Smithsonian

Contributions to Astrophysics

VOLUME 5, NUMBER 7

MAJOR FLARES AND

GEOMAGNETIC ACTIVITY

by BARBARA BELL

SMITHSONIAN INSTITUTION

Washington, D.C.

1961
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Major Flares and Geomagnetic Activity

By BARBARA BELL

 1

Since Hale (1931) and Newton (1930, 1943)

pointed out the striking tendency of great solar

flares to be followed by more or less severe

disturbance of the earth's magnetic field, it has

become generally accepted that nonrecurrent,

sudden-commencement (sc) geomagnetic storms

result from clouds of ionized gases (corpuscles)

ejected by the flaring solar region and interacting with the field of the earth. However, there

exists no one-to-one correlation between major

flares and geomagnetic storms. A discouragingly high percentage of even bright and optically impressive flares are not followed by any

significant disturbance of the earth's magnetic

field.

Recent work has been concentrated largely

on a search for one or more properties of flares

that will permit forecasters to distinguish stormproducing from non-disturbing flares. Radionoise studies have to date provided the most

useful criteria for this purpose, and substantial

progress has been made by Dodson and Hedeman (1958), Simon (1956, 1959), Maxwell,

Thompson and Garmire (1959), and others.

It is only reasonable that radio outbursts should

provide the most reliable criteria, since these

radiations arise at higher levels in the solar atmosphere than do the optical flares; if a flare

cannot disturb the outer solar atmosphere, it

would not seem to have much chance to disturb

the earth.

Significant and useful as the radio noise properties of a flare are, however, a radio outburst

would appear more in the nature of evidence of

corpuscular emission rather than a cause of it.

Casual factors more probably reside in the details of the magnetic field and the hydromagnetic

forces that produce the flare itself. Unfortunately, we cannot yet study these forces observa1

 Harvard College Observatory, Cambridge, Masr

tionally in anything like the necessary detail.

The only data available are the Mount Wilson

Observatory's classification of the magneticfield types of sunspot groups, a classification

that has been carried out systematically since

1917.

Bell and Glazer (1958) used these Mount

Wilson data to investigate the geomagnetic consequences of the central meridian passage

(CMP) of large sunspot groups. They found

the magnetic-field type to be the most useful

optical criterion we yet have for separating geomagnetically disturbing spots from others of

equally large area. In the present paper, the

geomagnetic consequences of major flares will

be discussed, with emphasis on the magnetic

class of the flaring spot group. Location of the

flare on the solar disc will also be considered,

and evidence presented that a flare in the

northern solar hemisphere has a substantially

greater probability of producing a great magnetic storm than does a southern flare.

Data

For the purposes of this investigation, a major

flare is one rated importance 3 or 3 + by at least

one observatory, or 2+ by two or more observatories. The IAU Quarterly Bulletin of Solar

Activity yielded 580 major flares in the 23

years, 1937-1959, each associated with a

specified active region. By matching active

regions with spot groups, I was able to link

most major flares with a particular Mount

Wilson sunspot. About two percent of the

flares (12 flares), however, occurred in the

absence of any identifiable sunspot, while

another two percent (11 flares) occurred in the

region of an unborn or an extinct sunspot.

Most of the flares occurred in association with

sizable sunspots, but about five percent (32

69
70 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

flares) were associated with either no spot or a

small spot of maximum observed field strength

less than 1000 gauss.

The Mount Wilson classification of sunspots

according to their magnetic properties (Hale

and Nicholson, 1938) recognizes four basic

categories of sunspot groups: unipolar (a),

bipolar (&), semicomplex (/3-y), and complex

(7). Unipolar (a) groups are single spots or

groups of spots having the same magnetic

polarity. Bipolar (0) groups 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 (7) groups

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. Semicomplex (#y) groups show

bipolar characteristics but lack a clearly marked

dividing line between the regions of opposite

polarities; this category includes groups whose

preceding or following members are accompanied by small spots having an opposite

polarity.

