A Planetary-Scale Magnetic Field Can Enhance, Rather than Inhibit, Ionospheric Escape

There are many pathways by which the solar wind interacts with the ionospheres of both magnetized and unmagnetized planets. For unmagnetized planets, such as Venus, the solar wind interacts directly with the ionosphere, and this direct interaction between the solar wind and the ionosphere is a major pathway for ionospheric, and, ultimately, atmospheric loss. Oxygen ion loss, in particular, is frequently treated as a proxy for loss of water from the atmosphere, and the direct solar wind interaction with the ionosphere is often assumed to be why Venus has lost its water, while Earth has not. This is also assumed to be the case for Mars, although, with its lower gravity, neutral processes such as dissociative recombination can also result in atmospheric loss. The Earth, on the other hand, has a strong intrinsic magnetic field, which forms the primary obstacle to the solar wind. But, by presenting a larger obstacle to the solar wind, Earth's magnetosphere can focus the available solar wind energy into the polar ionosphere, where heating by both precipitating electrons and Joule dissipation increases the scale height of the ionosphere. Plasma-physical processes, e.g., wave heating, further energize the upwelling plasma, allowing the heavier ions to flow out into the magnetosphere. Whether or not these ions ultimately escape from the magnetosphere is a complicated process, involving continued energization combined with the reconnection-dependent magnetic field topology, ultimately providing pathways for escape. Given the comparable contemporary outflow rates at Earth, Venus, and Mars, the presence of a planetary scale magnetic field does not appear to be sufficient to explain why Earth has retained its water, while Venus and Mars have not. But in order to understand the evolutionary implications of what we see today requires that we also consider how the overall scenarios for atmosphere escape might have changed in the several billion years since impacts and hydrodynamic outflows likely dominated. Are the processes that dominate escape today the same, or have their relative and absolute contributions varied over time as the Sun and the atmospheres themselves (and possibly the planetary fields) evolved? Remembering that the present is but one point in time is critical to making defensible progress on this long-standing debate.