4. Results and discussion
For each measurement we obtain the average radiance in the observed area. From 5 March 1996 to 8 August 1999 we have made 34 observations. A summary of all the results is presented in Fig. 1. It shows the evolution of the absolute radiance of each line in the entire time period starting well before the minimum of the sunspot activity between solar cycles 22 and 23. On 26 August 1996 a comparison between detector A and B has been made, and from thereon, for most of the time the B detector has been used. After the recovery from the SOHO accident, that occurred between June and September 1998, both detectors have been used alternately. Thus we can exclude any variation due to changes of response of one detector. A loss of responsivity associated with the SOHO accident phase has been determined in the analysis of these data. By minimizing the difference between a linear least squares fit to the data before and after the accident, for each line individually a change of responsivity between 25% and 45% has been compensated.
All lines show an increase of radiance during the observed time period. The He I 584 Å line shows the smallest variation, perhaps because this chromospheric emission originates at a lower level in the solar atmosphere, and, contrary to the other lines, it is optically thick. For the other lines the increase of radiance amounts to more than 100% and probably will continue as we reach the next solar maximum. The H I Lyman continuum at 880 Å shows an increase of nearly 75%.
We estimate overall uncertainties of the radiance values to be within 15% (1) for all lines observed. The variations of data points however are much larger due to variation of the radiance inside the sampled area. This has been proven by a comparison with data measured in coregistration by the CDS (Coronal Diagnostics Spectrometer) instrument on SOHO. These data have been evaluated for the first 150 days, and they agree on the absolute scale within the uncertainty margins given by both instruments. The data show in particular that the variations are in phase for both instruments (Pauluhn et al. 1999). Thus the scatter of data is not instrumental and would perhaps be reduced if a larger area were sampled. Despite the scatter on the short time scale, the long-term trend is clearly discernible.
Next we investigate the spatially resolved images. We try to separate the network areas from the cell interior locations (areas of lower radiance) by integrating for each line those locations separately where the intensity is 25% above and below the average. In Fig. 2 we have plotted the radiance variations of the lines Mg X 609 Å and 624 Å, N V 1238 Å, and He I 584 Å on a normalised scale. We plotted the network locations, i. e. the highest 25% of the intensities, and the areas of less brightness, i. e. the lower 25% intensities. A linear fit was applied to the data to represent the variations. In each of these lines we see the increase of the radiance in both, the network and cell regimes. For the N V line only data after solar minimum are available. But for this line, which is formed at regions of lower temperature with more detailed network structure than the Mg X lines, we see that there is no difference in the variability of the network and the cell interior areas.
We conclude that the radiances of the lines of N V , Mg X , and Ne VIII , which originate in the transition region and the lower corona, show an increase in quiet-Sun areas that seems to be associated with the solar activity cycle. This result implies that the variation of the irradiance from the full Sun cannot simply be modelled by the number of active regions and plage areas visible on the solar disk and their variation throughout the solar cycle. Instead, there is an additional contribution to the variability from the quiet solar network. We find that the increase of radiances from solar minimum to August 1999 amounts to between 45% and 100%. The separation into bright and dark areas reveals that both, the network regions and the darker regions show the same relative amount of increase. Thus it seems that the variability is a global brightening of the emission of these lines in quiet-Sun regions, or the variation is not resolved by the SUMER instrument.
We stress that the scatter of data points is much higher than photon counting statistics and is entirely of solar origin, as a result of the limited size of the sampled area. Also, as we approach the maximum of the solar activity cycle, it becomes more difficult to select regions on the solar disk that are not influenced by active regions nearby, and the term "quiet Sun" may become questionable. We must notice also that the maintenance of the radiometric calibration over such a long period is a difficult task, and any error in this procedure will influence these results. It is therefore extremely valuable to have such observations from several independent instruments on SOHO. A joint evaluation of the data from SUMER, UVCS, and CDS instruments is underway.
© European Southern Observatory (ESO) 2000
Online publication: February 9, 2000