The equivalent width of the high excitation photospheric line of C 5380A has been measured 3 to 4 times monthly in the solar irradiance spectrum since 1978. C 5380 behaves differently than other spectral lines in that it is unmodulated by the activity cycle, yet has increased in strength by 0.081 plus or minus .008 mA in 12 years. Nominal equivalent width is 22.25 mA. Implied is a temperature increase of 4.6 K which is an order of magnitude greater than constraints allowed by Active Cavity Radiometer Irradiance Monitor (ACRIM) results

Livingston, William C.

English

NASA Technical Reports Server

SECULAR CHANGE IN EQUIVALENT WIDTH OF C 5380,1978 - 90. 
William C. Livingston 
National Solar Observatory, Tucson, Arizona 85726 
Abstract 
The equivalent width of the high excitation photospheric line of C 5380A has been measured 3-4 times monthly in 
the solar irradiance spectrum since 1978. C 5380 behaves differently than other spectral lincs in that it is 
unmodulated by the activity cycle, yet has increased in strength by 0.081 + .008 mA in 12 years. Nominal 
equivalent width is 22.25 mA. Implied is a temperature increase of 4.6 K which is an order of magnitude greater 
than constraints allowed by ACRIM results. 
Introduction 
Twelve years of monthly measurements of selected Fraunhofer lines in the full disk irradiance spectrum now provide 
us with a good picture on the response of various spectral features to a changing Sun. Chromospheric lines such as 
Ca K. H alpha and Ca 8542 all weaken a percent or so with an increase of solar activity, Fig. 1. Resolved disk 
observations suggest this weakening is due to active region plage. In such areas chromospheric lines become 
shallow or even display emission components. 
He 10830, on the other hand, is a maverick that strengthens in plage. From solar minimum to maximum the depth 
of this line can double in the irradiance spectrum.' It has been demonstrated that the strengths of these 
chromospheric lines correlate well with Ly alpha emission and thus can serve as surrogates for the EUV-UV solar 
output.2 
At the photospheric level the pattern is not so simple. One might expect strong photospheric lines to show the 
greatest variability. Eclipse "flash" spectra? which clearly represent the chromosphere, tell us that emission strength 
goes hand-in-hand with disk equivalent width. If we think of this emission component as "filling in" the cores of 
strong lines, then they should weaken at solar maximum. The central depths of such lines do display a slight 
chromospheric weakening with activity (not reproduced herein). Equivalent width, on the other hand, seems constant 
in our data, for example Fe 5250.6 in Fig. 2. Probably this is a matter of line saturation4 combined wilh 
insufficient instrumental sensitivity. 
The equivalent widths of most moderate to weak photospheric lines track nicely the chromospheric lincs in cyclic 
variability, see Mn 5394 in Fig. 2. In terms of percentage change they are in fact comparable indicators, although 
observational error is greater owing to their relatively small central depths. 
Like He 10830, the "photospheric" oxygen triplet at 7774A is another loner. Unlike all other photospheric lincs 
0 7774 increases in strength with activity. Probably there is some NLTE effect at work here. 
Finally we come to the behavior of C 5380A. The equivalent width of this line shows no detectable cycle 
variability, but curiously has increased in strength over our observing time interval of 12 years. 
C 5380 on the Resolved Disk 
CI 5380.3231A has an excitation potential of 7.65 V. The species has an ionizing potential of 11.26 V and is 
always completely neutral in the photosphere (compared to Fe, which is not). At disk center C 5380 has a ccntrnl 
