SMITHSONIAN

CONTRIBUTIONS

to

ASTROPHYSICS

Smithsonian Institution

Astrophysical Observatory

Volume 10, Number 4

Some Additional Aspects of

the Unequal Activity of the

Northern and Southern

Solar Hemispheres:

Microwave Radio Bursts

and SWFs

by Gladys A. Harvey

and Barbara Bell

Smithsonian Institution Press

Smithsonian

Contributions to Astrophysics

VOLUME 10, NUMBER 4

SOME ADDITIONAL ASPECTS OF THE UNEQUAL ACTIVITY OF THE

NORTHERN AND SOUTHERN SOLAR HEMISPHERES:

MICROWAVE RADIO BURSTS AND SWFs

by GLADYS A. HARVEY and BARBARA BELL

ISSUED

MAR 28 1968

SMITHSONIAN INSTITUTION PRESS

Washington, D.C,

1968
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Among these are Smithsonian Physical Tables, Smithsonian Meteorological

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Some Additional Aspects of the Unequal Activity of the

Northern and Southern Solar Hemispheres: Microwave

Radio Bursts and SWFs

Gladys A. Harvey

 l and Barbara Bell2

1. Introduction

In a study of the geomagnetic effects of major

solar flares observed from 1937 through 1959,

Bell (1961) found that northern sunspot

groups produced 62% of all observed major

flares and a significantly larger proportion

(86%) of those followed by a great geomagnetic

storm. Subsequent investigations (Bell, 1962;

Bell and Wolbach, 1962) showed that the

Northern Hemisphere of the sun has been the

more frequent source of great storms for most

of this century, while the Southern Hemisphere

was the more frequent source in the latter

half of the 19th century. Detailed study of

several active-sun phenomena observed from

1957 through 1960 (Bell, 1963a,b) provided

additional information on the unequal activity

of the two solar hemispheres, but left unanswered certain questions of potential interest

to those concerned either with forecasting solar

activity and its terrestrial consequences or

with theoretical understanding of active-sun

phenomena.

Bell (1961, 1963a) showed that major flares

were more concentrated to the solar North

than were flares in general. In the present

paper we investigate each category of flare

importance to see whether the north-south

asymmetry increases progressively with flare

importance, or whether it increases abruptly

for major flares.

We also investigate here the hemispheric

distribution of microwave bursts. In a study

of meterwave bursts of spectral types II and

• National Research Council, Ottawa, Canada.

* Harvard College Observatory, Cambridge, Mass.

IV, Bell (1963a,b) showed that long-lived

bursts of each type are more concentrated to

the North than are short-lived bursts. However, duration alone is not the ideal measure

of the "importance" of a burst. (No quantitative measures of intensity or energy content

are available from present spectrum records.)

Microwave-burst emission is observed much

more frequently than the relatively rare emission of spectral types II and IV, and covers a

wide range of magnitudes for which quantitative

measures are available.

The magnitude of the microwave burst is

investigated by hemisphere in relation to three

terrestrial effects: polar-cap-absorption (PCA)

events and geomagnetic storms, arising respectively from high- and low-energy corpuscular

radiation, and sudden ionospheric disturbances,

arising from shortwave electromagnetic radiation. The ionospheric events have been

examined for their relationship to solar flares

and microwave bursts (Hachenberg and Kriiger,

1959; Harvey, 1964) without consideration of the

hemispheric location of the source of the

disturbance. We examine the hemispheric

distribution of the sources of shortwave fadeouts

(SWF) to determine whether a disproportionate

number of major SWFs, like great magnetic

storms, arose from northern active regions.

In a study of 2800-MHz bursts, Harvey

(1965) showed that only major bursts having a

peak intensity >100 flux units and a broad

burst profile (half-width>8 min) were strongly

associated with major geomagnetic storms. We

investigate here whether the dearth of great

storms from southern active regions parallels

197
198

SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

a dearth of southern broad major bursts, or

whether even broad major bursts failed to be

followed by a great storm when they arose

from a southern center.

In the next section we discuss each manifestation of solar activity individually, while in the

following section we consider various solarterrestrial relationships according to the solar

hemisphere from which the activity originates.