Through 1958 the average magnetic classification for each spot group was published by the

Mount Wilson Observatory in Publications of

the Astronomical Society of the Pacific. For

the present study, however, the magnetic type

on the flare day was needed, since the magnetic

type of many groups changes during the 14 days

of a disc passage. With the cooperation of

Mount Wilson, J. G. Wolbach obtained for me

the daily classifications of the relevant sunspots. Because of gaps in the observations,

no reasonable estimate could be obtained for

the flare-day magnetic type for about 12 percent of the flares. About 15 percent had a

flare-day type significantly different from the

average classification of the associated spot.

Each flare was assigned to one of four categories of geomagnetic "success," according to

the behavior of the Kp and Ap magnetic indices in the three days following the flare: (1)

no storm (Ap<25 on all three days following

the flare); (2) small storm (Ap and Kp both >

25 on at least one day); (3) moderate storm

(one or more Ap>50 and/or a 3-hr Kp>7+);

and (4) great storm (one Ap> 100 and/or one

3-hr Kp>9~). While the daily Kp is used

below in the superposed epoch analyses, Ap

has been given the greater weight in classifying

the magnetic storms. Ap, having a linear

scale, is a more sensitive indicator of differences

in the magnitudes of the storms; however, because it departs radically from a normal distribution, one or two large storms could distort

a superposed epoch curve. Kp, a logarithmic

measure of disturbance, also lacks a normal

distribution but does come much closer to it

than does Ap; thus Kp seems to me preferable

for use in superposed epoch analyses. Most

flares could be assigned to a geomagneticsuccess category without difficulty; the few

borderline cases should not significantly distort the results.

If a given day had two or more major flares,

this day entered two or more times into the

analysis.

Figure la shows a superposed epoch diagram

of Kp where the flares have been sorted solely on

the basis of their geomagnetic success. Day

zero is the flare day. Figure la illustrates the

differences between the geomagnetic classes

and may be useful in interpreting other superposed epoch curves in this and other papers.

Figure lb shows only the great storms, subdivided into great (4) and extra great (4+

), the

latter having at least two 3-hr Kp>9~ or one

Kp=9°. Note that this 4 + curve has its

maximum one day after the flare; the class 3

and 4 curves have their maxima on day +2, and

the class 2 curves on day +3 . This progression

would suggest that the more violent the storm,

the shorter the time lag. Note also the two

strong subsidiary peaks in the 4+ curve. A

spot having one flare vigorous enough to give

rise to a 4+ magnetic storm seems particularly

likely to have other storm-producing flares

during its disc passage. Spots having central

meridian passage (CMP) on 18 January 1938,

26 March 1940, 27 March 1946, 19 September

1946, 23 January 1949, and 14 July 1959, each

produced two or more great storms, and several

other spots produced one great and one or more

lesser storms. This tendency accounts for the

lack of sharpness in most of the superposed

epoch curves in this paper.
MAJOR FLARES AND GEOMAGNETIC ACTIVITY

4 8 DAYS -4 0

2 0 -

2 0 -

FIGURE 1.—Average geomagnetic conditions (Kp) after major flares followedjwithin three days by (in a)

a great geomagnetic storm (0—#) , a moderate storm (O—O)»

 a small storm ( ), no storm

(X X), and (in b) by a great storm (A—A) and an extra-great storm (A—A)-
72 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL. 5

Location of Flares

Newton (1943) and several subsequent

writers have pointed out that those flares that

are followed by great storms generally occur

within about 45° of the solar central meridian

(CM). Figure 2a shows the distribution of

the present 580 major flares with solar central

meridian distance. The shadings correspond

to the four categories of geomagnetic success.

Figure 2b shows the longitude distribution in

terms of the ratio of successes to the total

number in the particular longitude interval.

The longitude distribution shows no castwest asymmetry. Indeed, exactly half (290)

of the flares occurred east of the CM, and half

of them west of it. Nor do great-storm flares

60

N

4 0

20

show any significant asymmetry, 37 of them

being west of the CM, and 36 east of it; of

small-storm flares, we find 75 east and 77 west.

