Heavy Ion Escape from Terrestrial Exoplanets Hilary Egan 1 , Riku Jarvinen 2 , Dave Brain 1 1. University of Colorado, Boulder 2. Finnish Meteorological Institute
Solar System as a Laboratory Mars and Venus may have been habitable in the past, but have undergone significant atmospheric evolution over billions of years much of it through loss to space Hilary Egan | AAS 2019
Heavy Species (O, O 2 , CO 2 , …) are commonly lost as Ions - Light species (H) may escape via thermal motion but heavier species need additionally energy sources such as electric fields to reach escape velocity - Ion escape is observed occurring at all terrestrial solar system planets today [NASA's Scientific Visualization Studio and the MAVEN Science Team] Hilary Egan | AAS 2019
My Work Applying Planetary Models to Exoplanets Magnetic Field Strength Tail Plasma Environment Weak Magnetic Fields & Ion Escape Topology of weak intrinsic fields ‣ Plasma environment for weak dipoles and ‣ ion morphology R0 : Nominal R1 : Parallel-IMF Influence of global planetary magnetic R0: Nominal R1 : Parallel-IMF ‣ fields on ion escape Ion Escape from Planets around M-Dwarfs Stellar properties relevant for escape ( stellar ‣ M-Dwarf Influence magnetic field , stellar wind pressure, EUV R1 R2 flux) and influence on escape processes R0 : Nominal R1 : Parallel-IMF E s R2 : Total-Pressure Stellar driving of loss asymmetries, with ‣ R3 : Density R4 : EUV u s atmospheric implications 1 1 0 1 = . . 0 0 k = = k k Coupling of stellar properties and escape ‣ rates Hilary Egan | AAS 2019
Why Consider Weakly Magnetized Planets? - Terrestrial around M-dwarfs planets are likely to be unmagnetized or weakly magnetized - Even a weakly magnetic field can change the overall morphology of the system - Ion escape is incredibly dependent on magnetic fields - Prevailing wisdom says magnetic fields act as a shield for atmospheric erosion Hilary Egan | AAS 2019
“Quick” Ion Escape Paradigm Overview Velocity Magnetic Field Unmagnetized Planets Hilary Egan | AAS 2019
“Quick” Ion Escape Paradigm Overview Velocity Magnetic Field Unmagnetized Planets Hilary Egan | AAS 2019
“Quick” Ion Escape Paradigm Overview Electric Field = - v x B Magnetic Field Velocity Unmagnetized Planets Hilary Egan | AAS 2019
“Quick” Ion Escape Paradigm Overview Pickup Ion/Plume Outflow Electric Field = - v x B Magnetic Field Velocity Unmagnetized Planets Hilary Egan | AAS 2019
“Quick” Ion Escape Paradigm Overview Pickup Ion/Plume Outflow Electric Field = - v x B Magnetic Field Cold Tail Outflow Velocity Unmagnetized Planets Hilary Egan | AAS 2019
“Quick” Ion Escape Paradigm Overview Magnetic Field Magnetized Planets Hilary Egan | AAS 2019
“Quick” Ion Escape Paradigm Overview Closed Field Lines Open Field Lines Magnetized Planets Hilary Egan | AAS 2019
“Quick” Ion Escape Paradigm Overview Polar Wind Escaping Plasma Plasmasphere Trapped Plasma Magnetic Field Magnetized Planets Hilary Egan | AAS 2019
“Quick” Ion Escape Paradigm Overview Magnetic Field Weakly Magnetized Planets Hilary Egan | AAS 2019
Hybrid Modeling of Weakly Magnetized Planets - Hybrid model treats ions as macroparticles evolved under the Lorentz equation, electrons as a fluid - Validated by observations at Mars, Venus - Ionospheric production implementation via Chapman profiles (not self-consistent) - Magnetic fields of 0-150 nT Hilary Egan | AAS 2019
Magnetic Fields Drive Escape Before Inhibiting Relative Ion Escape Rate - Peak escape rate for B P ~ 75 nT with both species - Factor of 2 difference between strongest and weakest escape Planetary Magnetic Field Strength (nT) Hilary Egan | AAS 2019
