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Nathanaël Schaeffer - One of the best experts on this subject based on the ideXlab platform.
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Turbulent convective length scale in Planetary cores
Nature, 2019Co-Authors: Céline Guervilly, P. Cardin, Nathanaël SchaefferAbstract:Convection is a fundamental physical process in the fluid cores of planets. It is the primary transport mechanism for heat and chemical species and the primary energy source for Planetary Magnetic Fields. Key properties of convection—such as the characteristic flow velocity and length scale—are poorly quantified in Planetary cores owing to the strong dependence of these properties on Planetary rotation, buoyancy driving and Magnetic Fields, all of which are difficult to model using realistic conditions. In the absence of strong Magnetic Fields, the convective flows of the core are expected to be in a regime of rapidly rotating turbulence1, which remains largely unexplored. Here we use a combination of non-Magnetic numerical models designed to explore this regime to show that the convective length scale becomes independent of the viscosity when realistic parameter values are approached and is entirely determined by the flow velocity and the Planetary rotation. The velocity decreases very rapidly at smaller scales, so this turbulent convective length scale is a lower limit for the energy-carrying length scales in the flow. Using this approach, we can model realistically the dynamics of small non-Magnetic cores such as the Moon. Although modelling the conditions of larger Planetary cores remains out of reach, the fact that the turbulent convective length scale is independent of the viscosity allows a reliable extrapolation to these objects. For the Earth’s core conditions, we find that the turbulent convective length scale in the absence of Magnetic Fields would be about 30 kilometres, which is orders of magnitude larger than the ten-metre viscous length scale. The need to resolve the numerically inaccessible viscous scale could therefore be relaxed in future more realistic geodynamo simulations, at least in weakly magnetized regions. Numerical modelling of rotating turbulent convective flows shows that the length scale of convection in Planetary cores is set by the flow speed and not by the fluid viscosity.
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Precessing spherical shells: flows, dissipation, dynamo and the lunar core
Geophysical Journal International, 2019Co-Authors: David Cebron, Raphaël Laguerre, Jerome Noir, Nathanaël SchaefferAbstract:Precession of planets or moons affects internal liquid layers by driving flows, instabilities and possibly dynamos. The energy dissipated by these phenomena can influence orbital parameters such as the planet's spin rate. However, there is no systematic study of these flows in the spherical shell geometry relevant for planets, and the lack of scaling law prevents convincing extrapolation to celestial bodies. We have run more than 900 simulations of fluid spherical shells affected by precession, to systematically study basic flows, instabilities, turbulence, and Magnetic field generation. We observe no significant effects of the inner core on the onset of the instabilities. We obtain an analytical estimate of the viscous dissipation, mostly due to boundary layer friction in our simulations. We propose theoretical onsets for hydrodynamic instabilities, and document the intensity of turbulent fluctuations. We extend previous precession dynamo studies towards lower viscosities, at the limits of today's computers. In the low viscosity regime, precession dynamos rely on the presence of large-scale vortices, and the surface Magnetic Fields are dominated by small scales. Interestingly, intermittent and self-killing dynamos are observed. Our results suggest that large-scale Planetary Magnetic Fields are unlikely to be produced by a precession-driven dynamo in a spherical core. But this question remains open as Planetary cores are not exactly spherical, and thus the coupling between the fluid and the boundary does not vanish in the relevant limit of small viscosity. Moreover, the fully turbulent dissipation regime has not yet been reached in simulations. Our results suggest that the melted lunar core has been in a turbulent state throughout its history. Furthermore, in the view of recent experimental results, we propose updated formulas predicting the fluid mean rotation vector and the associated dissipation in both the laminar and the turbulent regimes.
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convective lengthscale in Planetary cores
arXiv: Geophysics, 2018Co-Authors: Céline Guervilly, P. Cardin, Nathanaël SchaefferAbstract:Convection is a fundamental physical process in the fluid cores of planets because it is the primary transport mechanism for heat and chemical species and the primary energy source for Planetary Magnetic Fields. Key properties of convection, such as the characteristic flow velocity and lengthscale, are poorly quantified in Planetary cores due to their strong dependence on Planetary rotation, buoyancy driving and Magnetic Fields, which are all difficult to model under realistic conditions. In the absence of strong Magnetic Fields, the core convective flows are expected to be in a regime of rapidly-rotating turbulence, which remains largely unexplored to date. Here we use a combination of numerical models designed to explore this low-viscosity regime to show that the convective lengthscale becomes independent of the viscosity and is entirely determined by the flow velocity and Planetary rotation. For the Earth's core, we find that the characteristic con-vective lengthscale is approximately 30km and below this scale, motions are very weak. The 30-km cutoff scale rules out small-scale dynamo action and supports large-eddy simulations of core dynamics. Furthermore, it implies that our understanding of Magnetic reversals from numerical geodynamo models does not relate to the Earth, because they require too intense flows. Our results also indicate that the liquid core of the Moon might still be in an active convective state despite the absence of a present-day dynamo.
