Universität zu Köln

Elekes, Filip ORCID: 0000-0002-7258-3386 (2025). Magnetospheric energetics in stellar wind-planet interactions: Implications for close-in extrasolar planets. PhD thesis, Universität zu Köln.

PDF Dissertation_Filip_Elekes.pdf - Published Version Bereitstellung unter der CC-Lizenz: Creative Commons Attribution Non-commercial No Derivatives. Download (27MB)

Abstract

The interaction between exoplanets close to their host stars and the surrounding stellar wind can potentially convert large amounts of electromagnetic energy. The converted powers are believed to exceed the strongest star--planet interactions in the solar system by orders of magnitude. Apart from the strong emissions produced by such interactions, they can have an impact on the evolution of planets, their space environment, atmosphere and interior. Planetary magnetic fields play a crucial role in the amount of energy converted during such interactions. A large number of known exoplanets are in close orbits around their host stars. In addition, many exoplanets have been discovered around sun-like and cool stars that exhibit significant magnetic activity. As magnetic activity increases, so does the possible influence of stars on the space environment of planets through frequent eruptions and flares, known as space weather. The sun's most energetic eruptive events are coronal mass ejections, which can contribute significantly to the erosion of atmospheres, for example. The Earth's magnetic field protects us to some extent, but whether this is a universal property of planetary magnetic fields is currently being debated. All in all, the strength of interactions between planets and their space environment is significantly influenced by the energy content of the stellar wind plasma and that of the stellar eruptive events. A planetary magnetic field increases the size of the planetary obstacle to intercept the plasma. In this work we aim to better understand the energetics of stellar wind-planet interactions and role of planetary magnetic fields in controlling the dissipation of energy contained in coronal mass ejections within the planets. We focus on two exoplanetary systems that are of particular importance for the field of exoplanet science. In the solar system, most gas planets and terrestrial planets have, or had, a magnetic field. Magnetic fields of exoplanets have not yet been measured conclusively. Longstanding efforts have been made to measure the magnetic fields of exoplanets by means of radio emissions originating from the auroral regions of the exoplanets. Recently, the first promising observation of radio emission from the Hot Jupiter exoplanet Tau Boötis b was reported, but to date this measurement has not been confirmed by follow-up observations. With auroral radio emissions from exoplanets the magnetic field strength of the planet can be deduced. In this work we make use of the proposed magnetic field strength of Tau Boötis b to constrain its space environment and to dissect the energetics of the systems by means of magnetohydrodynamic simulations. We study the detailed electromagnetic energy fluxes of the interaction that ultimately carry the energy to power the auroral emissions. Due to the uncertainty of stellar wind predictions we also study the role of stellar wind density and magnetic field orientation on the energetics. In our simulations find that Poynting fluxes converted in the magnetosphere of the planet reach up to 1e+18 W. We find that exceptionally high Poynting flux-to-radio efficiencies are needed that exceed those measured at Jupiter by orders of magnitude. Furthermore, the magnetic activity of the host star, Tau Boötis A, can cause magnetic polarity reversals which strongly affect magnetic reconnection between stellar wind and planetary magnetic field. Due to the stars known magnetic cycle we expect periodic increases and decreases in radio power. In a closed magnetosphere the estimated radio flux falls below the observational threshold of the LOFAR telescopes, nearly regardless of the stellar wind density. Despite the high energetics of the interaction, radio fluxes generated in the auroral regions of the planet are, according to our model, too weak to be observable on Earth. These results make it clear that the search for radio emission requires stronger targeted stellar wind-planet interactions and better stellar wind predictions. The Trappist-1 system of consists of seven terrestrial exoplanets orbiting an active cool star in close distances. In this work we study the space environment of Trappist-1b and e that is exposed to stellar coronal mass ejections (CMEs) by means of time-dependent magnetohydrodynamic simulations. We aim to better understand how planetary magnetic fields influence the intake of energy carried the CMEs. Furthermore, we study the interaction with CMEs dominated by different kinds of energy. We consider purely mechanical CMEs consisting of enhanced density and velocity and magnetically dominated CMEs that posses intrinsic flux ropes. We aim to understand the CME energy dissipation within the planet's interior by calculating inductive heating of the interior due to variations of magnetospheric magnetic fields. In our simulations we find that, for flux rope CMEs, planetary magnetic field only have insignificant effects on the energy dissipated in the interior as the magnetic variability contained in the flux rope translates to the magnetosphere via reconnection. Mechanical CMEs, however, need to convert the mechanical energy to magnetic variability by deforming and perturbing the planetary magnetic field. The magnetospheric dynamo action responsible for the conversion of kinetic energy to magnetic variability, strongly depends on the planetary magnetic field strength. Thus, dissipation rates in the interior scale strongly with the planetary magnetic field. Above a planetary magnetic field near half of Earth's magnetic field strength, the dissipation rates saturate in bot CME models and decrease slowly for stronger magnetic fields. Heating rates for single CME events are in the order of 1-10 TW while Joule heating the a hypothetical ionosphere amounts to several thousands of TW. Atmospheric dissipation rates that high can drive significant atmospheric outflows. In regard to the Poynting fluxes oriented towards the planet we find that the electromagnetic energy converted within the magnetosphere scales with the magnetic field cubed. Thus, planetary magnetic fields amplify the intake of external electromagnetic energy although only a smaller amount is eventually dissipated within the planet or in its atmosphere.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Elekes, Filip UNSPECIFIED orcid.org/0000-0002-7258-3386 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-753206
Date:	2025
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Geosciences > Institute for Geophysics and Meteorology
Subjects:	Natural sciences and mathematics Physics Earth sciences
Uncontrolled Keywords:	Keywords Language Star-planet interactions English Magnetohydrodynamics English Exoplanets English Numerical simulation English Magnetic fields English Coronal mass ejections English Stellar winds English
Date of oral exam:	11 February 2025
Referee:	Name Academic Title Saur, Joachim Prof. Dr. Schilke, Peter Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/75320