Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p42b..05d&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P42B-05
Astronomy and Astrophysics
Astrophysics
[1510] Geomagnetism And Paleomagnetism / Dynamo: Theories And Simulations, [5430] Planetary Sciences: Solid Surface Planets / Interiors, [5734] Planetary Sciences: Fluid Planets / Magnetic Fields And Magnetism, [7524] Solar Physics, Astrophysics, And Astronomy / Magnetic Fields
Scientific paper
More than 360 extrasolar planets have been detected to date, with only 7 less massive than 10 Earth masses. Planets in the 1-10 Earth-mass regime orbiting within 3 AU with an Earth-like composition, referred to as Super-Earths, are expected to have large, mostly Iron cores that could sustain a convectively driven dynamo. Several techniques have been proposed to detect and measure the magnetic fields of extrasolar planets, including star-planet interaction, cyclotron radio emission, and Zeeman Doppler spectroscopy, and the presence of a magnetic field has been claimed in about 10. Strong magnetic fields can be used to infer the planetary rotation rate, may be easier to detect than terrestrial atmospheres, and can act as a barrier to stellar radiation which is often invoked as a requirement for habitability. We present a model to estimate the maximum self-sustained magnetic moment of a terrestrial dynamo given only the total mass and core-mass fraction. Assuming the magnetic field is self-sustained by a convectively driven dynamo we estimate the magnetic moment using a dynamo scaling law, which relies on dynamical properties of the planetary interior, such as the heat flux at the core-mantle boundary and size of the dynamo region. To estimate these interior properties we model the internal structure of the planet using a sub-solidus, mobile lid convection profile for the mantle and a thermal convection profile for the core. We present models for 1-10 Earth-masses and a range of core-mass fractions. For an Earth-mass planet with a core-mass of 32% of the total mass (similar to the Earth) we calculate an optimal heat flux of 29 TW, which implies an upper bound magnetic moment of 257 ZAm^2 (Z=10^21). For an Iron-rich Earth-mass planet with a core-mass fraction of 65% (similar to Mercury) we calculate an optimal heat flux of 25 TW, which implies an upper bound magnetic moment of 268 ZAm^2. Our estimated magnetic moments are large compared to the present-day geomagnetic virtual axial dipole moment of 80 ZAm^2, but several measurements in the paleomagnetic record approach this upper limit. For larger 10 Earth-mass planets the magnetic moment could be as much as 10 times larger than the estimates given here.
Driscoll Peter E.
Olson Peter
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