2. Observations and data reduction
The 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 Te=5.4 K), Mg IX (log Te=6.0 K) and Fe XVI (log Te=6.3 K). Data were obtained after the recovery of SOHO and so the lines show the characteristic broadened wings. This broadening has the effect of blending the Mg IX 368 Å line with that of the nearby Mg VII 367 Å line. Henceforth this blend will be referred to only as the dominant component of the blend, in this case Mg IX . To improve the signal-to-noise of this data we binned by three pixels along the slit, in effect creating new pixels of 54 arcsec2. The data for the coronal line Fe XVI 335 Å were very weak and are therefore not included in the present analysis.
Table 1. Details of the temporal series observations
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 `limb method ', where we assume that above the limb all (non-radial) wave or mass motions on average cancel out. In the absence of radiative transfer effects, above limb Doppler shifts must on average be zero. This method was also used by Doschek et al. (1976) and Peter & Judge (1999). Note that Doppler shifts in the coronal hole are independent of the rest wavelength; everything is calibrated relative to the limb.
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