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Title: Magnetospheric structure and atmospheric Joule heating of habitable planets orbiting M-dwarf stars

Abstract

We study the magnetospheric structure and the ionospheric Joule Heating of planets orbiting M-dwarf stars in the habitable zone using a set of magnetohydrodynamic models. The stellar wind solution is used to drive a model for the planetary magnetosphere, which is coupled with a model for the planetary ionosphere. Our simulations reveal that the space environment around close-in habitable planets is extreme, and the stellar wind plasma conditions change from sub- to super-Alfvénic along the planetary orbit. As a result, the magnetospheric structure changes dramatically with a bow shock forming in the super-Alfvénic sectors, while no bow shock forms in the sub-Alfvénic sectors. The planets reside most of the time in the sub-Alfvénic sectors with poor atmospheric protection. A significant amount of Joule Heating is provided at the top of the atmosphere as a result of the intense stellar wind. For the steady-state solution, the heating is about 0.1%-3% of the total incoming stellar irradiation, and it is enhanced by 50% for the time-dependent case. The significant Joule Heating obtained here should be considered in models for the atmospheres of habitable planets in terms of the thickness of the atmosphere, the top-side temperature and density, the boundary conditions for themore » atmospheric pressure, and particle radiation and transport. Here we assume constant ionospheric Pedersen conductance similar to that of the Earth. The conductance could be greater due to the intense EUV radiation leading to smaller heating rates. We plan to quantify the ionospheric conductance in future study.« less

Authors:
; ; ;  [1];  [2];  [3]; ;  [4]
  1. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
  2. NASA/GSFC, Code 673, Greenbelt, MD 20771 (United States)
  3. Center for Planetary Atmospheres and Flight Sciences, National Institute of Aerospace, Hampton, VA 23666 (United States)
  4. Center for Space Environment Modeling, University of Michigan, 2455 Hayward Street, Ann Arbor, MI 48109 (United States)
Publication Date:
OSTI Identifier:
22365565
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 790; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ALFVEN WAVES; BOUNDARY CONDITIONS; DENSITY; DWARF STARS; EXTREME ULTRAVIOLET RADIATION; HEATING RATE; JOULE HEATING; MAGNETIC FIELDS; MAGNETOHYDRODYNAMICS; ORBITS; PLANETARY IONOSPHERES; SATELLITE ATMOSPHERES; SATELLITES; SHOCK WAVES; SIMULATION; SPACE; STEADY-STATE CONDITIONS; STELLAR WINDS; TIME DEPENDENCE

Citation Formats

Cohen, O., Drake, J. J., Garraffo, C., Poppenhaeger, K., Glocer, A., Bell, J. M., Ridley, A. J., and Gombosi, T. I.. Magnetospheric structure and atmospheric Joule heating of habitable planets orbiting M-dwarf stars. United States: N. p., 2014. Web. doi:10.1088/0004-637X/790/1/57.
Cohen, O., Drake, J. J., Garraffo, C., Poppenhaeger, K., Glocer, A., Bell, J. M., Ridley, A. J., & Gombosi, T. I.. Magnetospheric structure and atmospheric Joule heating of habitable planets orbiting M-dwarf stars. United States. doi:10.1088/0004-637X/790/1/57.
Cohen, O., Drake, J. J., Garraffo, C., Poppenhaeger, K., Glocer, A., Bell, J. M., Ridley, A. J., and Gombosi, T. I.. Sun . "Magnetospheric structure and atmospheric Joule heating of habitable planets orbiting M-dwarf stars". United States. doi:10.1088/0004-637X/790/1/57.
@article{osti_22365565,
title = {Magnetospheric structure and atmospheric Joule heating of habitable planets orbiting M-dwarf stars},
author = {Cohen, O. and Drake, J. J. and Garraffo, C. and Poppenhaeger, K. and Glocer, A. and Bell, J. M. and Ridley, A. J. and Gombosi, T. I.},
abstractNote = {We study the magnetospheric structure and the ionospheric Joule Heating of planets orbiting M-dwarf stars in the habitable zone using a set of magnetohydrodynamic models. The stellar wind solution is used to drive a model for the planetary magnetosphere, which is coupled with a model for the planetary ionosphere. Our simulations reveal that the space environment around close-in habitable planets is extreme, and the stellar wind plasma conditions change from sub- to super-Alfvénic along the planetary orbit. As a result, the magnetospheric structure changes dramatically with a bow shock forming in the super-Alfvénic sectors, while no bow shock forms in the sub-Alfvénic sectors. The planets reside most of the time in the sub-Alfvénic sectors with poor atmospheric protection. A significant amount of Joule Heating is provided at the top of the atmosphere as a result of the intense stellar wind. For the steady-state solution, the heating is about 0.1%-3% of the total incoming stellar irradiation, and it is enhanced by 50% for the time-dependent case. The significant Joule Heating obtained here should be considered in models for the atmospheres of habitable planets in terms of the thickness of the atmosphere, the top-side temperature and density, the boundary conditions for the atmospheric pressure, and particle radiation and transport. Here we assume constant ionospheric Pedersen conductance similar to that of the Earth. The conductance could be greater due to the intense EUV radiation leading to smaller heating rates. We plan to quantify the ionospheric conductance in future study.},
doi = {10.1088/0004-637X/790/1/57},
journal = {Astrophysical Journal},
number = 1,
volume = 790,
place = {United States},
year = {Sun Jul 20 00:00:00 EDT 2014},
month = {Sun Jul 20 00:00:00 EDT 2014}
}
  • Dwarf stars are believed to have a small protostar disk where planets may grow up. During the planet formation stage, embryos undergoing type I migration are expected to be stalled at an inner edge of the magnetically inactive disk (a{sub crit} {approx} 0.2-0.3 AU). This mechanism makes the location around a{sub crit} a 'sweet spot' for forming planets. In dwarf stars with masses {approx}0.5 M{sub sun}, a{sub crit} is roughly inside the habitable zone of the system. In this paper, we study the formation of habitable planets due to this mechanism using model system OGLE-06-109L, which has a 0.51 M{submore » sun} dwarf star with two giant planets in 2.3 and 4.6 AU observed by microlensing. We model the embryos undergoing type I migration in the gas disk with a constant disk-accretion rate ( M-dot ). Giant planets in outside orbits affect the formation of habitable planets through secular perturbations at the early stage and secular resonance at the late stage. We find that the existence and the masses of the habitable planets in the OGLE-06-109L system depend on both M-dot and the speed of type I migration. If planets are formed earlier, so that M-dot is larger ({approx}10{sup -7} M{sub sun} yr{sup -1}), terrestrial planets cannot survive unless the type I migration rate is an order of magnitude less. If planets are formed later, so that M-dot is smaller ({approx}10{sup -8} M{sub sun} yr{sup -1}), single and high-mass terrestrial planets with high water contents ({approx}5%) will be formed by inward migration of outer planet cores. A slower-speed migration will result in several planets via collisions of embryos, and thus their water contents will be low ({approx}2%). Mean motion resonances or apsidal resonances among planets may be observed if multiple planets survive in the inner system.« less
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