A marked excess west of the CM appears, however, in moderate-storm flares, with 46 west

and only 28 east of the CM. A compensating

excess east of the CM occurs among the failures,

with 130 west and 151 east of the CM. According to the normal distribution law, the

probability that an asymmetry as large as that

shown by moderate-storm flares should arise

by chance is about 10~3.

It is beyond the scope of this paper to theorize

on possible causes of the anomalous distribution in moderate-storm flares. It is perhaps

relevant to note, however, that Hartz and

•

Hi

•

1

m

—

msgem

90° 3Oe

60°

o o80

60

40

FIGURE 2.—Distribution of 580

major solar flares with central

meridian (CM) distance, tf,

Number of flares; b, percent in

the given latitude interval.

Shadings indicate flares followed

within 3 days by a great storm

(black), by a moderate storm

(dark gray), by a small storm

(light gray), and by no storm

(white).

£ 90°
NO. 7 MAJOR FLARES AND GEOMAGNETIC ACTIVITY 73

McAlpine (1960) found a very pronounced

western excess in number of flares followed by

polar cap absorption and a weak Forbusb decrease in cosmic ray intensity, but no western

excess among flares followed by polar cap absorption and a strong Forbusb. decrease. Their

interpretation of this difference in terms of

the fields in interplanetary space might be

applied also to the differing distributions of

great- and moderate-storm flares; the possibility at least deserves further study.

Figure 3a shows the longitude variation when

flares in the eastern and western hemispheres

are combined and the data smoothed by running means over 30° intervals (20° at each end).

Figures 36 and 3c show the smoothed distributions in terms of percents of successes. Previous workers (Behr and Siedentopf, 1952;

Waldmeier and Bachmann, 1959) found the

visibility function to the limb of flares to be

roughly proportional to the cosine of the longitude from the CM. However, the distribution

of the major flares plotted in figure 3a does not

fit a cosine law, and indeed appears perhaps

best represented by a straight line. Both east

and west show fewer flares in the 0-9° zone than

in the adjacent zones. This dip can be only

partially explained by the fact that 9°E to 9°W

contains one degree less than the 10-19° zones;

however, its signifiance is doubtful since Waldmeier and Bachmann (1959) report a contrary

excess of flares of importance 2 and 3 in the

central zone as compared to the 11-20° zone

for the years 1945-1954.

Figures 2a and 3a confirm the previously reported tendency for great-storm flares to be

60° 90°

FIGURE 3.—Smoothed distribution of major flares with CM

distance, a, Number of flares (see legend of fig. 2). b,

Percent followed by various geomagnetic conditions (see

legend of fig. 2). c. Percent followed within 3 days by a

storm (see legend of fig. la).
74 SMITHSONIA

N CONTRIBUTIONS TO ASTROPHYSICS

concentrated toward the CM. The center-to-

limb decrease in the number of great-storm

flares fits neither a cosine nor a linear distribu-

tion law. At first, the frequency decreases rap-

idly with increasing distance but then levels off

around 50° and shows a slight rise at the limb.

If we consider the ratio of storm flares to all

major flares in the given longitude range, we

see (fig. 3c) that the probability of a great

storm reaches a minimum for a flare in the 60-

69° interval. The probability of a moderate

storm shows no significant dependence on the

distance from the CM, while the probability of

a small storm actually increases toward the

limb. (Some of these small-storm flares would

probably give rise to great storms if they were

located closer to the CM.) The probability of

failure (no storm) appears to be greatest for

flares around 35° (unshaded section of figure 36).

Figure 4a maps the location of all major flares

that were followed by great magnetic storms

within three days. Figure 46 similarly shows

the location of all major flares that were fol-

lowed by moderate storms. The symbols here

indicate the magnetic type of the flaring spot,

the actual flare-day type being used whenever

known. I originally plotted this figure to look

for differences in longitude distribution between

the magnetic types. Its striking and unex-

pected feature, however, is the strong prefer-

ence of great-storm flares for the northern solar

hemisphere over the southern hemisphere.

