Smithsonian

Contributions to Astrophysics

VOLUME 5, NUMBER 15

SOLAR RADIO RURSTS OF SPECTRAL TYPES H AND IV:

THEIR RELATIONS TO OPTICAL PHENOMENA AND

TO GEOMAGNETIC ACTIVITY

by BARBARA BELL

SMITHSONIAN INSTITUTION

Washington, D. C.

1963
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Washington 25, D.C. - Prim SO cento
Solar Radio Bursts of Spectral Types II and IV:

Their Relations to Optical Phenomena and to

Geomagnetic Activity

By Barbara Bell *

This paper will discuss solar radio bursts of

spectral types II and IV, with primary attention to the magnetic type of the associated

sunspot, and to the north-south asymmetry in

their numbers and durations. Their relation

to geomagnetic storms and to solar proton

events (PCA) is also explored. Before considering the radio bursts themselves, however, it

may be useful to summarize information on

other manifestations of solar activity with

respect to magnetic type and the north-south

asymmetry.

In a study of the geomagnetic effects of

major solar flares from 1937 through 1959,

Bell (1961) showed that a flare from a magnetically complex (/8y and y) sunspot group is

much more likely to be followed by a geomagnetic storm than is a flare from a simple bipolar

(0) or unipolar (a) spot group. Table 1

presents a convenient summary of previous

information on the distribution of various

active-sun phenomena among the four Mount

Wilson magnetic types of sunspots. The top

four lines are derived from Bell and Glazer

(1959), the remainder from Bell (1961).

The top line of table 1 shows that the complex

fiy and y types account together for only about

3.4 percent of all classified sunspots. The

second line includes only large spot groups, of

area greater than 500 millionths of the solar

disk (Greenwich, 1955). Here the complex /3y

and y types make up about 27 percent, while

they together comprise about hah7 of the spots

• Harvard College Observatory, Cambridge, Mass.

over 1000 millionths of the disk. The fourth

line shows the distribution of all flares reported

over the same years, 1937-1953. The percentages do not differ greatly from those for spots

larger than 500 millionths, as one might expect,

since most flares arise from the larger sunspot

groups.

The lower hah* of the table gives the distribution of major flares, of importance > 2+

,

from 1937 through 1959. Major flares are

more concentrated to large spots than are

flares in general. But also, on the average,

complex spots produce more major flares (and

more flares in general) than do j9 and a spots

of comparable size. This may be seen from a

comparison of the line for major flares with the

last line for spot groups producing one or more

major flares.

The major flares are also divided according

to their geomagnetic consequences, that is,

according to whether they were followed within

three days by no storm, a small (Ap> 25),

moderate (Ap > 50), or great (Ap > 100 and/

or Kp > 9") storm (see Bell, 1961). Failures

and small-storm flares are relatively more numerous among /9 and a major flares, while moderate and great storms are relatively more numerous among the complex (3y and y types.

Bell (1961) further found that about 62

percent of major flares arose from the northern

solar hemisphere, and that the northern hemisphere produced a disproportionate 86 percent

of the flares followed by a great geomagnetic

storm. Subsequently Bell (1962) showed that

239
2 4 0 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

TABLE 1.—Percentage of solar activity in each magnetic class, and in the northern solar hemisphere

Activity

All spot groups

Area > 500 x 10-«

Area > 1000 x 10"*

All flares

Major flares

followed by:

no storm

small storm

moderate storm

great storm

Major-flare spots

Years

1937-53

a

n

a

1937-59

a

a

a

H

II

Number

4668

248

59

8403

560

270

146

71

72

344

a

37. 8%

11.3

5.1

14.3

12.8

15.6

9.6

9.9

11.1

16.0

Magnetic class

P

58.8%

61.3

44.1

60.4

46.6

53.3

51.3

42.1

16.6

54.6

fiy

2.7%

19.8

35.6

16.7

24.5

21.0

22.0

24.0

43. 1

18.9

y

0.7%

7.6

15.2

7.5

16. 1

10.0

17.1

24.0

29.2

10.5

Percent

North

50. 4%

54.4

54

53.4

62

60

52

70

86

58

this tendency for great storms to arise preferentially from the northern solar hemisphere

extended back for two additional sunspot cycles

to about 1913. For cycles 10 to 14 (1856-

1912) the southern solar hemisphere appeared

to be the preferred source of great storms,

while in cycle 9 (1844-1856) the north was

again the more geoactive.

The final column of table 1 provides reason

to postulate that the north-south asymmetry

increases with the vigor of the solar event.

Since 1937 major flares are more concentrated

to the north than are flares in general. The

concentration to the north is greater for sources

of great geomagnetic storms than for sources

of lesser storms, and greater for ground-level

cosmic-ray events than for PCA events. We

shall investigate below whether it is greater

for major than for lesser radio bursts of spectral

types II and IV.

Slow-drift radio bursts were first studied

with a dynamic spectrum analyzer by Wild

and McGready (1950) in Australia. They

introduced the designation "spectral type II "

for these phenomena. Type II bursts commonly have a lifetime of a few minutes. They

begin at high frequencies and spread to lower

ones in a manner suggestive of a source of

excitation moving outward through the solar

atmosphere with a velocity of around 1000-

1500 km/sec. Various properties of type II

bursts have been described by Wild (1950),

Roberts (1959), Maxwell, Thompson, and

Garmire (1959), Maxwell and Thompson (1962),

and Wood (1961).

Continuum radiation of type IV was first

identified by Boischot (1957, 1958), and has

been further described by McLean (1959),

Thompson and Maxwell (1960a, 1960b), and

Pick (1961). Type IV emission has a duration

of a few minutes to a few hours, covers a broad

range of frequencies, varies more or less

smoothly in intensity, and, in accordance with

the suggestion of Boischot and Denisse (1957),

is generally believed to be synchrotron radiation

from relativistic electrons spiralling in magnetic fields.

Data

The observational data were obtained from

lists of bursts recorded with the dynamic

spectrum analyzer at the Harvard Radio

Astronomy Station, Fort Davis, Texas, from

1956 October through 1960 December 31, and

at the Radiophysics Laboratory of C.S.I.R.O.,

Sydney, Australia, from 1956 November

through 1960 August 11. Maxwell, Thompson,

and Garmire (1959) estimate that these two

stations cover an average of 14 hours per day.