2. Hemispheric distribution of active-sun

phenomena

We selected for study the 3.5 years from July

1957 through December 1960. Observations are

unusually complete for this period, and it is

long and active enough to provide sufficient

data to establish the similarity or contrast in

the hemispheric trends displayed by various

types of solar activity. Table 1 lists the types

and sources of the data that we analyzed.

Figure 1 shows the results for both the solar

events and the most probable sources of terrestrial events, the total number of events

observed in each importance category, and the

percentage occurring in the Northern Solar

Hemisphere.

TABLE 1.—Types and sources of data analyzed

Activity

Sunspots

Plages

Flares

Major microwave

bursts (2800 to

3750 MHz)

Microwave bursts

(2800 MHz)

Shortwave fadeouts

Geomagnetic storms

Polar-cap absorption

Sources of data

Photoheliographic Results,

Royal Greenwich Observatory Bulletins

McMath-Hulbert Observatory

McMath-Hulbert Observatory Working Lists of Flares

for the IGY, IGC, and 1960

A. D. Fokker (private communication)

National Research Council

CRPL-F Series Bulletins,

Part B, Solar-Geophysical

Data

CRPL-F Series Bulletins,

Part B

Bell list, unpublished, based

i

Bailey (1964)

a. Sunspots.—The distribution of sunspot groups

(58.9% N), shown in the first line of figure 1,

is the basic asymmetry for the period, a norm

to which the other percentages should be compared. That the two solar hemispheres should

have so unequal a number of spot groups has

itself a very small probability of chance occurrence (P<10~10

 for the observed distribution of

2690 spots, with 50% N "expected"). Of

greater interest here, however, is the fact that

the distribution of some spot-linked forms of

solar activity deviates strikingly from the spot

distribution.

Figure 1 shows also that when spot groups are

subdivided by mean area (in units of 10 ° of

the visible hemisphere), the categories up to an

area of 1000 have almost identical hemispheric

distributions. The percentage North is somewhat greater for spot groups of area >1000,

but the difference is not statistically significant.

The percentage North of the largest spot groups

remains less than that of several forms of spotlinked activity.

b. Plages.—Considering only those plage regions

that were identified with one or more flares in

the Working Lists, we tabulated the number of

flares listed as occurring in each region during

its disk passage and grouped the plages into

three categories according to the number of

flares. As expected, the percentage of plages

located in the Northern Solar Hemisphere is

very close to that for sunspot groups. However,

figure 1 shows that the percentage in the Northern Hemisphere increases with plage importance, when plage importance is rated by

the number of flares observed in the plage.

c. Flares.—When referred to the norm of spot

groups, the Northern Hemisphere is significantly (P=0.0003) more productive of flares

than the Southern. With an increase in flare

importance from 1 to 2 (as rated in the Working

lists), figure 1 shows a small, but not statistically significant, increase in percentage North.

With a further increase in flare importance to

3 and 3+, we see a marked increase in percentage from the Northern Solar Hemisphere.

When the expected asymmetry is taken as

60.6%, the average for all flares > 1 in these

years, the probability is 0.02 that an asymmetry

as great as the observed 72.7% N for
MICROWAVE RADIO BURSTS AND SWFS 199

ACTIVITY TOTAL PERCENT 50% N

NUMBER NORTH I

70

I

eo

i

100

I

SUNSPOT GROUPS 2692 58.9

AREA (10-6)

PLAGES

NO. FLARES

FLARES

IMPORTANCE

MICROWAVE

I I .

I

<IOO

I00-499

500-999

>IOOO

1-19

20-59

> 6 0

1

1 +

2

2 *

3

3 +

BURSTS

500 - 990

1000- 3000

3100 - 9900

0000 - 30000

31000- 99000

2800-MHz

(1) I < 10,

>IOOOOO

BURSTS

E <IOOO

(2) 10 <I < 100. E< 1000

(3)1 2 100 OR E 2 1000

(So) KIO O

(3b ) 1 > 100

( 3c ) I > 100

( 3d ) I > 100

SHORTWAVE

IMPORTANCE

, E> 1000

, E < 1000

, E> 1000, NAR.

, E > 1000, BR.

