Modeling the Upper Atmospheres of Exoplanets: EnergyDeposition and Escape

Alex Glocer (1,2), Ofer Cohen (3), Vladimir Airapetian (1,2), Katherine Garcia-Sage (1,2), Guillaume Gronoff (4), Suk-BinKang (1,2), and William Danchi (1,2)(1) Heliophysics Science Division, NASA GSFC, Greenbelt, MD 20771 (2) GSFC Sellers Exoplanet EnvironmentsCollaboration, NASA GSFC, Greenbelt, MD 20771 (3) Lowell Center for Space Science and Technology, U. of MassachusettsLowell, 600 Suffolk Street, Lowell, MA 01854 (4) Science Systems and Application, Inc. & NASA Langley Research Center,Hampton, Virginia 23681 (alex.glocer-1@nasa.gov)

AbstractThe upper atmospheres of exoplanets are subject totwo important energy sources derived form the hoststar. First, the stellar photon flux in the EUV andXUV ionizes and heats the upper atmosphere, driv-ing atmospheric heating, affecting the conductance,and enhancing atmospheric escape. Second, the stel-lar wind’s interaction with the planet’s intrinsic mag-netic field transfers energy to the atmosphere throughfield aligned currents and Poynting flux. That en-ergy is dissipated in the high latitude cusp and au-roral regions through Joule heating which can inflatethe atmosphere and also enhance the escape rate. Thispresentation will discuss recent advances in modelingthese energy inputs and there consequences for exo-planetary habitability.

1. IntroductionRecent Kepler observations revealed hundreds of ter-restrial type exoplanets around G to M dwarfs. Manyof the detected exoplanets are located close to theirhost stars and are exposed to large fluxes of ioniz-ing radiation. How do exoplanetary atmospheres re-spond to such harsh stellar environments? This pre-sentation discusses modeling the upper atmospheresof exoplanets with a focus on three key parts. Section1.1 provides an overview of atmospheric escape pro-cesses and discusses how enhanced EUV and XUVinputs, typical for close-in exoplanets around activeK-M dwarfs, may lead to significant atmospheric lossthrough the ionospheric outflow process. Section 1.2presents recent results demonstrating how the extremestellar wind conditions encountered by many exoplan-ets leads to enhanced Joule heating of those atmo-spheres and discusses the role of ionospheric con-ductivity in modulating the efficiencies of the energy

transfer from the wind to the planet. Finally, Sec-tion 1.3 discusses new model development to addressthese interesting problems including new calculationsfor ionospheric conductivity and upper atmosphericresponse.

1.1. Atmospheric EscapeFigure 1 presents a schematic summary of the differ-ent atmospheric escape processes. Many of these pro-cesses are independent of the planet’s magnetic field.For example Jeans escape, which considers the portionof the distribution function above escape velocity, andhydrodynamic escape, a strong thermally driven bulkflow, have no terms related to the planet’s field. On theother hand, the loss of atmosphere through collisionalinteraction with the stellar wind, known as sputtering,can be reduced when the planet has a strong field toshield the atmosphere from the influx of stellar windparticles. In contrast, ionospheric outflow can actuallybe enhanced by planetary magnetic fields as the fieldcan collect and funnel energy from the wind into theplanet.

In this presentation we will discuss atmospheric es-cape and it’s implication for atmospheric evolution. Inparticular, we will focus on recent results demonstrat-ing how EUV and XUV inputs lead to atmosphericloss for close-in exoplanets such as Proxima Cen b[3, 1]. We show the scaling of ionized particle escaperates with XUV input and discuss the importance ofthermospheric heating and magnetic field interactions.

1.2. Joule HeatingThe habitable zone around faint M-dwarfs is veryclose to the star. While such conditions are favor-able for detection, the extreme conditions encounteredby this class of planets is extreme. In the case ofTRAPPIST-1, planets e,f, and g are located at less than

EPSC AbstractsVol. 12, EPSC2018-589, 2018European Planetary Science Congress 2018c© Author(s) 2018

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Jeans Escape IonosphericOutflow

Atmospheric Escape

PhotochemicalEscape

HydrodynamicEscape

SputteringIon Pickup

Bulk Ion Escape

Figure 1: A schematic summary of the myriad of pro-cesses involved in atmospheric escape.

0.05 AU and are essentially inside the stellar corona.These stellar wind conditions may lead to heating andevaporation of the planetary atmosphere. Such con-ditions pose a challenge to whether such planets cantruly be considered habitable. We present results fromour recent paper examining the limits of the energytransmitted from the intense stellar wind to the upperatmosphere of the TRAPPIST-1 planets using an an-alytic formalism [2]. We explore the importance ofthe atmospheric conductivity in modulating the energytransmission and compare the energy transmission toother energy inputs from the star.

1.3. New Model DevelopmentsIn addition to the science results of the previous twosection, we discuss new developments required formodeling energy input and effects in the upper at-mospheres of exoplanets. In particular, we discussimproved approaches for calculating the conductivityand ohmic dissipation for exoplanet applications. Wealso present initial results of these new model devel-opments.

AcknowledgmentsThis research study was supported by funding fromthe NASA NExSS The Living Breathing Planet projectsupported by NASA Astrobiology Institute and by theSellers Exoplanetary Environments Collaboration atNASA GSGC & GISS.

References[1] Airapetian, V.S., Glocer, A., Khazanov, G.V., Loyd,

R.O.P., France, K., Sojka, J., Danchi, W.C. and Liemohn,M.W., 2017. How hospitable are space weather affectedhabitable zones? The role of ion escape. The Astrophys-ical Journal Letters, 836(1), p.L3.

[2] Cohen, O., Glocer, A., Garraffo, C., Drake, J.J. andBell, J.M., 2018. Energy Dissipation in the Upper Atmo-spheres of TRAPPIST-1 Planets. The Astrophysical Jour-nal Letters, 856(1), p.L11.

[3] Garcia-Sage, K., Glocer, A., Drake, J.J., Gronoff, G. andCohen, O., 2017. On the magnetic protection of the atmo-sphere of Proxima Centauri b. The Astrophysical JournalLetters, 844(1), p.L13.