Other data on type IV events (e.g. Pick, 1961)

might have been used, but in the interest of

a homogeneous sample I have restricted the

analysis to spectral observations. Records

at the two stations yielded 96 type IV events

and 197 type II.
SOLAR RADIO BURSTS 241

The flare lists in the I.A.U. Quarterly

Bulletin of Solar Activity were used to select the

flare most probably associated with each burst.

In general, the highest "importance" rating

was used for each flare, rather than a mean of

the often discordant estimates.

Each burst was classified according to its

geomagnetic success, that is, according to

whether it was followed within the interval of

10 hours to 3 days by no storm, a small

(Ap > 25), moderate (Ap > 50), or great

(Ap > 100 and/or Kp > 9~) storm.

Bursts of each type were also subdivided

according to their duration, the best available

criterion of their importance. Type II bursts

were divided into those lasting less than 10

minutes and those lasting 10 or more minutes.

In 1956-1958 the frequency range of the

Harvard equipment was 100-580 Mc/s; in

January 1959 it was extended to 25-580 Mc/s

(see Thompson, 1961). Accordingly, because

of the greater range of the equipment, type II

bursts in 1959-1960 have on the average a

longer duration, and the bursts from 1957-1958

and from 1959-1960 have where pertinent

been discussed separately.

Type IV events were divided initially into six

duration groups: < 15m, 16-29m, 30-59m, 60-119m,

2

h

-3h59m, and > 4h. For most purposes I

found that no significant information was lost

by combining the four middle intervals and

discussing only three categories of type IV

events.

Association with flares

Table 2 shows the relation between flare

importance and bursts of type II and IV.

About half of the type IV events are found to

be associated with a major flare of importance

2

+

 or greater, about 20 percent with class 2

flares, and 20 percent with class 1 flares.

Of the remainder, 3 occurred when there were

no flare observations, 4 apparently had no

observed flare, and 3 had two or more equally

likely flares. In six of the assigned cases

there is also some ambiguity, but the type IV

burst was assigned to the more important

flare.

There is a clear relation between the duration

of the burst and the importance of the associated flare. Over half of the short-lived

bursts are associated with class 1 flares, while

88 percent of the longest-lived bursts are

associated with major flares.

Type II bursts are less strongly associated

with major flares than are type IV bursts.

Only 28 percent of the 197 type II bursts are

associated with a major flare; 24 percent have

a class 2 flare; 12 percent a 1+ flare; 17 percent

a class 1 flare; and 8 percent al " flare. Thirteen bursts occurred when there were no flare

observations, and 7 apparently occurred without any flare. In 9 cases the assignment was

ambiguous but given to the more important

flare. The distribution found for type II

bursts is in satisfactory agreement with those

found by Roberts (1959), and by Maxwell and

TABLE 2.—Relation between radio bursts of spectral types II and IV and the importance of the associated flare

Activity

Type II

Type IV

of duration:

<15TM

16--59-

Xh_3i>3gm

> 4 h

No. flares of importance

< 1 +

73

20

8

8

3

1

2

48

18

1

6

10

1

> 2 +

55

48

4

15

15

14

No. uncertain

21

10

3

5

2

0

Total

bursts

197

96

16

34

30

16
242 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

Thompson (1962). The percentage of type IV

events associated with major flares is somewhat

lower than the 67 percent found by McLean

(1959) from a much smaller sample.

North-south asymmetry

Data on the north-south asymmetry for the

years 1957-1960 for various active sun phenomena are given in the last column of table 3. The

percentage of type IV events generated in the

north is only slightly greater than the percent

of major flares over the same period. The percentage of type II bursts coming from the north

is somewhat larger, but probably arises from

the greater completeness of Harvard type II

data in 1959-1960, years of more extreme concentration of solar activity to the north than

were 1957-1958. Type II bursts occurred 60

percent north in 1957-1958 and 80 percent

north in 1959-1960.

The years 1957-1960 exhibit a striking

tendency for the north-south asymmetry to

increase with increasing intensity of the solar

activity. In agreement with table 1 for a

longer time span, table 3 shows that major solar

flares are more concentrated to the north than

are flares in general. The percentage of flares

in the north significantly (P < 10~10) exceeds

that of spots. Major flares followed within

three days by a great geomagnetic storm are

more concentrated to the north than are flares

followed by lesser or no geomagnetic disturbance. The sources of ground-level cosmic-ray

increases are more concentrated to the north

than are the sources of the less energetic PCA

events.

Similar trends are clear also among both type

II and type IV events. Bursts associated with

major flares are more concentrated to the north

than are bursts associated with lesser flares.

TABLE 3.—Distribution of active-sun phenomena among the sunspot magnetic types, and between the two solar hemispheres

Activity

Major flares

followed by:

no storm

small storm

moderate storm

great storm

Type IV events

associated with:

flare >2+

flare <2

of duration:

<15-:

16- to59-

l

k

 to 3h59-

> 4 h

Type II bursts

associated with:

flare >2+

flare <2

Sunspot groups

All flares

Polar cap events

Ground level events

Years

1957-60

ft

"

ft

1957-60

tt

"

"

"

1957-60

"

1957-60

//

1956-60

1942-60

Number

309

128

83

44

54

86

48

38

13

29

28

16

166

52

113

2998

12979

46

12

a

13%

18

10

9

7

9

6

13

23

3

14

0

16

9

20

8

0

Magnetic type

43%

50

52

43

15

29

23

37

46

24

32

19

48

38

53

22

8

27%

19

24

23

54

37

44

29

23

35

29

69

23

30

20

37

34

y

17%

13

14

25

24

25

27

21

8

38

25

12

12

23

7

33

58

Percent

North

64%

60

57

68

83

67

75

58

38

69

64

88

68

80

63

56.5

59.8

70

92
SOLAR RADIO BURSTS 243

Thus a northern major flare is more likely to

have an associated radio burst than is a southern

flare of similar importance. We find 0.18 type

IV events per northern and 0.11 per southern

major flare, with a probability of 0.06 of finding

by chance two such large deviations from the

mean of 0.155 type IV bursts per major flare.

Similarly, we obtain 0.23 type II bursts per

northern and 0.10 per southern major flare,

with a probability of 0.003 of finding by chance

two such large deviations from the mean. Because the observations do not cover 24 hours,

these numbers are only relative and the actual

probability that a major flare will be accompanied by a type II or type IV event is estimated to be at least 50 percent greater.

