Observations of the Sun at Ultraviolet Wavelengths: 1 to 400 nm
Gary Rottman, Laboratory for Atmospheric and Space Physics

3. Solar Variability

The present state of our understanding of the solar irradiance in absolute units, that is on a scale related to the SI standard of irradiance, has an uncertainty of approximately 2 to 5%. The reality of this fact is that if we compare two different observations of the Sun, from two different instruments, they may provide an estimate of true solar variability with an uncertainty of roughly 3 to 7%. This may be sufficient, although not altogether desirable, at certain wavelengths where the variation is much larger than these numbers. For example, such a capability would suffice at Lyman alpha where the solar cycle variation is about a factor of two. But such comparisons would be completely inadequate, providing ambiguous information near 200 nm where the solar cycle variation is only on the order of 10%. Therefore, our inability to calibrate irradiance instruments better than a few percent, limits the usefulness of intercomparing data sets from different instruments. Nevertheless, we have been fortunate in establishing long-term solar variations by using data from single instruments. As long as the observations continue for periods of several years, and as long as there is a credible technique to account for changes in the instrument sensitivity; then the data comparison becomes straightforward. For if we compare data from a single instrument, the ratio of any two measurements causes many terms to cancel - the area of the entrance aperture probably has not changed, the wavelength bandpass of the spectrometer likely has not changed, and so forth. The confidence in the amount of variation attributed to the Sun is related to the time base between the two measurements, and as mentioned above, to our ability to track changes in instrument sensitivity. Since 1978, the handful of instruments described above, ranging from the NIMBUS-7 SBUV to the present day UARS and ERS-2 instruments, have provided precise measurements accurate to perhaps a few tenths of a percent over time bases of two to three weeks, and one to two percent over time bases of several years.

3.1. 27-day Variations

One of the dominant signals of solar variation is related to the 27-day rotation period of the Sun. As active regions appear and disappear on the solar disk, their occurrence is non-uniform. The resulting irregular distribution provides a striking signal modulated at the rotation period of the Sun. The amplitude of the signal is dependent on the summed strength of the activity on one side of the Sun, opposed by the signal on the other. This implies that in the unlikely occurrence of a near uniform distribution of activity, the 27-day signal could be quite small even for a very active Sun. Likewise, for a very non-uniform distribution - an active hemisphere and a quiet hemisphere could provide a strong 27-day signal even at moderate solar activity.

Figure 3 is a typical variation of the Sun during a 27-day period (i.e., measurements separated by roughly 13 days) shown as a function of wavelength. The ratio is plotted twice, once for the scale at the left and then magnified by a factor of ten for the scale to the right. Notice how details of this curve correspond with the spectral features of Figure 2. All of the strong emission lines at the blue end of the spectrum have much higher variability than their neighboring "continuum." As our attention moves across the aluminum edge at 208 nm, the variation drops by a factor of two. The amplitude of the 27-day variation continues to fall toward longer wavelength, becoming only a small fraction of a percent with the exception of the strong Fraunhofer lines of Mg II at 280 nm and Ca II at 390 nm. In fact, we notice that longward of about 300 nm the variation becomes negative. That is, the "active" phase is now dimmer than the "inactive" phase. This is reminiscent of measurements of TSI where often the more active Sun is accompanied by large sunspots which block more radiation than can be filled in by the surrounding bright faculae. Thereby, the ratio of the two irradiance values falls below "one," and we see the spectral signature of the "sunspot blocking" phenomenon. It should be noted that the 27-day variation shown in Figure 3 is typical, but by no means standard. Each rotation of the Sun will most likely provide a magnitude, and perhaps a shape, different from that shown in this figure.

FIGURE 3: The amplitude of a 27-day variation of the Sun shown as a function of wavelength. This curve corresponds to a single rotation period in early 1992. Although this curve is typical of the 27-day modulation of solar radiation, it should not be considered "standard" in either amplitude or shape.

3.2. Solar Cycle Variations

As we extend our observations of the Sun over a longer and longer time base, we hope to establish correspondingly longer time scales of solar variability. If a single instrument has been used to make the measurements, we must establish any and all changes in the instrument sensitivity before we can extract from the data the inherent variation in the Sun. The two UARS instruments, SOLSTICE and SUSIM, were both designed with the specific goal of measuring long-term solar variations. SUSIM uses standard lamps as in-flight calibration source; and moreover, it uses redundant lamps and optical channels to confirm all instrument changes. SOLSTICE uses a completely different calibration technique relying on bright blue, early-type stars as in-flight calibration standards. This unique approach relies on the assumption that the stars are very stable (inherent variability of a small fraction of one percent over time periods of thousands of years), and that the ensemble average of twenty or so stars form an even more stable reference standard. The validation paper by Woods et al. (1996) documents the initial comparisons of these two instruments, and additional comparisons are in progress. Both techniques appear to work well, and the six and one half year data record from either, or both instruments will be accurate to better than one percent. The UARS time period spans conditions from near solar maximum in early 1992 through solar minimum in 1996, and now on toward the next solar maximum. Both instruments together with the entire UARS spacecraft are working well, and there is every reason to believe that the mission will continue through the maximum of cycle 23.

Figure 4 provides a preliminary estimate to the solar cycle variation from early 1992 to late 1996. Similar to Figure 3, it is the ratio of the maximum Sun to the minimum Sun, and it is interesting to see the similarity of this curve to the rotational variation of Figure 3. We note the strong variation - as much as a factor of two in Lyman alpha and in the other strong chromospheric lines short of 140 nm, and generally stronger than the neighboring continuum. The amount of variability steadily decreases toward longer wavelengths where again we see a drop of about a factor of two moving across the aluminum edge at 208 nm. At the present time we feel that these data sets have a precision and relative accuracy (uncertainty in the ratio of two values) limited at one to two percent, and therefore as we move toward longer wavelengths, especially at 250 nm and above, we are becoming limited by the observations. It is encouraging to see the Mg II doublet at 280 nm rising from the noise floor, but as we move on to 300 nm and above the present state-of-the-art is just not up to the task of resolving true solar variability. Both the SUSIM and SOLSTICE Science Teams continue to refine their data processing algorithms, and we are optimistic that in the final analysis the detection limit for solar variability will be at, or slightly better than, the one percent level. This precision and relative accuracy will hopefully be adequate to establish solar variability at wavelengths less than 300 nm. At wavelengths longward of 300 nm it appears that the solar variation will remain hidden below our present detection limits, and will await the next generation of solar irradiance techniques.

FIGURE 4: The amplitude of solar cycle variation (early 1992 divided by 1996) as a function of wavelength. Estimate of the uncertainty in this ratio is 1 to 2%, and there is presently only an upper limit (< 1%) of solar cycle variations at wavelengths longer than about 300 nm.

3.3. Flare Enhancements

Short-term solar variations in particular flares are very dramatic when observed in the ultraviolet, and these transient occurrences have important stellar counterparts. Brekke et al. (1996) discuss an observation of a Class 3B solar flare for which the emitted flux density from the entire solar disk increased by more than an order of magnitude for many of the chromospheric emission lines in the spectral range 120 to 170 nm. Such a flare phenomenon would be easily detected in other stars. For example, if a star is observed in the Si IV line at 139 nm during the impulsive phase (approximately 5 minutes) of a large stellar flare, its brightness may be expected to rise a factor of ten or more.

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