The yearly variability of the sodium exosphere of Mercury: A toy model.

Observations of the sodium exosphere of Mercury show a peculiar yearly variability, with two intensity maxima at aphelion and perihelion. Here we present an analytical model for the total Na exosphere content, and we compare our results with ground-based observations. The model is able to reproduce the observed data, both in magnitude and in the seasonal variability. The combined effect of the planetary rotation with the modulation of sources and losses magnitude along the orbit, is able to produce a source of Na at dawn, which is needed to explain the observed maximum at aphelion. Also, we demonstrate that a process producing a consistent Na supply rate at the nightside, which can either be plasma or micrometeoroid precipitation, is needed as well. With the help of the model, we also propose a possible explanation for the dusk enhancement of Na that was seen in the MESSENGER data during the inbound leg of Mercury's orbit. • The analytical model of total exospheric Na content reproduces observations, both in magnitude and in seasonal variability. • A source of Na at dawn, driven by planetary rotation is needed to explain the peak of observed Na column density at aphelion. • A process producing Na supply rate in the nightside, either plasma and/or micrometeoroid precipitation, is needed. • We provide a possible explanation for the "dusk enhancement" observed by MESSENGER in the inbound leg of Mercury's orbit. [ABSTRACT FROM AUTHOR]