If we consider the importance of the radio

bursts themselves, as measured by duration,

we find that the percent generated in the north

is greater for the long-lived than for shortlived events. In table 3 the type IV bursts

exceeding 4 hours duration show an especially

Btrong preference for the northern solar hemisphere. Among type II bursts the percentage

from the north is greater for events lasting 10

or more minutes than for shorter ones, in both

1957-1958 and 1959-1960. The percentages

north are 75 and 52 for long and short bursts

in the earlier years, and 88 and 71 in the later

years.

We have already noted that the sources of

ground-level cosmic-ray increases appear more

concentrated to the north than do the sources

of PCA events. It is of interest to explore

the north-south asymmetry in relation to the

intensity of the PCA events themselves. Unfortunately, the answer depends on whether

one employs Bailey's (1962) VHF forward

scatter signal, or Reid and Leinbach's (1959)

riometer intensity measures. I used the list

of PCA's and their most probable associated

flares compiled by Warwick and Haurwitz (1962)

which gives intensity measures from both

sources. Using Bailey's scale we find 9 "large

to very large" PCA's, of which 8 appear to

have come from the north; his 33 "moderate

to very small" events come 64 percent from

the north. Using Reid and Leinbach's intensity measures, however, I find no relation

between the north-south asymmetry and the

intensity of the PCA.

From the delay times between flare and

PCA determined by Warwick and Haurwitz,

I find average values of 3.7 hours for northern

and 5.2 for southern PCA's. Due to the large

dispersion in delay times this difference has no

statistical significance, but it seems nevertheless suggestive when viewed with the other

evidence on north-south asymmetry.

In summary, the data on type II and type IV

radio events, and with less certainty on PCA

events, lend further support to the hypothesis

that the north-south asymmetry is a phenomenon whose magnitude increases with the energy

of the solar event. It should not be expected,

of course, that the northern hemisphere will

remain the more energetic indefinitely (see Bell,

1962). However, there is at present no theoretical basis for a prediction of when it may

cease to be so.

Magnetic type

Data on the distribution of active-sun phenomena among the four basic types of sunspot

magnetic field pattern also appear in table 3

for the years 1957-1960. The distribution for

major flares does not differ significantly from

that presented for a longer interval in table 1.

In agreement with previous results (Bell, 1961),

sunspot groups with complex 0y and y field

patterns contribute a proportion of geomagnetic

storms that increases with the intensity of the

storm.

Table 3 shows a marked contrast between

type II and type IV radio events. Complex

spot groups produced only 36 percent of the

type II bursts as compared with 61 percent of

the type IV events.

Complex spots produced 54 percent of the

type II bursts associated with a major flare.

We find 0.20 bursts per major 187 or y flare, and

0.14 bursts per major o or (S flare, with a

probability of 0.07 of obtaining by chance two

such large deviations from the mean. The

difference is therefore of marginal statistical

significance. Complex spots produced 28 percent of the lesser-flare bursts, as compared with

23 percent of all flares over the same time

interval.

In striking contrast, complex spots produced

a disproportionate 71 percent of the type IV

events associated with a major flare, and 44
244

SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

percent of those associated with a lesser flare.

We find 0.25 bursts per major 07 or y flare, and

only 0.08 bursts per major a or 0 flare, with a

probability of only about 10"* of obtaining by

chance two equally large deviations from the

mean. The contribution of complex spots is

at a minimum for the shortest-lived bursts and

maximal for those lasting over 4 hours.

Finally, table 3 shows that complex spot

groups contributed 70 percent of all PCA

events and, over a longer time span, 92 percent

of the ground-level increases in cosmic-ray

intensity. (See also Noyes, 1962, and Ellison

et al., 1961)

Because of the parallelisms between percent

in the north and percent complex, one may

wonder about the percentage of complex spots

and flares in the solar north. Table 4 provides

information on this question. Comparison

with tables 1 and 3 indicates that a relatively

higher percentage of complex spots and of their

associated active phenomena occur in the north.

Simple spots are more evenly divided between

the two hemispheres. Table 4 also shows that

northern complex spots are more active

than their southern counterparts in producing

major flares and radio bursts. The activity of

a and /3 spots is more nearly equal in the two

hemispheres, that is, more closely in proportion

to the numbers of spot groups.

TABLE 4.—Percentage of complex (fiy and y) magnetic

types occurring in the northern solar hemisphere

Activity

1937-53:

Sunspots

Great spots

Flares

Major flares

1937-59:

Sunspots

Flares

Major flares

1957-60:

Sunspots

Flares

Major flares

Type IV bursts

Type II bursts

No.

155

75

1996

115

228

4714

226

82

2974

136

53

57

Percent

North

57.5%

57.5

59.5

77.5

58.3

60.0

73.4

67.0

66.7

73.5

83

81

The data presented in this section provide

further support for the earlier conclusion (Bell

and Glazer, 1958; Bell, 1961) that the magnetic

type of the sunspot is a criterion that should

receive more attention in efforts to forecast

major flares, geomagnetic storms, and increases

in cosmic-ray intensity.

Geomagnetic effects of type II and type IV

bursts

Since the first suggestion by Sinno and Hakura

(1958) that type IV events have a significant

probability of being followed by geomagnetic

disturbance, additional evidence favoring this

hypothesis has been presented by McLean

(1959), de Feiter et al. (1960), Simon (1960),

and Mustel and Yegorova (1961). The geomagnetic consequences of type II bursts are

more controversial. Because of their spectral

characteristics, Wild (1950) suggested that they

might be caused by the ejection of a corpuscular

stream of the sort believed responsible for geomagnetic storms. Some evidence for a relation

between type II bursts and geomagnetic storms

was found by Roberts (1959) and by Maxwell,

Thompson, and Garmire (1959). Other authors,

however, find type II bursts to be of negligible

geomagnetic significance, and Maxwell et al.

also pointed out that type II with type IV had

a greater probability of geomagnetic success

than did type II alone.

The bursts discussed in this paper have been

analyzed for geomagnetic success. They form

a larger sample than previously studied. Table

5 gives the percentages of type II (left) and of

type IV (right) events followed within three

days (with a minimum allowed time delay of

ten hours) by various levels of geomagnetic

activity. The bursts are subdivided by several

associated properties in an attempt to evaluate

the relative importance of these features.

The last line of table 5 shows the percentages

for a random sample of days. The random

sample was determined from the total number

of days and the total number of storms at each

intensity level within the four years. For

example, 26 great storms occurred within the

four years. The probability that any day

chosen at random will be followed within three

days by a great storm is 1460/26X3=0.054.

