Over the five years double-pass spectrometer observations of the Sun-as-a-star revealed significant changes in line intensities. The photospheric component weakened linearly with time 0 to 2.3%. From a lack of correlation between these line weakenings and solar activity indicators like sunspots and plage, a global variation of surface properties is inferred. Model-atmosphere analysis suggests a slight reduction in the lower-photospheric temperature gradient corresponding to a 15% increase in the mixing length within the granulation layer. Chromospheric lines such as Ca II H and K, Ca II 8543 and the CN band head weaken synchronously with solar activity. Thus, the behavior of photospheric and chromospheric lines is markedly different, with the possibility of secular change for the former

Holweger, H.

Livingston, W.

White, O. R.

English

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N82' 17023
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PRECEDINGPAGEBLANK HOT FILMED
OBSERVED VARIABILITY IN THE F_%UNHOFER LINE
SPECTRUM OF SOLAR FLUX, 1975-1980
W. Livingston
Kitt Peak National Observatory*
H. Holweger
Institut fur Theoretische Physik
und Sternwarte, Kiel, West Germany
O. R. White
High Altitude Observatory
National Center for Atmospheric Research
ABSTRACT
Over the past five years double-pass spectrometer observations of the
"sun-as-a-star" have revealed significant changes in line intensities. The
photospheric component has weakened linearly with time 0 to 2.3%. From a lack
of correlation between these line weakenings and solar activity indicators like
sunspots and plage we infer a global variation of surface properties. Model-
atmosphere analysis suggests a slight reduction in the lower-photospheric
temperature gradient corresponding to a 15% increase in the mixing length
within the granulation layer. Chromospheric lines such as Ca II H and K,
Ca II 8542 and the CN band head weaken synchronously with solar activity.
Thus the behavior of photospheric and chromospheric lines is markedly differ-
ent, with the possibility of secular change for the former.
INTRODUCTION
Since 1975/1976 systematic observations have been routinely made at Kitt
Peak in an attempt to quantify any temporal variation in the Fraunhofer lines.
The original hope was that temperature sensitive lines might provide an indi-
cation of luminosity variation, assuming F = OT _ (ref. I). When line strengths
are measured near disk center variations of several percent are the rule, both
in time and space, owing to a hierarchy of surface Inhomogenelties--the gran t
ulation, supergranulation, faculae, plage, and so on. But if we observe the
whole disk at once, in "integrated light", this fine structure averages out
and the variance in line equivalent widths between spectrum scans reduces to
* Operated by the Association of Universities for Research in Astronomy, Inc.,
under contract with the National Science Foundation. _
95 i   Imlmm
1982009140-096
https://ntrs.nasa.gov/search.jsp?R=19820009149 2020-03-21T16:15:18+00:00Z
-0.02%. This very low noise azises because the equivalent width parameter
refers to the integral of absorption across the entire line profile, and so
is invariant to instrument resolution, and because equlvalent width is taken
relative to the local continuum and thus is Invariant to the transmission
fluctuations of the earth's atmosphere. In terms of the predicted change in
the high excitation llne of CI 5380 our sensitivity is 1 K, or 0.1% in the
solar constant assuming the sun is a black body.
After 2 years of data were acquired we noted that CI 5380 was 0.6% weaker
than at the beginning and, following the above, we concluded the sun had cooled
6 K (ref. 2). At that point the equivalent width values for the other lines
were really inconclusive. Today, however, with 5 years of data before us, the
simple picture of temperature change is no longer tenable (ref. 3). As will
be shown below all photospheric lines have either weakened or remain fixed,
and the degree of weakening obviously depends on other factors besides temper-
ature.
In the sections which follow, the ful" d&ta set for both photospherlc and
chromospheric lines is presented. A tentative explanatfon of photospheric llne
behavior emerges from a full atmospheric-model analvslb The interpretation
of the chromospheric Ca II H and K lines appears sr_alghtforward in terms of
plage.
PHOTOSPHERIC LINES
OBSERVATIONS
