For several months in early 1965 the sun appeared to have large scale magnetic field directed outwards at nearly all latitudes and longitudes. Several independent observations that lead to this puzzling situation are discussed

Wilcox, J. M.

English

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SUIPR Report No. 469
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May 1972
Office of Naval Research
Contract N00014-67-A-0112-0068
National Aeronautics and Space Administration
Grant NCR 05-020-559
and
National Science Foundation
Grant GA-31138
INSTITUTE FOR PLASMA RESEARCH
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https://ntrs.nasa.gov/search.jsp?R=19720020184 2020-03-11T18:25:55+00:00Z
WHY DOES THE SUN SOMETIMES LOOK LIKE A MAGNETIC MONOPOLE?
by
John M. Wilcox
Office of Naval Research
Contract N00014-67-A-0112-0068
National Aeronautics and Space Administration
Grant NGR 05-020-559
and
National Science Foundation
Grant GA-31138
SUIPR Report No. 469
May 1972
Institute for Plasma Research
Stanford University
Stanford, California
Abstract
For several months in early 1965 the sun appeared to have
large-scale magnetic field directed outwards at nearly all latitudes and
longitudes. Several independent observations that lead to this puzzling
situation are discussed.
WHY DOES THE SUN SOMETIMES LOOK LIKE A MAGNETIC MONOPOLE?
John M. Wilcox
Institute for Plasma Research, Via Crespi
Stanford University, Stanford, California 94305
A puzzling situation exists in the observations of the sun's
magnetic field during the first part of 1965. For three or four consecu-
tive months the observations obtained with solar magnetographs show that
the solar field was predominantly directed out of the sun at all solar
latitudes. Although it is true that we only observe one hemisphere of
the sun at a given time, the continuing predominant polarity of observa-
tions during several consecutive months makes it almost certain that the
backside solar hemisphere had the same predominant polarity. These sur-
prising observations with the solar magnetograph are at least partially
validated by spacecraft observations of the polarity of the interplane-
tary magnetic field observed near the earth, which show during this same
interval that the interplanetary field was very predominantly directed
away from the sun.
This interval in the first part of 1965 is near the minimum of
the sunspot cycle. The surprising magnetic observations mentioned above
can be discussed with regard to some further information. Solar activ-
ity has been strongly associated with large-scale solar regions with
field directed predominantly into the sun, while the observations men-
tioned above seem to show that just near solar minimum there is almost
no field observed to be directed into the sun. A recent model of mag-
netic fields in the sunspot cycle, based on interplanetary field polar-
ities inferred from geomagnetic observations during an interval of
1.
forty-five years, suggests that an interval of several months at sunspot
minimum during which the solar field is very predominantly directed out
of the sun may be a characteristic of each sunspot cycle. In the present
note we will attempt to describe and develop these interesting circum-
stances.
Consider first the observations obtained with the solar mag-
netograph at Mount Wilson Observatory. Figure 1 shows synoptic charts
of the photospheric magnetic field during Bartels rotations 1800-1802,
which cover the interval February 3, 1965 to April 24, 1965. Contours
with dark shading represent field directed out of the sun and contours
with light shading represent field directed into the sun. It is clear
that at nearly all of the latitudes and longitudes shown in Figure 1,
the observed solar field was predominantly directed out of the sun. The
solar magnetograph observations during Bartels Rotation 1799 were simi-
lar to the situation shown in Figure 1. Thus for several consecutive
months the observed solar field had a predominant outward polarity at
the latitudes shown in Figure 1 (poleward of about 40 or 50 latitude
the field contours are not shown in Figure 1, because of the projection
effect). This is a very unusual situation, because the synoptic charts
usually show approximate equality between areas of outward and of inward
field polarity.
Upon looking at Figure 1 and considering Maxwell's equation
57'B = 0 (integrated over the solar surface), we might come to either of
two conclusions. First, perhaps the polar regions of the sun for which
the fields are not shown in Figure 1 had field directed predominantly
inward to the sun. Second, perhaps during this interval of several
2.
months, the zero-level of the solar magnetograph at Mount Wilson was
consistently in error by several gauss such as to bias the observations
in the direction shown in Figure 1. We will examine each of these
possibilities.
Figure 2 shows ten-day averages of the observed longitudinal
component of the solar fields poleward of about 60 latitude during the
years 1964, 1965 and 1966, obtained with the Mount Wilson solar magneto-
2
graph. A letter N represents the average field magnitude in the north-
ern polar region, and a letter S represents the average field magnitude
in the southern polar region. The interval in early 1965 is shown with
a bracket in Figure 2. During most of this interval the field in the
northern polar region is nearly zero and the field in the southern polar
region has field directed away from the sun with a magnitude of about
1/3 gauss. There is no indication in the observations shown in Figure 2
of the many magnetic field lines directed into the sun in the polar
regions that would be needed to balance the predominantly outward
directed field lines shown at the lower latitudes in Figure 1.