The similar probability that a given day will
SOLAR RADIO BURSTS 245

60

50

4 0

3 0

2 0

10

CM

A l

LU

= Al

IIIII,, *

I- u. * • a>.

ex

' >

•T..

CM

Al

Small

Storms

4 0

\

3 0

\

20

10

0

40

30

20

10

I

-I-

— 3o-l Moderate

— 2 <r[ Storms

I I T

I I

I I

40 60 80 100 120 140 160 180 200 N

FIGURE I.—The heavy vertical fines indicate the percentage of various active sun

phenomena followed within three days by a great (solid lines), moderate (broken

lines), and small (dotted lines) geomagnetic storm. The curved horizontal lines

represent standard errors (<r) above the percentage (p) expected from a random

sample.

60O7M
246 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

TABLE 5.—Percentage of type II and type IV bursts followed within three days by various levels of geomagnetic activity,

subdivided by properties of the solar event. Values deviating less than 2a and more than 4" from the random sample

are designated by parentheses and bold face type, respectively.

Properties of events

a) All events

b)

 iVi «" •

>w-\ 1959"

60

<15»

l6m_29»

30m-59m

Ih_!h59m

2h_3h59-

>4h

c) Flare >2+

Flare <2+

d) Py, y sunspots

a, P sunspots

e) North hemisphere

South hemisphere

f) East

West

g) <30° from CM

30°-59°

>60°

h) Flare >2+ lPy'

y

l«> P

Flare <2+ [0y>

y

[<*> P

i) Flare >2+ (nor*^

I south

Flare <2+ ( nor ^

[south

j) <15-^ Y

16"^3h59- lfiy'

y

\<*,P

> 4 k l*.y

{<*, P

Type II bursts

No.

197

73

33

47

44

55

121

58

104

118

52

75

97

59

64

49

28

24

30

80

45

11

73

42

33*

63*

25**

41**

Percent followed by

Great

storm

12

(8)

15

(ID

16

24

(7)

21

(9)

15

(6)

13

12

15

(9)

12

32

17

(10)

(6)

24

(18)

(10)

(2)

18

(6)

24

(12)

Moderate

storm

(11)

(13)

(21)

(6)

(9)

(16)

(9)

(13)

(11)

(12)

(13)

(12)

(12)

(10)

(25)

(12)

(11)

25

(17)

(8)

(14)

(27)

(10)

(10)

(12)

(10)

(16)

(15)

Small

storm

33

34

(28)

(32)

36

35

34

37

33

32

35

35

32

38

(32)

(31)

39

(25)

(33)

35

38

(28)

(29)

37

(37)

(32)

(36)

(34)

Type IV bursts

No.

96

16

14

20

17

13

16

48

38

53

33

58

28

40

46

44

29

13

34

14

19

19

36

12

22

16

4

9

36

21

13

3

Percent followed by

Great

storm

23

(6)

28

(10)

17

23

56

35

(8)

27

18

31

(7)

25

22

30

21

(8)

38

29

(14)

(10)

44

(8)

(9)

(6)

(25)

14

24

62

(33)

Moderate

storm

19

(0)

(15)

30

(17)

(23)

(25)

23

(13)

22

(12)

22

(ID

(15)

22

(20)

(20)

(7)

(24)

(21)

(12)

(6)

23

(25)

23

(0)

(0)

25

(14)

(22)

(33)

Small

storm

34

(44)

(36)

(35)

(42)

(31)

(19)

(28)

47

(32)

43

(32)

46

42

(30)

(34)

(28)

62

(24)

(36)

48

47

(25)

(34)

(41)

57

(25)

56

39

(38)

(16)

(34)
SOLAR RADIO BURSTS 247

TABLE 5 (continued).—Percentage of type II and type IV bursts followed within three days by various levels of geomagnetic activity, subdivided by properties of the solar event. Values deviating less than 2<r and more than 4o from the

random sample are designated by parentheses and bold face type, respectively.

Properties of events

,. _ _ /flare >2+

k)

 ~

b

 lflare<2+

m Jflare >2+

[flare <2 +

Jflare >2+

~* [flare <2+

r . / QOrt h

l; _io i n | soutl l

Jnorth

1 6 m- 3 59 JgQy^-Jj

h Jnorth

— [south

Random sample

Type II bursts

No.

24*

76*

31**

39**

1460

Percent followed by

Great

storm

21

(7)

26

(8)

5.4

Moderate

storm

(17)

(9)

(16)

(13)

11.3

Small

storm

(37)

(30)

(32)

(35)

23.2

Type IV bursts

No.

4

9

30

27

14

2

5

8

39

18

14

2

1460

Percent followed by

Great

storm

(25)

—

30

(4)

50

(100)

(20)

—

21

(11)

64

—

5.4

Moderate

storm

—

23

(18)

28

—

—

23

(17)

(22)

(50)

11.3

Small

storm

(25)

56

(30)

49

(22)

(20)

(50)

41

(39)

(14)

(50)

23.2

• <9>» for type n

•*>10» for type n

be followed within three days by a moderate

storm is 0.113, and by a small storm is 0.232.

Figure 1 shows the percentage of type II and

type IV events that are followed within three

days by the three levels of geomagnetic activity,

and provides also a guide for evaluating the

significance of the difference from the random

sample. The curved horizontal lines indicate

deviations of two, three, and four standard

errors above the random values, where a =

(Npq) * and N is the number of cases, p is the

random probability (0.054, 0.113, and 0.232)

for obtaining a given level of geomagnetic

activity, and g = 1 — p. Figure 1 also shows

the probability of a geomagnetic storm after

all major (3y and y flares, and all major /3 and a

flares without regard for radio bursts; and after

type II bursts without type IV. Further data

comparing type II with and without type IV

appear in table 6 and figure 2. The a-lines in

figure 1 were used to estimate the statistical

significance of all values in tables 5 and 6; the

three different type faces distinguish values

less than 2<r, 2 to 4<r, and more than 4<r above

the random values.

Figure \c shows a highly significant probability of obtaining a great geomagnetic storm

in the three days following a type IV event,

and also following a major flare from a /9y or

7 sunspot. The probability of a great storm

after type II events is also significantly above

the random; however, it is not significant for

type IPs that are unaccompanied by type IV,

nor for a and 0 major flares. The more detailed

data in table 6 support this conclusion. In the

absence of type IV, type II bursts appear to

have only slightly more than the random

probability of being followed by a great geomagnetic storm. The most successful of type

II alone are those accompanied by a major

flare and arising from a complex sunspot

(table 6, line h).