All dat_ used in this study were taken with the 13.5 m double-pass grating
spectrometer of the McMath Telescope on Kitt Peak. Details of the physical
instrument and reduction procedures are given elsewhere (refs. 4, 5, 6).
Table I lists the observed lines together with pertinent line parameters
such as excitation potential, Zeeman sensitivity, and temperature sensitivity.
Figure 1 gives examples of the run of eq1_ivalent width with time. Within the
noise band a linear fit adequately represents the data up to the present. The
5 year intercept of this fit, converted to fractional change, is tabulated in
the last column of table I.
What is the cause of the scatter in equivalent widths displayed in figure
I? If the scatter were solar in origin, say caused by facular line weakening
which at high resolutlon was discovered by Chapman and Sheeley (ref. 7) then
the weakening of different lines should correlate on a day-to-day basis. None
is found (ref. 8). We conclude this scatter is instrumental in origin, oe.rhaps
arising from several causes such as spectrograph misalignment, grating drive
screw error and at_ospherlc scintillation. Because these errors are all
stochastic in nature, with no known secular trend, we accept the scatter as
unavoidable with the present technique, but find _o reason to doubt the long
...... 96
1982009140-097
term changes. Note that the equivalent width of Si I 10827 has remained
conbtant (fig. 1).
INTERPRETATION
In figure 2 we explore a number of mechanisms to explain the ob_ _
tions, (ref. 8). First we evoke a step change in temperature through
line-forming region but immediately find a conflict in sJ:_ between ca_; "_
the other lines (fig. 2a). Microturbulance can affect t_ _I-ength of wea_
lines, but proves ineffectual here (fig. 2b). Likewise _eema_ broadening
alters llne strength but doesn't work either (fig. 2c). ._ must fit between
theory and observations follows from a 2.3% change in surface chemical abun-
dance (fig. 2d) but this is considered unacceptable in principle.
Having disposed of the more simple possibilities we turn to the fact t_mt
our various lines are formed at differing levels in an atmosphere which
possesses a temperature gradient. As shown by the respective contribution
functions, C 5380 is formed low, Fe 5250 high, while Si 10827 is common to
both regions (fig. 3). The positive thermal response of the carbon line can
be combined with the negative response of the iron lines (table I) if we intro-
duce a cooling of the low photosphere while heating up the higher layers. This
can be accomplished by a change in the ratio of mixing length tc pressure scale
height £/H from 2 to 2.3 (fig. 4). This initial value of £/H=2 is In accord "
with the recent evolutionary calculations for the sun by Mazzitelli. If w_
assume this 15% change in £/II is confined to the granulatlon layer, £he luml-
nosity effect is negligible (ref. 9).
CHROMOSPHERIC LINES
OBSERVATIONS
Figure 5 displays the essence of the variation seen in the calcium K pro-
file, which occurs in the line core primarily, if not wholly, and arises from
the contribution of plage over the visible disk. As described elsewhere
(ref. 6), we measure both the central intensity of the line (K. intensity) but
also, for comparison with stellar observers, the 1 A equivalen_ width (K-index).
The temporal behavior of K]-intensity and K-index are essentially the same,
figure 6, except the modulition of the K-index somewhat less than the former.
The proof that the Ca K-line variation is due to chromospheric plage is given
by the correlation of K1-intensity with Mr. Wilson plage area index, figure 7.
Similar results are fotu_d for Ca II 8542, and CN 3883, although the correla-
tion coefficient is less (ref. i0).
97
1982009140-098
CONCLUDING REMARKS
We have seen that over our 5 year time base photospheric lines have
weakened linearly with time %bile chromospheric indicators mimic the activity
cycle. Remaining unanswered are the following questions:
i. Is the photospheric variation truly global, or does it reflect un-
resol.ed elements of solar activity (faculae, for instance)?
2. Is the time scale of the photospheric variatio_ tied to the (22 y)
solar cycle or is it evidence of some secular change of tu%known dura-
tion?
3. In effect wc are observing a change in line-blanketing. Will this
affect solar irradiance?
REFERENCES
I. Livingston, W.; Milkey, R.; and Slaughter, C.: Solar Luminosity Variation
