  The exosphere, the tenuous collisionless cloud of gas surrounding Mercury is
still a poorly known object because it is the result of many various
interactions between the surface, the interplanetary medium (Solar wind,
photons and meteoroids), the planetary and the interplanetary magnetic fields.
Many ground-based observations have allowed the detection of intense and
variable sodium emissions at global and local spatial scales, the latter being
mostly concentrated in the polarmid latitude regions. These regions are indeed
the preferred location of solar wind precipitation on the surface of the
planet. In the present paper, by using high resolution Na observations obtained
at the Canary Islands with the THEMIS solar telescope, we analyze the
variability of the sodium exosphere on time-scale of 1 hour and investigate the
possible mechanisms that could explain the exospheric sodium emission
distribution and its dynamics. Our interpretation relates the observed sodium
asymmetries to the combined effects of plasma and photons impacts onto the
Mercury's surface and of sodium diffusion through the upper layer of the
surface. The comparison between data and simulations seems to evidence that,
similarly to what occurs at the Earth, both the magnetic reconnection regimes
of pulsed or quasi-steady reconnection could occur on Mercury, and be
responsible for the observed Na short term variations. In addition to this, a
progressive broadening of the peak regions together with an increase of the
equatorial region seem to corroborate the idea of the role of photon stimulated
desorption, in association with ion sputtering and with global sodium migration
around Mercury as the cause of the observed evolution of the Na exosphere.
