## 2. Observations and data reductionThe Coronal Diagnostic Spectrometer (CDS) onboard the Solar Heliospheric Observatory (SOHO) is a dual extreme ultraviolet spectrometer, covering the wavelength range 150 to 780 Å, comprising of a normal incidence and a grazing incidence spectrometer (Harrison et al. 1995). The normal incidence spectrometer (NIS), whose data is the subject of this paper, gives spectral images in two wavebands (308-381 Å and 513-633 Å). In order to get good time resolution, we used the NIS in a sit-and-stare mode. For the data reported here, the 4x119 arcsec slit was used. Although CDS has the ability to compensate for solar rotation, this was turned off since we did not want to introduce any possible variations due to instrument movements. These observations were obtained at a pointing of X=-6.5, Y=+981 and thus for these plume observations rotational compensation is not an important consideration. Fig. 1 shows an image of the north polar coronal hole region taken with EIT in Fe XII 195 Å at 22:12 UT on July 15, 1999 with the slit superimposed.
The data discussed here were obtained on the 15 July 1999 at 15:33
UT. The observations are summarised in Table 1. Two temporal
series datasets were obtained for the three lines of O
V 629 Å (log T
Using the standard CDS software procedure VDS-CALIB, we debiased and flat-fielded the data. The resulting data after running this procedure were in units of photon-events/pixel/sec. Multiplying by the exposure time yielded units of photon-events/pixel. In the result section counts means photon-events in the binned pixel. The data was cleaned of cosmic ray hits by using the CDS software procedure CDSCLEAN. Details of the Fourier analysis can be found in Doyle et al.
(1999). Power spectra are obtained from the Fourier transforms of the
auto-covariance functions, multiplied by a window function to reduce
the variance of the noise. For the smoothed spectra we used the Tukey
window. Power spectra are normalized in such a way that the expected
mean noise level equals 2. Because the mean noise level and its
variance are known we are able to derive confidence limits for
spectral features. For both intensity and velocity power spectra we
use confidence levels of 99.9%. To determine the Doppler shifts,
wavelength calibration is needed. We use the ` In order to find the most reliable periods, we have also performed wavelet analysis on the data. To this end we used the methods, techniques and software provided by Torrence & Compo (1998). This wavelet software uses the Morlet wavelet and moreover allows the calculation of confidence levels. Again we chose a confidence level of 99.9%. The benefits of using wavelet analysis is that the localised (in time) nature of the wavelet transform allows us to study the duration of any statistically significant oscillations as well as their period. For further details on wavelet analysis see Torrence & Compo (1998) and references therein. © European Southern Observatory (ESO) 2000 Online publication: March 21, 2000 |