Physics of the Solar Transition Region

A long-standing challenge in Solar Physics has been to understand the nature of the so-called transition region (TR) connecting the corona to chromosphere. The problem is that EUV observations imply the presence of much more plasma at temperatures below 100,000 K than coronal loop models would predict. Two types of theories have been proposed to account for the discrepancy: missing structure (unresolved fine structure unrelated to coronal loops dominates the emission at these temperatures) and missing physics (the standard Spitzer thermal conductivity central to the loop models breaks down at low temperatures). We use IRIS and SDO observations of flare loop ribbons to distinguish between the two theories. The key point is that these ribbons correspond only to the footpoints of flare coronal loops and not to any small-scale fine structure. We present a detailed analysis of the observations and, in particular, examine the evolution of the emission in Si IV, Fe IX, and Fe XXI, thereby covering the full temperature range from the lower TR to the flare corona. We conclude from this analysis that the discrepancy is due to missing physics, perhaps due to the effects of turbulence at these TR temperatures. We discuss the implications of our results for understanding the corona, in general.

This work was supported by the NASA Living With a Star Program.