Space environment of Mercury at the time of the first MESSENGER flyby: Solar wind and interplanetary magnetic field modeling of upstream conditions

Abstract

The first flyby of Mercury by the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft occurred on 14 January 2008. In order to provide contextual information about the solar wind (SW) properties and the interplanetary magnetic field near the planet, we have used an empirical modeling technique combined with a numerical physics‐based SW model. The Wang‐Sheeley‐Arge (WSA) method uses solar photospheric magnetic field observations (from Earth‐based instruments) in order to estimate inner heliospheric conditions out to 21.5 solar radii from the Sun. This information is then used as input to the global numerical magnetohydrodynamic model, ENLIL, which calculates SW velocity, density, temperature, and magnetic field strength and polarity throughout the inner heliosphere. The present paper shows WSA‐ENLIL conditions computed for the several week period encompassing the first flyby. This information is used in conjunction with MESSENGER magnetometer data (and the only limited available MESSENGER SW plasma data) to help understand the Mercury flyby results. The in situ spacecraft data, in turn, can also be used iteratively to improve the model accuracy for inner heliospheric “space weather” purposes. Looking to the future, we discuss how with such modeling we can estimate relatively continuously the SW properties near Mercury and at the cruise location of MESSENGER now, for upcoming flybys, and toward the time of spacecraft orbit insertion in 2011.