MAGNETIC FIELDS OF EXOPLANETS. FEATURES AND DETECTION UCM, 27th May 2014 Enrique Blanco Henríquez.
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Transcript of MAGNETIC FIELDS OF EXOPLANETS. FEATURES AND DETECTION UCM, 27th May 2014 Enrique Blanco Henríquez.
MAGNETIC FIELDS OF EXOPLANETS. FEATURES AND
DETECTION
UCM, 27th May 2014Enrique Blanco Henríquez
OUTLINE
• Magnetospheres of Earth-like exoplanets• Dynamo mechanism
• Hot Jupiters magnetospheres• Atmospheric escape from Hot Jupiters• Magnetodisks
• Radio emission related to magnetic fields• far-UV transits
• Bow-shocks
Magnetospheres in Earth-like exoplanets• Magnetic field sustained by a dynamo mechanism• In spite of major differences in structure, composition, and history, most of these dynamos
are thought to be maintained by similar mechanisms: thermal and compositional convection in electrically conducting fluids in the planet interiors
• Tarter et al. 2007 and Scalo et al. 2007 recommended M-dwarfs as best targets to search for exo-Earths.
• M-dwarfs more active than Sun-like stars planets will be exposed to denser winds. However, Planets are tidally locked, are in synchronous rotation and have weak magnetic moments (maybe not as weak as we thought)
• Early model attempts
• Olson & Christensen (2006), independent of rotation rate
Magnetospheres in Earth-like exoplanets
• Nowadays, it is not know if F and D change with time• However, rotation rate can play an important role in the nature of the magnetic field
- Fast rotators dipole - Slow rotators multipole
Magnetospheres in Earth-like exoplanets
• Magnetic moment depends on its rotation rate, but also on it’s chemical composition and the efficiency of convection in its interior (F)
• Ω only marks if the dynamo is dipolar or multipolar, but magnetic moment strength will not explicitly depend on rotation.
• Planets under extreme conditions, i.e. highly inhomogeneous heating or under very strong stellar winds, may have their magnetic field affected.
• This is still work in progress and a better understanding of the interior structure and energy transportation mechanisms in rocky planets is still necessary.
Hot Jupiters Magnetospheres
usual Giants
Super-Earths
Hot Jupiters
Hot Jupiters Magnetospheres
• Upper atmospheres subjected to intense heating and tidal forces• Magnetic pressure dominates gas pressure (gas rarified)• High temperatures generated by EUV heating
• Soft X-ray and EUV induced expansion of the upper atmosphere
• Thermal escape: • Jeans escape – particles from tails• Hydrodynamic escape – all particles
• Non-thermal escape:• Ion pick-up• Sputtering• Photo-chemical energizing & escape• Electromagnetic ion acceleration
Hot Jupiters Magnetospheres- importance of magnetodisk
• Huge amount of Hot Jupiters are efficiently protected against extreme plasma and radiation conditions.
• All estimations were based on too simplified model.• It was considered a planetary dipole dominated magnetosphere only• Dipole magnetic field balances stellar wind ram pressure
• However, big M is needed for efficient protection:
big tidal locking small M
• Specifically for close-in exoplanets, new model is required• Strong mass loss of a planet should lead to formation of a plasma disk • A magnetodisk domaining magnetosphere• More complete planetary magnetosphere model, including the whole complex of the magnetospheric electric current systems
Hot Jupiters Magnetospheres- importance of magnetodisk
• Formation of magnetodisk for Hot Jupiters
• “Sling” model:
Dipole magnetic field drives plasma in
co-rotation regimen inside the
Alfvenic surface.
• “material-escape driven” modelsHydrodynamic escape of plasma.Dipolar magnetic field provoques a charge separation which causes an electric field Hall current inequator plane.
Hot Jupiters Magnetospheres- importance of magnetodisk
• Paraboloid Magnetospheirc Model (PMM) for Hot Jupiters • Key assumption: magnetopause is approximated by paraboloid of revolution along
planet-star line
- Planetary magnetic dipole
- Magnetotail
- Magnetodisk
- Magnetopause currents
- Magnetic field of stellar wind
Radio emission from exoplanets
• Interaction between the stellar wind and the magnetised planet provoques a reconnection that releases energetic electrons: radio emission
Detection of cyclotron radio emission (CRE) would prove thatthe exoplanet is magnetised
Electron cyclotron emission frequency:
Radio Bode’s Law
The radio flux observed at the Earth
Radio emission from exoplanetsOptimal dynamos in the cores of terrestrial exoplanets:Magnetic field generation and detectability. Driscoll and Olson 2011
- CRE for 32% and 65% CMF exoplanets
- The ionospheric cutoff at 10 MHz sets the lower frequency limit for ground-based radio telescopes
such as LOFAR. - LOFAR (LOw Frequency ARray)
- It’s is possible to detect CRE?- Small fluxes - To be detectable with LOFAR, emission power must increase by a 1e3 factor
Measuring planetary magnetic field with transition observations
• Asymmetry between the ingress and egress times can be observed in the near-UV light curve compared to the optical observations (eg. WASP-12b)
• Led to suggest the existance of a bow-shock surrounding the planet’s atmosphere.
• For a shock to develop, the relative velocity between the planet and the stellar corona must be greater than local sound speed
• For a shock to be detected, it must compress the local plasma to a density high enough. For a hydrostatic, isothermal corona, the local density is
• Suppose that coronal material from the star is not magnetically confined, so it can escape in the form of a wind
• Monte Carlo simulations for WASP-12b (early ingress)
Measuring planetary magnetic field with transition observations
• Measuring the planetary magnetic field (Vidotto et al. 2010)
Measuring planetary magnetic field with transition observations