Individual K-line profiles from elements of fine structure on the surface of the sun are calibrated absolutely. The continuum calibrations of Labs and Neckel and of Houtgast and Namba are considered, and the average K-profile is scaled to that of White and Suemoto. The ranges of intensities across a high-resolution spectrogram are tabulated for various parts of the line profile. Although the spatially-averaged value for K3 of 4.2% of the continuum corresponds to a brightness temperature of 4155 deg K, minimum and maximum values were 3980 and 4360 K, respectively. Similarly, K2v ranges from 4200 to 4560 K, and K2r from 4180 to 4460 K in small elements about 1 arc sec across

Pasachoff, J., M.

English

NASA Technical Reports Server

BIG BEAR
SOLAR OBSERVATORY
C A L I F O R N I A I N S T I T U T E OF T E C H N O L O G Y
ABSOLUTE INTENSITY CALIBRATIONS OF
SOLAR K LINE PROFILES.
By
Jay M. Pasachoff
x"
COPFILE
OPERATED BY
HALE OBSERVATORIES
https://ntrs.nasa.gov/search.jsp?R=19720006109 2020-03-11T21:48:39+00:00Z
ABSOLUTE INTENSITY CALIBRATIONS OF
SOLAR K LINE PROFILES.
By
Jay M. Pasachoff
BIG BEAR SOLAR OBSERVATORY,, HALE OBSERVATORIES
CALIFORNIA INSTITUTE OF TECHNOLOGY
CARNEGIE INSTITUTE OF WASHINGTON
Received
ABSTRACT
Individual K-line profiles from elements of fine
structure on the surface of the sun are calibrated
absolutely. The continuum calibrations of Labs and
Neckel and of Houtgast and Namba are considered, and
the average K-profile is scaled to that of White and
Suemoto.
The ranges of intensities across a high-resolution
spectrogram are tabulated for various parts of the line
profile. Although the spatially-averaged value for K~
of 4.2% of the continuum corresponds to a brightness
temperature of 4155°Kj minimum and maximum values were 3980
and 4360°Kj respectively. Similarly, K~ ranges from 4200
to 4560°K,, and K2r from 4180 to 4460°K in small elements
about 1 arc sec across.
-2-
I. INTRODUCTION
Studies of the surface of the sun often suffer from
the sheer volume of data available. Yet as the only star
on which many surface details can be observed, it is
important to understand this single accessible example
of the stars beyond. Even with the sun's great apparent
brightness, current spectral observations expand both
spatial and spectral dimensions until we are again limited
by the amount of available light. Thus to gain precision
in measuring the fluxes in different spectral lines, spatial
information is often sacrificed by averaging over large areas
A number of studies, including my own (Pasachoff, 1969a,
I969b, 1970), show that the fluctuations in the intensities
and shapes of line profiles of elements of fine structure
are so substantial that they cannot be ignored. My quanti-
tative reduction was performed on spectrograms obtained
with the McMath Solar Telescope at the Kitt Peak National
Observatory by N. R. Sheeley. The 242 spectral profiles
of individual elements of the sun were averaged to give a
mean K-line profile which I scaled to match the photo-
electric double-pass profile of White and Suemoto (1968).
Their work is" presented with respect to a calculated con-
tinuum value in that spectral region. The establishment
of an absolute intensity scale for the mean profiles and
for the line profiles for fine-structure elements is the
subject of this paper. Such absolute values are important
-3-
for understanding the temperature structure of the sun
and as a parameter in stellar atmospheres calculations.
Previous research on the formation of the K-line has
recently been reviewed by Linsky and Avrett (1970) .
