Characterizing the specific energy and pressure in near-Earth magnetic clouds

Abstract

Context. The pressure and energy density of the gas and magnetic field inside solar coronal mass ejections (in relation to that in the ambient solar wind) is thought to play an important role in determining their dynamics as they propagate through the heliosphere. Aims. We compare the specific energy (erg g−1), comprising kinetic (Hk), thermal (Hth) and magnetic field (Hmag) contributions, inside magnetic clouds (MCs) and the solar wind background. We examine whether the excess thermal+magnetic pressure and specific energy inside MCs (relative to the background) are correlated with their propagation and internal expansion speeds. We consider whether the excess thermal+magnetic specific energy inside MCs might cause them to resemble rigid bodies in the context of aerodynamic drag. Methods. We used near-Earth in situ data from the WIND spacecraft to identify a sample of 152 well-observed interplanetary coronal mass ejections and their MC counterparts. We compared various metrics based on these data to address our questions. Results. We find that the total specific energy (H) inside MCs is approximately equal to that in the background solar wind. We find that the excess (thermal+magnetic) pressure and specific energy are not well correlated with the near-Earth propagation and expansion speeds. We find that the excess thermal+magnetic specific energy is greater or equivalent to the specific kinetic energy of the solar wind incident in 81–89% of the MCs we study. This might explain how MCs retain their structural integrity and resist deformation by the solar wind bulk flow.