Further data on latitude distribution and the

north-south asymmetry are shown in figures

5-7 and in table 1.

In the 23 years studied, about 56 percent of

all observed flares, regardless of importance,

occurred in the northern solar hemisphere. Of

the 580 major flares in this study, 62 percent

occurred in the north (see table 1). Northern

spot groups, however, produced 86 percent of

the 74 major flares that were followed by a

great storm (64 north and 10 south), and 64

percent of the 70 that were followed by moder-

ate storms, but only 52 percent of the 153 that

were followed by small storms and 60 percent

of the 284 failures. Thus 64 percent of the

flares that were followed by at least a small

storm occurred in the north. The north domi-

nance increases strongly with increasing magni-

tude of the geomagnetic disturbance. The

-I _l 1_ cP

o:

 o Oo o * >P o %K

§

x

:

= 4 5 a. 6 3

lsS

.9. B
MAJOR FLARES AND GEOMAGNETIC ACTIVITY 75

o .

i

0

D

• *

• c

3}

o

x x

o

s •'

,-—r

•

>••

0

*

t

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O

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- t 1 Vo '

T3

if) -S

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D76077—61-
76 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

probability of a great storm after a northern

major flare is 0.176, contrasted with only 0.045

after a southern major flare. The probability

of so large an asymmetry occurring by chance

is less than 10~3.

If we consider storms instead of flares, we

find 46 great storms in the data, of which 37

were preceded by northern, 6 by southern, and

3 (on 2 and 4 September 1957, and 15 July 1959)

by both northern and southern major flares.

The data include 53 moderate storms, 32 of

them preceded by northern, 14 by southern

flares, and 7 by both a northern and a southern

100

N

80

60

40

20

flare. As figure 7 shows, this astonishing dominance of the northern solar hemisphere is present in all three sunspot maxima that were

studied and shows no apparent relation to the

22-year cycle observed in the magnetic polarities of sunspot. Evidence for a longer cycle

will be explored in a subsequent paper.

Magnetic type of flaring spot

Figure 8 shows the number of major flares

observed in association with spot groups of the

four basic magnetic types, and the probability

of geomagnetic success of each type of major

100

80

60

40

20

S 30° 20° 10

OL

FIGURE 6.—Distribution of major

flares with solar latitude (see

legend of fig. 2). a, Number of

flares, b, Percentage in the given

latitude interval followed by each

geomagnetic condition.

S 20° 10° j 0° 10° 20
MAJOR FLARES AND GEOMAGNETIC ACTIVITY

CYCLE ] 7 J8 19

77

flare. Bipolar spot groups are much more

numerous than complex ones, although the

latter on the average produce more flares per

group (Hale and Nicholson, 1938; Giovanelli,

1939; Bell and Glazer, 1959). The distribution

in figure 8a arises from a combination of these

two factors. Figure 86 illustrates the much

larger probability of a great or moderate geomagnetic disturbance following a flare associated

with a complex group, as compared with a bipolar group.

Superposed epoch curves appear in figure 9,

with the flares grouped according to (a) average

magnetic type, and (b) flare-day magnetic type

N

240

200

160

120

80

4 0 -

OL 1i

r

•

100

°/

80

60

40

20

0 -

FIGURE 7.—Percentage of observed major

flares in each spot cycle (NOB. 17-19)

occurring in the northern solar hemisphere. Left: Heavy line indicates major

flares; thin line, all flares. Right: See

legend of figure la.

of the associated spot group. Flares of all CM

distances are included here. The mean value

of Kp is 20.75, derived from the annual mean

values of Kp, with each year weighted according

to the number of major flares observed in that

year. For each magnetic type, the peak value

of Kp is higher when the flare-day type is used,

presumably because in this case those flares

whose flare-day type could not be estimated

with reasonable confidence were omitted. Most

of the differences between figure 9a and 96, however, are minor. The only conspicuous difference appears in the unipolar or a curve,

where the flare-day a's have better geomagnetic

success than those whose average type is a.

