Smithsonian

Contributions to Astrophysics

VOLUME 2 , NUMBER 8

SUNSPOTS AND GEOMAGNETISM

by BARBARA BELL AND HAROLD GLAZER

SMITHSONIAN INSTITUTION

Washington, D. C.

1958
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Sunspots and Geomagnetism

By Barbara Bell1 and Harold Glazer2

Geomagnetic storms are commonly believed

to be of two types: (1) nonrecurrent, great

storms, which have a sudden commencement

(sc) and follow the general cycle of solar activity in frequency of occurrence; and (2) recurrent or M-region storms, which are of lesser

magnitude and more gradual onset and which

are most prevalent in the declining phase of

the solar cycle. Both types of storm presumably result when electrically neutral streams of

ionized particles ejected by the sun reach the

earth and disturb its magnetic field, but they

appear to differ in the solar source of the corpuscular stream. Disturbances of the first type

are often found to be associated with central

meridian passage (CMP) of a large sunspot or

the occurrence of a major flare (Newton, 1943;

Allen, 1944). Recurrent storms, on the other

hand, appear to avoid active solar centers. In

a recent paper (Bell and Glazer, 1957) we discussed in some detail the recurrent type of

storms and their apparent inverse relation to

the intensity of the 5303 coronal line. In the

present paper we are concerned primarily with

the active-center type of storms.

Newton (1949) found, for the period 1914-44,

that CMP of large sunspot groups (area > 1500

millionths of the solar disc) tends to be associated with a rise in geomagnetic activity and

that this relation diminishes for the group with

area 1000-1500 and virtually disappears for

spots of area less than 1000 millionths. For

the period 1935-44, Newton found also that

CMP of spot groups of moderate area that produce a large number of flares tend to be associated with a rise in geomagnetic activity.

Becker (1953) found that CMP of paired

sunspots, one north and one south of the solar

' Harvard College Observatory.

» Formerly at Harvard College Observatory; now at Sylvanla Waitham Laboratories, Waltham, Mass.

equator, tends to be followed by relatively quiet

geomagnetic conditions. Unpublished results

by workers at the Central Radio Propagation

Laboratory suggest that CMP of large round

stable spots tends to reduce the level of geomagnetic activity. Denisse (1952, 1953) found

that 158 me radio noise provided a useful criterion for separating geomagnetically active and

inactive spot groups, that CMP of radio-noisy

spots tends to be followed by enhanced geomagnetic activity, and that CMP of radio-quiet

spots tends to be followed by decreased geomagnetic activity.

The radio noise results of Denisse (1952,

1953) and of Simon (1956a, 1956b), and other

work by Bell and Glazer (1954a, 1954b, 1956,

1957) and Pecker and Roberts (1955) have

indicated that active centers on the sun can

affect geomagnetism in two contrary ways.

An active center may appear to emit solar

corpuscles to produce a geomagnetic storm or

it may appear to inhibit the emission of solar

corpuscles to produce a geomagnetically quiet

condition. On the other hand, it may fail to

cause any detectable effect; that is, it may

seem neither to increase nor inhibit the emission

of corpuscles. The essential problem now is

how to predict what a given individual active

center will do. At present, the best criteria

for identifying a storm producer seem to be:

radio noise, presence of the 5694 coronal line

(Denisse and Simon, 1954; Bell and Glazer,

1957), large area, and numerous flares. Inhibitors of corpuscular emission seem to be

characterized by radio quiet, pairing, few flares,

and round stable spots of large size.3

The distribution of magnetic field in a spot

group may be important in determining whether

the spot will predominantly eject corpuscles or

i For a more detailed discussion of the literature, see Bell and Glazer

(1967).

161
162 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

will inhibit their escape from the sun. In this

connection, Nicholson (1940) wrote that "although only about 2 percent of all spot groups

are magnetically complex, great magnetic

storms are usually associated with groups of

that type." The relation between geomagnetic

conditions and the magnetic classes of sunspots, however, has not previously been systematically investigated.

Observational data

As a measure of the solar corpuscular radiation reaching the earth,4

 or of the geomagnetic

activity, we have used the daily Kp index.

The Kp index of geomagnetic activity is measured for each 3-hour interval, on a scale from

0 (very quiet) to 9 (very disturbed); the daily

index is the sum of the 3-hour values, and thus

has a theoretical range of 0-72. The largest

daily sum so far observed, however, is 62.

Our investigation covers the years 1937-53, the

major part of two solar cycles. The mean Kp

for this interval is 19.6, a value derived from

all the Kp's in the 17 years, with each yearly

mean Kp weighted by the annual sunspot

number. (The unweighted mean is 19.5.)

For each sunspot group listed by the Mount

Wilson Observatory in the Publications of the

Astronomical Society of the Pacific, the following quantities were tabulated and punched

on IBM cards: Julian date of CMP; latitude

of spot group; its magnetic classification; its

maximum magnetic field strength, H, in units

of 100 gauss; a duration factor to indicate

whether the spot was in existence at CMP; the

number of flares, F, observed in the region, as

published in the I. A. U. Quarterly Bulletin of

Solar Activity; and Kp's from 8 days before to

12 days after the CMP of the spot group. No

area data were punched on the cards, but we

used the list of spot groups having a mean

area equal to or greater than 500 millionths of

the solar disc published by the Royal Greenwich Observatory in 1955.

The Mount Wilson classification of sunspots

according to their magnetic properties provides

the basic data used in our investigation. This

system, developed by Hale and Nicholson

(1938) from observation of hundreds of spot

• For a more detailed discussion of the .Kp-index of planetary magnetic

activity, see, for example, Bartels and Veldkamp (1049).

fields and the distribution of their associated

calcium plages, recognizes three basic categories

of sunspot groups: unipolar (a), bipolar (/S),

and complex (7).

Unipolar (a) groups are single spots or groups

of spots having the same magnetic polarity.

Bipolar groups (/S) in their simplest form consist of two spots of opposite polarity. Often,

however, a bipolar group is a stream of spots,

those in the preceding and those in the following

parts of the group having opposite polarities.

Complex or multipolar groups (7) are those

having polarities so irregularly distributed that

they cannot be classified as bipolar; sharply

bounded regions of opposite polarities sometimes exist within the same penumbra.

Intermediate between the complex and the

bipolar groups are the semicomplex (Py) groups

in which bipolar characteristics appear but no

clearly marked dividing line occurs between the

regions of opposite polarities. This category

includes groups whose preceding or following

members are accompanied by small spots

having an opposite polarity.

The results presented here were obtained by

the superposed epoch method of analysis

(Chree and Stagg, 1927), with zero days

defined by the CMP of selected groups of

sunspots.

Magnetic class and area of sunspots

Table 1 presents some properties of sunspot

groups subdivided according to magnetic class

and to area. This table, which includes only

long-lived, limb-to-limb sunspots, is based on

the Greenwich spot groups with mean areas

greater than 500 millionths of the solar disc,

and includes also the following smaller spot

groups: 7's greater than 300 millionths; /Vs

with 77>1000 gauss; /S's and a's with #>2000

gauss. Columns 7, 8, and 9 provide some clues

to the relative importance of spot area and of

magnetic class. Within each magnetic class,

the rows in table 1 are arranged in order of

decreasing area. It is immediately apparent

that the Kp-conditions in our sample depend

more strongly on magnetic class than upon the

area of the spot group. We may consider each

magnetic class separately and examine within

each the area dependence of Kp averaged

over five days, CA1P (0)_ to ( + 4), a quantity

referred to hereafter as Kp(0, +4).
SUNSPOTS AND GEOMAGNETISM 163

TABLE 1.—Geomagnetic conditions associated with CMP of large- sunspot groups, 1937-53

(For explanation of symbols see below)

(1)

Magnetic

class

hi

Iffyt

101

ted

(2)

Area rang

>1500

750-1500

500-750

300-500

>300

>1500

1000-1500

750-1000

600-750

500-«)0

>500

<500

>1500

1000-1500

750-1000

600-750

500-600

>500

<500

<500

<500

>1000

750-1000

600-750

500-600

>500

<500(a)

<500(ap)

<500(ap)

<500

(3)

! N

8

6

6

4

24

9

9

12

10

6

46

20

9

15

26

36

31

117

43

130

173

3

3

5

18

29

50

61

163

274

(4)

