SPRG Seminars
October 8, 2013:
"On Cross-Sectional Properties of Coronal Loops"
Anna Malanushenko, Lockheed Martin Solar and Astrophysics Laboratory, LMSAL
Coronal loops have been observed for several decades, yet some of their properties remain a mystery. These in particular include the lack of apparent expansion of coronal loops and the increased pressure scale height in loops compared to the diffuse background. Various theories have been proposed to explain these phenomena. All of them involve models of flux tubes with a circular cross-section. I will show that solely lifting the assumption about circular cross-sectional shape of flux tubes is alone sufficient to explain the lack of observed expansion and the increased pressure scale height.
While magnetic flux tubes expand in the corona, they do so in a highly anisotropic manner, which I will discuss in details. I will also demonstrate how, and why, this leads towards (1) selection bias which might make some loops stand out if they expand mostly along the line of sight, due to their increased column depth; (2) principal limitations on measuring expansion of coronal loops, even if they are resolved and (3) the apparent increased pressure scale height. I will also demonstrate how loops, oblate in cross-section, might appear circular in an analysis similar to the ones that were done in the past.