depth with respect to the continuum of about 0.155 and an equivalent width of 26.3 mA. At the limb (p = 0.1) the 
central depth reduces to about 0.04. According to the HSRA model atmosphere, the peak in h e  physical height of 
the response function for C 5380 lies only slightly above (-25 km) the contribution function for the h5380A 
https://ntrs.nasa.gov/search.jsp?R=19910003189 2020-03-24T06:54:35+00:00Z
Fig. 1. Observed cycle modulation of a variety of chromospheric (top panel) and photospheric lines (bottom). 
Strength of the photospheric lines in an eclipse flash spectrum is indicated in the middle panel. Eclipse intensity of 
zero means the line is not seen. The lines shown are the same as in the lower panel except for 0 7774, which is off 
scale, and the 372 mA line which was outside the wavelength coverage. All chromospheric lines are off scale in this 
representation. 
-5 - Cycle 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ~ ~ ~ ~ ~ ~ ~ -  variation of chromospheric lines 986-90 
-4 
- 
R 
w 
h -3 
2 !z 
n 
. -2 
- 
- 
- 
- 
- 
- He 10830 - 
- 
- 
- 
- 
- 
- 
- ( 
- +96% - 
r Ca K - 
- 
- 
- 
- 
- 
- 
- 
h 
$ 
-lL 1 1  1 1  1 1  1 1  1 1  1 1  1 1  1 H alpha Ca 8542 0 .  
0 .1 .7 .8 .9 1 
central depth 
"rhotospheric" line intensity in eclipse flash spectrum 
2 4 -  
V) 
$ 
4 
d 
.CI 
2 -  
V) 
a 
.4 A
0 
0 
0 
l l l ~ ' l l ~ l l l ~ l l l ~ l l l ~ l l l ~ l l l ~ l l l  
- - 
- - 
- - 
- 
- - 
- - 
- - 
- 
- - 
- - 
- - 
a a - - 
- - 
0 20 40 60 80 100 120 140 160 
disk equivalent width (d) 
I 1  I  I I I I I I I  
2 
I  I  I  I l l  
1 1 1  
I I I I I I I  
Cycle variation of photospheric lines 1986-90 
I  I  I 
n 
1 
K 
w 
h 
5 o -  3 
(d 
.r( 
2 
> 
-1 
- 1 l 1 ~ l l l ~ l l l ~ l l l ~ l l l ~ l l l ~ l l l  
- 
- 
- 
- 
- 
- 
- 
- 
- 
- 
- 0 3774 - 
- 
- 
C 5380 - 
1 x / I \  - T 
1 
- - 
- 
- 
- I 1 372 d- - - 
- 1 
- 
- 
- 
- 
- I 1 1 - 
- 
- 
- 2 t 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1  
- 
0 20 40 60 80 100 120 140 160 
equivalent width (d) 
continuum. At disk center AW/W = + .00086/K, i.e. the line should strengthen with increasing tempcraturc. 
Compare this with AW/W = -.00063/K for the typical moderate strength Fe line such as Fe 5250.6. Scc 
Livingston, et. al.,s and Livingston and H~lweger,~ for more details and references. From 105 days of observation 
we obtain at disk center, Wk = 25.024 + .0117 f .0025 (year) mA, or an increase of 0.14 f .03 mA/12 year. 
106 I I Photospheric I I I I I [ I  Fe 5250.6 I I I ew fd I I (no I cycle) I I I I 
77 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1  Photospheric Mn 5394 ew fd (weak cycle) 
.11 Chromospheric I I I 1 1 1 1 1 1 1  Ca K index fd I (strong I I I I cycle) I I I I 
.08 I  I  I  I  I  I  I  I  I  I  I  I I l l  I l l  I 
78 80 82 84 86 8 8 9 0 
year 
Fig. 2. Examples of solar cycle behavior of selected lines in Fig. 1. The Mn 5394 plot is inverted to agrec in scnsc 
with Ca K. 
C 5380 Observed Full Disk 
Although our C 5380 full disk archives extend back to 1976, various changes in the instrument and observing 
technique contaminate the data before November 1978. Since then, however, all aspects of the observations and data 
reduction have remained fixed. Annually the telescope mirrors are recoated and once, in April 1980, the spectrograph 
mirrors were also re-aluminized. Otherwise the system has remained immutable (a beneficial aspect of poor 
funding). 
Fig. 3 displays the temporal record for individual observations. A least square fit to the 487 days of data gives 
Wh = 21.67 + .00675 f .00066 (year) mA. The formal probable error in the slope is one-tenth the value. Implied 