Escape Decreases Due to Plasmasphere Trapping Test Particles Test Particles Injected Injected Solar Wind Velocity B = 50 nT B = 100 nT Open Magnetic Field Lines Closed Magnetic Field Lines As a larger area of the planet becomes wrapped in stronger, closed magnetic field lines, it becomes more difficult for ions to escape the plasmasphere Hilary Egan | AAS 2019
Escape Increases Due To Shielding of Southern Hemisphere Test Particles Test Particles Injected Injected B = 10 nT B = 50 nT Solar Wind Velocity Open Magnetic Field Lines Closed Magnetic Field Lines Particles travel along open field lines farther from the planet before being exposed to tailward oriented v x B forces, because of magnetic field standoff Hilary Egan | AAS 2019
Escape Increases Due To Shielding of Southern Hemisphere Test Particles Test Particles Injected Injected E = -v x B B v B = 10 nT B = 50 nT Solar Wind Velocity Open Magnetic Field Lines Closed Magnetic Field Lines Particles travel along open field lines further from the planet before being exposed to tailward oriented v x B forces, because of magnetic field standoff Hilary Egan | AAS 2019
Escape Increases Due To Shielding of Southern Hemisphere Test Particles Test Particles Injected Injected B E = -v x B B v E = -v x B B = 10 nT B = 50 nT v Solar Wind Velocity Open Magnetic Field Lines Closed Magnetic Field Lines Particles travel along open field lines further from the planet before being exposed to tailward oriented v x B forces, because of magnetic field standoff Hilary Egan | AAS 2019
Magnetic Stand-off Distance Controls Peak Escape B-Field - Both increase and decrease are B Max dependent on the magnetic stand off distance (R S ) in comparison to the altitude of the planetary ions R S Hilary Egan | AAS 2019
Peak Escape Magnetic Field Depends on Solar Wind Pressure B Max ~P sw1/2 - For a dipole: R s = R p ( 1/6 P sw ) P B 0 → B Max ∼ P 1/2 SW - This will not scale indefinitely , very strong fields change escape scale lengths and introduce new physics (e.g. polar wind) Hilary Egan | AAS 2019
M-Dwarf Habitable Zone Likely Has More Radial Magnetic Field - Everything so far has been under assumptions of present day solar conditions - Stellar environment around M-Dwarfs challenging because habitable zone is closer ‣ More intense solar wind ‣ Higher EUV input ‣ More variable, space weather ‣ Radially oriented stellar magnetic field Hilary Egan | AAS 2019
IMF Orientation Drives Asymmetric Ion Outflow R0 : Nominal R1 : Parallel-IMF R0: Nominal R1 : Parallel-IMF - Radial magnetic field case introduces asymmetry - Plume ions are accelerated from side due to unstable shock May introduce/enhance - compositional atmospheric Electric Field = - v x B 10 1 asymmetry, especially for tidally n(O 2+ ) [cm -3 ] locked planets Perpendicular 10 0 Magnetic Field 10 -1 Velocity 10 -2 Hilary Egan | AAS 2019
Come talk to me during coffee about… Conclusions Magnetic Fields & Ion Escape Topology of weak intrinsic fields ‣ - Ion escape is important for habitability! Plasma environment for weak dipoles ‣ Can change both atmospheric and ion morphology composition and overall mass Influence of global planetary magnetic ‣ fields on ion escape rates - Planetary magnetic fields can enhance ion escape before inhibiting it, reflecting Ion Escape from Planets around M-Dwarfs a balance between increased ion pickup Stellar properties relevant for escape ‣ and plasmasphere trapping (IMF, stellar wind pressure, EUV flux) and influence on escape processes - The planetary plasma environment Stellar driving of loss asymmetries, with ‣ atmospheric implications around M-Dwarfs can vary in a variety of Coupling of stellar properties and ‣ ways, making systematic studies escape rates important Or contact me at: hilary.egan@colorado.edu
Recommend
More recommend