Observations obtained with the solar magnetograph at the
Crimean Astrophysical Observatory appear to be consistent with the re-
3
suits shown in Figure 2. Severny says "in the first half of the year
1965 (February - June) we observe the appearance at both poles of a
field of the same outward* polarity". Severny goes on to observe that
"At times, if we judge by the polar field, the sun acts as a 'monopole'
of one sign". Thus, all of the available solar magnetograph observations
* 3
We note that in this paper Severny uses the convention that a negative
sign represents field directed out of the sun.
3.
consistently show that the sun's polar regions had field predominantly
directed outward at both poles during this interval in early 1965.
Let us examine the second possibility mentioned above, that
during several consecutive months in this interval the Mount Wilson
solar magnetograph had an error in the zero-level of several gauss such
as to bias the observations and produce the predominant outward polarity
shown in Figure 1. This appears to be unlikely for several reasons. We
note that each day's observations were monitered in real time at Mount
Wilson through the use of an X-Y plotter. During the considered inter-
val almost every daily solar magnetogram showed field directed out of
the sun in most areas. This surprising situation resulted in immediate
attention to the optical and electronic components to try to detect an
error in the zero-level. . For several months these efforts were unsuccess-
ful, and finally the "difficulty" mysteriously went away.
The solar observations with the solar magnetograph at the
Crimean Astrophysical Observatory were in agreement with the Mount Wilson'
observations as described above. Thus if the Mount Wilson observations
had an instrumental error of several gauss, then the Crimean observations
4
would have had a similar instrumental error. Kotov and Stenflo have
shown that during the summer of 1968 simultaneous measurements of the
i
polar magnetic fields made at the Crimea and at Mount Wilson were in good
agreement.
A strong independent validation of at least some of the Mount
Wilson observations shown in Figure 1 comes from observations made with
spacecraft of the polarity of the interplanetary magnetic field near the
earth. This interplanetary field polarity is usually very similar to the
4.
5,6
large-scale photospheric field polarity. The observed interplanetary
field was very predominantly directed away from the sun during the first
part of 1965, that is, during most of Bartels Rotations 1799 through
1802. These spacecraft observations are represented by the bars drawn
at the bottom of the synoptic charts in Figure 1, and are plotted four
days later to allow for the transit time of the solar wind plasma that
brings the magnetic lines from near the sun to the earth. (One of the
clearest individual examples of the solar magnetic sector boundary
7
structure inferred by Wilcox and Howard can be seen in the narrow
region near the start of rotation 1802 with field directed into the sun
from 40 north to 40 south. The resulting narrow inward polarity
interplanetary sector can be seen in the spacecraft observations. The
fact that this inward polarity region shows so clearly in the solar mag-
netograph observations is some additional evidence against the presence
of a large error in the instrumental zero-level at this time.)
A broader perspective on the spacecraft observations of pre-
dominant outward polarity can be gained by inspection of Figure 2 of
8 °
Wilcox . This figure shows that these four consecutive Bartels Rotations
have the most concentrated predominantly outward polarity of any such
3
interval that has been observed with spacecraft. Severny has noted
that "if the interplanetary field is a continuation of the solar field,
which is indicated by the close connection between the data on the inter-
planetary field and the appearance on the sun of regions of heightened
magnetism, then the fine structure of the interplanetary field is a
reflection of the fine structure of the sun's general field and the pre-
ponderance of a field of one sign indicates a continuation into inter-
5.
planetary space of the effect of the disruption of balance observable in
the photosphere and chromosphere." Thus it appears that the predominant
polarity situation existing in early 1965 cannot easily be dismissed as
an instrumental effect.
Since the solar magnetograph actually measures the product of
the line-of-sight components of the field and the brightness, the mag-
netograph observations of a predominantly outward polarity might be ex-
plained by postulating a configuration in which the inward field lines
were concentrated in small dark filaments and therefore not properly
measured by the magnetograph. The outward field lines would be postu-
lated to be much more spread out and therefore fully measured by the
magnetograph. (If such a situation were to obtain over the entire sun,\
it would be quite interesting in itself.) However, it appears to be very
unlikely that any such configuration could cause the spacecraft magneto-
meters to miss any appreciable amount of inward polarity field.
The predominant outward polarity observed at the minimum of
the sunspot cycle can be related to some other considerations. Solar
activity has been strongly associated with large-scale solar regions of
predominantly inward field polarity, so that the interval of predomin-
antly outward field polarity just at the minimum of the activity cycle
Q
may have fundamental significance. Bumba says "Again in the present
cycle, as in the previous one, the concentrations of solar activity
inevitably coincide with concentrations of negative polarity fields on
the large scale" and also discusses the "close relation of the position
of very large flares (flares with cosmic ray and PCA events) to the regu-
larities in the internal structure of hugh features formed in inward
6.
polarity regions." Similar results have been discussed by Ambroz, et
al. It is important to note that the association of solar activity
with inward polarity regions has been observed during at least two sun-
spot cycles and in both the northern and the southern solar hemispheres.
II such an association existed in a single solar hemisphere during a
single sunspot cycle, the association might be related to the dynamical
differences between preceding and following.magnetic regions and sun-
spots, but when activity remains associated with just the inward polar-
ity through two sunspot cycles and in both solar hemispheres, then this
association assumes a more fundamental importance.
