7,427 research outputs found
Thermal evolution and lifetime of intrinsic magnetic fields of Super Earths in habitable zones
We have numerically studied the thermal evolution of various-mass terrestrial
planets in habitable zones, focusing on duration of dynamo activity to generate
their intrinsic magnetic fields, which may be one of key factors in
habitability on the planets. In particular, we are concerned with super-Earths,
observations of which are rapidly developing. We calculated evolution of
temperature distributions in planetary interior, using Vinet equations of
state, Arrhenius-type formula for mantle viscosity, and the astrophysical
mixing length theory for convective heat transfer modified for mantle
convection. After calibrating the model with terrestrial planets in the Solar
system, we apply it for 0.1-- rocky planets with surface
temperature of 300~\mbox{K} (in habitable zones) and the Earth-like
compositions. With the criterion for heat flux at the CMB (core-mantle
boundary), the lifetime of the magnetic fields is evaluated from the calculated
thermal evolution. We found that the lifetime slowly increases with the
planetary mass () independent of initial temperature gap at the
core-mantle boundary () but beyond a critical value
() it abruptly declines by the mantle viscosity
enhancement due to the pressure effect. We derived as a function of
and a rheological parameter (activation volume, ).
Thus, the magnetic field lifetime of super-Earths with
sensitively depends on , which reflects planetary
accretion, and , which has uncertainty at very high pressure. More
advanced high-pressure experiments and first-principle simulation as well as
planetary accretion simulation are needed to discuss habitability of
super-Earths.Comment: 19pages, 15 figures, accepted for publication in Ap
Saturated torque formula for planetary migration in viscous disks with thermal diffusion: recipe for protoplanet population synthesis
We provide torque formulae for low mass planets undergoing type I migration
in gaseous disks. These torque formulae put special emphasis on the horseshoe
drag, which is prone to saturation: the asymptotic value reached by the
horseshoe drag depends on a balance between coorbital dynamics (which tends to
cancel out or saturate the torque) and diffusive processes (which tend to
restore the unperturbed disk profiles, thereby desaturating the torque). We
entertain here the question of this asymptotic value, and we derive torque
formulae which give the total torque as a function of the disk's viscosity and
thermal diffusivity. The horseshoe drag features two components: one which
scales with the vortensity gradient, and one which scales with the entropy
gradient, and which constitutes the most promising candidate for halting inward
type I migration. Our analysis, which is complemented by numerical simulations,
recovers characteristics already noted by numericists, namely that the viscous
timescale across the horseshoe region must be shorter than the libration time
in order to avoid saturation, and that, provided this condition is satisfied,
the entropy related part of the horseshoe drag remains large if the thermal
timescale is shorter than the libration time. Side results include a study of
the Lindblad torque as a function of thermal diffusivity, and a contribution to
the corotation torque arising from vortensity viscously created at the contact
discontinuities that appear at the horseshoe separatrices. For the convenience
of the reader mostly interested in the torque formulae, section 8 is
self-contained.Comment: Affiliation details changed. Fixed equation numbering issue. Biblio
info adde
Modification of Angular Velocity by Inhomogeneous MRI Growth in Protoplanetary Disks
We have investigated evolution of magneto-rotational instability (MRI) in
protoplanetary disks that have radially non-uniform magnetic field such that
stable and unstable regions coexist initially, and found that a zone in which
the disk gas rotates with a super-Keplerian velocity emerges as a result of the
non-uniformly growing MRI turbulence. We have carried out two-dimensional
resistive MHD simulations with a shearing box model. We found that if the
spatially averaged magnetic Reynolds number, which is determined by widths of
the stable and unstable regions in the initial conditions and values of the
resistivity, is smaller than unity, the original Keplerian shear flow is
transformed to the quasi-steady flow such that more flattened (rigid-rotation
in extreme cases) velocity profile emerges locally and the outer part of the
profile tends to be super-Keplerian. Angular momentum and mass transfer due to
temporally generated MRI turbulence in the initially unstable region is
responsible for the transformation. In the local super-Keplerian region,
migrations due to aerodynamic gas drag and tidal interaction with disk gas are
reversed. The simulation setting corresponds to the regions near the outer and
inner edges of a global MRI dead zone in a disk. Therefore, the outer edge of
dead zone, as well as the inner edge, would be a favorable site to accumulate
dust particles to form planetesimals and retain planetary embryos against type
I migration.Comment: 28 pages, 11figures, 1 table, accepted by Ap
TOWARDS REGIONAL DIFFERENTIATION OF RURAL DEVELOPMENT POLICY IN THE EU
In this study a comparative analysis of the Rural Development Plans (RDPs) in four intermediate rural regions (Northern Netherlands, Lower Saxony, Wales and Emilia Romagna) and four most urban regions (Southern Netherlands, North Rhine-Westphalia, Flanders and Lombardia) is made. Such plans are designed in the scope of the second pillar of the Common Agricultural Policy (CAP). In particular, the focus was on the question whether the menu approach of the second pillar enables EU member states and regions to design Rural Development Plans with a tailor-made set of measures which address their specific rural development needs. The findings of this study suggest that the current menu of rural development measures is sufficient to suit the wide range of socio-economic, ecological and physical circumstances in the EU regions. The analysis also revealed that there is some overlap between the three rural development priorities of the second pillar. Therefore, an outline of future rural development priorities and measures in the EU is designed, in which it is attempted to avoid overlap between the various rural development priorities and in which each measure contributes to the achievement of one development priority only. In addition, it is proposed that regions would select only those measures in their Rural Development Plan which really address the rural development needs in their region, even if this results in a Rural Development Plan with only one or two rural development measures. Such an approach of selecting rural development measures according to regional needs will result in a large variation in rural development measures implemented and may be considered regional differentiation of EU rural development policy.Agricultural and Food Policy,
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