WSA-ENLIL Cone Extension: Improving Solar Wind Forcing Parameter Estimates at Mercury

Understanding magnetospheric and exospheric processes at Mercury requires knowledge of solar wind 'forcing' conditions. This forcing includes both the background quasi-steady solar wind and the effects of transient solar eruptions, most notably coronal mass ejections (CMEs). The departures from background solar wind due to CMEs often correspond to more than an order of magnitude greater ram pressure and dynamo electric field applied to the magnetosphere. Observations from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft were previously combined with the Wang-Sheeley-Arge (WSA)-ENLIL solar-wind modeling tool to calculate such solar wind forcing parameters as the interplanetary magnetic field (IMF) strength (B); solar wind velocity (V), density (n), and temperature (T); ram pressure (~nV²); cross-magnetosphere electric field (V×B); and Alfvén Mach number (M_A). Previous efforts relied only on the background solar wind, however, and constituted an incomplete model of solar wind forcing given that the effects of transient solar phenomena were not included. The WSA-ENLIL model with the Cone extension permits inclusion of the effects of CMEs and related transient solar phenomena, and thus characterization of the effect of strong solar wind perturbations on the Mercury system. The Cone extension utilizes the heliocentric location, velocity, and radial size of a CME to propagate it through the inner solar system under the assumption of constant angular and radial velocity. This more complete approach provides a firmer basis with which to study magnetospheric and exospheric processes at Mercury and thereby better understand how the solar wind drives the Mercury system. Comparisons of WSA-ENLIL-Cone model outputs with measured properties from the MESSENGER Magnetometer (MAG), Neutron Spectrometer (NS), and Energetic Particle and Plasma Spectrometer (EPPS) permit quantification of the improvement in solar wind/IMF specification, particularly during times of the largest solar eruptive events over the period 2011-2013.