I. CI 5380 as a Temperature Indicator and a Search for Global Oscilla-
tions. Ap. J. 211, 1977, pp. 281.-287.
2. Livingston, W. C.: Cooling of the Sun's Photosphere Coincident with In-
creased Sunspot Activity. Nat. 272, 1978, pp. 340-341.
3. Hill, Henry A.; Livingston, William C._ and Caudell, Thomas, P.: Empirical
Sensitivity of the CI 5380 Line to Temperature Changes in Static and
Oscillating Systems. Ap. J. 214, 1977, pp. LI37-LI40.
4. Brault, J. W.; Slaughter, C. D.; Pierce, A. K.; and Aikens, R. S.: True
Intensities of Fraunhofer Lines. Solar Phys. 18, 1971, pp. 366-378.
5. Brault, J. W. and White, O. R.: The Analysis and Restoration of Astronomi-
cal Data via the Fast Fourier Transform. Astron. and Astrophys. 13, 197_
pp. 169-189.
6. White, 0. R. and Livingston, W.: Solar Luminosity Variation. II. Behavior
of Calcium H and K at Solar Minimum and the Onset of Cycle 21. Ap. J.
226, 1978, pp. 679-686.
7. Chapman, G. A. _d Sheeley, N. R., Jr.: The Photospherlc Network. Solar
Phys. 5, 1968, pp. 442-461.
8. Livingston, W. and Holweger, H.: Solar Luminosity Variation. IV. The
Photospheric Lines, 1976-1980, suh_,itted to Ap. J.
98
1982009140-099
!'
9. Mazzitelli, I.: Solar Models, Helium Content and Mixing Length. Astron.
and Astzophys. 79, 1979, pp. 25]-253.
10. White, O. R. and Livingston, W. C.: Solar Luminosity Variation. III.
Calcium K Variation from Solar Minimum to Maximum in Cycle 21, submitted
to Ap. J.
99
1982009140-100
• iI
Wavelength Width W EP Land_ AW/W* _W/W**
(A) (mA) (eV) _ K-X (%) obs. (%) !
Fe I 5249.1 42. 4.5 0.9 -0.07 -0.45 1
Fe I 5250.2 74. 0.I 3.0 -0.09 -0.50 ,_
Fe I 5250.6 108. 2.2 1.5 -0.05 -0.17
Fe I 5379.6 62. 3.7 1.0 -0.05 -0.60
C I 5380.3 22. 7.7 1.0 +0.12 -2.30
Ti II 5381.0 65. 1.6 0.9 -0.00 -0.71
Si I 10827.1 423. 4.9 1.5 -0.04 0.00
* Predicted from HSRA model
** Total change over 5 years
I00
1982009140-101
}
!
a) C[ 5580 &W/W=-25%
E
• • _ • ]_3.o :. : . . ";.
T /
% * "'' t /
E3 ?2.0
0
I.iJ
19"15 19"76 1977 ! 9"/8 1979 1980
Figure la. Observed equivalent width in mA over 4-5 year time
span: Cl 5380
lOl
i
1982009140-102
b) Tirr 5:381 &W/W=-071%
A
E
66.0
7-
"11 • • • •t--
S.. e" .." • "" t_'e • I_ I
_: • . ._,..,_ .to,, _ , _.; ;,..""" -.
64.0
0
W
LLL_.__L_J__U.___ I,' I L, II l, ,l L_ u_u.I ,, l, li l | l U.J_LU_l ,Lll ll.[
1975 19"16 1977 |978 1979 1960
Figure lb. Observed equivalent width in mA over 4-5 year time
span: Ti II 5381
I02
i
1982009140-103
c) SiI 10827 AW/W =0,0%
_' (Stand.Dev. =0.2%)
E
q30.
7-
I--
c__ 425, *•
• ; _
420.i
:E) •
0
ILl
Ll|lllllltllltititlllltllltllllillllllllllltltlltlllllllllilllllllllll|l
1975 1976 1977 1978 1979 1980
Figure Ic. Observed equivalent width in mA over 4-5 year time
span: Si I 10827
103
1982009140-104
2 (a)
-' [• -2 8TEFF - - 20K
-1
8 MICRO=-SOM/S
-2
_ 0.8t"(c)
L 6B=100G 9[_ a4,": tt I0 o 9 ?
-1
6 LOG N = -0.01
-2
0 c FE FETr*FE eE oSjL
-I
-2
q, OBSERVED
• * i m i m J J * I I I • I I
0 20 40 60 80 I00 "400SO0
w(m_)
Figure 2. Predicted _W/W (t) compared with observed
AW/W (t) for differing mechanisms: a) a step change
of temperature, b) a change of microturbulence, c)
introduction of a 100-gauss uniform magnetic field,
and d) a 2.3t reduction in the abundance of observed
species, and (bottom) the observations.
104
1982009140-105
105
1982009140-106
o- , ..... _ _ ,,_ ,(o)
"' I
I
-2 I
I
o 51I/H) =0.3
0 i
8 (I/H): 1
(c) a
-1
-2
t 6 (VH)= 0.3-3 (_ MODIFIED
C FE FETI" FE o, FE 51
o ' ° o_
-1
- 2 _ OBSERVED
...... * , _ .I , , _1 I I
0 20 /,0 60 80 100 /,00 500
W(m}_)
Figure 4. Equivalent width change in percent due to
an increase of t/H a) from 2.0 to 2.3, b) 2 to 3, c)
same as (a) but corresponding AT(Ts_nn) shifted on T
scale by factor of two towards latter-(squares), or
smalle_ (circles) depths
106
1982009140-107
I_
Q) Full Ditk
C,4H K Minln_m vS. Idottmum Activlly
b) Ploge vs QuHIt Svn
Co][ K
\/
Figure 5. The variability of calcium K profiles: a) full disk K
at minimum and maximum superposed and b) average profiles for an
active region (plage) and a n_arby quiet region at the same limb
distance.
107
i
1982009140-108
108
1982009140-109
! I ! • "
o) Full Disk
Ploge vs. K3 Intensity " ,." •
u,O. a •
eo •
O • •
20• •o •
.
• )0 Q •
0,
I I
.057 .063 ,069
K3 Intensity
Figure 7. Correlation between K3-ir,tensity and the Mr. Wilson
plage area index
109
1982009140-110


Observed variability in the Fraunhofer line spectrum of solar flux, 1975 - 1980

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