A

2535

900

630

445

1300

2610

1270

860

660

545

1200

2320

1210

850

670

545

870

(5)

n

3680

2860

2.*>00

2050

2910

3560

3120

2620

2220

2800

2840

2090

3140

3280

3260

3080

3050

3140

>3000

20-2900

1290

850

685

545

675

>2000

3440

3800

2780

3350

3310

>2000

>3000

20-2900

Mean or "expected" values

>2000

(6;

F

36.6

22.8

19.0

13.2

24.8

27.7

29.7

16.0

10.0

9.7

18.8

4.1

25.2

15.0

12.2

6.9

4.9

10.0

5.1

3.6

4.0

10.3

9.0

4.0

4.5

5.5

1.1

1.3

1.9

U)

27.0

27.0

26.0

26.7

26.6

24.3

18.6

25.1

18.0

23.0

21.9

21.0

18.8

19.2

19.2

19.7

18.9

19.3

20.6

19.2

19.6

18.4

22.8

24.5

19.4

20.6

19.1

20.9

20.4

20.2

19.6

(8)

Kp(max)

29.0(0)

29.8(1)

29.8(2)

36.8(2)

28.3(2)

25.6(2)

19.3(0)

27.7(1)

21.3(2)

24. 5(1)

23. 2(2)

23.9(1)

22.9(2)

21.3(3)

20. 5(3)

21.1(0)

21.9(0)

19.8(0)

27.3(3)

27.3(3)

30.0(1)

21.5(3)

22.1(3)

(9)

AKp

+7.4

+7.4

+6.4

+7.1

+7.0

+4.7

-1. 0

+5.5

-1. 6

+3.4

+2.3

+1.4

-0. 8

-0. 4

-0. 4

+0.1

-0. 7

-0. 3

+1.0

-0. 4

0.0

-1. 2

+3.2

+4.9

-0. 2

+1.0

-0. 5

+1.3

+0.8

+0.6

0.0

(10)

Ti_/'-3 to -IN Kp\ +5, +6 )

24.3

25.2

22.7

23.6

24.0

18.7

17.8

18.9

18.3

23.6

19.1

18.0

19.4

19.7

20.2

19.6

19.8

19.8

19.3

22.8

22.1

18.7

19.8

19.6

(11)

T-_/-6 to -4 \

M +7, +8;

25.4

21.4

20.2

20.4

22.2

15.9

19.3

19.2

19.3

21.7

18.9

19.4

18.4

21.8

19.3

19.2

19.3

19.5

18.5

19.1

16.4

18.3

18.1

19.6

(12)

N'

8

6

6

4

24

9

9

12

10

6

46

20

9

15

25

34

30

113

38

115

153

3

3

4

16

26

44

60

146

250

(13)

+7.4

+7.4

+6.4

+7.1

+7.0

+4.7

-1.0

+5.5

-1.6

+3.4

+2.3

+1.4

-0. 8

-0.4

-1.4

-0.4

-0. 8

-0. 6

-0.6

-1. 2

-1. 1

-1.2

+3.2

+3.2

-1.9

-0. 5

-1. 8

+1.1

-0.8

-0. 7

0.0

(14) (15) (16)

Percent .Kp(0,+4)

>40 30—39 <10

5.0

13.3

6.7

20.0

10.0

15.6

9.0

10.0

6.0

6.7

9.6

2.0

8.9

5.3

5.6

2.4

1.3

3.7

2.6

2.0

2.1

6.7

0

0

3.8

3.1

0.5

2.4

1.0

1.2

3.7

37.5

30.0

25.7

25.0

30.9

20.0

2.1

18.4

8.0

23.4

13.9

19.0

4.4

10.7

7.2

9.4

10.0

8.8

9.7

11.1

10.9

6.6

13.3

25.0

3.7

10.8

10.4

19.0

13.1

14.1

12.1

0

3.3

3.3

10.0

3.3

11.1

20.0

8.3

24.0

13.3

15.2

16.0

28.9

13.3

14.4

18.2

14.0

16.5

14.2

18.7

17.8

20.0

0

0

22.5

16.1

16.8

11.3

17.2

15.7

15.0

Explanation of symbols

A: Mean area of spot groups.

C: An index of geomagnetic disturbances based on

activity scale of 0.0 (very quiet) to 2.0 (very

disturbed).

C(max): Highest average value of the C index in the

5-day interval (0, +4), with the day of its

__ occurrence given in parentheses.

C(0, +4): C index of geomagnetic activity averaged

over the five days (0) through (+4), around

_ central meridian passage of spot groups.

C( —3 to —1, +5, +6): The average values of the

__ geomagnetic C index for the five days indicated.

C( —6 to —4, +7, +8): The average values of the

geomagnetic C index for the five days indicated.

AC: The quantity C(0, +4) minus the mean, 0.70.

dc.v'- Spots born around or after central meridian

passage of region.

d,: Spots born on disc east, before central meridian

__ passage of region.

F: Mean number of observed flares in spot groups.

/: Spots in the favorable solar hemisphere.

/': Spots in favorable solar hemisphere, Februar3'-

April and August-October.

H: Mean value of the maximum magnetic field strength

of spot groups.

Kp: An index of geomagnetic activity designed to

measure the solar corpuscular radiation reaching

the earth.

used in tables 1-8

ifp(max): Highest average value of the Kp index in

the 5-day interval (0, +4), with the day of its

occurrence given in parenthesis.

Kp(0, +4): Kp index of geomagnetic activity averaged

over the five days (0) through (+4), around

central meridian passage of spot groups.

~Kp{ —3 to —1, +5, +6): The average values of the

geomagnetic Kp index for the five days indicated.

Kp{ — 6 to —4, +7, +8): The average values of the

geomagnetic Kp index for the five days indicated.

AKp: The quantity Kp(0, +4) minus the mean, 19.6.

AKp': The AKp based on the number of sunspot

groups adjusted as described on page 165.

N: Number of spot groups.

N': Number of sunspot groups adjusted as described

on page 165.

0: Radio-quiet spots.

R: Radio-noisy spots,

s: Spots in the solsticial months, May-July and

November-January.

u: Spots in the unfavorable solar hemisphere,

u': Spots in the unfavorable solar hemisphere

February-April and August-October.

+ (in col. 2 of table 4): Excess of flares.

— (in col. 2 of table 4): Dearth of flares.

0 (in col. 2 of table 4): Average number of flares.
164 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

In each area group of y spots, Rp(0, +4) is

conspicuously above average and shows only a

minor dependence on spot area, even for spots

smaller than 500 millionths. The larger y spots,

however, show an enhanced Kp on a larger

number of days, the rise being less concentrated

to the days around CMP (see columns 10 and

11), than for the smaller y spots. With the

fiy spots, "Rp(0, +4) is also above average, but

the rise is smaller and occurs less consistently

than for the y spots. The relation between

Kp(0, -f-4) and spot area is not systematic and

therefore would appear to be of minor importance. For both the P and a spots, Hp(0, +4)

appears to be independent of area. For the

/5's, Rp(0, +4) is slightly below average, while

for the a's it is slightly above average.

In figure 1 is plotted the average Kp values

associated with CMP of large C4>500 millionths) spot groups for each of the four magnetic classes of sunspots. Because area appeared to be relatively unimportant in our

sample, we combined the various area groups

FIGURE 1.—Average geomagnetic conditions (Kp) on days

around the central meridian passage (CMP) of large (A> 500

millionths) sunspots of the four magnetic classes, a, fi, Py,

and 7, for the years 1937-1953. The number of spot groups

used is given in parentheses.

above 500 millionths. To estimate the statistical significance of the results shown, we used

the standard error of the mean, a =10/VJV,

where AT is the sample size and 10 is the standard

error of a single Kp. In the fiy curve, only

Kp(-{-2) is more than 2 a above the mean.

In the y curve, 2<r lies at Kp—24.1, so that the

Kp on each day (—1 through +8, except for

-f 6) is more than 2 a above the mean of 19.6;

the maximum value, at Kp(0), lies about 3.5a

above the mean. The a and 0 curves show no

statistically significant deviations from the

mean.

The standard error for the average of the

means of n consecutive days is given by the

equation,

— A

where pi and pa are the autocorrelations of the

mean Kp's for 1-day lag and for 2-day lag,

respectively. From the values of the autocorrelation given by A. H. Shapley (1947), we

estimated that pt = 0.5 and p2=0.2. Substitution of these values in equation (1) gives

<TI=GA/TJN. This standard error may be used

to obtain an estimate of the significance of the

numbers in columns 7 and 13 of table 1.

