Predicting the long-term solar wind ion-sputtering source at Mercury

Maps of the precipitating solar wind proton flux onto Mercuryʹs surface are constructed using a modified Toffoletto–Hill (TH93) model of the Hermean magnetosphere. Solar wind and IMF conditions around Mercuryʹs orbit near aphelion and perihelion, respectively, were estimated by reanalyzing the Helios 40-s data for times when the spacecraft as in Mercuryʹs orbital range (0.31–0.47 AU). Probability density estimates obtained in this way allow us to quantitatively predict the likely range of the ion-sputtering source as a function of true anomaly angle of the planet. Results indicate that the sputtering source along open fieldlines increases fourfold from aphelion to perihelion, and that significant precipitation along closed fieldlines is twice as likely at perihelion due to finite Larmor radius effects. We conclude that ion sputtering is comparatively more important as a source for the Hermean exosphere at perihelion.