Shrinking and Cooling of Flare Loops in a Two-Ribbon Flare

Abstract

We analyze the evolution of the flare/postflare-loop system in the two-ribbon flare of November 3, 2003, utilizing multi-wavelength observations that cover the temperature range from several tens of MK down to 10⁴ K. A non-uniform growth of the loop system enables us to identify analogous patterns in the height-time, h(t), curves measured at different temperatures. The "knees," "plateaus," and "bends" in a higher-temperature curve appear after a certain time delay at lower heights in a lower-temperature curve. We interpret such a shifted replication as a track of a given set of loops (reconnected field lines) while shrinking and cooling after being released from the reconnection site. Measurements of the height/time shifts between h(t) curves of different temperatures provide a simultaneous estimate of the shrinkage speed and cooling rate in a given temperature domain, for a period of almost ten hours after the flare impulsive phase. From the analysis we find the following: (a) Loop shrinkage is faster at higher temperatures - in the first hour of the loop-system growth, the shrinkage velocity at 5 MK is 20 - 30 km s⁻¹, whereas at 1 MK it amounts to 5 km s⁻¹; (b) Shrinking becomes slower as the flare decays - ten hours after the impulsive phase, the shrinkage velocity at 5 MK becomes 5 km s⁻¹; (c) The cooling rate decreases as the flare decays - in the 5 MK range it is 1 MK min⁻¹ in the first hour of the loop-system growth, whereas ten hours later it decreases to 0.2 MK min⁻¹; (d) During the initial phase of the loop-system growth, the cooling rate is larger at higher temperatures, whereas in the late phases the cooling rate apparently does not depend on the temperature; (e) A more detailed analysis of shrinking/cooling around one hour after the impulsive phase reveals a deceleration of the loop shrinkage, amounting to ā ≈ 10 m s⁻² in the T < 5 MK range; (f) In the same interval, conductive cooling dominates down to T ≈ 3 MK, whereas radiation becomes dominant below T ≈ 2 MK; (g) A few hours after the impulsive phase, radiation becomes dominant across the whole T < 5 MK range. These findings are compared with results of previous studies and discussed in the framework of relevant models.