(Because columns 10 and 11 include nonadjacent days, for them <T6=5.8/T/N.) The

statistically significant (>2<rs) deviations of

K]p(0,+4) from the mean of 19.6 are confined

to the y and &y spots. For the 46 py spots,

the deviation of S^(0,+4) from the mean is

about 2<r6, or barely significant at the 5 percent level. For the individual y area-groups

the deviation from the mean ranges from 2 to

3a6; and for the total of 24 spots it is 5<r»,

significant well beyond the 0.1 percent level.

There is no evidence for any significant areadependence of i£p(0,+4), in any magnetic

class.

The dependence of Kp on sunspot magnetic

class may be at least partially reconciled with

the apparent area-dependence found by Newton (1949) by considering the frequency of the

various magnetic classes in each area grouping.

With area greater than 1500 millionths, we

have 87, IO/37, 9/3 and la; whereas in the area

range 500-750 we have 6y, 21/3-y, 67/3 and 23a,
SUNSPOTS AND GEOMAGNETISM 165

that is, a much lower percentage of the geomagnetically more disturbing 7 and fiy spots.

The superior storm-producing efficacy of 7

and fiy spot groups may be further seen by considering the problem conversely. Greenwich

(1955, p. 79) lists eight "very great" storms

since 1917; four of these appear to be associated

with y spot groups and four with fiy spot groups.

Of 66 Greenwich "great" storms from 1917 to

1953, 18 can be attributed to 7 spots, 20 to fiy,

21 to fi, and 4 to a; 10 lack any spot of area

greater than 200 millionths. Some of these

latter are probably not true "great" storms,

but rather storms of the recurrent or M-region

type. (In assigning the spots, we have taken

account of the "notes" and not restricted ourselves to the spots given in the table itself.

The total exceeds 66 because a few storms could

be assigned equally well to two or more spots.)

The geomagnetic index Ap, because of its

approximately linear scale (Kp is a more nearly

logarithmic measure of the amplitude of the

disturbance), appears especially suited for distinguishing "great" storms. Use of this index

yields 33 storms with ^>10 0 between 1937

and 1953. Of these we can attribute 9 to 7,

11 to fiy, 11 to fi, and 1 to a groups, while only

one lacks any notable spot group.

Thus we conclude that the great storms

appear to be produced in about equal numbers

by 7, /S7, and fi spot groups. Since fiy, and

particularly 7, are relatively uncommon spot

types (Hale and Nicholson, 1938; also Bell and

Glazer in a forthcoming paper), a given 7 or fiy

spot group is more likely to be accompanied by

a geomagnetic storm than is a simple bipolar

or unipolar group of similar or even much

larger area.

Taking this point as established, we next

checked the individual cases of large storms

(arbitrarily defined as Kp>40) occurring near

CMP of a large bipolar or unipolar sunspot;

we discarded each case where, from geometrical

considerations, the storm could belong about

equally well to a 7 or a fiy spot, and recomputed the fi and a Kp-curves. For this recomputation, we also removed the cases of fi and

a spots whose storms were members of prominent M-sequences; and in the case of spots

with area less than 500, we removed spots

whose storms could equally well be attributed

to a substantially larger fi or a spot group.

The results make up columns 12 and 13 in

table 1, which correspond to the data given in

columns 3 and 9 for the complete sample. The

net effect is a slight decrease in 1^(0,-f-4) for

the fi and a groups. These new curves are

plotted in figure 2 for spots of area greater than

500 millionths. The 7 curve is narrowed by

the addition of the four spots in the 400-500

area range; the peak is shifted to 2£p(+2),

which lies about 4<r above the mean. The

fiy curve is unchanged from that shown in

figure 1.

We pointed out earlier that sunspots appear,

geomagnetically, to be of three types, which we

may call geomagnetically active or "storm producing," "inhibiting," and "inert." (We use

these terms for convenience in discussing apparent relations, and do not intend to imply

thereby that causal connections have been

established between these spots and geomagnetic conditions.) In any superposed epoch

FIGURE 2.—Average geomagnetic conditions (Kp) on days

around central meridian passage (CMP) of selected large

(^7>400, others>500 millionths) sunspots of the four

magnetic classes for the years 1937-1953. The number of

spot groups used is given in parentheses.
166 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL. z

analysis, as Denisse first pointed out, the effects

of the storm-producers and those of the inhibitors will tend to neutralize one another, and

leave a featureless curve. To explore this situation, for each area group of each magnetic class

we have determined the frequency of Kp in

various intervals for the five days (0, +4),

shown in the last three columns of table 1.

For all Kp's, 1937-53, the mean percentages

are: 3.7 percent Kp>40, 12.1 percent 30-39,

and 15.0 percent less than 10.

Columns 14, 15, and 16 provide several clues

toward understanding the differences in Kpcurves associated with the various spot groupings. The 7 and 07 spots appear about equal

in percentage of Kp >40, containing respectively

2.7 and 2.6 times the average or expected frequency. Although not shown in the table, the

7 spots have 3.3 times, and the By spots 5.8

times the expected frequency of Kp>50. The

frequency of Kp>40 shows no relation to spot

area for the y spots, with some suggestion of a

relation in the By spots. The difference between the i£p-curves associated with CMP of

7 spots and those associated with CMP of By

spots appears to arise from the large (2.5 times

the expected) frequency of Kp 30-39, and the

very low (0.22 of the expected) frequency of

Kp<C.10 with 7 spots, especially the larger ones.

For 07 spots these frequencies are highly variable and show no systematic relation to area,

but on the average they are close to the expected

values.

In the case of 8 and a spots, the percentages

of Kp >40 show some relation to spot area, with

the over-all frequencies close to the expected

values. The frequency of Kp 30-39 is 0.73 of

the expected number for B and 0.9 for a spots,

and shows no systematic relation to area. The

frequency of i£p< 10 is 1.1 of the expected value

for these spots. For the very large (A > 1500)

B spots, both Kp>40 and i£p< 10 appear substantially in excess of the expected frequency,

and tend to neutralize one another in any

superposed epoch analysis.

Having obtained the above results for spots

in the period 1937-53, we thought it desirable

to test them further, using the Greenwich spots

of area equal to or greater than 500 millionths

for the years 1917-36. Since the Kp index is

not available prior to 1937, it is necessary to

use the older C index of geomagnetic activity.

The scale of the C index is 0.0 (very quiet) to

2.0 (very disturbed). The results are exhibited

in table 2 and figure 3. The mean value of C

used in the table and the figure, 0.70, was

derived as the mean of the 15 points on the B

superposed epoch curve; it is therefore less

exact than the mean .Kp=19.6, which was derived from all the Kp's in the 17 years. The

expected frequencies in the various intervals of

C have not been determined. Twenty-two

bipolar spots of variable magnetic class have

been omitted from figure 3 and table 2 because

of the uncertainty of their classification; they

giveC(0, +4)=0.68.

In general, the results of 1937-53 are supported by those from the earlier period,

although the 1917-36 results show somewhat

more evidence for an influence of spot area on

geomagnetic activity. In terms of geomagnetic

consequences, the fly spots in the earlier period

appear indistinguishable from the B and a spots.

FIGURE 3.—Average geomagnetic conditions (C) on days

around central meridian passage (CMP) of large (^7>300,

others > 500 millionths) sunspots of the four magnetic classes,

a, /3, 0y, and y, for the years 1917-1936. Number of spot

groups used is given in parentheses.
5UNSP0TS AND GEOMAGNETISM 167

TABLE 2.—Geomagnetic conditions associated with CMP of large sunspot groups, 1917-86

(For explanation of symbols see p. 163)

(1)

Magnetic

class

hi

Ifiyl

W

lal

(2)

Area range

>1500

1000-1500

750-1000

500- 750

290- 500

>290

>1500

1000-1500

750-1000

600- 750

>500

>1500

1000-1500

750-1000

600- 750

500- 600

>500

>1000

750-1000

500- COO

>500

(3)

N

3

2

3

G

5

19

4

8

7

13

32

3

12

19

21

28

83

•>

4

11

17

(4)

A

1970

1335

820

595

370

2350

1240

875

til 5

1840

1130

850

075

545

1225

805

530

(5) (6)

5 F

3260 ... .

3250

3230 ... .

2700 ...-

2840 ... .

3420 ... .