David J Stevenson - One of the best experts on this subject based on the ideXlab platform.
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A Complex Dynamo Inferred From the Hemispheric Dichotomy of Jupiter's Magnetic Field
Nature, 2018Co-Authors: Kimberly Moore, Jeremy Bloxham, Rakesh K. Yadav, Laura Kulowski, Hao Cao, John E. P. Connerney, Stavros Kotsiaros, John Leif Jørgensen, José M.g. Merayo, David J StevensonAbstract:The Juno spacecraft, which is in a polar orbit around Jupiter, is providing direct measurements of the planet's Magnetic field close to its surface1. A recent analysis of observations of Jupiter's Magnetic field from eight (of the first nine) Juno orbits has provided a spherical-harmonic reference model (JRM09)2 of Jupiter's Magnetic field outside the planet. This model is of particular interest for understanding processes in Jupiter's magnetosphere, but to study the field within the planet and thus the dynamo mechanism that is responsible for generating Jupiter's main Magnetic field, alternative models are preferred. Here we report maps of the Magnetic field at a range of depths within Jupiter. We find that Jupiter's Magnetic field is different from all other known Planetary Magnetic Fields. Within Jupiter, most of the flux emerges from the dynamo region in a narrow band in the northern hemisphere, some of which returns through an intense, isolated flux patch near the equator. Elsewhere, the field is much weaker. The non-dipolar part of the field is confined almost entirely to the northern hemisphere, so there the field is strongly non-dipolar and in the southern hemisphere it is predominantly dipolar. We suggest that Jupiter's dynamo, unlike Earth's, does not operate in a thick, homogeneous shell, and we propose that this unexpected field morphology arises from radial variations, possibly including layering, in density or electrical conductivity, or both.
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Planetary Magnetic Fields: Achievements and Prospects
Space Science Reviews, 2010Co-Authors: David J StevensonAbstract:The past decade has seen a wealth of new data, mainly from the Galilean satellites and Mars, but also new information on Mercury, the Moon and asteroids (meteorites). In parallel, there have been advances in our understanding of dynamo theory, new ideas on the scaling laws for field amplitudes, and a deeper appreciation on the diversity and complexity of Planetary interior properties and evolutions. Most Planetary Magnetic Fields arise from dynamos, past or present, and Planetary dynamos generally arise from thermal or compositional convection in fluid regions of large radial extent. The relevant electrical conductivities range from metallic values to values that may be only about one percent or less that of a typical metal, appropriate to ionic fluids and semiconductors. In all Planetary liquid cores, the Coriolis force is dynamically important. The maintenance and persistence of convection appears to be easy in gas giants and ice-rich giants, but is not assured in terrestrial planets because the quite high electrical conductivity of an iron-rich core guarantees a high thermal conductivity (through the Wiedemann-Franz law), which allows for a large core heat flow by conduction alone. This has led to an emphasis on the possible role of ongoing differentiation (growth of an inner core or “snow”). Although Planetary dynamos mostly appear to operate with an internal field that is not very different from (2 ρ Ω/ σ )^1/2 in SI units where ρ is the fluid density, Ω is the Planetary rotation rate and σ is the conductivity, theoretical arguments and stellar observations suggest that there may be better justification for a scaling law that emphasizes the buoyancy flux. Earth, Ganymede, Jupiter, Saturn, Uranus, Neptune, and probably Mercury have dynamos, Mars has large remanent magnetism from an ancient dynamo, and the Moon might also require an ancient dynamo. Venus is devoid of a detectable global field but may have had a dynamo in the past. Even small, differentiated planetesimals (asteroids) may have been capable of dynamo action early in the solar system history. Induced Fields observed in Europa and Callisto indicate the strong likelihood of water oceans in these bodies. The presence or absence of a dynamo in a terrestrial body (including Ganymede) appears to depend mainly on the thermal histories and energy sources of these bodies, especially the convective state of the silicate mantle and the existence and history of a growing inner solid core. As a consequence, the understanding of Planetary Magnetic Fields depends as much on our understanding of the history and material properties of planets as it does on our understanding of the dynamo process. Future developments can be expected in our understanding of the criterion for a dynamo and on Planetary properties, through a combination of theoretical work, numerical simulations, Planetary missions (MESSENGER, Juno, etc.) and laboratory experiments.
