Do Intrinsic Magnetic Fields Protect Planetary Atmospheres from Stellar Winds? - Lessons from Measurements in the Solar System

Exoplanets in our Backyard Houston, 2020-02-06

ROBIN RAMSTAD1 AND STAS BARABASH2

1LABORATORY FOR ATMOSPHERIC AND SPACE PHYSICS, UNIVERSITY OF COLORADO, BOULDER

2SWEDISH INSTITUTE OF SPACE PHYSICS, IRUNA, WEDEN K S 1 Exoplanets in our Backyard Houston, 2020-02-06

Do Intrinsic Magnetic Fields Protect Planetary Atmospheres from Stellar Winds? - Lessons from Measurements in the Solar System

ROBIN RAMSTAD1 AND STAS BARABASH2

1LABORATORY FOR ATMOSPHERIC AND SPACE PHYSICS, UNIVERSITY OF COLORADO, BOULDER

2SWEDISH INSTITUTE OF SPACE PHYSICS, IRUNA, WEDEN K S 1 Magnetospheres

Without global dipole With global dipole

2 ’Hypothetical’ star system

Middle-aged, main sequence G-type star ~0.01 bar Extreme ultraviolet Energetic stellar (EUV) radiation winds

~100 bars ~1 bar

The innermost planet is subjected to the strongest stellar winds and EUV, yet retains the thickest atmosphere. How?! 3 ’Hypothetical’ star system

Middle-aged, main sequence G-type star ~0.01 bar Extreme ultraviolet Energetic stellar (EUV) radiation winds

~100 bars ~1 bar

The innermost planet is subjected to the Maybe protected by a magnetic field? strongest stellar winds and EUV, yet retains …how about core composition, metals… the thickest atmosphere. How?! 3 Not so ’Hypothetical’ star system

Middle-aged, main sequence G-type star ~0.01 bar Extreme ultraviolet Energetic stellar (EUV) radiation winds Mars!

Venus! Earth!

~100 bars ~1 bar

What factors might we need in a general model to understand atmospheric escape Similar gravity from exoplanets? Diff. magnetic Diff. atm. Planetary properties - Gravity/escape velocity - Magnetization - Atmospheric composition - Atmospheric mass Diff. gravity Diff. gravity Similar magnetic Stellar/upstream properties Diff. magnetic Diff. atm. pressure - Stellar wind properties Diff. atm. - Energy input to thermosphere (EUV power) Similar atm. composition - Photoion production rate (EUV photon flux) No −1 푏 푐 푑 푒 푓 푄 s = 푎푀푝 푀푎푡푚푝푠푤퐵푠푤퐹퐸푈푉 … orbiters

4 Solar conditions over deep time

Mars Express observations 2007-2017

Ramstad et al. [2018] Based on Ribas et al. [2005] & Wood et al. [2006]

5 Gravity and escape

Single Escapeparticle escape velocity condition:

2 푚|퐯0| 퐺푀 Venus+ න 푭 Earth풓 − 푚 풓ො ∙Mars푑풔 > 0 , 퐶: 풓0 → ∞ 2 other 푟2 10.4 km/s퐶 11.2 km/s 4.9 km/s Initial energy Acceleration Gravity (at exobase) Escape energy

H 0.6 eV For ions:0.7 eV푞 ≠ 0 0.14 eV O 8.9 eV 10.3 eV 2.1 eV 퐅other = 퐅lorentz(퐫) = 푞(퐄(퐫) + 퐯(퐫) × 퐁(퐫)) O2 17.8 eV 20.6 eV 4.2 eV

For neutrals: 푞 = 0

2 푚|푣0| 퐺푀푀 2퐺푀 − 푚 2 > 0 ⇒ 푣0 > 2 푟 푟0 6 Ion escape in induced/intrinsic magnetospheres

−훻푝 푒 −퐯 × 퐁

−퐯 × 퐁 퐉 × 퐁 −훻푝푒 퐉 × 퐁

~2 × 1025 s−1 퐽푐푡

[Seki et al., 2001] Ramstad et al. [2017c] 1 1 퐄 = −퐯 × 퐁 + 퐉 × 퐁 − 훻푝푒 푛푒푒 푛푒푒 7 Ion escape in induced/intrinsic magnetospheres

First view of the global current systems

The magnetized solar wind induces ”ionospheric currents” that enhance the magnetic field until −훻푝푒

푝B = 푝dyn −퐯 × 퐁 퐉 × 퐁

퐽푐푡 ~2 × 1025 s−1

[Seki et al., 2001] Ramstad et al. [2019], Nat. Astro, accepted Ramstad et al. [2017c] 1 1 퐄 = −퐯 × 퐁 + 퐉 × 퐁 − 훻푝푒 푛푒푒 푛푒푒 7 States of atmospheric escape processes

Poynting (energy) flux Ion escape requires: • Energization • Ionization of neutrals

Planet

8 Measuring atmospheric escape

MAVEN/ STATIC

Mars

9 Atmospheric escape at Venus, Earth, Mars

Fluxes of escaping O+ ions from Venus, Earth and Mars in cylindrical coordinates.