3140 ... .

2790 ... .

2680

3060 ..-.

2840 ... .

3110 .-.-

2940 ... .

2860 ... .

2900 ... .

2920 ... .

3140 ... .

(7)

C(0,+4)

1.15

1.33

0.90

0.67

1.25

1.00

0.94

0.68

0.70

0.61

0.68

0.55

0.66

0.75

0.69

0.78

0.73

0.97

0.67

0.78

0.77

(8)

C(max)

1.33(3)

1.75(2)

1.13(3)

0. 73(2)

1.48(2)

1.16(3)

1.15(2)

0.95(2)

0.97(0)

0.66(3)

0. 78(2)

0.83(1)

0.73(1)

0.91(4)

0.71(2)

0. 85(0)

0.76(0)

1.30(3)

0.80(0)

0.87(1)

0.85

(9)

AC

+0.45

+0.63

+0.20

-0.03

+0.55

+0.30

+0.24

-0.02

0.00

-0.09

-0.02

-0.15

—0.04

+0.05

-0.01

+0.08

+0.03

+0.27

-0.03

+0.08

+0.07

(10)

H +5, +6 )

0.60

1.22

0.59

0.82

0.69

0.76

0.80

0.70

0.75

0.73

0.74

1.10

0.61

0.65

0.67

0.74

0.70

0.90

0.71

0.79

0.78

(11)

C

( +7t,°+84)

0.50

1.01

0.66

0.73

0.64

0.68

0.67

0.64

0.64

0.75

0.68

0.67

0.63

0.61

0.68

0.78

0.69

0.74

1.02

0.77

0.78

(12)

2.0

13.3

30.0

0

0

12.0

8.4

5.0

2.6

2.9

0.0

1.9

6.7

1.7

1.0

0

2.8

1.7

10.0

0

0

1.2

(13) (14)

Percent C(0, +4)

>l-7

40.0

30.0

20.0

10.0

32.0

24.2

25.0

5.0

5.7

1.4

6.2

6.7

3.3

4.2

1.0

5.7

3.9

10.0

10.0

7.3

8.2

1.2—1.6

0

30.0

13.0

13.3

10.0

13.7

5.0

17.5

14.3

4.3

10.0

6.7

15.0

22.1

16.2

21.4

18.8

40.0

10.0

10.0

14.1

(15)

<0.2

6.7

0

20.0

30.0

0

13.7

15.0

27.5

31.4

24.3

26.2

40.0

20.0

21.1

18.1

27.2

22.9

30.0

25.0

11.7

17.6

The 7 spots again appear to be storm producers,

and their superiority over the /Sfy's is more

striking in the earlier period.

In addition to the limb-to-limb or II spots

discussed above, a number of young (dl) groups

are found among the larger spots. In the

category with area >500 millionths, the dl spots

are all bipolar, /3 and /S7; there are no large

young 7 or a spots. Table^ 3 gives some results

for the larger dl spots. i£p(0,+4), in column

7, shows no significant (>2a8) deviations from

the mean. There is some indication that dl

spots are more commonly inhibitors than II

spots of equal area (cf. column 16), but both

the samples and the differences are too small to

justify any definite conclusions.

Effect of flare abundance

Newton (1949) found that spots producing large

numbers of flares are more likely to produce

geomagnetic disturbance than spots having a

subaverage number of flares. To investigate

this result with our more extensive data,

for each magnetic class we plotted area of

spot group (column 4) against number of

observed flares (column 6 of table 1) and

drew a smooth curve through the points.

(The relation between flare production, magnetic class, and spot area will be more fully

discussed in another paper, in preparation.)

We divided the spots of each magnetic type into

three groups of approximately equal size: those

having an average number of observed flares

within about ± 5 flares of the curve, those

having an excess, and those having a dearth of

observed flares, designated by the symbols o,

+ , and —, respectively.

TABLE 3.—Geomagnetic conditions associated with CMP of young sunspot groups of large area, 1937-53

(For explanation of symbols see p. 163)

(1)

Magnetic

class

d0yl

d£l

Mean or

(2)

Area

range

>1000

500-1000

>500

<500

750-1100

600-750

500-600

500-1000

"expected"

(3) (4)

N "A

2

5

7

18

7 ... .

12 ... .

12 ... .

14 ... .

values

(5)

77

2300

2540

1000

3020

2850

2680

2460

(6)

f

40.5

15.0

14.1

6.8

9.1

6.4

a)

25.1

18.3

20.2

18.0

18.0

17.8

18.4

17.7

10.6

(8)

) JCp(max)

38.0(4)

22.8(2)

24.7(4)

19. 6(4)

19.8(2)

22.2(3)

20. 5(1)

18.8(4)

(9)

AKp

+5.5

-1.3

+0.6

-1. 6

-1. 6

-1. 8

-1. 2

-1. 9

0.0

(10)

—• / g ^ j

Kp\+5, +6

20.1

17.8

18.5

16.1

19.9

18.3

19.8

20.5

19.6

(11)

\ ^_/-6 to -4\

) KP{ +7f +8 )

19.2

23.0

21.9

17.8

20.0

23.0

19.5

20.0

19.6

(12)

N'

6

11

10

(13)

AKp'

-3. 8

-3. 2

-2. 5

0.0

(14) (15)

Percent Kp

>40

10.0

4.0

5.7

0

1.8

0

0

1 7

30-39

40.0

16.0

22.8

3.3

7.3

6.0

12.1

(16)

<10

20.0

28.0

25.7

26.6

25.5

12.0

15.0
168 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

f -4 3 S

FIGURE 4.—Average geomagnetic conditions (ATp) on days around central meridian passage (CMP) of large sunspots of the

magnetic classes a, ft, fiy, and y, having an excess (left), an average number (upper right), and a dearth (lower right) of

observed flares. Number of spot groups used is given in parentheses.
SUNSPOTS AND GEOMAGNETISM 169

TABLE 4.—Geomagnetic conditions associated with CMP of large sunspots subdivided by magnetic class and by apparent

flare productivity

(1)

Magnetic

class

hi

Ifiyl

101

lal

(2)

Flares

+

0

—

+

0

—

+

0

—

+

0

—

(3)

N

8

8

8

24

14

16

IB

46

25

51

36

112

5

10

11

26

(4)

A

1210

1390

1310

1300

1270

1200

1130

1200

1010

840

830

870

620

810

610

690

(5)

F

37.5

23.8

13.2

24.8

32.7

16.4

8.6

18.6

23.4

9.1

3.2

10.0

10.8

7.7

1.5

5.6

(6)

WF/A

.310

.172

.101

.191

.258

.137

.076

.156

.232

.109

.039

.115

.174

.095

.024

.082

(For explanation of symbols see p. 163)

(7)

*pft+4

31.5

24.0

24.6

26.6

24.6

21.8

19.6

21.9

18.5

20.5

17.2

19.3

22.1

19.9

17.1

19.1

(8)

) Xp(max)

36.8(4)

25.8(2)

30.0(2)

28. 2(3)

23. 9(2)

20.4(2)

19. 7(2)

21.6(2)

18.5(3)

26.2(3)

25. 5(4)

19.0(3)

(9)

Jfp(min)

28.7(0)

21.9(0)

19.5(4)

22.6(0)

18. 7(4)

18.6(4)

16.9(0)

19. 7(4)

16.2(2)

17.2(0)

17.6(0)

15.2(0)

(10)

>40

20.0

2.5

7.5

10.0

12.9

11.3

6.0

4.0

5.1

1.7

0

4.0

3.6

(ID (12)

Percent Kp(0, +4)

30-39 <1O

42.5

25.0

25.0

30.8

15.7

15.0

11.3

9.6

7.8

10.0

12.0

14.0

7.3

2.5

2.5

5.0

3.3

10.0

20.0

15.0

16.8

13.0

21.6

0

18.0

21.8

(13)

>20

82.5

60.0

67.5

66.7

62.9

48.8

41.3

50.4

43.2

60.1

33.3

43.2

64.0

46.0

36.3

45.4

(14) (15)

Atleastoneirp(0,-f4)

>40 >30

62.5%

12.5

37.5

37.5

35.7

31.2

15.0

30.4

16.0

17.6

8.3

14.3

0

20.0

18.2

15.4

100.0%

62.5

87.5

83.4

57.2

62.5

50.0

56.5

48.0

45.2

50.0

47.4

40.0

60.0

36.4

46.2

From superposed epoch analyses we obtained the results plotted in figure 4. Because

some of the samples were small, these curves

have been plotted as 3-day running means.

