Summary: Astronomy & Astrophysics manuscript no. 8617lett c ESO 2007
October 4, 2007
Plasma condensation in coronal loops
E. O'Shea1, D. Banerjee2, and J.G. Doyle1
Armagh Observatory, College Hill, Armagh BT61 9DG, N.Ireland
Indian Institute of Astrophysics, II Block, Koramangala, Bangalore 560 034, India
Aims. Using temporal series data from the Coronal Diagnostic Spectrometer (CDS) on SOHO, we seek to investigate plasma conden-
sation 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 He I 584 Å and O V 629 Å, there are sharp increases in intensity, accompanied by large
blueshifted velocities. In the case of O V 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