By

•

m

1

•

i

•HUB]

FIGURE 8.—Magnetic type of sunspot groups associated with major

flares, a, Number of flares associated with (left to right) a, 0,

fiy, and y spots, b, Percentage

of each type of flare followed

within 3 days by a great storm

(black), moderate storm (dark

gray), small storm (light gray),

and no storm (white).

a
78 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

Figure 9 may be compared with figure 1 of

Bell and Glazer (1958), which shows the geomagnetic conditions associated with the CMP

of large sunspot groups of each magnetic type.

While the differences between these two figures

are not striking, the higher level of geomagnetic

disturbance is obtained from each magnetic

type when the dates of major flares, rather than

the CMP dates of large sunspot groups, define

the zero day. In the y case the difference between flare and spot curves is small; but for

the 07, j8, and a types the flares give substantially more disturbance. The systematic direction of the difference between flare and spot

curves supports the now prevalent opinion that

the emission of a corpuscular stream from the

sun is associated with the occurrence of a major

flare rather than with the CMP of the sunspot

region. The small difference between the y

curves from flares and from spots suggests

that a given y spot is more likely than any

other type to produce one or more major, and

geoactive, flares.

The double maximum in the 7-flare curves

probably arises from the tendency of an active

y spot to produce more than one disturbing

flare during its disc passage (cf. figure 16). I

have no explanation for the strange form of

the a-flare curves, with their major peak two

days before the flare day.

The influence of the disc position of the flare

on superposed epoch curves appears in figures

10 and 11. The former shows that the effect

of separating the flares of figure 96 according

to CM distance is minor. The greater probability of a small storm from a flare at CM distance beyond 45° in large measure compensates

for the reduced probability of a great storm

from a flare in these noncentral regions (fig. 3c).

The curves in figure 11 were computed at a

different time from those of figure 10 and include some additional flares of unknown flareday type that were assigned on the basis of the

average type of the spot. Note that in figure

11 the geomagnetic difference between northern

and southern flares is particularly striking for

/37 and 0 flares, and negligible for a and y flares.

In this connection it may be of interest that

T. Cragg (private communication via J. Wolbach) remarked that when an a spot becomes

complex it tends to become a y, but when a /3

becomes complex, it more commonly becomes j8?.

To investigate further the north-south asymmetry, I used the large sunspots previously

studied (Bell and Glazer, 1958), separated the

north and the south spots of each magnetic

type, and obtained the results shown in figure

12 and table 2. In essential agreement with

the flare results, northern y and f3y spot groups

appear conspicuously more disturbing than

southern ones. (Note that the spot data cover

1937-1953 and do not include the current

maximum.)

Denisse (1952) and Simon (1956) have

demonstrated that radio noise at meter wavelengths is a valuable criterion for picking out

geomagnetically disturbing spots (see also Bell

and Glazer, 1958). It is of interest to look for

north-south asymmetries in radio-noisy (R) and

radio-quiet (Q) spot groups. Using Simon's

list of R and Q spots, but including only those

of area greater than 500 millionths of a solar

hemisphere, I find that the distribution of R

and Q spots shows no marked north-south

asymmetry. However, as table 3 shows, the

geomagnetic difference between R and Q spots

is very much clearer for northern than for

southern spots. The Q spots act, geomagnetically, about the same in the north and in

the south. Southern R spots are on the average

geomagnetically inactive, while the northern

R spots are quite disturbing.

One may well ask whether northern active

centers seem to possess a greater disturbing

power because complex spots and associated

flares are relatively more numerous in the north,

TABLE 1.—Percentage of major flares in the northern

solar hemisphere

Magnetic

class

7

07

a

unclassified

Average

Followed within 3 days by

Great

storm

90%

100

92

12

86

Moderate

storm

71 of •

x

 /o

76

53

86

64

Small

storm

68%

47

51

36

53

No

storm

85%

63

56

45

60

Avg.