The other category, type IV without type

II, is shown in table 7. Thompson (private

communication) cautions that to be certain

that a type IV burst is not accompanied by a

type II it is necessary that the start of the

event be recorded completely over a frequency

range extending down to 50 Mc/s or lower.

This condition was fulfilled at Fort Davis only
248

SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

TABLE 6.—Percentage of type II with and without type IV followed within three days by various levels of geomagnetic

activity, subdivided by properties of the solar event. Values deviating less than 2<x and more than fa from the random

sample are designated by parentheses and by bold face type, respectively.

Properties of events

a) All bursts

b) >10»

<9»

c) Flare >2+

Flare <2+

d) fiy, y sunspots

a, p sunspots

e) North

South

f) East

West

g) <30°fromCM

30-59°

>60°

h) Flare >2+ \0y'

y

Random sample

Type II without type IV

No.

146

40

84

27

98

32

84

85

40

51

75

42

46

38

9

14

1460

Percent followed by

Great

storm

(8)

(2)

(10)

16

(5)

19

(4)

(8)

(5)

(6)

11

(10)

(2)

13

33

(7)

5.4

Moderate

storm

(8)

(10)

(9)

(10)

(9)

(9)

(9)

(10)

(10)

(12)

(6)

(7)

(9)

(13)

(22)

11.3

Small

storm

34

41

(30)

(37)

32

41

(32)

33

(33)

35

(32)

(35)

39

(24)

(45)

(21)

23. 2

No.

51

30

16

29

17

26

20

33

10

24

22

17

18

11

19

10

1460

Type II with type IV

Percent followed by

Great

storm

24

33

(12)

31

18

23

30

33

(8)

29

18

29

28

(9)

32

30

5.4

Moderate

storm

(17)

(20)

(19)

(21)

(17)

(19)

(20)

(19)

(23)

(13)

27

(18)

28

(9)

(15)

(30)

11.3

Small

storm

(32)

(27)

(44)

(34)

(30)

(31)

(35)

(30)

(38)

(33)

(32)

(41)

(11)

55

(37)

(30)

23.2

after January 1959. Therefore the bursts for

1959-60 are shown separately in table 7. Their

geomagnetic success does not differ from that

of the larger but less reliably defined four-year

sample. Table 7 suggests that type IV and

type II bursts should both be present for the

maximum probability of a great geomagnetic

storm, although, of the two, type IV is the more

essential.

Figure 16 and line a of tables 5 and 6 (i.e.,

lines 5a and 6a) show that type IV bursts have

a greater probability than type II of being

followed by a moderate geomagnetic storm.

In the probability of a small storm, on the other

hand, we find no significant difference between

type II and type IV bursts. The highest

probability of a small storm is given by type IV

without type II; however, the small sample in

table 7 limits the significance of the difference.

Because of the larger number of type II bursts

observed, about an equal number of type II and

type IV are followed by great and moderate

storms. About twice as many type IPs as

type IV's are followed by a small storm. Thus

the finding that type II alone has significant

geomagnetic success only at the small-storm

level does not necessarily disprove Wild's

(1950) attractive hypothesis that type II bursts

arise from the ejection of a corpuscular stream

which is related to geomagnetic storms. Indeed

the data tend to support Wild's (1960) later

suggestion that the presence of type IV radiation may be an indication of the amount of

matter transported from the flare by the type

II phase of the disturbance.
SOLAR RADIO BURSTS 249

40 80 100

pm

jjjt.V.v.:.Wv.M •• 1

LwW'&'-fc--':

No. Activity

1324 All Major Flares

173 a, /3 Flares

24

14

10

14

159

149

Type II

Type IV

II and IV

ll.no IV

No IV Obs'd

No II Obs'd

136 /3X,/Flares

Type II

Type IV

II and IV

ll.no IV

No IV Obs'd

No II Obs'd

1460 Rondom Sample

FIGURE 2.—Percentage of major flares followed within three

days by a great (black), moderate (dots), or small (gray)

geomagnetic storm, subdivided by magnetic type of the

flaring sunspot and by the types of accompanying spectral

radio bursts.

TABLE 7.—Percentage of type IV events without type II

followed within three days by various levels of geomagnetic

activity

No.

events

14

35

Years

1959-60

1957-60

Percent followed by

Great

storm

(14)

14

Moderate

storm

(15)

(20)

Small

storm

50

40

From an examination of the details of tables

5 and 6 we can draw some additional conclusions. Line 56 shows that type IV bursts

of long duration tend to be more geoactive than

short-lived ones, especially in the production of

great and moderate storms. With respect to

geomagnetic success the bursts appear to

divide into three duration categories: <15m,

effective only for small storms; 16m to 3* 59m;

and > 4b, with the latter especially effective in

the production of great storms.

Similarly, among typo II bursts the long-lived

surpass the short-lived in geomagnetic success.

The numbers of cases in line 66 show that a

much higher percentage of long- than of shortlived type II bursts are accompanied by type

IV. Line 66 also shows that, even among type

II's with type IV, the longer-lived type II's are

more successful, especially in producing great

storms. If matter is transported at all during

the type II phase of the disturbance, one would

expect that the amount of matter transported

would be related to the duration of the burst.

Thus the data of line 66 provide further support for Wild's (1960) suggestion that the

presence of type IV radiation may be an indication of the amount of matter transported.

Table 5 shows that association with a major

flare (line c), origin from a magnetically complex spot group (line d), and location in the

northern solar hemisphere (line e) are all factors

that increase the probability of success of a

type IV burst. The significance of these factors is substantial for great and moderate

storms, but becomes minimal, and indeed even

reversed, for small storms. Line 5e is in accord

with Bell's (1961) earlier finding that the northsouth asymmetry was significant only for

sources of great storms, not for lesser storms.

The association with a major flare and a complex sunspot also increases the probability that

a type II burst will be accompanied by one of

type IV (lines 6e-6e).

Line 5/ shows that a few more bursts of each

type occurred west than occurred east of the

solar central meridian. In agreement with

Bell's result from major flares, there is no

evidence of any east-west asymmetry in the

geomagnetic success of either type IV or type

II bursts. From line bg we can see that the

number of type IV bursts falls off to the solar

limb much more than does that of type II.

