Stellar ablation of planetary atmospheres

Abstract

[1] We review observations and theories of the solar ablation of planetary atmospheres, focusing on the terrestrial case where a large magnetosphere holds off the solar wind, so that there is little direct atmospheric impact, but also couples the solar wind electromagnetically to the auroral zones. We consider the photothermal escape flows known as the polar wind or refilling flows, the enhanced mass flux escape flows that result from localized solar wind energy dissipation in the auroral zones, and the resultant enhanced neutral atom escape flows. We term these latter two escape flows the ''auroral wind.'' We review observations and theories of the heating and acceleration of auroral winds, including energy inputs from precipitating particles, electromagnetic energy flux at magnetohydrodynamic and plasma wave frequencies, and acceleration by parallel electric fields and by convection pickup processes also known as ''centrifugal acceleration.'' We consider also the global circulation of ionospheric plasmas within the magnetosphere, their participation in magnetospheric disturbances as absorbers of momentum and energy, and their ultimate loss from the magnetosphere into the downstream solar wind, loading reconnection processes that occur at high altitudes near the magnetospheric boundaries. We consider the role of planetary magnetization and the accumulating evidence of stellar ablation of extrasolar planetary atmospheres. Finally, we suggest and discuss future needs for both the theory and observation of the planetary ionospheres and their role in solar wind interactions, to achieve the generality required for a predictive science of the coupling of stellar and planetary atmospheres over the full range of possible conditions.

Effects of M dwarf magnetic fields on potentially habitable planets

“…If we consider that r M 2 r p can still offer a reasonable protection for the planetary atmosphere, as suggested by Lammer et al (2007), then such a planet would present an auroral oval aperture of α 0 45 o and the open field line region would cover ∼30% of the planetary surface: a significantly larger area of the planet would remain exposed to, e.g., incidence of particles from the star (generated in flares, CMEs, stellar wind) and from the cosmos (galactic cosmic rays), as well as escape of planetary atmosphere through polar flows (Grießmeier et al 2005(Grießmeier et al , 2009Moore & Horwitz 2007;Khodachenko et al 2007;Lammer et al 2007). …”

Mechanisms of ionospheric mass escape

“…[17] In contrast, many observations have been made of "transversely accelerated ions" or TAI, and their close relatives, ion conics [Moore and Horwitz, 2007]. Conics develop as transversely accelerated ions move upward while slower ions remain relatively stationary.…”

“…[2] Heating and ablation of ionospheric plasma by solar wind energy is an important atmospheric loss process that shapes Earth's magnetosphere during space storms, adding substantial plasma pressure to the magnetosphere [Moore and Horwitz, 2007]. The rate of removal of the atmosphere is nonthreatening over human time scales, but is representative of a widely applicable space plasma process that played a role in removing much of the atmosphere of Mars.…”

“…High-altitude heating is effective in raising the flow speed of ion outflows. However, the escaping mass flux is determined by energy inputs at lower altitudes [Leer and Holzer, 1980;Moore and Horwitz, 2007] in the topside F region ionosphere, about 250-1000 km altitude.…”

Survey of cold ionospheric outflows in the magnetotail

“…Moore et al, 1997;Su et al, 1998;Chappell et al, 2000;Lennartsson et al, 2004;Liemohn et al, 2005;Huddleston et al, 2005;Peterson et al, 2006). More details on the previous measurements of the high-latitude ion outflows can be found in the recent review articles by Yau et al (2007) and Moore and Horwitz (2007).…”