We shall use the following notation, slightly modified
from that of Hale and Ellerman (1903) , for the central
parts of the K line. K., is the central dip; K2 and K~
are the small peaks on the violet and red sides of the
line, respectively, and K, and K, are the dips just
shortward of K_ and longward of K_ , respectively. The
line wings, which extend for dozens of Angstroms, are
often referred to in the literature as K,, the notation
we have used for the dips. Here the wings have no
specific names, although they would logically be called
II. CALIBRATION OF THE ABSOLUTE INTENSITY SCALE
It is difficult to measure directly the absolute
value of the flux in the centers of the H and K lines
because of the uncertainties in the transmissions of
the earth's atmosphere and in the instrumentation. Further,
no satisfactory continuum exists nearby in the spectrum
with which one could compare. The problem is further
complicated by the relative importance of scattered light,
for 1 per cent of the continuum is equivalent to 25 per
—4—
cent of the central intensity of the K line.
Only recently have we fully appreciated the impor-
tance of the variations in instrumental and atmospheric
sensitivity through this region of the ultraviolet
spectrum (Linsky, 1968; Pasachoff, Noyes and Beckers,
1968; White and Suemoto, 1968; Pasachoff 1969b). Thus
the basic work of Goldberg, Mohler and Muller (1959,
revised by Teske [1967]), which was calibrated on the
4000 A window alone, has been superseded.
The White and Suemoto calibration is based on the
work of Michard (1950), which indicates that the observed
continuum near 3900 A is depressed by 15 per cent, and the
work of Beckers (1962), which indicates that the observed
continuum near 3953 A is depressed by 16.3 per cent. A
smooth curve is drawn through these corrected points and
through supposed windows between 4000 A and 4050 I.
Carbon, Gingerich and Kurucz (1968) have found that the
4000 A window (actually 3999.89 A) has a contribution
from line blanketing less than 1 per cent. White and
Suemoto make a consistency check of this continuum
that is good to about 5 per cent.
Zirker (1968) accepts the uncorrected intensity peaks
at 3909 A and 4013 A as a continuum, and draws a straight
line between them on the tracings to represent a continuum
level with which to compare the intensity at each point.
-5-
He assigns a color temperature of 6500°K to this con-
tinuum, fixed at the 4013 A point (Zirker, 1969), to give
absolute intensities. The error at 3909 A is small,
A log I = 0.013.
Zirker compares his values with those of Houtgast
(1965, 1968) and finds that
A flog T\ ] ' =0.052
I4013 4 Houtgast-Zirker, maximum
and is about 0.02 near the centers of the H and K lines.
Figure 1 shows the trend of the disparity between the
measures of Houtgast and Zirker. Log (IHoutgast/IZirker)
varies roughly linearly from -0.05 at 3900 I to +0.01 at
4000 A, i.e. (IHoutgast/Izirker) = 0.891 at 3900 A and
1.025 at 4000 A, a variation of over 10 per cent. Zirker
finds agreement between his I/IC values and those of White
and Suemoto for (_i < 0.7, but tney have used different
continua.
Linsky (1968) finds that I(K3)/I(H3) = 1.061 and
1.041 for his two observing days, compared with White and
Suemoto's value of 1.00 and Zirker's value of 1.075.
Linsky accepts the observed continuum at 4000 A as a
window, and follows typical values from his model atmos-
pheres in assigning a drop of 2.2 per cent in the con-
tinuum to 3968 I, and 4.4 per cent from 4000 & to 3933 A.
His results for the absolute values of the intensities
of the cores at u, = 1 are 4.34 per cent and 4.09 per
-6-
cent of the continuum for H_ and K., respectively.
The Utrecht Atlas (Minnaert, Mulders and Houtgast,
1940) presents tracings of the H and K lines with respect
to a value, not necessarily the continuum, set equal to
100. This 100-level is merely approximately set to
provide full-scale deflections for the lines in a mod-
erate spectral region. The value for the center of the
K line is about 6.5 on this scale.
There is undoubtedly a major contribution from stray
light in the spectra traced for the Utrecht Atlas, and it
is unfortunate that we must refer to it for intensity
information in the cores of strong lines. Since it is a
standard reference, however, several studies of the con-
tinuum in this region have referred to it.
