Effect of the solar wind density on the evolution of normal and inverse coronal mass ejections

Abstract

Context. The evolution of magnetised coronal mass ejections (CMEs) and their interaction with the background solar wind leading to deflection, deformation, and erosion is still largely unclear as there is very little observational data available. Even so, this evolution is very important for the geo-effectiveness of CMEs. Aims. We investigate the evolution of both normal and inverse CMEs ejected at different initial velocities, and observe the effect of the background wind density and their magnetic polarity on their evolution up to 1 AU. Methods. We performed 2.5D (axisymmetric) simulations by solving the magnetohydrodynamic equations on a radially stretched grid, employing a block-based adaptive mesh refinement scheme based on a density threshold to achieve high resolution following the evolution of the magnetic clouds and the leading bow shocks. All the simulations discussed in the present paper were performed using the same initial grid and numerical methods. Results. The polarity of the internal magnetic field of the CME has a substantial effect on its propagation velocity and on its deformation and erosion during its evolution towards Earth. We quantified the effects of the polarity of the internal magnetic field of the CMEs and of the density of the background solar wind on the arrival times of the shock front and the magnetic cloud. We determined the positions and propagation velocities of the magnetic clouds and thus also the stand-off distance of the leading shock fronts (i.e. the thickness of the magnetic sheath region) and the deformation and erosion of the magnetic clouds during their evolution from the Sun to the Earth. Inverse CMEs were found to be faster than normal CMEs ejected in the same initial conditions, but with smaller stand-off distances. They also have a higher magnetic cloud length, opening angle, and mass. Synthetic satellite time series showed that the shock magnitude is not affected by the polarity of the CME. However, the density peak of the magnetic cloud is dependent on the polarity and, in case of inverse CMEs, also on the background wind density. The magnitude of the z-component of the magnetic field was not influenced by either the polarity or the wind density.