FADEOUTS

1 -

1

1 +

2 -

2

2 +

3 -

3

3 *

GEOMAGNETIC STORMS

INTENSITY

4

POLAR-CAP

EOUIV. ABS

(db)

200-299

300-499

500-999

1 1000

ABSORPTION

< 10

!0<E.A.<20

>20

1934

6 0 6

1 I 2

4 0

1 1 1 1

9 2 6

16 1

2 4

952 7

731 6

1 301

6 9 1

1 3 1

7 1

1 7

2 8 9

1 0 1

7 9

4 5

3 8

12

1 4

1171

301

6 0 0

2 5 5

7 2

8 6

6 3

3 4

1441

7 7

3 4 0

268

8 7

3 4 7

1 4 2

3 3

M l

3 6

7 8

2 6

2 4

2 0

8

31

2 0

4

7

58.7

59

5 9

6 8

58.4

5 7

6 6

7 1

60.6

60.6

58.3

6 3

5 9

72

7 7

69.2

6 7

6 8

73

63

6 7

9 3

60.2

55

6 1

6 4

5 4

7 1

6 2

7 4

62.5

5 2

6 1

6 8

7 1

6 0

6 8

6 4

6 0

5 6

74.4

6 9

7 1

8 5

8 8

71.0

6 0

7 5

100

1—i

i

1

1

r ]

1

i

I

5 0 % 6 0 70

I

80

I

90

I

100

FIGURE 1.—Percentage of events occurring in the Northern Solar Hemisphere, July 1957 to December 1960: Active-sun phenomena

and probable sources of terrestrial events.
200 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL. 10

the 88 major (3 and 3+) flares would occur

by chance. Figure 1 also shows an unexpected,

below-average concentration in the Northern

Hemisphere for both groups of flares (1+ and

2+) rated as having above-average intensity

for their area.

d. Microwave bursts.—The list of bursts available

to us from 24-hour coverage includes only the

larger events, with a time-integrated intensity

I. /.>500 (see Fokker, 1963). Figure 1 shows

that 69% of these bursts arose from the solar

North, a percentage significantly (P = 0.002)

larger than the 60.6% for all flares. Although

it is recognized that the larger the flare the

greater the likelihood of a large microwave

burst, this contrast between bursts and flares

does not arise, as might be expected, from the

high percentage of major flares in the North.

The 289 microwave bursts in figure 1 show

almost as high a percentage North (69%) as

that displayed by the many fewer (88) major

flares (72.7%). Investigation shows that a

higher percentage of northern than southern

flares in each importance class <2 + is associated with a microwave burst of 7./.>500.

Figure 1 shows no significant trend in hemispheric distribution with an increase in burst

magnitude below /./. = 100,000. Above this

limit there are 14 superbursts, 13 of which

arose from the Northern Solar Hemisphere.

Their average /./. was 200,000, while the

single southern superburst attained only

110,000. (Suprisingly, five of the northern

superbursts are associated with flares rated

only 2 or 2+ in the Working lists.)

e. 2800-MHz bursts.—Although not covering the complete 24 hours, the singlefrequency microwave bursts observed at Ottawa

provide a representative sample of microwave

bursts through the entire magnitude range

from the smallest observable to the larger

events considered above. Moreover, the availability of the burst profiles permits us to use

the characteristics of the bursts in assessing

their importance and minimizes the uncertainty

in magnitude determinations.

Of a total of 1953 bursts recorded at Ottawa

during the 3.5 years, 1171 were associated with

flares in the Working Lists. Unlike the distribution of the larger bursts in Fokker's list, the

percentage of Ottawa bursts from the solar

North does not differ from that of all flares

recorded during the Ottawa observing hours,

each being 60%. The percentage of flares

accompanied by a burst (of any magnitude) is

the same for both solar hemispheres.

However, active centers in the two hemispheres are not equally productive of bursts of

all magnitudes and profile characteristics. The

subdivision of bursts in figure 1 is based on the

two measures of magnitude, peak intensity (/)

and total energy (E), used previously by

Harvey (1964). A coarse division by magnitude—omitting only 15 bursts so incompletely

recorded that magnitude could not be determined—shows that the percentage North

increases with increasing magnitude (1, 2, 3 in

fig. 1).

