Plasma condensation in coronal loops

Abstract

Aims.Using temporal series data from the Coronal Diagnostic Spectrometer (CDS) on SOHO, we seek to investigate plasma condensation through the use of spectroscopic analysis. Methods.Using a space-time (X-T) type plot we show, for a number of transition region (TR) and coronal lines, the variation of intensity (radiant flux) with time in an off-limb loop system. Noting a number of pixels where the intensity shows a sharp jump in value for the TR lines and a corresponding sharp decrease for the coronal lines, we investigate further and plot the intensity and velocity for these pixels as a function of time. Using standard Fourier techniques, together with a pre-whitening method, we measure all statistically significant frequencies present in the time series so created. Results.We find that, for the cooler lines of $\ion{i}$ 584 Å and $\ion$ 629 Å, there are sharp increases in intensity, accompanied by large blueshifted velocities. In the case of $\ion$ there is a substantial rebound to redshifted velocities when the strong brightening fades. The sharp increase in intensity and blueshifted velocity in the cooler lines is accompanied by a corresponding decrease in the intensity of the coronal lines ( $\ion{x}$ 624 Å, $\ion{xvi}$ 360 Å, $\ion{xii}$ 520 Å), but with no corresponding change in their velocity values implying an evacuation of plasma. From the Fourier analysis, evidence is found for possible 1st and 2nd harmonics in the $\ion{i}$ and $\ion$ lines, indicative of standing waves in loops. Conclusions.We conclude that what we are seeing is the first spectroscopic evidence of plasma condensation taking place in coronal loops.