Further particulars appear in table 4, in the

last two columns of which are given the percents of spots associated with at least one daily

Kp>40, and >30, in the five-day interval

CMP (0 to +4). As a rough estimate of the

expected frequencies, we obtain 15 percent for

column 14 and 52 percent for column 15. For

column 13, the expected value has not been

computed; the mean for the 208 spot groups is

47.8 percent Kp>20.

In figure 5 we have plotted ~Kp{Q, +4)

against the ratio 10F/A, i. e., the data of

column 7 against that of column 6 from table 4.

We also plotted Kp(0, +4) against area for

individual spot groups, with flare frequency distinguished by the three symbols given above.

These plots, not includedhere, show no evidence

of a relation between Kp(Q, +4) and spot

area, but do show a moderate tendency for the

+ symbols to be more frequent among the

larger Kp(0, +4) values.

The data in table 4 and figures 4 and 5 tend

to support Newton's finding that flare-active

spot groups are more likely to be geomagnetically disturbing than spots deficient in flares or

having only an average number of flares.

1

_

I

10

1

«

tr

1

2 0

1

1

1 1

*

1 |

1

1

tOflk

-

_

FIGURE 5.—Average geomagnetic conditions (Kp) for day$ 0

through +4, plotted against flare productivity (1O7%7) for

spot groups of the magnetic classes a, 0, /9-y, and y.

When we consider the separate spot classes, we

find that the relation appears fairly clearly for

the o, /S7, and y spots, but seems negligible

for the /? spots. However, the only statistically

significant deviations of Kp(0, +4) (column 7)

from the mean of 19.6 are y(+) c~5as, Py(+)

c~2.5<rs and 0(— )^2cr5. The y spot groups

tend to be more flare-active than do the other

typesj_ but for a given relative flare-activity,

10F/A, the 7 spots appear to be more disturbing

geomagnetically than do the other classes.

Therefore it does not appear likely that the

greater storm-producing tendency of y spots

arises solely from their tendency to greater

flare activity.
170 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

TABLE 5.—Some particulars of flare productivity and area of sunspots associated with various geomagnetic conditions

(For explanation of symbols see p. 163)

(1)

Magnetic

class

hi

Ifiyl

W

lal

(2)

Kp

range

>30

25-30

20-25

15-20

>30

25-30

20-25

15-20

10-15

<10

>30

25-30

20-25

15-20

10-15

<10

25-30

20-25

15-20

10-15

<"10

(3)

N

9

4

6

8

9

7

10

9

S

6

6

10

32

30

30

4

6

6

7

3

4

(4)

Kp

33.7

27.4

22.9

18.0

35.6

27.5

22.0

17.3

13.2

8.5

33.2

26.9

22.4

17.3

12.8

9.0

27.9

22.1

18.1

13.3

7.7

(5)

F

26.9

29.5

27.6

14.2

22.4

25.2

15.5

17.1

13.6

17.0

17.6

13.0

6.0

12.7

8.7

19.8

5.8

6.3

5.3

10.0

1.8

(6)

A

1210

1200

1870

870

1400

800

1530

1170

840

1160

1690

790

750

830

830

1480

710

640

620

1120

530

(7)

10 F/A

.224

.246

.148

.164

.160

.314

.101

.146

.161

.146

.104

.165

.080

.153

.105

.134

.082

.099

.085

.089

.033

(8) (9)

Percent spots with

excess

of flares

56

50

17

0

44

57

20

22

20

33

17

30

12

30

17

50

16

33

29

0

0

dearth

of flares

22

25

50

40

11

0

70

56

40

17

0

10

44

20

50

25

33

33

43

33

75

(10) (11)

Percent spots in

favorable

hemisphere

22

50

50

0

56

29

40

50

80

67

33

50

62

43

60

75

17

33

43

33

100

equinoctial 6

months

78

7.r>

50

80

7H

57

40

33

80

67

67

10

53

53

53

25

50

67

29

100

75

We also examined the problem inversely,

subdividing each magnetic class of spots according to the value of Kp(0, -f-4) and computing

the average values of flares and area, with the

results shown in table 5. Columns 8 and 9

give the percent of spots with an excess and

with a dearth of flares.

Plotting Kp(0, +4) (column 4) against the

data in columns 7, 8, and 9 does not reveal anyclear relation between flare-activity and the

geomagnetic effects of spots. On the other

hand, as we have seen, there is a marked

inverse relation between geomagnetic storms

and the magnetic class of sunspots. Column 6

provides further evidence that no relation exists

between spot area and geomagnetic conditions,

and agrees with the evidence presented in

table 1.

We conclude that, although the data of table

4 and of figures 4 and 5 do indicate some degree

of positive relation between flare-activity and

geomagnetic conditions, the relation between

sunspot magnetic class and geomagnetic conditions is the more pronounced, and probably the

more basic. On the other hand, magnetic class

alone is not a sufficient basis for accurate prediction, and the use of flare-activity as well as

other criteria remains important.

Seasonal effects

The March and September maxima in geomagnetic and auroral activity are well known,

but not understood. The semiannual variation in magnetic activity was first demonstrated by Bartels (1932) from the w-figures,

and appeared to be present at all phases of the

sunspot cycle. We present here some graphs

to illustrate the nature of the seasonal variation

in the more modern daily Kp-sums.

Figure 6 shows the average monthly value of

Kp for three phases of the sunspot cycle: curve

(A) covers the periods of high sunspot activity,

January 1937 to June 1940 and January 1946

to June 1950; (B) that of declining sunspot

activity, June 1940 to June 1944 and June 1950

through December 1953; and (C) that of minimum and early rise in activity, June 1944

through December 1945 and January 1954

through December 1955. Figures 7 and 8 show

the average monthly frequencies of Kp>SO and

Kp>40 respectively, with curves (A) and (B)

covering the same periods used in figure 6. (For

statistics on the seasonal variation in the 3-hour

Kp's, see Bartels and Veldkamp, 1951, table

21.)

The exact intervals which represent the different phases of the sunspot cycle are of course
SUNSPOTS AND GEOMAGNETISM 171

FIGURE 6.—Average monthly values of the geomagnetic

index (Kp) for three phases of the sunspot cycle. Curve

(A) covers the periods of maximum sunspot activity, curve

(B) those of declining activity, and curve (C) those of minimum activity (see p. 170).

T 1 1

FIGURE 7.—Average monthly frequency (N) of geomagnetic

index Kp>30: curve (A), around the maximum; curve

(B), in the declining phase of the sunspot cycle.

somewhat arbitrary. In a general way, however, if we follow the ideas of Newton and Milsom (1954), we may consider that curve (A)

covers the periods dominated by the nonrecurrent, great storms of sudden commencement

(sc); whereas curve (B) covers the periods dominated by the recurrent and moderate-intensity

storms. Although it is often difficult to classify

an individual storm, it does appear that the corpuscles causing the two types of storms arise

from quite different types of solar regions (Bell

and Glazer, 1957). The great storms seem to

arise from active solar centers, i. e., from large

sunspots and/or major flares; whereas the recurrent storms arise from the more obscure Mregions. Because of these different mechanisms

of storm-production, one would not necessarily

expect that Kp would show a similar seasonal

variation at all phases of the solar cycle. Figures

6, 7, and 8, however, each show a clear seasonal

variation, and except for a generally lower level

of activity around sunspot minimum (figure 6,

curve (C)), they show no significant difference

with phase of the sunspot cycle, a result in agreement with Bartels' findings from the tt-figures.

Figure 9 (top) shows the monthly total numbers of sunspots of area >500 millionths, 1874-

1954, and (bottom), the monthly total numbers

of great magnetic storms, 1840-1954 (Greenwich, 1955). The great storms show the same

semiannual variation in frequency as did Kp

in figures 6, 7, and 8 (8 being the most comparable), whereas no such variation appears in

the frequency of sunspots. We included in each

FIGURE 8.—Average monthly frequency (N) of geomagnetic

index Kp>40: curve (A), around the maximum, and

curve (B), in the declining phase of the sunspot cycle.

FIGURE 9.—Top: Monthly total number (N) of sunspot

groups of area>500 millionths, for the period 1874-1954.