A further subdivision of those bursts having

large / or E (3 in fig. 1) illustrates the variation

found in percentage North when the profile of

the burst is considered. Bursts observed at

2800 MHz may be impulsive or long-enduring,

or they may combine impulsive and longenduring components (Covington, 1959); the

profile of the impulsive component may be

narrow or broad (Harvey, 1965). Bursts with

large energy show a progressively greater percentage North with an increase in the proportion of that energy contributed by the impulsive

component:

(3a) minor impulsive component, 54% N;

(3c) impulsive component relatively intense

but not maintained, 62% N;

(3d) impulsive component relatively intense

and maintained, 74% N.

Bursts of lesser energy show a similar increase

in percentage from the solar North with increased importance of the impulsive component

(1, 2, 3b). The Southern Hemisphere appears

to produce a disproportionate share of bursts

in which the long-enduring component predominates over the impulsive component (3a).

The larger microwave bursts and the Ottawa

2800-MHz bursts together show a progressive

increase in the percentage from the solar

North with an increase in burst importance,

ranging from about 55% to the 93% N displayed by the microwave superbursts. (Since

only 4 of the 14 superbursts were observed at
HO. 4 MICROWAVE RADIO BURSTS AND SWFS 201

Ottawa, this category is not shown in the 2800-

MHz subdivision.)

/. Shortwave fadeouts (SWF).—Of the 1892

SWFs recorded during the 3.5 years, we find

that 1441 (76%) are associated in time with

flares in the Working Lists. Our investigation

is restricted to these SWFs. Figure 1 shows

that the percentage of identified SWF sources

in the solar North is greater than that of the

sunspot groups, but does not differ significantly

from the average value for all flares.

However, the trend in hemispheric distribution with SWF importance differs strikingly

from the trends for other events appearing in

figure 1. As the importance of the SWF increases from 1— to 2—, the percentage arising

from the solar North increases. As the SWF

importance increases further from 2+ to 3+,

the percentage from the North decreases,

quite contraiy to expectation. None of the

deviations from the SWF average of 62.5% N

attains the 0.05 level of probability that is

generally required for significance. However,

the 59% N for major (3, 3+) SWFs does differ

significantly (P=10-*) from the 72.7% N

for major flares.

Although the Central Radio Propagation

Laboratory (CRPL) lists of SWFs are as

complete as current information can make

them, Dodson and Hedeman (1964) have

pointed out observational difficulties that

affect both the number of SWFs detected and

the rating of their importance. These factors

doubtless have a bearing on the total number of

SWFs recorded in each importance class and

may influence the trends seen in the percentage

North. Nevertheless, it seems unlikely that

these factors could account for the decrease in

the percentage of large SWFs associated with

flare events in the Northern Solar Hemisphere.

g. Geomagnetic storms.—As a measure of the

intensity of a geomagnetic storm we used

y ^ C the sum of the four largest consecutive

values of the 3-hour ap index during the storm.

Figure 1 includes only those storms that

could be assigned unambiguously to one solar

hemisphere, in most cases to a single most

probable solar event; a few are ambiguous

as to the particular flare, but not with respect

to the active center or hemisphere. The systematic increase in the percentage of storms

originating from the Northern Hemisphere

with an increase in storm intensity, established

by Bell (1961, 1963b) from a longer time

period, shows clearly in the present data.

h. Polar-cap-absorption (PCA) events.—To compare the solar hemispheres in their production

of high-energy protons that cause polar-cap

absorption, we used Bailey's (1964) "Catalog of

Principal Events" and his intensity measure,

the equivalent absorption for a 30-MHz riometer. We divided the events (believed to be

quantitatively uniform) into three importance

categories. Figure 1 shows that the Northern

Hemisphere produced a higher percentage of

the more intense events (>10 db) than of the

weaker PCAs, and produced all the most intense

(>20 db) events.

3. Solar-terrestrial relationships

The data presented in the previous section show

that, for most forms of solar activity and for

terrestrial disturbances caused by solar-particle

radiation, the percentage of events produced in

the Northern Solar Hemisphere becomes greater

as the activity increases in importance. However, the major shortwave fadeouts do not accord with this general picture. The difference

between SWFs and the other major events in

this respect suggests that both our knowledge of

the relationships among solar phenomena and the

accuracy of forecasting may be extended by

including the parameter of hemispheric location.

