A study of the heliocentric dependence of shock standoff distance and geometry using 2.5D magnetohydrodynamic simulations of coronal mass ejection driven shocks

Abstract

We perform four numerical magnetohydrodynamic simulations in 2.5 dimensions (2.5D) of fast coronal mass ejections (CMEs) and their associated shock fronts between 10Rs and 300Rs. We investigate the relative change in the shock standoff distance, Δ, as a fraction of the CME radial half-width, D OB (i.e., Δ/D OB). Previous hydrodynamic studies have related the shock standoff distance for Earth's magnetosphere to the density compression ratio (DR; ρu/ρ d) measured across the bow shock. The DR coefficient, k dr, which is the proportionality constant between the relative standoff distance (Δ/D OB) and the compression ratio, was semi-empirically estimated as 1.1. For CMEs, we show that this value varies linearly as a function of heliocentric distance and changes significantly for different radii of curvature of the CME's leading edge. We find that a value of 0.8± 0.1 is more appropriate for small heliocentric distances (<30Rs) which corresponds to the spherical geometry of a magnetosphere presented by Seiff. As the CME propagates its cross section becomes more oblate and the k dr value increases linearly with heliocentric distance, such that k dr= 1.1 is most appropriate at a heliocentric distance of about 80Rs. For terrestrial distances (215Rs) we estimate k dr= 1.8± 0.3, which also indicates that the CME cross-sectional structure is generally more oblate than that of Earth's magnetosphere. These alterations to the proportionality coefficients may serve to improve investigations into the estimates of the magnetic field in the corona upstream of a CME as well as the aspect ratio of CMEs as measured in situ. © 2012. The American Astronomical Society. All rights reserved.