Active region dynamics

It has been frequently observed that in solar active regions the measured line widths are larger than those based on thermal equilibrium widths. This excess width (characterised as non-thermal velocity, V_nt) has been proposed as a signature of the heating mechanism. The behaviour of the V_nt at coronal temperatures has not produced consistent results with values ranging from 0 to 100 km/s. We investigate this problem by using joint observations from Norikura Solar Observatory, Japan and the Coronal Diagnostic Spectrometer (CDS) on SOHO. We find that values of V_nt range between 10-20 km/s. We analyse the dynamical bahaviour of different temperature loops and find that for loops with log T_e < 5.8, the variability ∝T_e^0.39. This is comparable to the dependence of non-thermal velocity on temperature which we have determined to be V_nt ∝ T_e^0.35. This suggests that the excess line broadening is caused by highly dynamical behaviour in the transition region for active regions. These results have significant implications for potential heating mechanisms and these are discussed.