We therefore compare the northern and southern microwave bursts for their terrestrial consequences—SWFs, magnetic storms, and PCA

events.

a. Shortwave fadeouts.—SWFs are closely correlated with microwave bursts (Hachenburg and

Kruger, 1959; Harvey, 1964), with respect both

to their production by solar events and to their

importance ratings. The surprising contrast in

the hemispheric distribution of the two phenomena, described in the preceding section,

suggests that this correlation is not the same

for the two hemispheres. Table 2 compares the

microwave-burst events (/./.> 500) from the

two hemispheres with respect to their success

in producing SWFs.
202 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

TABLE 2.—Relation between microwave bursts and SWFs*

VOL. u

Microwave Bursts

/./.> 100,000

31000-99000

10000-30000

3100- 9900

1000- 3000

500- 990

Total (/./. >500)

2800-MHz Bunts

7>100,.E> 1000 broad

/>100, £>1000 narrow

/ > 100, E< 1000

/<100

Total (7>100)

Northern Hemisphere

No.

of bursts

13

8

24

33

64

68

200

25

39

61

571

125

Number with SWF

3,3 +

13 (100)

5 (62)

13 (54)

9 (27)

5 (9)

8 (12)

53 (27)

13 (52)

4 (10)

1 (2)

4 (0.7)

18 (14)

3-,2+,2

:\

10

19

33

18

83 (41)

10

28

19

69 (12)

57 (46)

< 2

1

4

13

30

48 (24)

2

7

25

160 (28)

34 (27)

No.

without

SWF

1 (3)

3 (6)

12 (18)

16 (8)

16 (26)

338 (59)

16 (13)

•Numbers In parentheses represent percentages.

In each hemisphere, the greater the microwave burst, the higher the probability of an

associated SWF and the greater the average

magnitude of the SWF. Every superburst

(> 100,000) is associated with a major SWF.

But at all other levels of burst magnitude the

percentage of events producing a major SWF

is greater for the southern than for the northern

events. If the superbursts are excluded, 21.4%

of northern and 36.4% of southern microwave

bursts (>500) were associated with a major

SWF.

Table 2 also presents data on the SWF

success of the Ottawa 2800-MHz bursts, to

show the effects both of restricting the investigation to approximately those hours of the

day when SWF detection was most complete

(Dodson and Hedeman, 1964) and of including

events with weaker microwave-burst emission.

Here, too, we see the general correlation between burst and SWF magnitudes; the highest

production rate of major SWFs occurs in

association with the large broad-profile bursts,

whose average intensity and energy both

exceed those of the narrow-profile bursts

(broad: average /~1230, average 2?~25,000;

narrow: average /~475, average 2?~8000; see

also Harvey (1965, p. 2963)). However, events

in the Southern Hemisphere remain the more

successful producers of major SWFs in each

burst category.

The relatively high percentage of major

SWFs associated with events in the Southern

Hemisphere is not the only evidence for a

hemispheric difference in the production of

ionizing radiation. While the success ratio for

major SWFs is higher for southern than for

northern microwave bursts, this does not hold

true for lesser SWFs. Table 2 shows that events

in the Southern Hemisphere have a lower

percentage of association with minor (<2)

SWFs than do events in the Northern Hemisphere. This tendency is most conspicuous for

the small (2800 MHz: 7<100) burst events,

but it appears also with the larger burst

events (microwave burst: /./.>500; 2800

MHz: J>100). Table 3 shows yet another

reversal. Although the difference is not large,
MICROWAVE RADIO BURSTS AND SWFS

TABLE 2.—Relation between microwave burst* and SWFa*—Continued

203

Microwave Bursts

/./. > 100,000

31000-99000

10000-30000

3100- 9900

1000- 3000

500- 990

Total (/./.> 500)

2800-MHz Bunts

7>100, E> 1000 broad

/>100, E> 1000 narrow

7>100, £<1000

7<100

Total (7>100)

Southern Hemisphere

No.

of bursts

1

4

14

12

25

33

89

9

24

25

402

58

Number with SWF

3,3+

1 (100)

3 (75)

9 (64)

9 (75)

3 (12)

8 (24)

33 (37)

7 (78)

11 (46)

3 (12)

5 (1.2)

21 (36)

3-,2+,2

1

4

2

14

12

33 (37)

1

9

9

37 (9)

19 (33)

< 2

1

4

8

13 (15)

1

4

6

78 (19)

11 (19)

No.

without

SWF

1 (8)

4 (16)

5 (15)

10 (11)

7 (28)

282 (70)

7 (12)

•Numbers in parentheses represent percentages.

a consistently higher percentage of southern

than of northern SWFs occurs in association

with flares that lack any detected 2800-MHz

burst.

