Analysis of Deformation and Erosion during CME Evolution

Abstract

Magnetised coronal mass ejections (CMEs) are quite substantially deformed during their journey form the Sun to the Earth. Moreover, the interaction of their internal magnetic field with the magnetic field of the ambient solar wind can cause deflection and erosion of their mass and magnetic flux. We here analyse axisymmetric (2.5D) MHD simulations of normal and inverse CME, i.e., with the opposite or same polarity as the background solar wind, and attempt to quantify the erosion and the different forces that operate on the CMEs during their evolution. By analysing the forces, it was found that an increase of the background wind density results in a stronger plasma pressure gradient in the sheath that decelerates the magnetic cloud more. This in turn leads to an increase of the magnetic pressure gradient between the centre of the magnetic cloud and the separatrix, causing a further deceleration. Regardless of polarity, the current sheet that forms in our model between the rear of the CME and the closed field lines of the helmet streamer, results in magnetic field lines being stripped from the magnetic cloud. It is also found that slow normal CMEs experience the same amount of erosion, regardless of the background wind density. Moreover, as the initial velocity increases, so does the influence of the wind density on the erosion. We found that increasing the CME speed leads to a higher overall erosion due to stronger magnetic reconnection. For inverse CMEs, field lines are not stripped away but added to the magnetic cloud, leading to about twice as much magnetic flux at 1 AU than normal CMEs with the same initial flux.

Interaction of coronal mass ejections and the solar wind

“…16 shows two peaks at the front of the flux rope in both cases. This contrasts with the CME structure simulated and analysed by Hosteaux et al (2021), which had only one peak at about the same distance from the Sun (at ≈35 R ). The first increase seen in our simulations can be attributed to the overlying magnetic field at the eruption time and to the streamer that is stripped away along with the CME.…”

“…The first increase seen in our simulations can be attributed to the overlying magnetic field at the eruption time and to the streamer that is stripped away along with the CME. This would not be present in the case of Hosteaux et al (2021), who just injected a plasma blob into the solar wind and did not model the eruption. This suggests that different triggering mechanisms can affect the structure of CMEs, including during propagation, and therefore should be taken into consideration when interpreting CME structure.…”

“…Chen & Garren 1993;Cargill et al 1995). The few more recent studies of these physical processes include Shen et al (2012), who analysed the forces causing acceleration and deceleration of CMEs, and Hosteaux et al (2021), who studied the deformation and erosion of CMEs during their evolution. Furthermore, Kay & Nieves-Chinchilla (2021) presented the first results of their numerical code that simulates the propagation, expansion, and deformation of a CME in the interplanetary medium, and quantified the contributions of each of the imposed forces to these processes.…”

Analysis of Voyager 1 and Voyager 2 in situ CME observations

“…Apart from expansion, magnetic clouds can experience erosion during their propagation, stripping the ICME from magnetic field lines and causing a lower average magnetic field (Dasso et al, 2006;Ruffenach et al, 2015). Hosteaux et al (2021) showed that, in case of an inverse ICME (i.e. a CME with the same magnetic polarity as the background coronal magnetic field), the opposite occurs and magnetic field lines are added to the ICME, increasing the average magnetic field strength.…”

Interaction of coronal mass ejections and the solar wind. A force analysis

“…Chen & Garren 1993;Cargill et al 1995). The few more recent studies of these physical processes include Shen et al (2012), who analysed the forces causing acceleration and deceleration of CMEs, and Hosteaux et al (2021), who studied the deformation and erosion of CMEs during their evolution. Furthermore, Kay & Nieves-Chinchilla (2021) presented the first results of their numerical code that simulates the propagation, expansion, and deformation of a CME in the interplanetary medium, and quantified the contributions of each of the imposed forces to these processes.…”