For comparison, Bell's 1937-1959 data give a

decrease by a factor of two in the number of

major flares from the central to the limb zone—

with 260, 190, and 130 major flares in the three

zones—a decrease intermediate between that

for type IV and that for type II radio bursts.

For type II the level of geomagnetic success

appears independent of meridian distance. The

type IV behavior parallels that for major flares,

with a striking decrease in the probability of a

great storm and a compensating increase in the

probability of a small storm toward the limb.

Lines hh to 5/ indicate that the various factors

contributing to the geomagnetic success of a
250

SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

solar-activity event cannot be readily separated.

Duration being equal, some advantage remains

to major flares (line 5k) and to northern bursts

(line 51), but the magnetic class of the spot

(line bj) appears unimportant. Figure 2 also

shows that the probability of geomagnetic success following a major flare with a type IV

burst does not depend significantly on the

magnetic type of the sunspot. However, lines

5^ to 51 reemphasize the point made earlier,

that complex spot groups—and the northern

hemisphere—generate a disproportionate share

of the longest-lived type IV events.

The principal conclusions of this section may

be summarized as follows. The data clearly

indicate a very strong association between radio

bursts of spectral type IV and great geomagnetic storms. The success of a type II burst

appears to depend on the presence of a type IV.

Type II bursts that are unaccompanied by type

IV are significantly related only to small

storms. However, type IV and type II bursts

occurring together give the highest probability

of a great storm.

Among the attributes considered in this

paper, a major associated flare, location in the

north, and long duration of the burst appear to

be the most important in raising the probability

of a great or moderate storm. Type IV events

lasting less than 15 minutes are unlikely to be

followed by more than a small storm.

When radio spectrum observations are available, the magnetic class of the sunspot is of

minor value in forecasting the probable geomagnetic consequence of a given flare. A

major flare with type IV emission lasting more

than 15 minutes has a large probability of

success whatever its spot class. However, a

major &y or y flare is about three times more

likely to be accompanied by type IV emission

than is a similar /3 or a flare. Also, among the

spot groups producing at least one major flare

(table 1), the #y and y spots produced an

average of 1.7 times as many major flares as the

0 and a spots. These two factors together indicate that in a given time interval a complex

spot has at least five times the probability of

generating a type IV burst than does a similar

a or 0 spot group. Thus, when we are interested in forecasting over a longer term in advance of the observation of an actual flare with

or without radio bursts, the magnetic type of

the sunspots appears to be the most useful criterion known to date.

PCA and type FV events

The present data confirm previous findings

(Boischot, 1958; Kamiya and Wada, 1959;

Hakura and Goh, 1959; Thompson and Maxwell, 1960a, 1960b; Obayashi and Hakura,

1960a, 1960b; Pick, 1961) of a significant association between type IV radio events and PCA

proton events. Of the 96 type IV events observed at Sydney or at Fort Davis, 23 were

followed within a few hours by a PCA. Of

these 23 events, 13 had a type II burst also;

and only three (in 1959-1960) can be said with

confidence to lack a type II burst. All but one

of the 23 were associated with a major flare.

Thus about half (46 percent) of the type IV

events with a major flare were followed by a

PCA. Eighteen other PCA's were most probably associated with flares which occurred at

times when neither the Fort Davis nor the

Sydney spectrum analyzer was operating.

Table 8 shows the relation between the

duration of the type IV emission and the

intensity of the PCA as measured by Bailey

(1962). The final column indicates that the

percentage of type IV bursts that are followed

by a PCA increases with increasing duration

of the radio emission. Eeference to table 2

reveals, however, that the percentage of type

IV events associated with a major flare similarly

TABLE 8.—Relation between radio bursts of spectral type

IV (by duration) and PCA events (by intensity)

Duration of

type IV burst

<15m

16m-29m

3()m_59m

Ih_lh59m

2

h

-3"59»

> 4 h

Total

No spectrum

observations

No. followed by

PCA of intensity:

L-VL

_

—

—

—

2

4

6

2

M

1

—

—

1

2

2

6

5

s-vs

2

5

3

—

1

11

11

Percent

Total followed

No.

1

2

5

4

4

7

23

18

by PCA

6%

14

25

24

31

44

24

—
SOLAR RADIO BURSTS 251

increases with the duration of the burst. If we

consider only type IV bursts with a major

flare, the percentage followed by a PCA appears

to be independent of the duration of the burst,

at least for bursts lasting more than 15 minutes.

On the other hand, the intensity of the PCA

shows a rather striking relation to the duration

of the burst. Among PCA's of moderate (M)

or greater intensity (L, VL) in table 8, 83 percent were preceded by a burst lasting at least

two hours, while 91 percent of the small (S) and

very small (VS) PCA's were preceded by a burst

lasting less than two hours. A 2X2 chi-square

test gives a probability of less than 0.001 that

the observed distribution might occur by

chance. Thus in spite of the rather small

sample size, it seems probable that there is a

significant relation between the intensity of the

PCA and the duration of the preceding type

IV emission. It would, indeed, be rather

disturbing if no such relation existed.

We have already noted that table 3 shows

that a substantial majority of PCA events

originate from magnetically complex sunspot

groups.

Table 9 shows the relation between the intensity of the PCA and the intensity of the

associated geomagnetic disturbance. Only 5

(11 percent) small or very small PCA's were

not followed within three days by at least a

small storm. (If we characterize the intensity

of the storm by its maximum 3-hr Kp, then we

find that 28 percent of the PCA events were

followed by Kp > 9~, 50 percent by Kp > 8~,

and 72 percent by Kp > 7". Only 13 percent

failed to be followed by a Kp > 6~.) The

correlation, if any, between the intensity of the

PCA and the intensity of the subsequent geomagnetic storm is too weak to be statistically

meaningful. On the other hand, as we shall

see in the last section, the probability that a

given geomagnetic storm was preceded by a

PCA is strongly dependent on the intensity of

the storm.

Solar activity preceding geomagnetic storms

In a previous section we considered the geomagnetic activity following solar radio bursts.

Here we investigate the inverse problem. We

begin with a list of great, moderate, and small

geomagnetic storms, defined as before, and

determine the percentage preceded within three

days by a major flare, a type IV burst, a type

II burst, and various combinations of these

events. The results appear in tables 10 and 11.

