Spectroheliograms taken in the light of He I 10830 A at the National Solar Observatory Vacuum Telescope on Kitt Peak were used to identify coronal holes and bright points (BPs). Target points were identified, coordinates calculated, and spectra recorded. For each spectrum, the difference in wavelength between the Ca II K minimum and the FeI reference line was calculated. It was noteworthy that the overall effect is a blueshift. It should be noted that if material of chromospheric density moves outward at this velocity, it could supply the mass flux of the solar wind if this chromospheric flow was concentrated in a few dozen sources, each of a diameter of a few arc seconds

Holt, Rush D.

Mullan, D. M.

Park, Albert H.

Thompson, Joseph C.

English

NASA Technical Reports Server

N87:2092 )
OBSERVATIONS OF THE Ca II K LINE
IN HeI0830A DARK POINTS ON AUGUST 3, 1985
Rush D. Holt, Albert H. Park,
Joseph C. Thompson
Swarthmore College
Swarthmore, Pennsylvania 19081
D. M. Mullan
Bartol Research Foundation
Newark, Delaware 19716
INTRODUCTION
In this paper we address the question: Is the velocity field of the
solar chromosphere at the location of bright points (BP) different
from that of the chromosphere in other regions of the sun? We have
used dark points in HeI0830A spectroheliograms as ground-based proxy
indicators of BP both in coronal holes and non-hole regions of the
Sun, and we have obtained spectra of such points in the vicinity of
the CaII K 3933A line. As a measure of differential chromospheric
velocity, we have used the shift of the K line center relative to a
nearby photospheric FeI line. We presume the K line center to be
formed high in the chromosphere. We find that in helium dark points
in general, the distribution of K line shifts is skewed towards the
blue; in helium dark points which lie within coronal holes, the
blueward skewing is more pronounced. Our results are of some
interest in the context of spacecraft detections of outflows from
BPs, and in the context of correlations of high speed solar wind
streams from coronal holes.
On August 3, 1985 we observed five He-10830A dark points. Taken all
together, the spectra of the five points that day show an average
blue shift of 0.5 ±.3km/s with respect to the spectra from random
points on the Sun. Four of the five points showed blue shifts of
approximately 1 km/s, and the fifth point (observed late in the day
and less reliable than the others) showed a red shift of
approximately 1 km/s. The CaII K spectra on August 3 show a pattern
of blue shifts that is consistent with the overall pattern observed
in helium dark points observed on 25 days over a 14-month period
(Holt and Mullan, 1986). The overall pattern for the fourteen months
shows a small blue shift between 0.i and 1.2 km/s for average spectra
from He I0830A dark points which are not within coronal holes. There
is also greater dispersion of the wavelength shifts than for spectra
from random points. The points on Aug 3 were not within coronal
holes. For dark points within coronal holes a fraction of the
spectra, one-tenth, show a more pronounced blueshift--about 3.7 km/s.
DATA ACQUISITION: TARGET IDENTIFICATION
To identify coronal holes and BP, we use spectroheliograms taken in
the light of HeI I0830A at the National Solar Observatory Vacuum
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Telescope on Kitt Peak (Livingston et al, 1976). The He I0830A image
is relayed via telephone multiplexer (Colorado Video Expander 275) to
the Swarthmore/Bartol Observatory, where the image is recorded
digitally and also displayed on a video monitor. The resolution of
the digital image is approximately 8 arcseconds per pixel.
On the He I0830A images, small dark points frequently are observed.
Some lie within the coronal holes but their distribution is not
obviously biased towards or away from coronal holes. During the
Skylab mission, it was found that there was a close correlation
between the HeI0830A dark points and BP (Harvey et al, 1974; Harvey
and Sheeley, 1977). If Zirin's (1975) explanation of radiative
pumping of the triplet level of HeI of weak He10830 absorption in
coronal holes is correct, it would provide a natural explanation for
enhanced absorption in HeI0830A in the chromosphere underneath a BP.