Bottom: Monthly total number of great geomagnetic

storms observed at Greenwich, for the years 1840-1954.
172 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

curve the longest available series of data, but

the shape of the storm-curve would not be significantly altered if we used only the data from

1874 to 1954, corresponding to the available

sunspot information. It is apparent that the

monthly frequency of large sunspot groups

shows no seasonal variation corresponding to

that shown by the frequency of great storms.

The monthly ratio of storms per spot displays

also a clear seasonal variation.

In his original paper Bartels (1932) suggested

two possible causes of the seasonal variation in

geomagnetic activity, which, following his terminology, we call the equinoctial and the axial

explanations. Briefly, the former suggests that

the earth is more responsive to disturbances

from solar corpuscles around the times of the

equinoxes than around the times of the solstices.

The axial explanation derives from the fact that

the sun's equator is inclined 7.2° to the plane

of the ecliptic. The heliographic latitude of the

earth varies from 7.2° south around March 5 to

7.2° north around September 7, so that if solar

regions are assumed to eject corpuscles in a

radial direction, corpuscles from southern solar

centers should intercept the earth in greatest

numbers in March, and those from northern

centers in September.

In the years of predominantly M-region activity, 1950-53, we found evidence (Bell and

Glazer, 1956, 1957; Bhargava and Naqvi, 1954)

that the tendency to greater storminess around

March and September arose primarily from the

inclination of the ecliptic to the solar equator,

although there was also some indication of a

terrestrial component. We denoted solar centers on the same side of the solar equator as

the earth as "favorable," and those on the

opposite side as "unfavorable." We found that

the correlation between geomagnetism and the

intensity of the green-line corona was much

better for the corona in the favorable than in

the unfavorable solar hemisphere. The data for

these four years suggested that M-regions were

areas of weak 5303 emission (and hence of low

density) in the favorable solar hemisphere; and

that a reduction in geomagnetic activity tended

to follow CMP of bright 5303 regions in the

favorable solar hemisphere. This concept of

favorable and unfavorable solar hemispheres

did not appear to hold for active-center storms

associated with regions characterized by yellow

(X5694) coronal-line emission.

TABLE 6.—Geomagnetic conditions associated with CMP

of sunspots in the favorable or unfavorable solar

hemispheres

(For explanation of symbols, see p. 163)

(1)

Magnetic

class

A>800

w

WJ

M

d j «

-4<500,

Ifiyl

lot

(2)

1,0-

Cfttlon

/

u

/

H

Nf

Su

Sf

Nn

f

u

f

II

/

u

(3)

JV !?•

7

17

24

22

27

27

3.S

24

62

51

11

15

14

11

H>2000

/

u

/

II

/

u

11

9

82

71

120

129

l2(0-H-P+a)l

A>500

A<500

/

u

/

u

"Expected"

97

88

213

209

(4)

26.9

26.5

21 4

22.8

17.6

18.5

18.9

21.1

18.4

19.8

15.6

21.7

16.5

15.7

21.8

20.0

19.6

(«)

> 4 0

8.8

10 6

11.6

7.3

4.4

3.0

2.3

5.8

3.2

4.3

1.8

4.0

1.4

0

1.8

2.2

2.2

2.0

1.7

0.8

5.1

5.0

1.9

1.2

3.7

(fi) (7)

Percent Kp

30-39

34.2

29 4

10.0

16.4

7.4

9.6

9.2

9.2

8.4

9.4

5.5

14.7

5,7

3 .6

20.0

17.8

11.2

10.4

13.3

14.7

8.5

12.0

12.9

13.4

12 1

10-29

.17.2

f». 3

60.9

64. S

64.6

69.6

74.2

75.0

70.0

72.2

63 6

74.6

71.5

72.8

67.3

57.8

68.1

70.7

68.0

69.9

67.0

70.7

67. y

69.7

69.2

w

<10

n

4.7

17.5

11.8

23.7

17.8

14.3

10.0

18.4

14.1

29.1

6.7

21.4

23.6

10.9

22.2

18.5

16.9

17.0

14.6

19.4

12.3

17.3

15.7

15.0

(9) (HO

Percent flu

> 3 0

1.07

n.9i

0.93

0.77

0.85

0.39

1.97

1.09

1.08

0.97

0.80

1.01

<!•>

(

1 48

1 33

1 43

1.30

4 34

0.1)1

n.49

1.09

1.16

1.58

1.10

The present sunspot study was undertaken,

in part, to explore the possibility of a hemisphere effect in sunspots, either as apparent

storm-producers or as inhibitors of corpuscular

emission. For this purpose we divided the

spots of table 1 into those favorably (/) and

those unfavorably (u) located, and carried out

superposed epoch analyses. Figure 10 shows

the resulting curves for the larger sunspots

(area > 500, except y area>400) of the different magnetic classes. Figure 11 shows two

independent samples, for /3 spots occurring in

spring and in fall half years. Further particulars are set forth in table 6, where the last

two columns give the ratio percent favorable

to percent unfavorable for Kp>30 (from

columns 5 plus 6) and /£p<10 (from column
SUNSPOTS AND GEOMAGNETISM 173

8). The columns showing frequencies and

ratios are included in an effort to unscramble

the effects of storm-producers from those of

inhibitors, or at least to allow both to manifest

themselves.

We also divided the large spots in each magnetic class into three groups, those occurring in

the six solsticial months (s), May through July

and November through January; and those in

the six equinoctial months, subdivided into

favorable (/') and unfavorable (u'), with the

results shown in table 7. The last two columns

give percents of spots associated with disturbed

(from columns 5 and 9) and with quiet (from

columns 6 and 10) geomagnetic conditions.

Surprisingly, Kp(0, +4) in table 7 shows none

of the expected seasonal effect; however, the

effect appears clearly in 77(0, +4) for the

earlier period. The reason for this result is

unknown.

FIGURE 10.—Average geomagnetic conditions (Kp) on days around central meridian passage (CMP) of

large sunspots of the geomagnetic classes a, 0, fly, and y; solid line, in the favorable solar hemisphere;

broken line, in the unfavorable solar hemisphere.
174 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL I

TABLE 7.—Geomagnetic conditions associated with CMP of sunspots subdivided by location and season

(For explanation of symbols see p. 103)

(1)

Magnetic

class

hi

161

u

(2)

Location

r

$

u'

f

$

u'

r

u'

f

$

u'

f«)J

1

u'

(3)

A

r

4

7

13

12

20

14

33

57

22

6

11

9

61

88

45

1937-1953

(4)

Fp(0, +4)

27.2

26.5

26.7

21.8

20.9

23.2

18.7

19 2

18.6

13.9

20.1

21.3

18.8

19.7

20.6

(6) (6)

Percent 5rp(0. +4)

£25

75

43

64

42

25

43

9

19

9

0

27

33

1C

22

24

<15

0

0

0

SO

15

14

30

30

32

67

9

22

39

24

24

(7)

N

5

7

7

8

19

6

15

42

26

3

11

3

26

72

35

1917-1936

(8)

C(0, +4)

1.07

0.88

1.08

0.84

0.65

0.60

0.85

0.66

0.75

0.89

0.66

1.08

0.85

0.65

0.75

(9)

Percent

>1.0

60

14

57

38

16

0

47

14

19

33

18

67

42

15

20

(10)

C(0, +4)

<0. 5

0

0

14

25

28

33

26

29

27

0

45

0

23

31

26

1917-1953

(11) (12)

Percent geomagnetlcally

dlsturbec

67

29

55

40

20

30

21

17

15

11

23

42

25

19

22

L quiet

0

0

5

40

20

20

29

29

29

44

27

17

34

27

25

In the previous section we divided the spots

according to the value of Kp(0, +4), with results given in table 5. Columns 10 and 11 of

this table give the percents of favorable spots

and of equinoctial spots, respectively, and

provide an inverse approach to the problems

of hemisphere effects and of seasonal effects.