The statistical significance of each deviation

from the random probability, or percentage to

be expected by chance, was estimated as

before. (In determining the random probabilities a given day was counted only once even

when it had several bursts; a burst observed

on the day following another burst-day added

only one to the number of days following within

three days of a burst, etc.)

The first line of table 10 shows that with one

exception (1960 Oct. 6) each great storm was

preceded within three days by at least one

major flare. Just over three-fifths were preceded by type IV and by type II bursts. Since

31 percent were preceded by a major flare that

occurred at a time when neither the Fort Davis

nor the Sydney spectrum analyzer was operTABLE 9.—Relation between PCA events and geomagnetic activity following within three days

Intensity

of PCA

L-VL

M

&-VS

All PCA'B

Percent

Great

storm

5

5

8

18

39%

No. followed by

Moderate

storm

2

4

5

11

24%

Small

storm

2

3

7

12

26%

No

storm

5

5

1 1 %

Total

No.

9

12

25

46

100%
252 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

TABLE 10.—Percentage of geomagnetic storms preceded within three days by various manifestations of solar activity.

Values (in cols. 3-6) deviating less than 2<r and more than \.afrom the random sample are designated by parentheses and

bold face type, respectively.

Storm intensity

Great

Moderate

Small

Max Kp

> 9"

8" to 8+

7- to 7+

6° to 6+

6"

5" to 5+

2a,

4

>900

500-899

300-499

200-299

160-199

110-159

Random sample

G

M

S

G

G-M

M-S

M-8

S

S

G

M-G

M-S

M-S

S

S

No.

26

55

113

15

24

41

43

32

40

12

22

42

55

36

27

1460

Type IV

burst

62

(24)

(21)

73

(29)

29

33

(22)

(5)

75

41

31

29

(11)

(7)

16.0

Type II

burst

62

(35)

(27)

73

(33)

(44)

(42)

(47)

(20)

75

(36)

(45)

(40)

(31)

(30)

32.5

Percent preceded by

Major

flare

96

60

(40)

93

83

68

(44)

(34)

(28)

92

91

71

(42)

(31)

(30)

41.2

PCA

65

(16)

(11)

87

29

24

(9)

(9)

(7)

83

50

21

(7)

(6)

(11)

9

No activity

All Recurrent

4

33

44

7

17

24

37

41

60

8

9

19

40

58

56

—

4

29

24

7

17

20

23

28

30

8

9

14

35

25

26

—

Total

percent

recurrent

23

44

35

7

46

29

40

47

35

8

41

26

51

36

30

—

ating, it seems probable that most of the great

storms in table 10 were actually preceded by

type IV emission.

The equality of the percentages for type IV

and type II bursts is somewhat misleading, for,

as table 11 shows, only 35 percent of the great

storms were preceded by type IV and type II

in association with the same (major) flare.

(Two additional cases had type IV bursts in

progress at sunrise at Fort Davis, so there was

no possibility to observe type II if it occurred.)

Most of the great-storm type IV bursts were

associated with a major flare, but a substantial

portion of the type II bursts occurred independently, in association with some lesser

flare.

The percentage of moderate storms preceded

by these solar activities is sharply below that

for great storms. A significant number are

still preceded by a major flare, but the association with radio bursts hardly exceeds the

number expected by chance. Even major

flares are not associated with small storms more

often than one would expect to occur by chance.

The last columns of table 10 show that a

substantial proportion of moderate and small

storms are recurrent. As used here, "recurrent" means a member of a sequence with at

least three recurrences at an interval of 26-28

days; most of the sequences contain more than

the minimal three recurrences. The high percentage of recurrent storms in these intensitygroups, and the low association with solar

activity is in accord with the hypothesis proposed first by Allen (1944) that there are two

fundamentally different types of magnetic

storms—the sporadic sc-type which includes

most of the great storms, and the recurrent or

M-region type of lesser intensity. Allen found

that although the sporadic storms showed a

positive association with the central meridian

passage of large sunspot groups, the recurrent
SOLAR RADIO BURSTS 253

TABUS 11.—Percentage of the geomagnetic storms preceded urithin three days by a major flare with and

without associated radio bursts of types II and IV. Values (in cols. S-S) deviating less than to and

than 4& from the random sample are designated by parentheses and bold face type, respectively.

Storm intensity

Great G

Moderate M

Small S

Max Kp

>9~ G

8" to 8+ G-M

7- to 7+ M-S

6° to 6+ M-S

6" S

5- to 5+ S

S o ,

>900 G

500-899 G-M

300-499 M-S

200-299 M-S

160-199 S

110-159 S

All major flares

No. with

major

flare

25

33

45

14

20

28

19

11

11

11

20

30

23

11

8

309

Percent with associated

IV, II

36

(18)

20

36

(20)

25

32

(18)

(0)

45

25

(20)

26

(18)

(0)

9.4

IV

60

30

(24)

72

(30)

32

42

(27)

(0)

73

45

(27)

39

(18)

(0)

15.5

II

40

(24)

29

43

(20)

32

37

(36)

(9)

54

(25)

(27)

35

(36)

(0)

17.8

No spect.

obs'ns

32

58

38

14

65

50

37

27

45

9

55

57

39

27

38

~43*

No burst

observed

4

6

29

7

5

11

16

27

45

9

0

10

13

36

62

~33*

•Estimate derived from 103 storm-flares only; sum of the last two columns is 76% of the 309 flares.

storms showed a negative association. In addition, Bell and Glazer (1957) found a negative

relation between recurrent storms and the intensity of the green (X5303) emission line corona.

Table 12 compares the solar activity preceding the recurrent and the nonrecurrent storms

in my four-year sample. Nonrecurrent storms

at each intensity level show a stronger association with major flares and with radio bursts

than do recurrent storms. Small recurrent

storms show a significantly (>2<r) lower association with major flares and with type II bursts

than would be expected to occur by chance.

The tendency of small and moderate recurrent

storms to avoid major flares and radio bursts

is in accord with Allen's finding of a negative

relation between recurrent storms and large

sunspots from an independent sample of data,

and strikingly supports his hypothesis of two

fundamentally different types of geomagnetic

storm.

