Observational example of a flux rope observed by STEREO-A HI1. The image is derived with running differences. In the right panel, the same image is shown with the front and rear sheaths outlined with green lines. The dashed blue lines are extrapolations towards the Sun. The flux rope axis (red line) is defined at equidistance from the two sheaths. Four twisted structures are marked with orange lines. The coordinate system is the elongation angle in degree from the Sun. This figure is adapted from Möstl et al. (2009).