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Planetary Magnetic Fields
Earth and Planetary Science Letters, 2003Co-Authors: David J StevensonAbstract:The past several years have seen dramatic developments in the study of Planetary Magnetic Fields, including a wealth of new data, mainly from the Galilean satellites and Mars, together with major improvements in our theoretical modeling effort of the dynamo process believed responsible for large Planetary Fields. These dynamos arise from thermal or compositional convection in fluid regions of large radial extent. The relevant electrical conductivities range from metallic values to values that may be only about 1% or less that of a typical metal, appropriate to ionic fluids and semiconductors. In all planets, the Coriolis force is dynamically important, but slow rotation may be more favorable for a dynamo than fast rotation. The maintenance and persistence of convection appears to be easy in gas giants and ice-rich giants, but is not assured in terrestrial planets because the quite high electrical conductivity of iron-rich cores guarantees a high thermal conductivity (through the Wiedemann–Franz law), which allows for a large core heat flow by conduction alone. In this sense, high electrical conductivity is unfavorable for a dynamo in a metallic core. Planetary dynamos mostly appear to operate with an internal field ∼(2ρΩ/σ)^(1/2) where ρ is the fluid density, Ω is the Planetary rotation rate and σ is the conductivity (SI units). Earth, Ganymede, Jupiter, Saturn, Uranus, Neptune, and maybe Mercury have dynamos, Mars has large remanent magnetism from an ancient dynamo, and the Moon might also require an ancient dynamo. Venus is devoid of a detectable global field but may have had a dynamo in the past. The presence or absence of a dynamo in a terrestrial body (including Ganymede) appears to depend mainly on the thermal histories and energy sources of these bodies, especially the convective state of the silicate mantle and the existence and history of a growing inner solid core. Induced Fields observed in Europa and Callisto indicate the strong likelihood of water oceans in these bodies.
Dimitra Atri - One of the best experts on this subject based on the ideXlab platform.
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galactic cosmic rays on extrasolar earth like planets i cosmic ray flux
arXiv: Earth and Planetary Astrophysics, 2015Co-Authors: Jeanmathias Griesmeier, F Tabatabavakili, A Stadelmann, John Lee Grenfell, Dimitra AtriAbstract:(abridged abstract) Theoretical arguments indicate that close-in terrestial exoplanets may have weak Magnetic Fields, especially in the case of planets more massive than Earth (super-Earths). Planetary Magnetic Fields, however, constitute one of the shielding layers that protect the planet against cosmic-ray particles. In particular, a weak Magnetic field results in a high flux of Galactic cosmic rays that extends to the top of the Planetary atmosphere. We wish to quantify the flux of Galactic cosmic rays to an exoPlanetary atmosphere as a function of the particle energy and of the Planetary Magnetic moment. We numerically analyzed the propagation of Galactic cosmic-ray particles through Planetary magnetospheres. We evaluated the efficiency of magnetospheric shielding as a function of the particle energy (in the range 16 MeV $\le$ E $\le$ 524 GeV) and as a function of the Planetary Magnetic field strength (in the range 0 ${M}_\oplus$ $\le$ {M} $\le$ 10 ${M}_\oplus$). Combined with the flux outside the Planetary magnetosphere, this gives the cosmic-ray energy spectrum at the top of the Planetary atmosphere as a function of the Planetary Magnetic moment. We find that the particle flux to the Planetary atmosphere can be increased by more than three orders of magnitude in the absence of a protecting Magnetic field. For a weakly magnetized planet (${M}=0.05\,{M}_{\oplus}$), only particles with energies below 512 MeV are at least partially shielded. For a planet with a Magnetic moment similar to Earth, this limit increases to 32 GeV, whereas for a strongly magnetized planet ($M=10.0\,{M}_{\oplus}$), partial shielding extends up to 200 GeV. We find that Magnetic shielding strongly controls the number of cosmic-ray particles reaching the Planetary atmosphere. The implications of this increased particle flux are discussed in a companion article.
