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MAGNETIC FIELDS OF exoplanetS . FEATURES AND DETECTION

MAGNETIC FIELDS OF exoplanetS . FEATURES AND DETECTION. UCM, 27th May 2014 Enrique Blanco Henríquez. OUTLINE. Magnetospheres of Earth-like exoplanets Dynamo mechanism Hot Jupiters magnetospheres Atmospheric escape from Hot Jupiters Magnetodisks

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MAGNETIC FIELDS OF exoplanetS . FEATURES AND DETECTION

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  1. MAGNETIC FIELDS OF exoplanetS. FEATURES AND DETECTION UCM, 27th May 2014 Enrique Blanco Henríquez

  2. OUTLINE • Magnetospheres of Earth-likeexoplanets • Dynamomechanism • Hot Jupitersmagnetospheres • Atmospheric escape from Hot Jupiters • Magnetodisks • Radio emissionrelatedtomagneticfields • far-UV transits • Bow-shocks

  3. Magnetospheres in Earth-likeexoplanets • Magneticfieldsustainedby a dynamomechanism • In spite of majordifferences in structure, composition, and history, most of thesedynamos are thoughtto be maintainedby similar mechanisms: thermal and compositionalconvection in electricallyconductingfluids in theplanetinteriors • Tarter et al. 2007 and Scalo et al. 2007 recommended M-dwarfs as best targets tosearchforexo-Earths. • M-dwarfsmore active thanSun-likestarsplanetswill be exposedtodenserwinds. However, Planets are tidallylocked, are in synchronousrotationand haveweakmagneticmoments(maybenot as weak as wethought) • Earlymodelattempts • Olson & Christensen (2006), independent of rotationrate

  4. Magnetospheres in Earth-likeexoplanets • Nowadays, itisnotknowif F and D changewith time • However, rotationrate can playanimportant role in thenature of the • magneticfield • - Fastrotatorsdipole • - Slowrotatorsmultipole

  5. Magnetospheres in Earth-likeexoplanets • Magneticmomentdependsonitsrotationrate, butalsoonit’schemicalcomposition and theefficiency of convection in its interior (F) • Ωonlymarksifthedynamois dipolar or multipolar, butmagneticmomentstrengthwillnotexplicitlydependonrotation. • Planetsunder extreme conditions, i.e. highlyinhomogeneousheatingorunderverystrongstellarwinds, mayhavetheirmagneticfieldaffected. • Thisisstillwork in progress and a betterunderstanding of the interior structure and energytransportationmechanisms in rockyplanetsisstillnecessary.

  6. Hot JupitersMagnetospheres usual Giants Super-Earths Hot Jupiters

  7. Hot JupitersMagnetospheres • Upperatmospheressubjectedto intense heating and tidalforces • Magneticpressuredominates gas pressure (gas rarified) • High temperaturesgeneratedby EUV heating • Soft X-ray and EUV inducedexpansion of theupperatmosphere • Non-thermal escape: • Ion pick-up • Sputtering • Photo-chemicalenergizing & escape • Electromagnetic ion acceleration • Thermal escape: • Jeans escape – particlesfromtails • Hydrodynamic escape – allparticles

  8. Hot JupitersMagnetospheres- importance of magnetodisk • Hugeamount of Hot Jupiters are efficientlyprotectedagainst extreme plasma and radiationconditions. • Allestimationswerebasedontoosimplifiedmodel. • Itwasconsidered a planetarydipoledominatedmagnetosphereonly • Dipolemagneticfield balances stellarwindrampressure • However, big M isneededforefficientprotection: bigtidallockingsmall M • Specificallyforclose-in exoplanets, new modelisrequired • Strongmassloss of a planetshould lead toformation of a plasma disk • A magnetodiskdomainingmagnetosphere • More complete planetarymagnetospheremodel, • includingthewholecomplex of themagnetosphericelectriccurrentsystems

  9. Hot JupitersMagnetospheres- importance of magnetodisk • Formation of magnetodiskforHot Jupiters • “Sling” model: Dipolemagneticfield drives plasma in co-rotationregimeninsidethe Alfvenicsurface. • “material-escape driven” models • Hydrodynamic escape of plasma. • Dipolar magneticfield provoques a • chargeseparationwhich causes an • electricfield Hall current in • equatorplane.

  10. Hot JupitersMagnetospheres- importance of magnetodisk • ParaboloidMagnetospheircModel (PMM) forHot Jupiters • Key assumption: magnetopauseisapproximatedbyparaboloid of revolutionalongplanet-starline • Planetarymagneticdipole • Magnetotail • Magnetodisk • Magnetopausecurrents • Magneticfield of stellarwind

  11. Radio emissionfromexoplanets • Interactionbetweenthestellarwind and themagnetisedplanetprovoques a reconnectionthatreleasesenergeticelectrons: radio emission Radio Bode’sLaw Detection of cyclotron radio emission (CRE)wouldprovethat theexoplanetismagnetised Electroncyclotronemission frequency: Theradio flux observed at theEarth

  12. Radio emissionfromexoplanets Optimaldynamos in thecores of terrestrialexoplanets: Magneticfieldgeneration and detectability. Driscoll and Olson 2011 • CRE for 32% and 65% CMF exoplanets • Theionosphericcutoff at 10 MHz • sets thelowerfrequencylimitfor • ground-based radio telescopessuch as LOFAR. • LOFAR (LOwFrequencyARray) • It’sispossibletodetect CRE? • Small fluxes • To be detectable with LOFAR, • emissionpowermustincrease • by a 1e3 factor

  13. Measuringplanetarymagneticfieldwithtransitionobservations • Asymmetrybetweentheingress and egress times can be observed in thenear-UV light curve comparedtotheopticalobservations (eg. WASP-12b) • Ledtosuggesttheexistance of a bow-shock surroundingtheplanet’satmosphere. • For a shock todevelop, therelativevelocitybetweentheplanet and thestellar corona must be greaterthan local soundspeed • For a shock to be detected, itmustcompressthe local plasma to a densityhighenough. For a hydrostatic, isothermal corona, the local densityis • Supposethat coronal material fromthestarisnotmagneticallyconfined, so • it can escape in theform of a wind

  14. Measuringplanetarymagneticfieldwithtransitionobservations • Monte Carlo simulationsfor WASP-12b (earlyingress)

  15. Measuringplanetarymagneticfieldwithtransitionobservations • Measuringtheplanetarymagneticfield (Vidotto et al. 2010)

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