An assessment of the length and variability of Mercury's magnetotail

► A simple model of solar wind-magnetosphere coupling at Mercury is considered. ► This magnetotail model is driven by Mariner 10 observations of the IMF near Mercury. ► The magnetotail is shown to be highly dynamic, varying in flux content and length. ► The tail increases in length (100 s Mercury radii) during periods of northwards IMF. ► The magnetotail is considerably shorter (10 s Mercury radii) during southwards IMF.