78%

69

55

44

01

02
MAJOR FLARES AND GEOMAGNETIC ACTIVITY 79

or because, regardless of magnetic type, northern active centers are simply more vigorous

ejectors of corpuscles. The 7 regions decisively

favor the north in the years under study, with

14 northern and 6 southern large spots; 7 spots

contribute 21.5 percent of the northern major

flares and 8.7 percent of the southern. The /S7

spots, on the other hand, are somewhat more

numerous in the south, although fty spots contribute 28 percent of the northern but only 20.2

percent of the southern flares. Southern (3y

spots and flares, on the average, are geomagnetic

failures, while northern ones rival the 7's in

storm-producing power. Thus a greater proportion of the northern flares do arise in magnetically complex regions; but /S and £7 data

clearly suggest that northern flares of a given

type are far more likely to produce significant

geomagnetic disturbance than are corresponding

southern flares. For all types except the a,

north dominance increases with increasing

storm intensity. The probability that a given

flare region will eject enough corpuscles to produce at least a minor storm is only slightly

greater for northern flares than for southern;

but the probability of an ejection vigorous

enough to produce a great storm is much greater

for a northern flare.

These facts suggest that northern centers,

when they do eject corpuscles, may do so at

higher velocities than southern centers. Or,

insofar as one believes that storm magnitude is

determined by the number rather than by the

velocity of the corpuscles, one would conclude

that a northern flare region ejects on the average

a larger number of corpuscles than does a southern region.

Another solar-geomagnetic relation might be

mentioned in connection with this north-south

asymmetry. Bell and Glazer (1957) found that

in the declining years of cycle 18, geomagnetic

activity showed a negative correlation with the

brightness of the green X5303 coronal line in the

solar hemisphere on the same side of the solar

equator as the earth, the so-called favorable

hemisphere. They also briefly considered north

and south coronal intensities simultaneously.

In the autumn, with the northern solar hemisphere favorable (Nf), the correlations appeared

not to be influenced by southern coronal intensities. In the spring, when the earth was south

of the solar equator, the data were more amKp

28

26

24

22

20

-

-

-

-

-

i

-4

I

, /

TV

i

0 4

A

K

-/I

1

1 1 i

AA

V\\/ x—\x7

1 1 1

8

i

S

DAYS

1

-

-

-

-

) i l •

a

-4

2 0 -

FIGURE 9.—Average geomagnetic conditions (Kp) on days around the occurrence of a major flare. Day

zero is the flare day. Flares are divided according to average magnetic type (a) and flare-day

magnetic type (b) of the associated spot group.
80

-4

SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

8 -4

4-r4 T—i—r

VOL. 5

DAYS

KP

28

24

20

I I I I I I I I I

fi

i i i i I j I I I I i I I i i i i i

FIGURE 10.—Average geomagnetic conditions (Kp) on days around the occurrence of a major flare, subdivided by flare-day magnetic type of the associated spot. Solid line indicates flares within 45° of

the CM; broken line, flares more than 45° from CM.

-4 AYS

2 0 -

FIGURE 11.—Average geomagnetic conditions (.£/>) on days around the occurrence of a major flare,

subdivided by flare-day magnetic type of the associated spot. Solid line indicates flares in the

northern solar hemisphere; broken line, flares in the southern solar hemisphere.

FIGURE 12.—Average geomagnetic conditions (Kp) on days around central meridian passage (0=CMP

day) of large (A >500 millionths) sunspot groups of the four magnetic types. Solid line indicates

northern spots; broken line, southern spots.
MAJOR FLARES AND GEOMAGNETIC ACTIVITY 81

biguous; the authors found "some indication

that Sf is more effective in giving the 'expected'

Kp results when paired with a similar Nu. The

data appear to suggest that the northern solar

hemisphere may have a more powerful 'control'

over Kp conditions than the southern; but in

view of the short time covered, we would not

propose that such an inherently unlikely suggestion to be taken too seriously" (Bell and

Glazer. 1957, p. 75 and fig. 18).

