Mercury exosphere I. Global circulation model of its sodium component

Based on these comparisons we derived the principal signatures for each ejection mechanism during a Mercury year and show that none of the ejection mechanisms dominates over the whole year. Rather, particular features of the annual cycle of the sodium intensity appear to be induced by one, temporarily dominant, ejection mechanism. Based on this analysis, we are able to roughly explain the annual cycle of Mercury's exospheric sodium emission brightness. We also derive a set of parameters defining those ejection mechanisms which best reproduce this cycle. For our best case, Mercury's exosphere content varies from ∼1.6 ± 0.1 × 1028 Na atoms at TAA = 140° and 70° respectively to ∼4.5 ± 0.3 × 1028 Na atoms at TAA = 180° and 0°. In addition, Mercury's exospheric surface reservoir contains ∼1 × 1031 Na atoms at TAA = 300° and at TAA = 170° with up to three times more sodium atoms trapped in Mercury's nightside than in its dayside surface.