A check of all the flare events, without

regard to bursts, showed that in every flare

category—with the exception of 3+, where

every flare was associated with a major SWF—

events in the Southern Hemisphere were more

productive of major (3, 3+) SWFs and less

productive of minor (<2) SWFs than were

seemingly similar northern events.

Huang (1964) has pointed out another manifestation of the unequal influence of northern

and of southern solar activity on the ionosphere.

For the years 1956 to 1960, he found that the

noon critical frequency (joF2) associated with

the disk passage of large northern sunspots

was smaller than that associated with large

southern spot groups. This result could be

interpreted as evidence that southern spot

groups produced more ionizing radiation than

did seemingly similar northern groups.

6. Geomagnetic storms and PCA events.—Figure

1 showed that the hemispheric distribution of

the sources of the microwave superbursts

strikingly resembles that of the sources of the

high- and low-energy corpuscular radiation

producing the larger FCAs and magnetic

storms. Unfortunately, the situation is less

straightforward than this parallelism might

suggest. Examination of the individual major

events reveals that microwave superbursts

preceded eight (73%) of the PCAs greater than

10 db, but only three (37%) of the storms with

eight (29%) of the storms

with ^a,,>500. Thus, the hemispheric distribution of the sources of the microwave superbursts appears to account adequately for that

of major PCAs, but not for that of great storms.

Table 4 gives data by hemisphere on the

geomagnetic conditions (described by y^.a)

subsequent to major microwave bursts in four

importance categories, and to major Ottawa

bursts subdivided into those with a broad and

those with a narrow profile. The greater the
204 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

TABLE 3.—Distribution of SWFs without microwave burst

SWF with no microwave burst >500

Percent of total SWF

SWF with no 2800-MHz buret

Percent of total SWF

Northern

3, 3+ 3- ,

34

(39)

1

(4)

Hemisphere

2 + , 2

242

(72)

14

(10)

< 2

441

(90)

87

(31)

Southern

3, 3+ 3 -

27

(45)

1

(4)

Hemisphere

2+, 2

164

(83)

15

(21)

< 2

270

(95)

65

(42)

importance of the microwave burst, measured

by the integrated intensity {l.L), the higher

the probability that it will be followed by a

great storm, or indeed by any storm >200.

The same holds true for 2800-MHz major

bursts with importance determined by profile

width, in agreement with previous results by

Harvey (1965).

However, the data indicate that a higher

percentage of northern than of southern bursts,

in each range of burst importance, is followed

by a great storm. Indeed, great storms are

significantly associated only with major burst

events originating in the Northern Solar Hemisphere. For northern bursts with /./.> 10,000

or a broad profile, the probability of a chance

association as high as that observed is <10~*,

while for bursts with 1000 < /./. < 9900, the

probability is ~10~4, and for 2800-MHz bursts

with a narrow profile, it is ~0.05 (see Bell

(1963a) for the procedure used to estimate

the probability of chance association). Major

bursts from the Southern Solar Hemisphere,

even those with a broad profile or with

/./.> 10,000, are not followed by a great

magnetic storm more often than would be

expected by chance.

The probabilities that a magnetic storm will

follow a broad major burst at 2800 MHz are

similar to those found by Bell (1963a, Table 5,

line i) for meterwave bursts of spectral type

IV subdivided by solar hemisphere. This agreement is not surprising in view of the association

between broad 2800-MHz bursts and type IV

bursts (Harvey, 1965).

