The extent to which a magnetosphere protects its planetary atmosphere from stellar wind ablation depends upon how well it prevents plasma from entering and how well it traps otherwise escaping plasma.
We focus on the entering, and provide a simple formalism for estimating upper limits on mass and energy capture, the latter of which determines maximum mass loss rates. Our approach quantifies a competition between two effects: a sufficiently strong planetary magnetic field deflects incoming plasma, but also increases the mass loading cross section when the wind and planetary fields are oriented for magnetic reconnection. The rate of mass loading can be larger for a magnetized versus an unmagnetized planet even if the rate of energy loading is correspondingly less.
We find this to be the case for Earth’s history since the lunar forming impact– the likely start of the geodynamo. However, polar focusing can increase the local energy flux beyond that without a magnetosphere for a strong enough planetary field and weak enough wind, and we find that this may be the case for Earth’s future.
The future protective role of Earth’s magnetic field would then depend on its ability to trap outgoing plasma. Generally, the competition between increased collection area and reduced inflow speed from a magnetosphere is likely essential in determining the net protective role of planetary fields and its importance in sustaining habitability. The competing effects may also help explain why current ion loss rates from Mars, Venus and Earth are similar.
Mass and energy capture from stellar winds for magnetized and unmagnetized planets: implications for atmospheric erosion and habitability
Eric G. Blackman (U. Rochester), John A. Tarduno (U. Rochester)
(Submitted on 3 Jan 2018)
Comments: 9 pages; 1 figure; submitted to MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1801.00895 [astro-ph.EP] (or arXiv:1801.00895v1 [astro-ph.EP] for this version)
From: Eric Blackman
[v1] Wed, 3 Jan 2018 04:21:38 GMT (330kb,D)
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