Recent contributions to the absolute calibration of
the continuum have been made by Labs and Neckel (1968)
and Houtgast and Namba (1968). They use their absolute
intensity data to calibrate each tracing in the Utrecht
Atlas.
Labs and Neckel feel that their measured continuum
intensities agree with recent model continua above 4000 A
to within 2 per cent, but are more reticent about values
below 4000 A. They estimate from the scatter that the
error for" the calibration of the tracings in the Utrecht
Atlas is about 3 to 4 per cent from 3600 A to 4200 &. The
points of Labs and Neckel and their adopted curve lie
-7-
slightly below those of Houtgast (1965) from 3400 I to
3500 A and slightly above (averaging perhaps 0.02 in the
logarithm) from 3600 A to 4100 A.
Labs and Neckel give the intensity value, in w cm
ster~ A , for Atlas level 100 (which is not necessarily
the theoretical continuum level). A 4 per cent error in
the 100-level would mean about an 0.25 per cent error at
the center of K. Since the values from the Utrecht Atlas
are severely affected by scattered light at the center of
the K line, we used the following procedure to calibrate
the data described in this paper:
For many wavelengths between 3900 A and 4000 A, we
computed the absolute intensity as the product of the
value we measured in the Utrecht Atlas as a fraction of
the 100-level and the absolute value for the 100-level
interpolated in the values of Labs and Neckel. This
should be equal to the residual intensity given by White
and Suemoto times the absolute value of their continuum.
That is:
100
aUtrecht L&N; Utrecht ~ rW&S W&S'
GW_ is graphed in Figure 2. Discarding the points near
the centers of H and K where stray light is a major con-
tributor, we adopt the value C-. _ = (4.5 +_ 0.1) x 10° erg
— 2 — 1 — 1 — 1 "
cm sec star A for the continuum. This agrees, within
-8-
our limits of error, with the continuum value deduced by
Houtgast and Namba (1968) .
The central intensity of White and Suemoto, rv = 4.2i\
per cent, then corresponds to the absolute value 1.9 x 10
—2 — 1 — 1 — 1
erg cm" sec ster I . The rest of the profile can be
similarly calibrated. We use the White and Suemoto values
for the line profiles, as the Utrecht Atlas values near line
center are greatly affected by scattered light.
We can assign brightness temperatures, T,, by the
—2 —1 — 1 o — T
Planck function in units of erg cm sec ster ft" ,
2
 1 _ = 1.27 x 109 _
exp(hc/AkTb)-l exp (3 .66xl04/T)
(1) B d = 2 he   1.27 x 10 _ 1
The scale is shown on the right-hand axis of Figure 3, which
shows our mean profile scaled to match that of White and
Suemoto. The average value of K, corresponds to (4155 +_ 25) °K.
In this analysis, we were forced to go through the
extra steps of referring to the continuum and to the Utrecht
Atlas only because other works refer to them. It would be
useful if observers rather than or in addition to referring
to a continuum, would refer to the intensity of some identifi-
able peak in the wavelength range under consideration. This
might be the window at 3999.9 A., or even the lower peak at
3950 A. New studies could thus draw on earlier work more
readily.
-9-
III. INTENSITY FLUCTUATIONS
We have presented graphs of the fluctuations in inten-
sity (Pasachoff, 1970) for the various K,. Those graphs
covered only half the data summarized in Table I, which
shows the ranges of values. Equation 1 has been used to
translate those values into equivalent black body temper-
atures. Thus we see that the value of the K_ minimum, some-
times quoted to hundredths of a per cent, actually varies
in our data from 2.8 per cent up to 6.2 per cent, equivalent
to a variation of the equivalent black body temperature of
380°K from 3980°K to 4360°K.
Similarly, K-2v varies from 4200°K to 4560°K, and K2r
from 4180°K to 4460°K. As we have previously discussed in
detail, all these fluctuations are not well correlated.