Shown to scale!

Fedorov et al. [2008] Nilsson et al. [2012]

Ramstad et al. [2017d; 2020] 10 Solar wind dependence

Ion escape dependence on solar wind dynamic pressure.

Mars – Negligible or inverse dependence.

Venus – Weak positive dependence.

Earth – Strong positive dependence.

Masunaga et al., [2019] Ramstad et al., [2018c; 2020]

Schillings et al., [2019] 11 EUV dependence

Ion escape from Venus and Mars displays opposite dependences on solar EUV/XUV, despite both interacting with the solar wind similarly.

Kollmann et al. [2016] Ramstad et al., [2017b] 12 Coupling dependence on EUV

Intrinsic magnetosphere (Earth) Induced magnetosphere (Mars)

MP-ionosphere field-aligned currents

(EUV proxy) [Ramstad et al., 2017b] [Ohtani et al., 2014] 13 Coupling dependence on SW Induced magnetosphere (Mars) Intrinsic magnetosphere (Earth)

Vasyliunas et al. [1982] Ramstad & Barabash, 2020, SSR, in review [Ramstad et al., 2017b] 14 Generalized atmospheric ion escape

Ramstad and Barabash [2020], SSR, in review 15 Conclusions • Earth’s magnetosphere makes the ion escape response sensitive to solar wind variations - Protects in weak SW, acerbates escape in strong SW - SW coupling increases with EUV • Venus and Earth escape are energy-limited, Mars is supply-limited • Induced magnetospheres efficiently screen atmospheres - Small SW interaction area - SW coupling decreases in response to SW and EUV • Weak gravity does not necessarily mean high ion escape rates. - System may transition to an ion supply-limited state (Mars) • Mars (probably) lost most of its atmosphere to photochemical neutral escape [Lillis et al., 2017] • Venus may have retained its atmosphere because it lacks a dipole 16 How strong B is strong enough?

Only one terrestrial planets with global dipole exists today - The parameter space needs to be explored with models.

Earth today: 7.7 x 1022 Am2

Cnossen et al. [2012] Egan et al. [2019]

22 Extra slides

2223 Venus is energy-limited

Persson et al. [2020], JGR, in review.

23 23 States of atmospheric escape processes

Ramstad and Barabash [2020], SSR, in review

24 Measuring atmospheric ion escape

2퐸 푗 푥, 푦, 푧, 휃, , 퐸, 푚 = 푓(푥, 푦, 푧, 푣 , 푣 , 푣 , 푚) 푚2 푥 푦 푧

25 Supply limit for Mars

+ + + 푄max O , O2 , CO2 = 4 − 12 × 1025 s−1

+ + + 푄measured O , O2 , CO2 ≈ 2.6 × 1024 s−1

[Fox et al., 1997]

26 Martian geological timeline

Ehlmann et al. [2011] Frey et al. [2008] Lillis et al. [2008]

Wordsworth et al.27 [2016] Influence of the crustal magnetic fields

−3 푛sw = 1 − 3 cm [Ramstad et al., 2016] 푣sw = 350 − 450 km/s 2 퐼EUV = 4.0 − 5.0 mW/m 28 Influence of the crustal magnetic fields

Enhancement in tail ion escape rate for strongest crustal field orientation SZA > 60° coincides with enhancement in ionosphere plasma scale height.

[Ramstad et al., 2016] [Andrews et al., 2015] 29 Primordial solar wind event

Inbound Outbound −3 −3 푛sw = 3.5 cm 푛sw = 39 cm 푣sw = 370 km/s 푣sw = 730 km/s

30 Solar dependences Solar wind Solar EUV

31 More planets

Ligher planets/thinner solar wind, low EUV - Pluto, Titan

Coupling to strong upstream magnetic field - Titan

Uranus/Neptune mission could be relevant - Similar escape energies: H2 escape from Uranus equivalent to O escape from Mars - Uranus obliquity possibly comparable to early Mars