The above considerations are further illuminated by
Svalgaard's recent model of the sunspot cycle. Svalgaard used inter-
12
planetary magnetic field polarities inferred from forty-five years of
polar geomagnetic observations. This method has been validated by
13
Friis-Christensen, et al. Portions of five sunspot cycles were avail-
able for the analysis. It appears that each of these sunspot cycles
ended with an interval of a few months of predominantly outward field
polarity. One such interval in early 1965 has been discussed above. In
July, August and September, 1954, Svalgaard has inferred that the inter-
14
planetary field polarity was almost completely outward. Thomson
examined the solar diurnal variation of cosmic rays at the 1954 solar
minimum, and says "it has generally been assumed that the away and the
toward magnetic flux near the ecliptic plane is roughly in balance. We
/
would like to suggest that this was probably not the case in the period
July-September 1954 and that at that time the field was predominantly in
one direction." Thus two very different methods of analysis have each
7.
suggested that a very unusual situation existed in July-September of
1954. Each strongly validates the other.
It is clear that the final word on the interesting situation
described in this note remains to be said. It seems very likely that
the resolution of this puzzle will be quite interesting.
This work was supported in part by the Office of Naval Research
under Contract N00014-67-A-0112-0068, by the National Aeronautics and
Space Administration under Grant NCR 05-020-559, and by the National
Science Foundation under Grant GA-31138.
8.
References
1. R. Howard, V. Bumba, and S. F. Smith, Carnegie Institution of
Washington Publication 626 (1967).
2. R. Howard, "Polar Magnetic Fields of the Sun: 1960-1971", submitted
to Solar Physics, 1972.
3. A. B. Severny, Iz. Krim. Ast. Obs. 38, 3 (1968).
4. V. A. Kotov and J. 0. Stenflo, Solar Physics 15, 265 (1970).
5. A. Severny, J. M. Wilcox, P. H. Scherrer, and D. S. Colburn, Solar
Physics 15, 3 (1970).
6. J. M. Wilcox, Space Science Reviews 8, 258 (1968).
7. J. M. Wilcox and R. Howard, Solar Physics 5, 564 (1968).
8. J. M. Wilcox, Comments on Astrophysics and Space Physics 3, 133
(1971).
9. V. Bumba, Proceedings of the Conference on the Solar Wind, Asilomar,
California, March 1971, to be published.
10. P. Ambro^, V. Bumba, R. Howard, and J. Sykora, in "Solar Magnetic
Fields," (R. Howard, Ed.) p. 696, D. Reidel Publishing Company,
Dordrecht-Holland, 1971.
11. L. Svalgaard, "interplanetary Sector Structure during 4' Solar
Cycles," Solar Physics Division Meeting, American Astronomical
Society, University of Maryland, College Park, Maryland,
April 4-6, 1972, to be submitted to Solar Physics.
12. L.' Svalgaard, Danish Meteorological Institute Geophysical Papers R-6,
(1968).
13. E. Friis-Christensen, K. Lassen, J. M. Wilcox, W. Gonzalez, and
D. S. Colburn, Nature Physical Science 233, 48 (1971).
14. D. M. Thomson, Planetary and Space Science 19, 1169 (1971).
9.
Figure Captions
Figure 1. Synoptic charts of the photospheric magnetic fields observed
with the solar magnetograph at Mount Wilson Observatory. The
dark contours represent field directed out of the sun and the
light contours represent field directed into the sun. The
first three contour levels are about 6, 10 and 16 gauss. The
charts are equal-area projections of the solar surface obtained
with a square aperture 23 arc-seconds on a side. The bars at
the bottom of the charts represent the polarity of the inter-
planetary magnetic field near the earth observed with space-
craft, and are plotted four days later to allow for the
transit time of solar wind plasma from near the sun to the
earth (after Howard, et al. ).
Figure 2. Ten-day averages of the polar fields observed above about 60°
latitude on the sun. A letter N represents the average field,
observed in the northern polar region, and a letter S repre-
sents the average field observed in the southern polar region.
The ordinates are in gauss, with a positive sign representing
2
outward field polarity (after Howard ).
10.
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ORIGINATING A C T I V I T Y (Cotpot»l» Mlthot)
Institute for Plasma Research
Stanford University
Stanford, California 94305
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3 REPORT TITLE
WHY DOES THE SUN SOMETIMES LOOK LIKE A MAGNETIC MONOPOLE?
4. D E S C R I P T I V E NOTES (Typ* ol roporf and Inclusive
Scientific Interim
; A U T H O R (51 (First nmm», e<lddt» Inltlel. If ml nam»)
John M. Wilcox
8 REPORT O A T E
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13 ABSTRACT
For several months in early 1965 the sun appeared to have large-scale magnetic
field directed outwards at nearly all latitudes and longitudes. Several independent
observations that lead to this puzzling situation are discussed.
DD ,FNr..l473 UNCLASSIFIED
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SOLAR MAGNETIC FIELD
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Why does the sun sometimes look like a magnetic monopole?

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