Magnetic flux ropes: Would we know one if we saw one?

There has been much debate lately about whether twisted magnetic flux ropes exist in the corona. When asked for observational evidence of them, the temptation is to show images of apparently twisted structures (e.g. see Figure 1). However, we must be very careful of projection effects in interpreting these observations. Two critical aspects of understanding how we might observe flux ropes are 1) the 3D nature of the flux rope, and 2) physically, which bits are visible and for what reasons? In this paper we will use a simple but physically reasonable 3D analytic model to address, these two issues, and develop techniques that can in future be used on more general models, both analytic and numerical.