Self-similar expansion of solar coronal mass ejections: implications for Lorentz self-force driving

We examine the propagation of several CMEs with well-observed flux rope signatures in the field of view of the SECCHI coronagraphs aboard the STEREO satellites using the GCS fitting method of Thernisien, Vourlidas & Howard (2009). We find that the manner in which they propagate is approximately self-similar; i.e., the ratio ($kappa$) of the flux rope minor radius to its major radius remains approximately constant with time. We use this observation of self-similarity to draw conclusions regarding the local pitch angle ($gamma$) of the flux rope magnetic field and the misalignment angle ($chi$) between the current density ${mathbf J}$ and the magnetic field ${mathbf B}$. Our results suggest that the magnetic field and current configurations inside flux ropes deviate substantially from a force-free state in typical coronagraph fields of view, validating the idea of CMEs being driven by Lorentz self-forces.