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 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 and Scalo et al 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” models Hydrodynamic escape of plasma. Dipolar magnetic field provoques a charge separation which causes an electric field Hall current in equator 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 that the exoplanet is magnetised Electron cyclotron emission frequency : Radio Bode’s Law The radio flux observed at the Earth

Radio emission from exoplanets Optimal dynamos in the cores of terrestrial exoplanets: Magnetic field generation and detectability. Driscoll and Olson 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