Astronomy & Astrophysics manuscript no. aa_16_2cols ©ESO 2022 November 1, 2022 On the specific energy and pressure in near-Earth magnetic clouds Debesh Bhattacharjee 1 Prasad Subramanian 1 Angelos Vourlidas 2 Teresa Nieves-Chinchilla 3 Niranjana Thejaswi 4 and Nishtha Sachdeva 5 1 Indian Institute of Science Education and Research, Pune Dr. Homi Bhabha Road, Pashan, Pune 411008, India 2 The Johns Hopkins University Applied Physics Laboratory, Laurel MD, USA. 3 Heliophysics science division, NASA-Goddard Space Flight Center, Greenbelt, MD (USA). 4 Department of Physics, The University of Arizona, Tucson, AZ 85721, USA 5 Department of Climate and Space Science and Engineering, University of Michigan, Ann Arbor, USA November 1, 2022 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 (H k ), thermal (H th ) and magnetic field (H mag ) contributions] inside MCs and the solar wind background. We examine if the excess thermal + magnetic pressure and specific energy inside MCs (relative to the background) is correlated with their propagation and internal expansion speeds. We ask if the excess thermal + magnetic specific energy inside MCs might make them resemble rigid bodies in the context of aerodynamic drag. Methods. We use 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 compute various metrics using 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 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  the specific kinetic energy of the solar wind incident on 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. Key words. magnetohydrodynamics (MHD) – statistical – data analysis – coronal mass ejections (CMEs) – solar wind 1. Introduction Earth-directed Coronal mass ejections (CMEs) originating from the solar corona are the primary drivers of geomagnetic storms. Realistic estimates of Sun-Earth CME propagation times and ar- rival velocities are therefore an important component of space weather forecasting. Understanding the dynamics of CMEs and the forces leading to their propagation and expansion is crucial to this endeavor. Approaches to this problem range from early analytical models for the entire Sun-Earth propagation (Chen 1996; Kumar and Rust 1996), semi-analytical models that ap- ply only to the aerodynamic drag-dominated phase of the prop- agation (Cargill 2004; Sachdeva et al. 2015; Vršnak et al. 2013) to detailed 3D MHD models (Linker et al. 1999; Odstrcil and Pizzo 2009; Keppens et al. 2020; Tóth et al. 2012). Some efforts have focussed on characterizing the internal magnetic structure of the interplanetary counterparts of CMEs (ICMEs) (Klein and Burlaga 1982; Nieves-Chinchilla et al. 2016) and others have fo- cussed on comprehensive characterizations of ICME peoperties (Richardson and Cane 2010; Temmer 2021; Forsyth et al. 2006). Despite these advances, there are still some fairly basic is- sues that remain to be addressed in this area. CME expansion provides a concrete window into some of these issues. It is well known that CMEs translate as well as expand as they travel through the heliosphere - CMEs are observed to expand in typ- ical coronograph fields of view (St. Cyr et al. 2000) and be- yond (Lugaz et al. 2010; Webb et al. 2009). CME expansion has also been confirmed using in-situ observations in the heliosphere (Bothmer & Schwenn 1998; Wang & Richardson 2004) and near the Earth (Dasso et al. 2007). The expansion is thought to occur because the interior of the CME is over-pressured with re- spect to its surroundings (e.g., von Steiger and Richardson 2006; Scolini et al. 2019; Démoulin and Dasso 2009; Verbeke et al. 2022), although some contend that the expansion is an outcome of CME magnetic field rearrangement (Kumar and Rust 1996). Some authors (e.g., Gopalswamy et al. 2014, 2015; Kassa Dag- new et al. 2022), ask if the abundance of halo CMEs during solar cycle 24 is because the ambient solar wind pressure is generally lower, leading CMEs to be more over-pressured (with respect to the surroundings) than usual. CME expansion speeds are also known to be lower than the Alfvén speeds in the ambient solar wind (Klein and Burlaga 1982; Lugaz et al. 2020) - this is an- other instance of comparison between the CME plasma and that of the surrounding solar wind. CME identification using in-situ data also relies on a comparison between the CME and the am- bient solar wind plasma - one of the well accepted criteria for Article number, page 1 of 18 arXiv:2210.16571v1 [astro-ph.SR] 29 Oct 2022