  The extent to which a magnetosphere protects its planetary atmosphere from
stellar wind ablation depends upon how well it prevents energy and momentum
exchange with the atmosphere and how well it traps otherwise escaping plasma.
We focus on the former, and provide a formalism for estimating approximate
upper limits on mass, energy, and momentum capture, and use them to constrain
loss rates. Our approach quantifies a competition between the local deflection
of incoming plasma by a planetary magnetic field and the increase in area for
solar wind mass capture provided by this magnetosphere when the wind and
planetary fields incur magnetic reconnection. Solar wind capture can be larger
for 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 case
throughout Earth's history since the lunar forming impact-- the likely start of
the geodynamo. Focusing of incoming charged particles near the magnetic poles
can increase the local energy flux 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. This 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, and can help explain the current ion loss rates from
Mars, Venus and Earth.