Other results

The 580 major flares were sorted by month,

with the results as shown in table 4 and figure

13. The well-known seasonal variation in

geomagnetic activity appears most clearly in

the variation in probability of a failure (top,

unshaded portions of figure 13) and much less

clearly in the probability of a great storm where

an October-December minimum is the most

conspicuous feature. The apparent seasonal

variation in the probability of a great storm in

figure 13 is distorted by observational selection,

as can be seen from the final column of table

4, which gives the total number of great storms

without regard for whether the storm was preceded by an observed major flare. (The definition of great storm remains the same as that

used elsewhere in this paper.)

The flares of each magnetic type were next

grouped according to whether they were on the

same (favorable) or opposite (unfavorable) side

of the solar equator as the earth. In agreement

with the results that Bell and Glazer (1958)

TABLE 2.—North-south asymmetry in geomagnetic conditions associated with major flares (1987-1959) and with

CMP of large sunspot groups (1937-1953)

JFMAMJJ A S 0 N D 100

80

60

40

20

OL

FIGURE 13.—Percentage of observed major flares in each month

followed by a great storm (black), moderate storm (dark

gray), small storm (light gray), and no storm (white).

Magnetic

class

7

7

e

a

Hemisphere

N

S

N

S

N

S

N

S

Major flares

No.

72

18

94

42

140

113

26

35

Kp

(max) *

29. 6(2)

28. 0(3)

29. 9(2)

22. 2(3)

23. 3(3)

21. 3(3)

23. 1(4)

24 7(1)

Large sunspots

No.

14

6

21

25

51

62

19

10

Kp

(max)*

29. 4(4)

24. 8(0)

27. 2(2)

20. 0(3)

20. 5(2)

19. 9(0)

23. 0(2)

22. 9(3)

Kp

(0,+4)t

2a 3

23.0

25.4

18.9

19.3

18.7

21.2

19.6

•Highest average value of the Kp index In the 5-day interval (0, +4),

with day of occurrence in parentheses.

t-Kp Index of geomagnetic activity averaged over the five days (0)

through (+4), around OMP of spot groups.

determined from spot groups, no significant

geomagnetic difference was found between the

favorable and the unfavorable flares of any

given type. Thus it seems clear that the axial

hypothesis is not an adequate explanation for

the seasonal variation in frequency of great and

other lesser nonrecurrent storms.

Mclntosh (1959) has found evidence for a

component in the diurnal variation of disturbance which seems clearly related to the obliquity of the earth's magnetic axis relative to

the earth-sun line, along which the solar corpuscles are presumed to travel. He revives a

long-neglected suggestion of Bartels to argue

that the semiannual variation in disturbance

also most probably arises from the seasonal

TABLE 3.—North-south asymmetry in geomagnetic conditions associated with CMP of large radio^noisy (R) and

radio-quite (Q) sunspots

Location

North

South

Radio

noise

R

Q

R

Q

No. spots

with £p(0,+4)

>25.0

19

1

7

2

20.0-

24.9

8

5

10

3

<20.0

1

14

13

10

Total

No.

28

20

30

15

Kp(0,+4)

27.8

17.8

20.4

17.8
82

SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL. 5

variation in the angle between the earth's

magnetic axis and the earth-sun line. In view

of the difficulties encountered by the equinoctial

and the axial hypotheses

 2 Mclntosh's suggestion would appear to deserve serious consideration.

TABLE 4.—Seasonal distribution of observed major flares

and their geomagnetic consequences, 1987-1959

Month

Jan

Feb

MaiApr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

No. flares followed

within 3

Great

storm

7

7

7

5

5

3

10

5

24

1

0

0

Moderate

storm

1

3

5

10

4

11

7

15

4

5

6

3

days by

Small

storm

10

17

18

11

15

8

13

11

18

11

8

10

No

storm

20

20

17

21

26

27

33

28

23

22

22

23

Total

major

flares

38

47

47

47

50

49

63

59

69

39

36

36

Total

great

mag.

