Solar Wind Forcing at Mercury: Comparison Between MESSENGER Observations and ENLIL Model Results for the Low-Altitude Campaign

Abstract:

Interpretation of observations from the MESSENGER spacecraft in orbit about Mercury benefit greatly from knowledge of solar wind “forcing” parameters. The Wang-Sheeley-Arge (WSA)-ENLIL solar wind modeling tool has been used to calculate the values of interplanetary magnetic field (IMF) strength (B), solar wind velocity (V) and density (n), ram pressure (~nV²), cross-magnetosphere electric field (VxB), Alfvén Mach number (M_A), and other derived quantities of relevance for solar wind–magnetosphere–exosphere interactions. Such parameters as solar wind dynamic pressure are key for determining the Mercury magnetopause standoff distance, as just one example. The relatively high-time-resolution B-field data from MESSENGER can be used to estimate the strength of the product of the solar wind speed and southward IMF strength (B_s) at Mercury. This electric field drives many magnetospheric dynamical processes and can be compared with general magnetic activity indices and with the occurrence of energetic particle bursts within the Mercury magnetosphere. Such parameters also serve as input to the global magnetohydrodynamic and kinetic magnetosphere models that are used to explore magnetospheric and exospheric processes. Such modeling can help assess near-real-time magnetospheric behavior for MESSENGER (as well as other mission analysis and/or ground-based observational campaigns). This solar-wind forcing knowledge is a crucial continuing step toward bringing heliospheric science expertise to bear on solar-planetary interaction studies.