From an examination of the assorted data in

tables 5 to 7 and figures 10 and 11, we conclude

that there is no evidence for any hemisphere

effect in storm-producing spots. Considering

the 7 spots, as the most important storm-producers, we find no significant difference between

the favorable and the unfavorable spots (figure

10, top left) either in the curves or in the tabulated figures. A similar lack of significant

difference appears in the fly curves and tabulations. For the /3 and a spots, table 6, column

9, suggests, if anything, a slight advantage

with the unfavorable spots, but this tendency

is not confirmed by the data of the earlier

period (table 7, right). The absence of any

hemisphere effect in sunspots which produce

great geomagnetic storms is in accord with

earlier results from the yellow coronal line, and

does not support any simple axial explanation

FIGURE 11.—Average geomagnetic conditions (Kp) on days around central meridian passage (CMP) of large bipolar or 0

sunspot groups in the favorable (solid line) and in the unfavorable (broken line) solar hemispheres, (a), For the

spring half-years; (b), for the autumn half-years.
SUNSPOTS AND GEOMAGNETISM 175

for the clustering of greatjstorms toward the

equinoctial months. This negative result suggests that Bartels' equinoctial hypothesis continues to merit consideration as at least one of

the factors which may contribute to the semiannual periodicity in geomagnetic and auroral

activity.

We conclude, secondly, that favorably located spots of area equal to or greater than 500

millionths are more likely to be associated with

geomagnetically quiet conditions than are unfavorably located spots. This tendency may

be seen in figures 10 (bottom) and 11 for the 0

and a spots, and in the consistent tendency of

the numbers in column 10 of table 6 to be

greater than unity for spots of the classes Ifiyl,

lal, and for the two independent groups of 101

spots. None of these curves shows any statistically significant deviations (>2o-) from

the mean. However, the two independent

samples of Ifil spots, from spring and fall,

shown in figure 11, agree in having a below

average Kp in the days following CMP of the

favorable spots. Without other evidence for

the existence of inhibiting solar regions, such

as the radio-noise data (#-spots) and paired

spots, and the apparent inhibiting effect of

bright regions of the green-line corona in the

favorable hemisphere, we should, however,

have less confidence than we do in the meaningfulness of these dips in the favorable Kp

curves for bipolar and unipolar spots of large

area.

Radio noise

Simon (1956a, 1956b) has recently extended

the work of Denisse on radio noise of meter

wavelengths as a criterion for picking out the

geomagnetically active sunspots. Simon has

investigated also the relation of radio noise to

optical properties of sunspots. He finds some

tendency for radio-noisy (R) spots to be larger,

more flare-active, and relatively more common

among magnetically complex groups and groups

of Zurich type F, than are radio-quiet (Q) spots.

But these tendencies are not sufficiently striking that R and Q spots can be distinguished from

one another by any of these optical properties

of the photospheric levels of the sun. The

meter-length radio noise arises from the lower

corona and may be considered as a manifestation of spot activity in the higher levels of the

solar atmosphere.

Simon's work indicates that geomagnetic

activity is more closely linked to the radionoise properties of sunspots than to any of

their optical properties on the photospheric

levels. Whichever optical subgrouping of spots

he used (area, flare activity, Zurich class, magnetic class, and field strength) Simon found

that radio-noise data permitted a marked separation of each subgroup into geomagnetically

active and inactive sunspots. His study of

geomagnetic effects emphasized area and flare

activity and dealt only slightly with magnetic

class. For the y and /S? classes combined, he

found that radio-noise data permitted a separation between geomagnetically active and inactive spot groups. However, he did not

investigate the magnetic classes individually.

Since the magnetic class of sunspots is the

principal criterion under study in the present

paper, we used Simon's (1956a, 1956b) lists of

R and Q spots to investigate whether the

amount of geomagnetic activity within the R

and Q categories appears to depend in any

degree on the magnetic class of the spot. We

obtained the following categories of R and Q

sunspots: area >500: lyl, Ifiyl, Ifil, led, defil;

area <500 and H> 2000 gauss: 101 and lad. For

each of these groups we made a superposed

epoch analysis. Figure 12 shows the average

Kp conditions associated with CMP of R and Q

sunspot groups for each of the four magnetic

classes. Table 8 gives further particulars. The

two area groupings of a and /3 spots have been

combined in the figure, but are listed separately

in the table. The 7 and fiy curves are plotted

as three-day running means, because of the

small size of the samples.

The results given in figure 12 and table 8 are

in clear accord with those obtained previously

by Denisse and by Simon, in indicating that

radio noise on meter wavelengths is an important criterion for separating geomagnetically

active and inactive sunspots. The inhibiting

effect of Simon's Q spots appears less than that

of Denisse's Q spots. By way of explanation,

Simon has pointed out that Denisse's Q spots

were entirely quiet, whereas he listed as Q spots

those which failed to attain a certain defined

level of noise, and thereby included some
176 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL. t

slightly noisy spots. Even so, very low values

of Kp (column 12) on days 0 through +4 are on

the average three times as frequent with Q

spots as with R spots. Conversely, values of

Kp>AQ (column 10) are 4.4 times more frequent with R than with Q spots, being respectively 2.2 and 0.5 times the frequency expected

by chance. From column 14, we see that 37

percent of R spots are associated with at least

one Kp>40 during the five days, 0 through +4,

whereas only 6.5 percent of Q spots are so

associated. By chance, one should expect an

association of about 17 percent. Similarly for

at least one Kp>30, the figures are 66 percent

and 38 percent for R and for Q spots, respectively, as compared with a random value of

about 53 percent. Moreover, of the 14 great

storms listed by Greenwich for 1947-51, the

period covered by Simon's radio-noise data,

nine can be associated with R spots, four with

unclassified spots, and one6 (19 August, 1950)

with Q spots. It seems clear that very few Q

spots are associated with storms of large magnitude, but on the other hand, not all R spots

appear to produce significant disturbance.

Almost every major storm can be associated

with an R spot, but by no means every R spot

is accompanied by a notable storm.

' According to a private communication from Helen Dodson Prince,

this storm was most probably related to a flare with an associated "major

early burst" of radio noise, occuring near a small y spot, No. 10444.

1 r- -f-H

FIGURE 12.—Average geomagnetic conditions (Kp) on days around central meridian passage (CMP)"of

(R, left) radio-noisy and (Q, right) radio-quiet sunspot groups of magnetic classes a, /3, fiy, and y

for the years 1947-1951. Number of spot groups used is given in parentheses.
MO, • 6UN6P0TS AND GEOMAGNETISM 177

TABLE 8.—Geomagnetic conditions associated with CMP of sunspots subdivided according to radio noise properties

(For explanation of symbols see p. 168)

(1)

Magnetic

class

hi

Ifiyl

A>500

w

Ud

dfl

w

led

TSRl

IXQl

(2)

Radio

noise

R

Q

R

Q

R

Q

R

Q

R

Q

R

Q

R

Q

R

0

"Expected"

(3)

N

7

2

8

5

28

16

5

5

5

6

20

26

32

54

100

108

(4)

A

1350

1150

1850

960

880

870

740

600

>500

>500

<500

<500

<500

<500

(5)

T

31.0

31.0

32.2

14.4

12.7

0.3

6.8

6.0

6.7

4.4

2.2

2.1

(6)

I07M

.229

.270

.174

.150

.145

.107

.002

.100

> 1 4

>.O9

>.O4

>.O4

(7)

BpQO, +4)

29.4

20.9

24.6

16.0

22.2

18.8

26.7

17.4

19.8

15.6

25.5

10.1

23.7

10.2

24.26

18.92

20.4

(8)

JTp(max)

31.6(2)

29.5(4)

27.7(3)

18.4(3)

23.3(2)

21.3(4)

30.2(3)

20.2(0)

25.0(0)

19.4(4)

28.3(2)

10.6(3)

26.0(4)

20.0(2)

(9)

JBTp(mln)

27.8(4)

11.5(1)

20.5(4)

13.8(4)

21.2(0)

17.2(2)

21.6(4)

13.8(1)

16.2(1)

12.0(0)

17.8(0)

18.6(4)

22.4(0)

18.6(3)

(10)

£40

11.4

0

7.5

0

7.2

2.5

8.0

0

4.0

0

14.0

2.3

10.6

2.6

10.0

2.2

4.4

(11) (12)

Percent Kv

30-30

37.2

20.0

30.0

4.0

10.0

10.0

28.0

4.0

8.0

8.0

15.0

12.3

17.5

12.6

17.8

11.7

12.1

<10

0

0

5.0

28.0

7.2

13.8

0

16.0

12.0

24.0

2.0

15.4

6.9

16.3

5.0

15.9

11.0

(13)

£20

sa 6

40.0

62.5

36.0

60.0

42.5

72.0

44.0

44.0

28.0

70.0

39.2

57.5

44.8

64.0

42.6

(14) (15)

Percent with at

least one Kv

£40 >30

57.2

0

87.5

0

25.0

6.2

40.0

0

20.0

0

40.0

7.7

40.6

7.4

37.0

6.5

17

85.7

50.0

75.0

20.0

57.1

50.0

100.0

20.0

60.0

40.0

65.0

34.0

62.5

38.9

66.0

38.0

53

Because of the small sample sizes, any further

inferences must be only tentative, and the

tendencies noted in this paragraph must not be

considered as established. In regard to magnetic class, the y spots retain their appearance

of superiority as geomagnetic-storm producers,

even the two quiet y spots showing a degree of

geomagnetic activity. Also it should be noted

that 7 spots contain a higher per cent of R

spots than do the other magnetic classes (cf.