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galactic cosmic rays on extrasolar earth like planets i cosmic ray flux
Astronomy and Astrophysics, 2015Co-Authors: Jeanmathias Griesmeier, F Tabatabavakili, A Stadelmann, John Lee Grenfell, Dimitra AtriAbstract:Context. Theoretical arguments indicate that close-in terrestial exoplanets may have weak Magnetic Fields, especially in the case of planets more massive than Earth (super-Earths). Planetary Magnetic Fields, however, constitute one of the shielding layers that protect the planet against cosmic-ray particles. In particular, a weak Magnetic field results in a high flux of Galactic cosmic rays that extends to the top of the Planetary atmosphere. Aims. We wish to quantify the flux of Galactic cosmic rays to an exoPlanetary atmosphere as a function of the particle energy and of the Planetary Magnetic moment. Methods. We numerically analyzed the propagation of Galactic cosmic-ray particles through Planetary magnetospheres. We evaluated the efficiency of magnetospheric shielding as a function of the particle energy (in the range 16 MeV ≤ E ≤ 524 GeV) and as a function of the Planetary Magnetic field strength (in the range 0 M ⊕ ≤ M ≤ 10 M ⊕). Combined with the flux outside the Planetary magnetosphere, this gives the cosmic-ray energy spectrum at the top of the Planetary atmosphere as a function of the Planetary Magnetic moment. Results. We find that the particle flux to the Planetary atmosphere can be increased by more than three orders of magnitude in the absence of a protecting Magnetic field. For a weakly magnetized planet (M = 0.05 M ⊕), only particles with energies below 512 MeV are at least partially shielded. For a planet with a Magnetic moment similar to that of Earth, this limit increases to to 32 GeV, whereas for a strongly magnetized planet (M = 10.0 M ⊕), partial shielding extends up to 200 GeV. Over the parameter range we studied, strong shielding does not occur for weakly magnetized planets. For a planet with a Magnetic moment similar to that of Earth, particles with energies below 512 MeV are strongly shielded, and for strongly magnetized planets, this limit increases to 10 GeV. Conclusions. We find that Magnetic shielding strongly controls the number of cosmic-ray particles reaching the Planetary atmosphere. The implications of this increased particle flux are discussed in a companion article.
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galactic cosmic ray induced radiation dose on terrestrial exoplanets
Astrobiology, 2013Co-Authors: Dimitra Atri, B Hariharan, Jeanmathias GriesmeierAbstract:This past decade has seen tremendous advancements in the study of extrasolar planets. Observations are now made with increasing sophistication from both ground- and space-based instruments, and exoplanets are characterized with increasing precision. There is a class of particularly interesting exoplanets that reside in the habitable zone, which is defined as the area around a star where the planet is capable of supporting liquid water on its surface. Planetary systems around M dwarfs are considered to be prime candidates to search for life beyond the Solar System. Such planets are likely to be tidally locked and have close-in habitable zones. Theoretical calculations also suggest that close-in exoplanets are more likely to have weaker Planetary Magnetic Fields, especially in the case of super-Earths. Such exoplanets are subjected to a high flux of galactic cosmic rays (GCRs) due to their weak Magnetic moments. GCRs are energetic particles of astrophysical origin that strike the Planetary atmosphere and produce secondary particles, including muons, which are highly penetrating. Some of these particles reach the Planetary surface and contribute to the radiation dose. Along with the Magnetic field, another factor governing the radiation dose is the depth of the Planetary atmosphere. The higher the depth of the Planetary atmosphere, the lower the flux of secondary particles will be on the surface. If the secondary particles are energetic enough, and their flux is sufficiently high, the radiation from muons can also impact the subsurface regions, such as in the case of Mars. If the radiation dose is too high, the chances of sustaining a long-term biosphere on the planet are very low. We have examined the dependence of the GCR-induced radiation dose on the strength of the Planetary Magnetic field and its atmospheric depth, and found that the latter is the decisive factor for the protection of a Planetary biosphere. Key Words: RadiationRadiation physicsHabitabilityHabitable zonePlanetary atmospheres.
Andrew Collier Cameron - One of the best experts on this subject based on the ideXlab platform.
