The presence or absence of a magnetic field determines the nature of how a planet interacts with the solar wind and what paths are available for atmospheric escape. Magnetospheres form both around magnetised planets, such as Earth, and unmagnetised planets, like Mars and Venus, but it has been suggested that magnetised planets are better protected against atmospheric loss. However, the observed mass escape rates from these three planets are similar (in the approximate (0.5−2) kg s −1 range), putting this latter hypothesis into question. Modelling the effects of a planetary magnetic field on the major atmospheric escape processes, we show that the escape rate can be higher for magnetised planets over a wide range of magnetisations due to escape of ions through the polar caps and cusps. Therefore, contrary to what has previously been believed, magnetisation is not a sufficient condition for protecting a planet from atmospheric loss. Estimates of the atmospheric escape rates from exoplanets must therefore address all escape processes and their dependence on the planet's magnetisation.

[1] Ions apparently emanating from the same source, the ionospheric polar cap, can either end up as energized to keV energies in the high-altitude cusp/mantle, or appear as cold ions in the magnetotail lobes. We use Cluster observations of ions and wave electric fields to study the spatial variation of ion heating in the cusp/mantle and polar cap. The average flow direction in a simplified cylindrical coordinate system is used to show approximate average ion flight trajectories, and discuss the temperatures, fluxes and wave activity along some typical trajectories. It is found that it is suitable to distinguish between cusp, central and nightside polar cap ion outflow trajectories, though O + heating is mainly a function of altitude. Furthermore we use typical cold ion parallel velocities and the observed average perpendicular drift to obtain average cold ion flight trajectories. The data show that the cusp is the main source of oxygen ion outflow, whereas a polar cap source would be consistent with our average outflow paths for cold ions observed in the lobes. A majority of the cusp O + flux is sufficiently accelerated to escape into interplanetary space. A scenario with significant oxygen ion heating in regions with strong magnetosheath origin ion fluxes, cold proton plasma dominating at altitudes below about 8 R E in the polar cap, and most of the cusp oxygen outflow overcoming gravity and flowing out in the cusp and mantle is consistent with our observations.

Abstract. We have investigated the total O + escape rate from the dayside open polar region and its dependence on geomagnetic activity, specifically Kp. Two different escape routes of magnetospheric plasma into the solar wind, the plasma mantle, and the high-latitude dayside magnetosheath have been investigated separately. The flux of O + in the plasma mantle is sufficiently fast to subsequently escape further down the magnetotail passing the neutral point, and it is nearly 3 times larger than that in the dayside magnetosheath. The contribution from the plasma mantle route is estimated as ∼ 3.9×10 24 exp(0.45 Kp) [s −1 ] with a 1 to 2 order of magnitude range for a given geomagnetic activity condition. The extrapolation of this result, including escape via the dayside magnetosheath, indicates an average O + escape of 3 × 10 26 s −1 for the most extreme geomagnetic storms. Assuming that the range is mainly caused by the solar EUV level, which was also larger in the past, the average O + escape could have reached 10 27-28 s −1 a few billion years ago. Integration over time suggests a total oxygen escape from ancient times until the present roughly equal to the atmospheric oxygen content today.

Abstract. We present a statistical study of the low (<1 Hz) frequency electric and magnetic field spectral densities observed by Cluster spacecraft in the high altitude cusp and mantle region. At the O + gyrofrequency (0.02-0.5 Hz) for this region the electric field spectral density is on average 0.2-2.2 (mV m −1 ) 2 Hz −1 , implying that resonant heating at the gyrofrequency can be intense enough to explain the observed O + energies of 20-1400 eV. The relation between the electric and magnetic field spectral densities results in a large span of phase velocities, from a few hundred km s −1 up to a few thousand km s −1 . In spite of the large span of phase velocity, the ratio between the calculated local Alfvén velocity and the estimated phase velocity is close to unity. We provide average values of a coefficient describing diffusion in ion velocity space at different altitudes, which can be used in studies of ion energization and outflow. The observed average waves can explain the average O + energies measured in the high altitude (8-15 R E ) cusp/mantle region of the terrestrial magnetosphere according to our test particle calculations.