Ion Outflows: Causes, Consequences, and Comparative Planetology

Both particle and electromagnetic energy flow into the Earth’s high latitude ionosphere. This energy results in heating of the topside ionosphere, causing ionospheric upwelling. The upwelling ions are subject to additional heating through wave-particle interactions. The hot ions then constitute a significant mass outflow as they are transported into the magnetosphere through the magnetic mirror force. Since these ions include oxygen, the mass density is increased, and this can affect to the time-scale for magnetospheric processes through lowering the Alfvén speed, which could reduce the reconnection rate at the magnetopause, for example. Depending on the energy and source location for the outflowing ions, the ions may be trapped within the magnetosphere, or may escape to interplanetary space. This escape could be either direct, along lobe field lines, or indirect, through transport to the dayside magnetopause. Integrated outflow rates can be as high as 1026 ions/s, although average rates are more typically of the order 1024 – 1025 ions/s. What is less clear is what fraction of these ions ultimately escape, but we note that oxygen outflow rates for the unmagnetized planets Venus and Mars are of the same order. It is conventional wisdom that oxygen outflows at these planets corresponds to water loss, and further this is one of the reasons why Venus and Mars are dry. Related to this is the idea that the Earth’s magnetic field shields the ionosphere from direct interaction with the solar wind, and the Earth has therefore not lost water from its atmosphere, unlike Venus and Mars. The magnetic shield is not total, however, as reconnection allows energy from the solar wind to flow into the polar ionosphere, driving the outflows. If a large fraction of the ions escaping from the Earth’s ionosphere are ultimately lost, then, given that the Earth clearly has retained its water, we may need to revisit the concept that oxygen outflows at the unmagnetized planets are equivalent to water loss