However, even without knowing the details of the physical process, we
adopt the empirical correlation (albeit imperfect) and assume that
HeI0830A dark points are proxy indicators of BP. We use the He10830
images to identify not only BPs, but also to identify the coronal
holes on any particular day.
The spectroheliograms are recorded once per day at Kitt Peak National
Observatory, conditions permitting. Each spectroheliogram requires
45 minutes to complete. We receive the image within minutes after
its completion. We identify the target points, calculate
coordinates, and begin recording spectra usually within 30 minutes.
The mean lifetimes of BPs are on the order of hours (Golub et al,
1977). The dark points on the HeI 10830 spectroheliograms have
comparable lifetimes. On August 3, 1985 clouds delayed our
observations for nearly three hours after we received the
spectroheliogram. Spectra were recorded between 19:44 and 23:10 UT.
For each spectroheliogram, we find the center of the solar disk by
triangulation from points on the circumference. We then locate our
target points and determine their positions with respect to the
center. Calculating from the solar tilt angles for each day, we then
account for solar rotation at the latitude of each point during the
time elapsed since that portion of the spectroheliogram was recorded.
After scaling the image coordinates to match our telescope image
guider, we position each point of interest on the entrance pinhole of
our spectrometer. Our procedure for positioning the targets is
uncertain by an amount which may be as large as about ' i0 arcsec.
Most of the He 10830 dark points are of order 10-30 arcsec in size.
We do not know whether or not the associated features in CaII K light
would have the same size.
DATA ACQUISITION: CaK SPECTRA
The telescope is a 61-cm Cassegrain reflector mounted in a fixed
equatorial position in a laboratory on top of a 50-foot building on
the campus of Swarthmore College in Pennsylvania. The telescope is
fed by a 91-cm polar heliostat mounted on the roof. This system
provides a full solar image with a scale of 16 arcsec/mm, and the
_ 378
stepping motors permit guiding movements as small as 0.i arcsec in
right ascension and 1 arcsec in declination.
The telescope feeds a high resolution spectrum scanner (Wyller and
Fay, 1972). For our observations, the spectrometer is used in the
Czerny-Turner configuration, and the echelle grating is rotated by a
stepping motor so as to sweep the spectrum past an exit slit, where
it is detected by a cooled photomultiplier tube (EMI 9789A). The
spectrometer entrance pinhole (0.I mm) is chosen to allow a spatial
resolution of 1.6 arcsec on the solar surface. However, local seeing
conditions usually degrade the resolution to worse than 2 arcsec.
The exit slit size (0.007 mm) is chosen so as to allow spectral line
positions to be determined to approximately 0.02 A.
The spectrum is sampled by a PAR-SSR photon counter driven by a
computer which also drives the grating motor. The spectrum is
sampled at typically i00 points with an integration time of either
0.I or 0.2 seconds per point, depending on light levels. The
spectral scan covers about 2A in the vicinity of CaK (3933A). The
FeI line at 3932.64A serves as a photospheric reference line, with
respect to which we measure the position of the central absorption
(K3) of the CaK line. In addition to spectra of BP candidates, we
also record spectra in the same wavelength region of random points on
the solar disk. These "random disk points" provide the references to
which we compare the wavelength separations of the FeI and CaK lines
of BP candidates. The random disk points are generally chosen to be
away from regions of strong magnetic activity (as determined from
daily magnetograms, also received from Kitt Peak). Preceding and
following the recording of the spectra of each target point, we
record a number of spectra, usually 6 or more, at random points on
the solar disk.
DISCUSSION
From each spectrum we calculate the difference in wavelength between
the CaII K minimum and FeI reference line. This wavelength
separation is compared to the average of the separations in five
random-point spectra taken most recently before and the next five