Theoretical calculations of stellar atmospheres that
compute absolute intensity values and try to deduce
physically-meaningful temperatures in the region of the
temperature minimum must take these fluctuations into
account.
ACKNOWLEDGEMENTS
I am continually indebted to Dr. Neil R. Sheeley for
permitting me to study his excellent data. I have had
many useful conversations with Dr. Eugene H. Avrett. Work
with Dr. Robert W. Noyes and Jacques M. Beckers, and dis-
cussions with Dr. Harold Zirin led to my involvement in
-10-
this aspect of the field.
I received assistance with photographic, artistic and
computing facilities from the Smithsonian Astrophysical Ob-
servatory. Mr. Robert Gardner of the Carpenter Center for
the Visual Arts of Harvard University made it possible for
me to produce an animation of the line profiles that was
of great use in the data reduction.
This research was largely carried out while I was at
the Harvard College Observatory and at the Radio Astronomy
Branch of the Air Force Cambridge Research Laboratories.
I thank Dr. Jules Aarons for his support. This work was
A/4-J-
also partially supported by NASA Grant No. 05-002-034
and NSF Grant GA-24015.
REFERENCES
Beckers, J. M. : 1962, Bull. Astron. I_nst. Netherlands 16,
No. 514, 133.
Carbon, D., Gingerich, O. and Kurucz, R. L.: 1968, Sjolar
Phys. 3_, 55.
Goldberg, L., Mohler, 0. and Muller, E.: 1959, Astrophys. J_.
129, 119.
Hale, G. E. and Ellerman, F.: 1903, Publ. Yerke_s_ Obs_. 3_, 1.
Houtgast, J.: 1955, Proceed. Kon. Netherlands AkacL van Weten.,
AmsterdajTL, Series B, 68, 306 .
-11-
Houtgast, J.: 1968, Solar Phys. 3_, 47.
Houtgast, J. and Namba, O.: 1968, Bu_l_l. Astron. Inst.
Netherlands. 20, 87.
Labs, D. and Neckel, H. : 1968, Z_e_it. fur Astrophys. 6_9_, 1
Linsky, J. L;: 1969, Thesis, Harvard Univ.; Smithsonian
Astrophys. Obs. Special Report. 274.
Linsky, J. L. and Avrett, E. H. : 1970, Publ. Astron. Soc.
Pacific, 82_, 169.
Michard, R.: 1950, Bull.Astron. Inst. Netherlands 11, 227
Minnaert, M., Mulders, G. F. W. and Houtgast, J.: 1940,
Photometric Atlas of the Solar Spectrum (Utrecht,
Sterrewacht "Sonnenborgh ") .
Pasachoff, J. M.:1969a, Bull. Am. Astron. S_oc. !_, 289.
Pasachoff, J. M.:1969b, Thesis, Harvard University.
Pasachoff, J. M. : 1970, Solar Phys. L2_, 202.
Pasachoff, J. M., Noyes, R. W. and Beckers, J. M.: 1968,
Sol_ar Plvy_s_. £, 131.
Teske, R. G.: 1967, Publ. Astron. Soc. Pacific 79, 110.
White, O. R. and Suemoto, Z.: 1968, SjxLar Phys. 3_, 523.
Zirker, J.: 1968, Solar Phy^s. _3_, 164.
Zirker, J.: 1969, private communication.
-12-
TABLE I
Fluctuations in the K line.
Low High Rms Range
Deviation
Tb(°K) I(°/o) Tb(°K)
Klv 4'2 416° 7.5 4460 0.59 300
4.5 4200 9.1 4560 0.60 360
2.8 3980 6.2 4360 0.49 380
K2r 4.3 4180 7.5 4460 0.58 280
3.9 4130 6.9 4410 0.56 280
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Absolute intensity calibrations of solar K line profiles

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