Do Intrinsic Magnetic Fields Protect Planetary Atmospheres from Stellar Winds?

Abstract

The accumulation of detailed ion flux measurements from long-lived spacecraft orbiting the solar system’s terrestrial planets have enabled recent studies to estimate the rate of solar wind driven atmospheric ion escape from Venus, Earth, and Mars, as well as the influence of solar wind and solar extreme ultraviolet (EUV) ionizing radiation on the atmospheric ion escape rates. Here, we introduce the basic forces and processes of ion escape, review the recent studies of ion escape rates, and provide a general framework for understanding ion escape as a process that can be limited by potential bottlenecks, such as ion supply, solar wind energy transfer, and transport efficiency, effectively determining the state of the ion escape process at each planet. We find that ion escape from Venus and Earth is energy-limited, though exhibit different dependencies on solar wind and EUV, revealing the influence of Earth’s intrinsic magnetic field. In contrast, ion escape from Mars is in a supply-limited state, mainly due to its low gravity, and has likely contributed relatively little to the total loss of the early Martian atmosphere, in comparison to neutral escape processes. Contrary to the current paradigm, the comparisons between the solar system planets show that an intrinsic magnetic dipole field is not required to prevent stellar wind-driven escape of planetary atmospheres, and the presence of one may instead increase the rate of ion escape. Anticipating the atmospheres of the exoplanets that will begin to be characterized over the coming decade, and the need to explain their evolution, we argue that a modern, nuanced, and evidence-based view of the magnetic field’s role in atmospheric escape is required.