Examining How Planets Interact With The Stellar Wind Using 3D Numerical Models
Citation:
Carolan, Stephen Dermot, Examining How Planets Interact With The Stellar Wind Using 3D Numerical Models, Trinity College Dublin.School of Physics, 2022Download Item:
Abstract:
The majority of planets do not exist in empty space, but are embedded in the stellar wind, which consists of mostly ionised Hydrogen ejected from the host star. Understanding stellar wind-planet interactions is of huge importance to the scientific community today, given the effect they have on the lifetime of a planet’s atmosphere, and consequently the planet’s habitability.
In this thesis I explore the various ways planets interact with the stellar wind using advanced 3D numerical models. I start with the interaction between Earth’s magnetic field and the solar wind. Given that the solar wind has evolved over the Sun’s main-sequence lifetime, I investigate how this evolving wind has affected Earth’s magnetosphere, the volume around the Earth dominated by
its magnetic field. To do this, I demonstrate the technique of using stellar wind and 3D magnetosphere simulations in tandem, changing the stellar wind properties to obtain the corresponding magnetospheric structure. I find that Earth’s magnetosphere in the young system was significantly smaller than it is today, and as the solar wind relaxes further, will continue to grow.
I then move to planets on close-in systems (< 0.1 au). The high-energy radiation that close-in planets receive from their host stars can lead to strong photoevaporation. The stellar wind exerts pressure on the expanding atmosphere and, if sufficiently strong, confines it close to the planet. Using 3D
isothermal models which ignore magnetic fields, I show that this confinement
can limit both the rate of escape and its observational signatures, placing importance on considering the conditions of the stellar wind when interpreting observational data.
I continue by applying these models to a test system: the newly discovered exo-planet AU Mic b. This young close-in system presents an interesting dichotomy.
While the planet is expected to be highly irradiated by the host star, which can cause significant photoevaporation, the stellar wind could be strong enough to reduce or even inhibit atmospheric escape. I show that stellar wind mass-loss rates on the upper end of those proposed in literature (< 1000 ̇M⊙, where M⊙ = 2 × 10−14M⊙/yr is the solar mass-loss rate) completely remove detection signatures of atmospheric escape, while those on the lower end (> 10 ̇M⊙) yield blue-shift dominant absorption. These models demonstrate that the wind-atmosphere interaction can also inform us of the stellar wind conditions, depending on what follow up observational missions detect.
I then investigate how magnetic fields change the wind-atmosphere interaction. Magnetic fields alter the previous picture, resulting in a new structure. I now find a dead-zone of low velocity material trapped in the closed field lines, and polar flows where escaping material is funnelled along the open field lines, resulting in the novel finding of a double-tail structure. This new structure affects the observational signatures of atmospheric escape, with larger magnetic field strengths yielding both increased line centre absorption, and more asymmetry between transits above and below mid-disc.
Finally, I present preliminary work on the inclusion of two processes in these 3D models: charge exchange and radiation pressure, both of which are thought to contribute to the high velocity absorption observations have found. Though it does not change the geometry of the escaping atmosphere, I find that charge exchange can greatly alter the absorption profile in Ly-α, with relatively small changes in stellar wind density and velocity causing large changes in blue wing
absorption. The current implementation of radiation pressure in my model yields a departure from the findings in literature. With a tail oriented more radially away from the star, the line profile is changed significantly, with increased blue-shifted absorption. This can be attributed to the current lack of self-shielding in my model, a process which attenuates the radiation as it passes through the atmosphere, reducing its effect on the dynamics of the atmosphere.
Although I investigate a variety of different wind-planet interactions in this thesis, my research consistently illustrates that considering the properties of the stellar wind is incredibly important in order to correctly interpret the structure of the planet’s local environment, and how it affects the planet’s atmosphere.
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Grant Number
European Research Council (ERC)
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https://tcdlocalportal.tcd.ie/pls/EnterApex/f?p=800:71:0::::P71_USERNAME:CAROLASTDescription:
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Author: Carolan, Stephen Dermot
Sponsor:
European Research Council (ERC)Advisor:
Vidotto, AlinePublisher:
Trinity College Dublin. School of Physics. Discipline of PhysicsType of material:
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