Mechanisms of ionospheric mass escape

Abstract

[1] The dependence of ionospheric O + escape flux on electromagnetic energy flux and electron precipitation into the ionosphere is derived for a hypothetical ambipolar pickup process, powered the relative motion of plasmas and neutral upper atmosphere, and by electron precipitation, at heights where the ions are magnetized but influenced by photoionization, collisions with gas atoms, ambipolar and centrifugal acceleration. Ion pickup by the convection electric field produces "ring-beam" or toroidal velocity distributions, as inferred from direct plasma measurements, from observations of the associated waves, and from the spectra of incoherent radar echoes. Ring beams are unstable to plasma wave growth, resulting in rapid relaxation via transverse velocity diffusion, into transversely accelerated ion populations. Ion escape is substantially facilitated by the ambipolar potential, but is only weakly affected by centrifugal acceleration. If, as cited simulations suggest, ion ring beams relax into nonthermal velocity distributions with characteristic speed equal to the local ion-neutral flow speed, a generalized "Jeans escape" calculation shows that the escape flux of ionospheric O + increases with Poynting flux and with precipitating electron density in rough agreement with observations.

Why an intrinsic magnetic field does not protect a planet against atmospheric escape

“…This energy contributes to heating of ionospheric particles leading to their subsequent escape. For low magnetic moments, the escape rate from the cusp is proportional to the area of the cusp and to the incident energy flux (Moore & Khazanov 2010), which increases with the cross section of the magnetosphere πr 2 c , where r c is the maximum distance to the planet-sun line of the dayside MP (Appendix A.1). Therefore, the escape rate increases with magnetic moment until a point where the maximum ion flux that the ionosphere can supply is reached (Barakat et al 1987) (Appendix A.2).…”

Seasonal variations and north–south asymmetries in polar wind outflow due to solar illumination

“…There are three main regions of outflow at high latitudes: the auroral oval, the cusp, and the polar cap. The auroral oval and the cusp are regions of intense ion outflow in response to strong energy inputs like Poynting flux, particle precipitation, and the work done by strong field-aligned electric fields accelerating ions upwards (Lockwood et al, 1985;Zheng et al, 2005;Moore and Khazanov, 2010;Nilsson et al, 2012). In the absence of such energy inputs, the main source of energy for ion outflow in the polar cap is solar illumination.…”

Magnetized Mars: Spatial distribution of oxygen ions

“…However, no fully self-consistent global model exists which could take into account various effects which play a role at the oxygen escape from the Earth-type magnetized ionosphere such as plasma waves, precipitating electrons, rotation of the planet, ion-neutral interactions and the conductivity of the ionosphere (see, for example, Moore and Khazanov, 2010).…”

“…In practice, however, a three dimensional planet-solar wind interaction model has to adopt various approximations and simplifications because physical processes of interest cover a wide time and space range (see, for example, discussion in Ledvina et al, 2008). This is the case for Earth (see the recent review of O + escape modeling from Moore and Khazanov, 2010; and also for Mars (see comparison of seven global Mars-solar wind interaction models from Brain et al, 2010).…”

“…Moreover, the role the planetary global intrinsic magnetic field plays in the planetsolar wind interactions is of a more general interest, because it is also linked to the escape of oxygen ions from Earth (see, e.g. Moore and Khazanov, 2010), as well as from Earth-like planets in our and other Solar Systems.…”