Coronal structure for solar eclipse of September 22, 196

Schatten, K. H.

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PREDICTION OF THE CORONAL STRUCTURE
FOR THE SOLAR ECLIPSE OF SEPTEMBER 22, 1 968
Kenneth Ti. Schatten
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(NASA CR OR TMX OR AD NUMBER)
(THRU)
(CO/DE)
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(CATEGORY)
Series 9, Issue 56
September- 30, 1968
UNIVERSITY OF CALIFORNIA, BERKELEY
iPREDICTION OF THE CORONAL STRUCTURE
FOR THE SOLAR ECLIPSE OF SEPTEMBER 22 9 1968
Kenneth H. Schatten
Technical Report
ONR Contract Nonr 3656(26), Project NR 021 101
NASA Grants NsG 243 and NGR 05-003-230
NSF Grant GA 1310
Distribution of this document is unlimited.
Reproduction in whole or in part is permitted for
any purpose of the United States Government.
*Submitted to Nature
Prediction of the Coronal Structure
for the Solar eclipse of September 22, 1968
f
Kenneth H. Schatten
.x
Space Sciences Laboratory
University of California
Berkeley, California
A method of determining the large-scale structure of the coronal and
interplanetary magnetic fields has recently been described by Schatten et
all . The model originated as an attempt to explain the observed large-
scale structure of the interplanetary magnetic field. The model has been
partially validated by successful comparisons of the observed intexplane.,
tary magnetic field with a field computed from photospheri.c observations
according to this formalism. Further evidence for support of the model
k
`,,as been established by Schatten 2
 by comparisons of the computed coronal
magnetic structure with coronal green line (7^ 5303) emission. The model
f
has been further investigated by comparisons with the coronal structures J,
revealed in eclipse photographs, and this process has suggested the pos-
sibility to make a prediction of the coronal structures to be expected at
the solar eclipse of September 22, 1968.
The model consists of a, "source surface", which in first approximation
is a sphere concentric with the sun. Between the photosphere and this
sphere a potential solution to Maxwell's equation is assumed to govern the
large-scale magnetic field. Near the source surface, 0.6 solar radii above
the photosphere, the transverse magnetic energy density is superseded by
2.
the plasma energy density and thus this solution becomes invalid. The
magnetic field above this region its "stretched" by the radial expansion
of the coronal plasma. It is this coronal plasma that develops into the
solar wind, transporting the imbedded source surface magnetic field with
it. Currents on the source surface are computed to approximate this effect.
'T'hese currents and those necessary to represent the observed photospheric
magnetic fields (Zeeman effect) comprise the sources of the magnetic field
in this potential solution.
The magnetic field is assumed to structure the coronal plasma in the
region between the source surface and the photosphere. This results in
magnetic loops or closed regions within the inner corona, predominantly
above small regions of opposing photospheric magnetic field polarity. Uni-
polar Magnetic Regions, observed in the photosphere, result in open field
line configurations within the corona,. The model calculates the effect of
this structuring in the potential solution. It has proved successful in
describing the transition from small scale features observed in the photo-
sphere to the much larger scale features observed in interplanetary space,
and in comparisons with coronal green line intensities.
A further evaluation of the model can be made using the coronal struc-
ture revealed in eclipse photographs. Figure la shows a photograph of the
November 12, 1966 eclipse obtained by S. Smith of NASA-Ames Research Center
using a radial transmission filter with a 3.3 second exposure time. The
radial transmission filter compensates for the large variation in brightness
present in the corona. This allows a single photograph to exhibit structure
throughout a large portion of the corona.._
Three large prominences near the solar limbs are indicated by the
letters F. Above these prominences are helmet streamers. A coronal
3.
condensation is indicated by the letter C in this figure. The ray structure
above the condensation is sometimes referred to as a coronal fan. Some polar
plumes are visible in the polar regions of the photograph. The general
	
