Variability of the hydrogen in the martian upper atmosphere as simulated by a 3D atmosphere–exosphere coupling

Abstract

We present the temporal variability of the atomic and molecular hydrogen density derived from a 3D General Circulation Model describing the martian atmosphere from the surface to the exobase. A kinetic exospheric model is used to compute the hydrogen density above the exobase. We use these models to study the diurnal and seasonal variations of the hydrogen density and the Jeans escape rate as well as their variations with solar activity, assuming a classic dust scenario. We find that the diurnal variations of the hydrogen density are important with a peak in the dawn region during equinoxes and a peak on the nightside during solstices. These features result from the dynamics of the martian upper atmosphere. The variations of the atomic hydrogen Jeans escape with seasons and solar activity are in the range 1.3×1025s−1–4.4×1026s−1. A factor ∼8 is due to the seasonal variations with a maximum during the winter solstice in the northern hemisphere and a minimum during the summer solstice in the northern hemisphere that we attribute to the variation of the Mars–Sun distance. A factor ∼5 is due to the solar cycle with a maximum escape rate at high solar activity. The variations of the molecular hydrogen Jeans escape with seasons and solar activity are in the range 3×1022s−1–6×1024s−1. A factor ∼10 is due to the seasonal variations with a maximum during the winter solstice in the northern hemisphere and a minimum during the summer solstice in the northern hemisphere. A factor ∼20 is due to the solar cycle with a maximum escape rate at high solar activity. If Jeans escape is the major escape channel for hydrogen, the hydrogen escape is never limited by diffusion. The hydrogen density above 10,000km presents seasonal and solar cycle variations similar to the Jeans escape rate at all latitudes and local times. This 3D temporal model of the hydrogen thermosphere/exosphere will be useful to interpret future MAVEN observations and the consequences of the hydrogen corona variability on the martian plasma environment.