storms

5

4

12

6

4

3

5

5

13

2

0

0

TABLE 5.—Geomagnetic conditions following major flares,

subdivided by area (in heliographic square degrees)

Area of

flare

< 1 0

10-19

20-29

30-39

40-49

>50

No.

flares

33

140

94

33

14

14

Avg.

CM

distance

34°

34

40

41

45

40

Kp (max)

27.9(1)

24. 4(2)

24 6(3)

23. 9(2)

27. 0(2)

34 2(2)

Percent

north

70%

60

55

64

50

64

Percent

followed by

Great

storm

6%

11

14

6

14

36

Moderate

storm

1 5%

13

13

12

0

21

For many of the flares observed since 1949,

the IAU Quarterly Bulletin gives data on the

intensity of Ha in units of the neighboring

continuum intensity, and on the area in heliographic square degrees. The largest reported

* For statements of these hypotheses, consult Bartels (1932) or Bell and

Glawr (1957,1958).

value was recorded for each flare. The flares

were sorted by area and by intensity, and the

geomagnetic superposed epoch curves computed, with the results summarized in tables

5 and 6. For the years 1949-1959 the weighted

mean of Kp is 21.8. The data give little

evidence for any systematic increase in geomagnetic effectiveness with either flare area or

intensity. However, flares of area greater than

50 square degrees, or Ha intensity exceeding

200 percent of the neighboring continuum, do

appear to be substantially more disturbing than

smaller and/or weaker flares. There is no clearcut relation between flare area or intensity and

the north-south asymmetry.

TABLE 6.—Geomagnetic conditions following major flares,

subdivided by intensity of Ha (in units of the neighboring continuum intensity)

Intensity

of Ha

<100

100-149

150-199

200-299

>300

No.

flares

52

73

34

35

11

Avg.

CM

distance

41°

43

37

33

34

Kp (max)

24 3(1)

24 2(1)

25. 0(2)

30. 1(2)

31.9(1)

Percent

north

60%

48

56

74

64

Percent

followed by

Great

storm

1 3%

8

15

20

27

Moderate

storm

12%

15

9

14

9

Acknowledgments

The work reported in this paper has been supported by the Air Force Cambridge Research

Center, Geophysics Research Directorate,

through contract AF19 (604)^962 with Harvard

University.

The cooperation of the Mount Wilson Observatory in granting access to its sunspot

classification records is gratefully acknowledged.

I wish particularly to thank John G. Wolbach

of Harvard, who copied the required daily

classifications from these records, and Thomas

A. Cragg of Mount Wilson for his valuable

cooperation. I am also grateful to G. Olivieri

and the Meudon Observatory for making the

1958 and 1959 flare data available to me in

advance of publication.
MAJOR FLARES AND GEOMAGNETIC ACTIVITY 83

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Abstract

Relations between geomagnetic activity and major (importance > 2 +) solar flares are studied, with primary attention

to magnetic type and location of the flaring sunspot group. The data cover the years 1937-1959 and include 580

observed major flares. It is found that a major flare occurring in association with a magnetically complex (y or

Py) sunspot group is much more likely to be followed by a major geomagnetic storm than is a similar flare in a unipolar (a) or bipolar (/3) group. Great-storm flares show the expected concentration toward the central regions of

the solar disc, and also an unexpected concentration in the northern solar hemisphere. In the 23 years studied,

northern spot groups produced 62 percent of all observed major flares, and 86 percent of those followed within 3 days

by a great geomagnetic storm. This north predominance of great-storm flares appears about equally in each of the

three sunspot maxima covered and is apparently not related to the 11-year or 12-year solar cycles.