Northern microwave bursts with /./. ^ 100,000

TABLE 4.—Relation between microwave bursts and geomagnetic activity*

Microwave Bursts

/./.> 100,000

31000-99000

10000-30000

1000- 9900

2800-MHz Bursts

2>100, ^>1000

broad

narrow

Number

of bursts

13

8

24

87

25

39

Northern Hemisphere

>500

10 (77)

1}««

16 (18)

11 (44)

6 (15)

Number followed by £a p

300-499

2

2

6

11

5

5

200-299

2

1

14

00 CO

<200

1 (8)

2}

 (34)

46 (53)

6 (24)

20 (51)

•Numbers In parentheses represent percentages.
MICROWAVE RADIO BURSTS AND SWFS 205

are more likely (77%) to be followed by

a great geomagnetic storm than is any other

solar event studied to date. Each of the great

storms of November 12 and 15, 1960, was

preceded by two microwave superbursts. However, as noted above, such bursts account for

less than half the greatest storms.

Table 5 shows the relationship between

microwave bursts and PCA events for the two

solar hemispheres, with each PCA attributed

to a single solar event. The greater the importance of the microwave burst, the greater the

probability of association with PCA. Highenergy protons were detected above the earth

after 12 of the 14 microwave superbursts.

Bursts in the range 31,000 </./.< 99,000 also

show a substantial though lesser association

with PCA events. Five of the seven largest

(>20 db) PC As and three of the four in the

10- to 19-db range followed microwave bursts

of /./. > 100,000. A broad profile characterized

all 2800-MHz bursts associated with major

PC As.

Unlike the situation with great magnetic

storms, the contrast between the solar hemispheres in PCA production seems adequately

explained by the hemispheric differences in

productivity of major microwave bursts. The

solar hemispheres do not differ significantly in

the percentage of microwave bursts associated

with PCA. Moreover, only four PCA events in

TABLE 4.—Relation between microwave

Bailey's (1964) catalog do not appear to be

associated with any microwave burst >1000;

the largest of these attained 10-db absorption

(March 25, 1958), and Bailey notes in two

cases that "the association with solar flare and

radio noise events is very uncertain or unknown."

The six ground-level cosmic-ray (GCR)

events recorded during the 3.5-year interval all

occurred in association with northern bursts,

four with /./.> 100,000, one with /./.=87,000,

and one with /./.=7500. Three were associated

with 2800-MHz bursts, all with broad profiles.

Fokker (1964) has pointed out that the largest

microwave bursts were not concentrated close

to sunspot maximum, but showed the same tendency as the larger PCA events and the GCR

events to avoid sunspot maximum.

4. Conclusions

In the 3.5 years from July 1957 through December 1960, the relative activity of the solar

hemispheres in producing solar events and

event-linked terrestrial disturbances differed

markedly from the basic north-south distribution (59% N) of the sunspot groups. For most

types of activity the inequality between the

hemispheres increased with increasing importance of the solar event. Distributions from

85% to 100% N occur for microwave bursts of

integrated intensity > 100,000, for PCA events

bursts and geomagnetic activity*—Continued

Microwave Bursts

/./.> 100,000

31000-99000

10000-30000

1000- 9900

2800-MHz Bursts

7>100, 2? > 1000

broad

narrow

Number

of bursts

1

4

14

37

9

24

Southern Hemisphere

>500

1 (7)

3 (8)

1 (11)

1 (4)

4

Number followed by £a p

300-499 200-299

1

3 4

5 6

2 3

<200

23 (62)

8 (89)

18 (75)

•Numbers In parentheses represent percentages.
206 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

TABLE 5.—Relation between microwave bursts and PCA*

Microwave Bursts

7.7. > 100,000

31000-99000

10000-30000

1000- 9900

2800-MHz Bursts

7>100, £>1000

broad

narrow

Number

of bursts

13

8

24

87

25

39

Northern Hemisphere

Number linked with PCA

>20

5

1

1

3

19-10

2

1

1

(db)

<io

4

2

2

4

3

2

Number

without

PCA

2

5

21 (88)

82 (94)

18 (72)

37 (95)

'Numbers In parentheses represent percentages.

of absorption > 10 db, for GCR events, and for

great \^yiv>500) geomagnetic storms. These

asymmetries substantially exceed even those of

the largest sunspot groups. (Moreover, many of

the major events did not originate in either the

largest sunspot groups or in the most flare-rich

plage regions.)

The hemispheric distribution of the greater

microwave bursts accounts adequately for the

distribution of the PCA sources, but not for

that of the great-storm sources. Even when possessing the burst attributes that appear most

favorable to a subsequent geomagnetic storm,

events in the Southern Solar Hemisphere did

not show any significant association with great

magnetic storms.