t=
	 appearance of the photograph is that of an inactive corona.
The corona is an optically thin, three-dimensional object and thus to
describe its appearance, one must project all of its features into the
plane of the sky. Figure lb is a composite drawing made from many commuter
plots of the coronal magnetic field structure on November 12, 1966. It was
made without prior reference to a coronal photograph from this eclipse.
Figure 2 shows a typical computer plot of a cross-section of the magnetic
field in the inner corona projected into the plane of the sky. The arrows
indicate the direction of the magnetic field and the spacing between the
arrows is inversely proportional to field strength. Figure lb was ,drawn
from about 10 such plots.
Closed loop structures in Figure lb appear on the east limb of the
drawing in the same region where coronal arches are seen in the photograph.
In the north-east quadrant, open field lines correspond to open coronal
rays. In the south-west quadrant, the helmet streamer appears to be in
agreement with the drawing. The coronal condensation and fan just north-
ward of the equator on the west limb correspond to diverging field lines
seen in the drawing. Magnetic loops northward of this feature also agree
with coronal arches. General features appear to be quite similar between
t
Figures la and lb. It should be kept in mind that coronal photographs
	
4'	 show the appearance of the corona rather than the structure. The structure
present can only be ascertained by examining the photograph very carefully.
I
i
Isophotes, lines of equal intensity, should not be identified with structure,
but rather with large-scale density variations.
4.
Figure 3 is a prediction made on September 20 of the structure of the
September 22, 1968 eclipse of the sun (which will be total over the U.S.S.R.
and a small portion of western China). Observations of the photospheric
magnetic field were unavailable during most of the month preceding the time
of this prediction due to malfunctioning of the Mount Wilson Observatory
magnetograph guiding system. Thus good magnetic observations for the limbs
of the sun on September 22 were unavailable. Utilizing magnetograms avail-
able from the previous solar rotation, and spectroheliograms of the calcium
K2 line and the Ha line, estimates of the photospheric field strengths and
directions at the time of the eclipse could be made. The coronal structure
in Figure 3 appears more complex than structure evident in the November 12,
1966 eclipse due to the greater solar activity at the present time.
Acknowledgements
I am appreciative of helpful discussions with John Wilcox, Sheldon
Smith and Robert Howard. I would also like to kindly thank Robert Howard
for the great deal of effort necessary to prepare the solar magnetograms.
I am also appreciative of the aid in reducing the spectroheliograms afforded
to me by Jan Olof Stenflo. I would also like to thank Robert Cannon for
help in data digitization and Jean Eilek for her artistic efforts in drawing
Figure lb.
I would also like to acknowledge support from a National. Science Founda-
tion graduate fellowship. This research was supported in part by the Office
of Naval. Research under contract Nonr 3656(26), and by the National Aeronautics
/VC A-Os=003-Of Z_
and Space Administration under granst -NsG 24) and NGR 05-003-230, and by the
National Science Foundation under grant GA 1319.
i
References
1. Schatten, K. FL, Wilcox, J. M. and Ness, N. F., SolarPhysics in press,
1968).
2. Schstten, K. H., Ph.D. Thesis, Department of Physics, University of
California, Berkeley (1968 .
Figure Captions
Figure la.	 Photograph of the corona of November 12, 1966 by S. Smith,
NASA-Ames Research Center,	 b. (bottom) Drawing of the magnetic struc-
ture of the corona according to the formalism outlined.
Figure 2.	 Magnetic field structure in one cross-section of the corona
computed from the photosphere to the source surface and projected into the
plane of the sky.
Figure 3.	 Prediction of the coronal structure for the September 22 9 19f
solar eclipse.
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4	 Figure 2
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Figure 3
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3. REPORT TITLE
PRE)ICTION OF THL COROUL 	 UCTURR FOR THE SOLAR ECLIPSE CW MMMM 22 1, 1968
4. DESCRIPTIVE NOTES (Typo of rape r. Out Inclusive dates)
Technical Report
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Schatten, Kenneth H.
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13• ABSTRACT
The coronal structure for the solar eclipse of September 22, 196
has been predicted utilizing a recent model for large-scale structure
within the inner corona.
	 Supporting evidence for the model has come
from comparisons With interplanetary magnetic field observations, coronal
green line emission and comparisons of coronal structures evident in
previous eclipses. (U)
FORM
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Prediction of the coronal structure for the solar eclipse of September 22, 1968

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