Dynamics of solar coronal loops

I. Condensation in cool loops and its effect on transition region lines

¹ Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany e-mail: peter@kis.uni-freiburg.de
² Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern 0315, Oslo, Norway e-mail: Viggo.Hansteen@astro.uio.no

Corresponding author: D. A. N. Müller, dmueller@kis.uni-freiburg.de

Received: 18 June 2003
Accepted: 22 August 2003

Abstract

We report numerical calculations of the condensation of plasma in short coronal loops, which have several interesting physical consequences. Firstly, we propose a connection between small, cool loops ( K), which constitute one of the basic components of the solar transition region, and prominences, in the sense that the same physical mechanism governs their dynamics: Namely the onset of instability and runaway cooling due to strong radiative losses. Secondly, we show that the temporal evolution of these loop models exhibits a cyclic pattern of chromospheric evaporation, condensation, motion of the condensation region to either side of the loop, and finally loop reheating with a period of 4000–8000 s for a loop of 10 Mm length. Thirdly, we have synthesized transition region lines from these calculations which show strong periodic intensity variations, making condensation in loops a candidate to account for observed transient brightenings of solar transition region lines. Remarkably, all these dynamic processes take place for a heating function which is constant in time and has a simple exponential height dependence.