spectroscopy - dynamics, energy production - characterising magnetospheres Medium-resolution spectroscopy - T,  ion mapping, energy flow - variability UKIRT

H 3 + and the Planets Steve Miller Medium resolution spectroscopy Uranus 1999 L window spectrum H 3 +  2 fundamental Temperature~600K H 3 + column density~ 5x10 15 m -2 Total emission~10 -5 W m -2 Q(1,0 - )  m

H 3 + and the Planets Steve Miller High resolution spectroscopy Jupiter 1998 Doppler shifting of Q(1,0 - ) v ion = 1 - 3km s -1

H 3 + and the Planets Steve Miller A Big Question Exospheric temperatures: CalculatedMeasured Jupiter 203K 940K Saturn 177K 420K Uranus 138K 800K Neptune 132K 600K Why are they so hot?

H 3 + and the Planets Steve Miller H 3 + as a tracer of energy inputs Particles (keV electrons) are accelerated along magnetic field lines H 3 + formation occurs Thermalisation then radiation What are the mechanisms that cause this? Connerney et al.

H 3 + and the Planets Steve Miller Planetary aurorae Earth- solar wind control Saturn - solar wind, rotation Jupiter - internal, rotation

H 3 + and the Planets Steve Miller The H 3 + thermostat In some spectral regions, H 3 + spectrum dominates  m Particle inputs to upper atmosphere ~ n mW m -2 But increased particle flux creates more H 3 + -H 3 + emission balances this for Jupiter and (probably) Uranus -but not for Saturn This does NOT help the high temperature problem BUT…

H 3 + and the Planets Steve Miller H 3 + in exoplanets Many large exoplanets found close to central star < 0.5a.u. Solar radiation >100 x jovian Will atmosphere heat up uncontrollably and boil off? More h creates more H 3 + More H 3 + more cooling Becomes less effective at d<0.4a.u. due to H 2  H + H Detection?

H 3 + and the Planets Steve Miller H 3 + heating - Joule heating H J = E eq 2  P  P  N(H 3 + ) Typical values: E eq = 1-3 Vm -1 ;  P = 1-10mho BUT … Downward field-aligned current Upward field-aligned current Equatorward electric field

H 3 + and the Planets Steve Miller H 3 + heating - ion winds v ion = -E eq x B J / |B J | 2 Typical values: B J = Tesla ; v ion = 1-2 km s -1 BUT … Magnetic field Ion drift Equatorward electric field

H 3 + and the Planets Steve Miller H 3 + heating - ion winds and ion drag v neut = k v ion k ~ 0.5 H J =[(1-k)E eq ] 2  P H drag =k(1-k)E eq 2  P H elec = H J + H drag Ion drift Neutral wind Typical values: H elec > W planetwide

H 3 + and the Planets Steve Miller Heating/cooling in an auroral event Sept. 8, 1998Sept. 11, 1998 T(H 3 + )940K1065K N(H 3 + )1.55x m x10 16 m -3 v ion 0.5 km s km s -1 H elec 67.0 mW m mW m -2 Precipitation10.8 mW m mW m -2 Conduction-0.3 mW m mW m -2 E(H 3 + )-5.1 mW m mW m -2 E(CH 4 )-65.0 mW m mW m -2 Net heating7.4 mW m mW m -2 Henrik Melin et al., Icarus Articles in press, 2006.

H 3 + and the Planets Steve Miller H 3 + heating - Saturn I Cassini: solar wind control of Saturn’s polar dynamics

H 3 + and the Planets Steve Miller H 3 + heating - Saturn II  ion = 0.34  Sat E(r ) = [  Sat -  ion ]r x B Sat Typical values: H elec = n x W planetwide

H 3 + and the Planets Steve Miller Uranus Solar cycle control of total H 3 + emission Auroral emission ~20% of total emission Effect of Sun-Magnetic Pole angle?

H 3 + and the Planets Steve Miller JupiterSaturnUranus Energy Tracer √ √ √ Thermostat √ √ Conductivity √ √ √ Heating √ √ ?

H 3 + and the Planets Steve Miller H the driver of planetary atmospheres George MillwardAlan Aylward Tom Stallard Makenzie Lystrup Henrik Melin Chris Smith Bob JosephJonathan Tennyson Tom Geballe Larry Trafton Oka