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The extent to which a magnetosphere protects its planetary atmosphere fromstellar wind ablation depends upon how well it prevents energy and momentumexchange with the atmosphere and how well it traps otherwise escaping plasma.We focus on the former, and provide a formalism for estimating approximateupper limits on mass, energy, and momentum capture, and use them to constrainloss rates. Our approach quantifies a competition between the local deflectionof incoming plasma by a planetary magnetic field and the increase in area forsolar wind mass capture provided by this magnetosphere when the wind andplanetary fields incur magnetic reconnection. Solar wind capture can be largerfor a magnetized planet versus an unmagnetized planet with small ionosphere,even if the rate of energy transfer is less. We find this to be the casethroughout Earth's history since the lunar forming impact-- the likely start ofthe geodynamo. Focusing of incoming charged particles near the magnetic polescan increase the local energy flux for a strong enough planetary field and weakenough wind, and we find that this may be the case for Earth's future. Thefuture protective role of Earth's magnetic field would then depend on itsability to trap outgoing plasma. This competition between increased collectionarea and reduced inflow speed from a magnetosphere is likely essential indetermining the net protective role of planetary fields and its importance insustaining habitability, and can help explain the current ion loss rates fromMars, Venus and Earth.

Mass, energy, and momentum capture from stellar winds by magnetized and unmagnetized planets: implications for atmospheric erosion and habitability