is an increase of 0.081 k .008 rnA112 years. This is slightly smaller than the center disk value, and for various 
reasons the probable error is less. If the HSRA model were applicable, a temperature increase of 4.6 K is deduccd. 
Assuming the Sun is a black body this leads to an increment in the solar constant of 0.3%. an amount incompatablc 
with ACRIM results. Evidently the HSRA model is inappropriate here. 
23 Equivalent Width C 5380 full disk I l l  I l l  " l ~ l l l ~ " ' ~ " ' ~ '  - 
- 
W, = 21.67 + .0068 k.00066 (year) - - 
- 
'- 
Equivalent Width C 5380 annual means 
I  I I  I I I J I I I I I I I I I I I I I I I I I  - 
- 
- 
- 
21.5 I I I I I I I I  I I I  I I  I t  I I  I  I ( 1 1 1 1 1  
7 8 8 0 82 84 8 6 8 8 9 0 
year 
Fig. 3. Temporal record of C 5380. Top panel is individual days, each an average of 6 observations. Bottom, the 
same data averaged into onc year bins. The straight line fit is from the un-bind data. 
Summary and Conclusions 
A comparison between a wide variety of Fraunhofer lines, and how they empirically respond to the solar activity 
cycle, leads us to conclude that C 5380 is a unique indicator of conditions in the low photosphere. Model 
calculations suggest it is more temperature sensitive (and should respond in an opposite sense) compared with other 
lines, yet it displays no significant cycle modulation. Its equivalent width appears to have increased by 0.081 It: .008 
mA over a time span of 12 years. Formal statistics suggest this trend is real, and implies a 4.6 K increase in 
surface temperature. Such a temperature rise translates to 0.3% in the solar constant, and is much too large for 
compatibility with ACRIM results. 
What is needed to understand C 5380? Obviously patience and a longer archival time base is desirable. We may also 
search for clues on the resolved solar disk. How does C 5380 behave in plage and faculae? Earlier work was 
inconclusive. Or perhaps the Sun is evolving in a non-thermal fashion on a time scale of 22 years, or longer. Othcr 
evidence has been proposed for long term excursions in solar temperature, but such ideas are speculative at 
pre~ent.~l~.9 
References 
1. Foukal, P., and Lean. J., 1988, "Magnetic Modulation of Solar Luminosity by Photospheric Activity", 
Astrophys. J.,  3 2 8, 347 - 357. 
2. White, 0. R., Rottman, G. J., and Livingston, W. C., 1990, "Estimation of the Solar Lyman Alpha Flux From 
Ground Based Measurements of the Ca 11 K Line", Geophysical Res. Let., (in press). 
3. Dunn, R. B., Evans, J. W., Jefferies, J. T., Orrall, F. Q., White, 0. R., and Zirker, J. B., 1968, "The 
Chromospheric Spectrum at the 1962 Eclipse", Astrophys. J. Suppl. 15,275 - 458. 
4. Mitchell, W. E. Jr., and Livingston, W. C., 1990, "Line-blanketing Variations in the Irradiance Spectrum of the 
Sun from Maximum to Minimum of the Solar Cycle", Astrophys. J. (submitted). 
5. Livingston, W., Milkey, R., and Slaughter, C., 1977, "Solar Luminosity Variation. I. CI 5380 as a Temperature 
Indicator and a Search for Global Oscillations", Astrophys. J. 2 11,281 - 287. 
6. Livingston, W., and Holweger, H., 1982, "Solar Luminosity Vairation. IV. The Photospheric Lines, 1976 - 
1980", Ap. J. 2 5 2,375 - 385. 
7. Kuhn, J. R., Libbrecht, K. G., and Dicke, R. H., 1988, "The Surface Temperature of the Sun and Changes in thc 
Solar Constant", Sci. 242,908 - 91 1. 
8. Schatten, K. H., and Orosz, J. A., 1990, "Solar Constant Secular Changes", Solar Phys., 1 2  5 ,  179. 
9. Kroll, R. J., Hill, H. A., and Beardsley, B. J., 1990, "A Correlation Between Changes in Solar Luminosity and 
Differential Radius Measurements. (this conference). 


Secular Change in Equivalent Width of C 5380, 1978-1990

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server