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radio emission from exoplanets the role of the stellar coronal density and Magnetic field strength
Astronomy and Astrophysics, 2008Co-Authors: M Jardine, Andrew Collier CameronAbstract:Context. The search for radio emission from extra-solar planets has so far been unsuccessful. Much of the effort in modelling the predicted emission has been based on the analogy with the well-known emission from Jupiter. Unlike Jupiter, however, many of the targets of these radio searches are so close to their parent stars that they may well lie inside the stellar magnetosphere. Aims. For these close-in planets we determine which physical processes dominate the radio emission and compare our results to those for large-orbit planets that are that are immersed in the stellar wind. Methods. We have modelled the reconnection of the stellar and Planetary Magnetic Fields. We calculate the extent of the Planetary magnetosphere if it is in pressure balance with its surroundings and determine the conditions under which reconnection of the stellar and Planetary Magnetic Fields could provide the accelerated electrons necessary for the predicted radio emission. Results. We show that received radio fluxes of tens of mJy are possible for exoplanets in the solar neighbourhood that are close to their parent stars if their stars have surface field strengths above 1–10 G. We show that for these close-in planets, the power of the radio emission depends principally on the ratio (Nc/B 1/3 � ) 2 where N
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radio emission from exoplanets the role of the stellar coronal density and Magnetic field strength
arXiv: Astrophysics, 2008Co-Authors: M Jardine, Andrew Collier CameronAbstract:The search for radio emission from extra-solar planets has so far been unsuccessful. Much of the effort in modelling the predicted emission has been based on the analogy with the well-known emission from Jupiter. Unlike Jupiter, however, many of the targets of these radio searches are so close to their parent stars that they may well lie inside the stellar magnetosphere. For these close-in planets we determine which physical processes dominate the radio emission and compare our results to those for large-orbit planets that are immersed in the stellar wind. We have modelled the reconnection of the stellar and Planetary Magnetic Fields. We calculate the extent of the Planetary magnetosphere if it is in pressure balance with its surroundings and determine the conditions under which reconnection of the stellar and Planetary Magnetic Fields could provide the accelerated electrons necessary for the predicted radio emission. We show that received radio fluxes of tens of mJy are possible for exoplanets in the solar neighbourhood that are close to their parent stars if their stars have surface field strengths above 1-10G. We show that for these close-in planets, the power of the radio emission depends principally on the ratio (Nc/B^{1/3})^2 where Nc is the density at the base of the stellar corona, and B is the stellar surface Magnetic field strength. Radio emission is most likely to be detected from planets around stars with high-density coronae, which are therefore likely to be bright X-ray sources. The dependence of stellar coronal density on stellar rotation rate and effective temperature is crucial in predicting radio fluxes from exoplanets.
Jeanmathias Griesmeier - One of the best experts on this subject based on the ideXlab platform.
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galactic cosmic rays on extrasolar earth like planets i cosmic ray flux
arXiv: Earth and Planetary Astrophysics, 2015Co-Authors: Jeanmathias Griesmeier, F Tabatabavakili, A Stadelmann, John Lee Grenfell, Dimitra AtriAbstract:(abridged abstract) Theoretical arguments indicate that close-in terrestial exoplanets may have weak Magnetic Fields, especially in the case of planets more massive than Earth (super-Earths). Planetary Magnetic Fields, however, constitute one of the shielding layers that protect the planet against cosmic-ray particles. In particular, a weak Magnetic field results in a high flux of Galactic cosmic rays that extends to the top of the Planetary atmosphere. We wish to quantify the flux of Galactic cosmic rays to an exoPlanetary atmosphere as a function of the particle energy and of the Planetary Magnetic moment. We numerically analyzed the propagation of Galactic cosmic-ray particles through Planetary magnetospheres. We evaluated the efficiency of magnetospheric shielding as a function of the particle energy (in the range 16 MeV $\le$ E $\le$ 524 GeV) and as a function of the Planetary Magnetic field strength (in the range 0 ${M}_\oplus$ $\le$ {M} $\le$ 10 ${M}_\oplus$). Combined with the flux outside the Planetary magnetosphere, this gives the cosmic-ray energy spectrum at the top of the Planetary atmosphere as a function of the Planetary Magnetic moment. We find that the particle flux to the Planetary atmosphere can be increased by more than three orders of magnitude in the absence of a protecting Magnetic field. For a weakly magnetized planet (${M}=0.05\,{M}_{\oplus}$), only particles with energies below 512 MeV are at least partially shielded. For a planet with a Magnetic moment similar to Earth, this limit increases to 32 GeV, whereas for a strongly magnetized planet ($M=10.0\,{M}_{\oplus}$), partial shielding extends up to 200 GeV. We find that Magnetic shielding strongly controls the number of cosmic-ray particles reaching the Planetary atmosphere. The implications of this increased particle flux are discussed in a companion article.