Simon 1956a, 1956b). The R spots of the

other magnetic classes appear about equal in

geomagnetic activity, and inferior to the y

spots. Young spots, of the de0l type, appear

less disturbing geomagnetically than do mature,

limb-to-limb spots of corresponding radio-noise

properties. In regard to area (see table 8), the

large a spots appear more effective than the

smaller ones, both as storm-producers (R) and

as inhibitors (Q); however, the smaller P spots

appear to be better storm producers (R) than

the larger ones. This supports our earlier

finding that area of a sunspot group plays a

relatively minor role in its geomagnetic activity.

We find the geomagnetic activity of R spots

to be independent of the location of the spot in

the favorable or the unfavorable solar hemisphere, in accord with the findings of Simon

(1956a) and with our results of the previous

section. Nor do we find evidence for any hemisphere effect among the Q spots. The tendency

noted in the previous section, for favorably

located spots to be more effective inhibitors

than unfavorably located spots, is therefore of

doubtful significance and requires further exploration as more data become available.

Discussion

The data presented in this paper indicate that

no known property of active solar centers is

sufficient, in itself, for accurate prediction of

geomagnetic activity. They support, however,

the conclusion of Denisse and Simon that the

level of radio noise on meter wavelengths appears to be the most important single criterion

for distinguishing geomagnetically active sunspots. This result appears the more plausible

when we consider that the meter radio noise

arises in the region of the solar corona, whereas

most of the other properties considered arise in

lower levels of the solar atmosphere. If a sunspot does not produce a disturbance in the solar

corona, there seems little reason to expect it

to disturb the vicinity of the earth. For use

in forecasting, the radio-noise criterion is imperfect mainly because not all R spots produce

storms, rather than because any significant

number of storms occur in association with Q

spots.
178 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

The present paper has investigated primarily

the magnetic class of sunspots as a criterion of

geomagnetic activity. Among the optical criteria of the photospheric levels, we find the magnetic class of the spot to be the most significant.

The CMP of magnetically complex (7) sunspot

groups shows a markedly greater tendency to be

associated with a great geomagnetic storm than

does the CMP of spot groups with more regular

magnetic fields. The very greatest geomagnetic storms, in particular, show a tendency to

associate with magnetically complex sunspots.

We find also, in confirmation of earlier results

by Newton (1949), that flare-active spots tend

to be more disturbing geomagnetically than do

spots deficient in flares. Sunspot area, however, appears to be less important than previously believed, and to play an insignificant

role except for the fact that magnetically complex spots are most common among groups of

large area.

Great storms, like recurrent storms, show a

marked seasonal periodicity. The evidence,

however, appears to be clearly against any

hemisphere effect or axial explanation, such

as we found earlier for M-regions, in these

storms arising from active solar centers. Sunspots in die unfavorable solar hemisphere

appear to be as much associated with great

geomagnetic storms as do spots in the favorable

hemisphere. The cause for the seasonal variation in great storms thus remains obscure, and

Bartels' equinoctial hypothesis would appear to

continue to merit consideration.

It has for some time seemed probable that

the magnetic fields of active solar centers

must play a basic role in the ejection and/or

inhibition of solar corpuscles, as well as in the

direction of the ejected beam. Because of

the complexity of the forces arising from the

interplay of general and local solar magnetic

fields, any one-to-one relation between geomagnetic conditions and an observed solar phenomenon is scarcely to be hoped for. While

we may expect additional improvement in

the accuracy of geomagnetic forecasting from

futher analysis of observational criteria of

active centers, it appears doubtful that the

most reliable predictions can be made without

the aid of a comprehensive theoretical understanding of the behavior of ionized gases in

magnetic fields, in conjunction with the various

observational criteria. In the meantime, however, the results presented here suggest that

increased attention could profitably be given

to the patterns of the magnetic fields of sunspot

groups.

Acknowledgments

The work reported in this paper has been

supported by the Air Force Cambridge Research

Center, Geophysics Research Directorate,

through contract AF 19 (604)-146 with Harvard

University.

We wish to thank P. Simon for making his

thesis and his list of R and Q sunspots available

to us in advance of publication.

References

ALLEN, C. W.

1944. Monthly Notices Roy. Astron. Soc, vol. 104,

p. 13.

BARTELS, J.

1932. Journ. Terr. Magn., vol. 37, p. 1.

BARTELS, J., AND VELDKAMP, J.

1949. Journ. Geophys. Res., vol. 54, p. 296.

1951. Int. Assoc. Terr. Magn. Electr., No. 12f.

BECKER, U.

1953. Zeitschr. Astrophys., vol. 32, p. 195.

BELL, B., AND GLAZER, H.

1954a. Journ. Geophys. Res., vol. 69, p. 551.

1954b. Harvard Sci. Report, No. 17.

1956. Journ. Geophys. Res., vol. 61, p. 179.

1957. Smithsonian Contr. Astrophys., vol. 2, No.

5, p. 51.

BHARGAVA, B. N., AND NAQVI, A. M.

1954. Nature, vol. 173, p. 498.

CHREE, C, AND STAQO, J. M.

1927. Phil. Trans. Roy. Soc. A, vol. 227, p. 21.

DEXISSE, J. F.

1952. Ann. Geophys., vol. 8, p. 155.

1953. Comptes Rendns Acad. Sci., Paris, vol. 236,

p. 1856.

DENISSE, J. F. AND SIMON, P.

1954. Comptes Rendus Acad. Sci., Paris, vol. 238,

p. 1775.

GREENWICH, ROT. OBS.

1955. Sunspots and geomagnetic storm data, 1874-

1954. Her Majesty's Stationery Office,

London.

HALE, G. E., AND NICHOLSON, S. B.

1938. Magnetic observations of sunspots. Carnegie Inst. Publ. No. 498.

NEWTON, H. W.

1943. Monthly Notices Roy. Astron. Soc, vol. 103,

p. 244.

1949. Monthly Notices Roy. Astron. Soc, Geophys. Suppl., vol. 5, p. 321.
NO. 8 STJNSPOTS AND GEOMAGNETISM 179

NEWTON, H. W., AND MILSOM, A. S. SHAPLEY, A. H.

1964. Journ. Geophys. Res., vol. 59, p. 203. 1947. Trans. Amer. Geophys. Union, vol. 28, p.

NICHOLSON, S. B. 715.

1940. Publ. Astron. Soc. Pacific, vol. 52, p. 169. SIMON, P.

PECKER, J. C. AND ROBERTS, W. O. 1956a. Thesis, Obs. de Meudon.

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Abstract

This paper investigates the relation of geomagnetic conditions to the magnetic class, area, flare productivity,

radio noise, and location of sunspots. The primary emphasis is on the magnetic class, which is found to be the optical property of sunspots most significant in relation to geomagnetic conditions, although less significant than radio

noise. Magnetically complex (7) sunspots appear to be the most consistently disturbing, geomagnetically, and give

a marked rise in the superposed epoch Kp-curve for several days around central meridian passage. Semicomplex

(f)y) spot groups give a moderate rise in the geomagnetic index, Kp, while the simple unipolar (a) and bipolar (0)

spots give no significant deviation from the mean Kp. Flare productivity appears to be of moderate importance,

while sunspot area is relatively unimportant. Earlier results indicating area to be significant can be reconciled with

the present findings because the magnetically complex 7-spots are relatively most numerous among spot groups of

very large area.

The seasonal variation in geomagnetic activity is also investigated and is shown to occur in the daily Kp-snms

at all phases of the sunspot cycle. The axial hypothesis to explain the March and September maxima in great geomagnetic storms is not supported by the evidence. The geomagnetic activity of a sunspot appears to be independent

of its location in the favorable or the unfavorable solar hemisphere. The cause of the seasonal variation in great

storms remains unknown.