random-point spectra taken after the target point spectrum. If the
separation is smaller than this 10-point average, we say that the K
minimum has been shifted to the blue with respect to the spectrum of
a random point. Conversely, we take a larger separation to be
evidence of red shift.
We recorded 47 spectra of BP on August 3, 1985 (Table I.). The
histogram of the wavelength shifts shows a small average wavelength
shift of 0.5 ± 0.3 km/s and also an asymmetry toward the blue. Four
of the five points have spectra showing blue shifts of approximately
Ikm/s, which is between 1 and 3 standard deviations for the four
points. (A different o is calculated for the reference random point
spectra for each BP spectrum.) The fifth point shows a red shift of
about ikm/s which is greater than 20 from the mean of zero. This
fifth point was taken late in the
379
Table 1.
CaK Spectra of He 10830A Solar Dark Points Aug 3, 1985 Swarthmore
Dark Point Location Time (UT) Shift
A N5.3E14.7
B N18.1E20.2
C N10.4E14.2
D N14.1W21
E $5.2Wl1.1
19:44-19:56
22:44-23:10
20:11-20:19
20:34-20:44
20:57-21:14
21:31-21:54
Ii spectra
spectra
spectra
spectra
10 spectra)11 spectra)
-.8km/sec+.3km/sec
-l+lkm/sec
-1 km/sec+ 1km/sec
-1.0-+.8km/sec
+l+.4km/sec
o
(=6
uJ_-
N
N
o'
CORONAL HOLES
TWO GAUSS F]T
3.2q 20.20 20. 16 -O.12 -'0. Oe -'0. oq -'0,00
0ELTR WAVELENGTH IA]
O'.Oq O'. OIB 0.12 0'.16
Fig. la. Wavelength shift of spectra of BP in coronal holes, relative to
spectra of random points
_=
a-O. 2q
DARK POINTS
NON-COR.HOLE
ITWO CRU55 F T
' oo. o ooooo. oo.
-0,20 -'0, 0'.12 _.16
QELTR HRVELENGTH [RI
Fig. lb. Wave length shift of spectra of BP not in coronal holes relative
to spectra of random points. Superimposed is the histogram of the
wavelengths from BP not in coronal holes observed Aug. 3, 1985
380
day, more than five and a half hours after the NSO I0830A
spectroheliograms had been made, and it is less likely than the other
points to have been a dark point.
In Fig.la we show a two-gaussian curve fitted to the histogram of the
wavelength shifts of the K minima of spectra taken of BP in coronal
holes during the full 14-month observing program. In Fig. ib we show
a two gaussian fit to the histogram of wavelength shifts for BP not
in coronal holes, and along with the smooth curve, we also show a
histogram of K minima shifts for 47 spectra taken on August 3, 1985.
The skewness in the distributions is small. We are operating close
to the limit of resolution of our system. However, it is
noteworthy that the overall effect is a blueshift, especially in the
BP's in coronal holes. For BP's not in coronal holes, like the
samples of August 3, the blueshifts are less distinct. For BP's in
coronal holes the amplitude of the blueshift is several kilometers
per second. One should note that if material of chromospheric density
moves outward at this velocity, it could supply the mass flux of the
solar wind if this chromospheric flow were concentrated in a few
dozen sources, each of diameter of a few arcsec.
ACKNOWLEDGMENTS
This work is supported in part by the National Science Foundation
(ATM-82-10936) and by the Research Support Committee and the Lang
Faculty Fellowship of Swarthmore College.
REFERENCES
Gollub,L., Krieger,A.S., Vaiana, G.S., and Harvey, J.W., 1977,
"Magnetic Properties of X-Ray Bright Points," Solar Physics,
53, III
Harvey, J.W., , Krieger, A.S., Timothy, A.F., and Vaiana,G.S.,
1974, "Comparison of Skylab X-Ray and Ground-Based Helium
Observations," Observ. Mem. Arcetri, 104, 50
Harvey, J.W., and Sheeley, N.R., 1977, "Comparison of HeII 304A and
HeII0830A Spectroheliograms," Solar Physics, 54, 343
Holt, Rush D., and D.M. Mullan, 1986, "Observations of the CaII K
Line in HeI0830A Dark Points," submitted to Solar Physics
Livingston, W.C., Harvey, J.W., Slaughter, C.D., and Trumbo, D.,
1976,"Solar Magnetograph Employing Integrated Diode Arrays,"
Applied Optics, 15, 40
Wyller, A., and Fay, T.d., 1972, "URSIES: An Ultravariable
Resolution Single Interferometer Echelle Scanner," Applied
Optics, 11, 1152
Zirin, H., 1975, "The Helium Chromosphere, Coronal Holes, and Stellar
X-Rays," Ap.J. Lett., 199, L63
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Observations of the Ca II K line in Hel0830A dark points on August 3, 1985

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