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galactic cosmic rays on extrasolar earth like planets i cosmic ray flux
Astronomy and Astrophysics, 2015Co-Authors: Jeanmathias Griesmeier, F Tabatabavakili, A Stadelmann, John Lee Grenfell, Dimitra AtriAbstract:Context. Theoretical arguments indicate that close-in terrestial exoplanets may have weak Magnetic Fields, especially in the case of planets more massive than Earth (super-Earths). Planetary Magnetic Fields, however, constitute one of the shielding layers that protect the planet against cosmic-ray particles. In particular, a weak Magnetic field results in a high flux of Galactic cosmic rays that extends to the top of the Planetary atmosphere. Aims. We wish to quantify the flux of Galactic cosmic rays to an exoPlanetary atmosphere as a function of the particle energy and of the Planetary Magnetic moment. Methods. We numerically analyzed the propagation of Galactic cosmic-ray particles through Planetary magnetospheres. We evaluated the efficiency of magnetospheric shielding as a function of the particle energy (in the range 16 MeV ≤ E ≤ 524 GeV) and as a function of the Planetary Magnetic field strength (in the range 0 M ⊕ ≤ M ≤ 10 M ⊕). Combined with the flux outside the Planetary magnetosphere, this gives the cosmic-ray energy spectrum at the top of the Planetary atmosphere as a function of the Planetary Magnetic moment. Results. We find that the particle flux to the Planetary atmosphere can be increased by more than three orders of magnitude in the absence of a protecting Magnetic field. For a weakly magnetized planet (M = 0.05 M ⊕), only particles with energies below 512 MeV are at least partially shielded. For a planet with a Magnetic moment similar to that of Earth, this limit increases to to 32 GeV, whereas for a strongly magnetized planet (M = 10.0 M ⊕), partial shielding extends up to 200 GeV. Over the parameter range we studied, strong shielding does not occur for weakly magnetized planets. For a planet with a Magnetic moment similar to that of Earth, particles with energies below 512 MeV are strongly shielded, and for strongly magnetized planets, this limit increases to 10 GeV. Conclusions. We find that Magnetic shielding strongly controls the number of cosmic-ray particles reaching the Planetary atmosphere. The implications of this increased particle flux are discussed in a companion article.
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galactic cosmic ray induced radiation dose on terrestrial exoplanets
Astrobiology, 2013Co-Authors: Dimitra Atri, B Hariharan, Jeanmathias GriesmeierAbstract:This past decade has seen tremendous advancements in the study of extrasolar planets. Observations are now made with increasing sophistication from both ground- and space-based instruments, and exoplanets are characterized with increasing precision. There is a class of particularly interesting exoplanets that reside in the habitable zone, which is defined as the area around a star where the planet is capable of supporting liquid water on its surface. Planetary systems around M dwarfs are considered to be prime candidates to search for life beyond the Solar System. Such planets are likely to be tidally locked and have close-in habitable zones. Theoretical calculations also suggest that close-in exoplanets are more likely to have weaker Planetary Magnetic Fields, especially in the case of super-Earths. Such exoplanets are subjected to a high flux of galactic cosmic rays (GCRs) due to their weak Magnetic moments. GCRs are energetic particles of astrophysical origin that strike the Planetary atmosphere and produce secondary particles, including muons, which are highly penetrating. Some of these particles reach the Planetary surface and contribute to the radiation dose. Along with the Magnetic field, another factor governing the radiation dose is the depth of the Planetary atmosphere. The higher the depth of the Planetary atmosphere, the lower the flux of secondary particles will be on the surface. If the secondary particles are energetic enough, and their flux is sufficiently high, the radiation from muons can also impact the subsurface regions, such as in the case of Mars. If the radiation dose is too high, the chances of sustaining a long-term biosphere on the planet are very low. We have examined the dependence of the GCR-induced radiation dose on the strength of the Planetary Magnetic field and its atmospheric depth, and found that the latter is the decisive factor for the protection of a Planetary biosphere. Key Words: RadiationRadiation physicsHabitabilityHabitable zonePlanetary atmospheres.