The hemispheric distribution of the sources of

major SWFs differs markedly from that of the

sources of other major events—flares, microwave bursts, magnetic storms, PCAs—as only

59% were produced by northern solar activity.

The tendency for southern flares to produce

more than their share of major SWFs appeared

consistently, whether the data were subdivided

by flare importance or by burst magnitude. The

evidence suggests that solar activity in the

Southern Hemisphere produced a greater effect

on the ionosphere than did seemingly similar

northern activity but, on the other hand, produced a smaller geomagnetic effect.

It would thus appear that, in recent years, a

greater proportion of the energy released by

the larger southern flare events tends to go into

shortwave electromagnetic radiation producing

SWFs, while more of the energy released by the

larger northern events tends to go into the ejection of corpuscular streams producing magnetic

storms.

Acknowledgments

The research reported in this paper has been

sponsored in part by the Air Force Cambridge

Research Laboratories, Office of Aerospace

Research, under Contract AFl9(628)-3322

with Harvard University.

The 2800-MHz solar records used in our

analysis were obtained by the solar patrol group

of the National Research Council under the

direction of A. E. Covington.

We are indebted to A. D. Fokker of the

Utrecht Solar Observatory for making available to us his unpublished list of major microwave bursts.

References

BAILEY, D. K.

1964. Polar-cap absorption. Planet. Space Sci.,

vol. 12, pp. 495-541.

BELL, B.

1961. Major flares and geomagnetic activity.

Smithsonian Contr. Astrophys., vol. 5,

pp. 69-83.
MICROWAVE RADIO BURSTS AND SWFS

TABLE 5.—Relation between microwave bursts and PCA*—Continued

207

7.7. > 100,000

31000-99000

10000-30000

1000- 9900

7>100, E>U

broad

narrow

Microwave

2800-MHz

KM)

Bursts

Bursts

Southern Hemisphere

Number

of bursts

1

4

14

37

9

24

Number linked with PCA (db)

>20 19-10 <10

1

2

2

4

3

1

Number

without

PCA

2

12 (86)

33 (89)

6 (67)

23 (96)

•Numbers In parentheses represent percentages.

BELL, B.—Continued

1962. A long-term north-south asymmetry in the

location of solar sources of great geomagnetic storms. Smithsonian Contr.

Astrophys., vol. 5, pp. 187-194.

1963a. Solar radio bursts of spectral types II and

IV: Their relations to optical phenomena

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239-257.

1963b. Type IV solar radio bursts, geomagnetic

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BILL, B., and WOLBACH, J. G.

1962. On short-period relations between northsouth asymmetry in spottedness and in

great-storm sources. Smithsonian Contr.

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COVINGTON, A. E.

1959. Solar emission at 10-cm wavelength.

Paris Symp. on Radio Astronomy, ed.

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DODSON, H. W., and HEDEMAN, E. R.

1964. Problems of differentiation of flares with

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HABVEY, G. A.

1964. Some relationships between 10.7-centimeter

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208 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS TOL. 10

Abstract

The relative activity of the Northern and Southern Hemispheres in the production of solar and solar-linked terrestrial events is investigated for the period July 1957 through December I960, with particular attention to shortwave

fadeouts (SWFs) and to microwave radio bursts, including those observed at 2800 MHz at Ottawa. The magnitude of the

microwave bursts is investigated, by hemisphere, in relation to three terrestrial effects: polar-cap-absorption (PCA)

events, geomagnetic storms, and SWFs.

As with solar flares, geomagnetic storms, and PCA events, the percentage of microwave bursts associated with the

Northern Solar Hemisphere increases with increasing importance of the event, to around 90% for bursts of integrated

intensity >10*, great magnetic storms, and PCA events with absorption >10 db. This substantially exceeds the percentage of all sunspot groups (2692—59% N) and of the largest spot groups with mean area > 1000 millionths of the

solar disk (40—68% N). In unexpected contrast, a relatively low percentage of major (3, 3+ ) SWFs are found to come

from the Northern Solar Hemisphere (59% N).

The hemispheric distribution of the greatest bursts appears to account for that of the PCA sources, but not for

that of the sources of great storms or of major SWFs.

o