It seemed unlikely that my rather arbitrary

choice of Ap > 100 and/or Kp > 9~ as the

criteria for a great storm should define as well

as possible the boundary between storms that

seem well accounted for in terms of preceding

major flares and radio bursts and those that are

not accounted for. The first three lines of

tables 10 to 12 invited a refinement of the storm

classification, particularly a subdivision of the

class of moderate storms. Two additional

measures of the intensity of the storm were

therefore considered—the maximum value of

Kp attained during the storm; and 2 ap, the

sum of the four largest consecutive 3-hr ap

values. In tabulating the data I kept separate

each value of Kp; for 2 <*»> I used steps of 100

4

above 700, of 50 from 200 to 700, and of 20

below 200. The groupings in table 10 were

suggested by inspection of these initial tabulations; to the best of my judgment they represent
254 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

TABLE 12.—Comparison of solar activity preceding recurrent and nonrecurrent geomagnetic storms. Values in cols.

3-5, 7, 9-11 deviating less than Sar and more than 4<r from the random sample are designated by parentheses and

bold face type, respectively.

Storm intensity

Great

Moderate

Small

M&xKp

>9~

8"to8+

7- to 7+

6°to6+

6-

5-to5+

2 op

4

>900

500-899

300-499

200-299

160-199

110-159

Random sample

Nonrecurrent storms

No.

20

31

73

14

13

29

26

17

26

11

13

31

27

23

19

Type

IV

burst

65

32

26

79

(31)

35

46

(18)

(8)

82

46

36

37

(17)

(11)

16.0

Percent preceded by

Type

II

burst

65

(48)

48

72

(31)

55

54

71

(27)

82

(31)

55

67

(35)

(37)

32.5

Major

flare

100

84

(49)

100

100

86

(54)

(41)

(35)

100

100

84

67

(30)

(37)

41.2

No activity

0

6

32

0

0

7

23

24

46

0

0

6

11

52

42

—

PCA

75

26

(14)

93

39

31

25

(12)

(12)

91

77

26

(11)

(22)

(16)

9

Recurrent storms

No.

6

24

40

12

29

[ 29

10

39

21

Percent preceded by

Type

IV

burst

(50)

(12)

(12)

(25)

(14)

(14)

(30)

(20)

(0)

16.0

Type

II

burst

(50)

(17)

17

(33)

(21)

14

(40)

15

19

32.5

Major

flare

83

(29)

22

(58)

(28)

21

(70)

23

(24)

41.2

No activity

17

67

68

42

62

72

30

64

76

—

groups homogeneous in degree of association

with solar activity. By these new criteria

there is a considerable range of overlap between

great and moderate, and between moderate and

small storms. The bottom categories are not

necessarily complete; they include only those

events also satisfying the requirements for a

"small storm" (Ap > 25).

Great storms, with Kp > 9~ or 2a, > 900,

show the expected very strong association with

all three types of solar activity. Table 11

shows that the association with type IV remains

stronger than that with type II bursts. With

the new criteria the percentage of storms preceded by radio bursts falls off rather abruptly

below this intensity limit, while the percentage

preceded by a major flare declines more gradually. Serious doubt is cast on the reality of the

sharp decline in percentage of storms preceded

by radio bursts by the next-to-last column of

table 11, which shows that a particularly high

percentage of the major flares preceding storms

in theranges 7" < Kp < 8+ and 300 < 2a, < 900

occurred at times without radio spectrum observations. At least down to Kp = 8~ and

2a, = 500 it remains possible, indeed even probable, that most nonrecurrent storms were preceded by type IV emission as well as a major

flare. Even down to Kp = 6° or 2a, = 200

the percentage preceded by a major flare without an observed burst of either type is small.

The inverse analysis tends to confirm the

previous finding that type IV emission is more

essential than type II for a great storm. Study

of the percentages in tables 10 and 11 indicates

that a large proportion of type II bursts, especially for the weaker storms, were associated

with minor flares. Not infrequently a type II

burst with a minor flare appeared to be the

only "explanation" for a small or moderate

storm. Table 12 shows that more nonrecurrent
SOLAR RADIO BURSTS 255

storms of medium intensity were preceded by

type II than by type IV bursts, although in

terms of deviation from the random probability

the type IV association remains the more

significant.

In table 10 the percentage of storms accounted for by a major flare falls off smoothly

with decreasing storm intensity, whichever

storm criterion is used. The percentage of

storms preceded by no activity (of the three

kinds considered here) rises. The percentage

belonging to recurrent sequences also rises, but

less smoothly, and is actually greater for moderate than for small storms. Most of the moderate storms without preceding solar activity

can be accounted for as members of M-region

sequences. Recurrent storms with maximal

Kp < 8~ or XaP < 500 show (table 12) a striking avoidance of major flares and radio bursts.

A similar, although statistically less significant,

avoidance is shown by the weaker nonrecurrent

storms. A substantial proportion of even nonrecurrent storms with maximal Kp < 7~ or

2ap < 200 are not accounted for by solar activity. It seems reasonable to postulate that

some of these are M-region storms whose

sources were disrupted before they had produced the minimal three recurrences required

for identification.

The PCA's, as we would expect, are almost

entirely associated with storms of the nonrecurrent type. Their association with recurrent storms is no greater than might be

expected to occur by chance. Most of the

greatest storms were preceded by a PCA (see

tables 10 and 12). The percentage drops

rapidly with diminishing storm intensity, which

might suggest that the percentage of storms

preceded by type IV emission also actually

drops about as shown in the tables. Since

less than half of the type IV bursts with a major

flare were followed by a PCA, such an argument does not seem to be compelling. The

anomaly in the completeness of the radio

spectrum observations makes it impossible to

set more precisely the limits of storm intensity

above which almost all of the storms are

preceded by a major flare and type IV emission,

at least until nonspectrum data on type IV

bursts are added to the analysis. This work is

in progress.

Note in tables 10 and 12 that 2ap appears

to be a better guide than the maximal Kp

to the possibility that a given storm was preceded by a PCA. 2a, also provides a sharper

boundary below which a substantial proportion

of nonrecurrent storms are unaccounted for.

For these reasons 2ap is probably the more

useful measure of the intensity of a storm.

Acknowledgments

The research reported in this paper has been

sponsored by the Geophysics Research Directorate of the Air Force Cambridge Research

Laboratories, Office of Aerospace Research,

under contract AFl9(604)-4962.

I am indebted to Dr. Alan Maxwell and

Dr. A. R. Thompson of the Harvard Radio

Astronomy Station, Fort Davis, Texas, for

several helpful and stimulating discussions

during the progress of the work. I am indebted

to Dr. C. S. Warwick and Mrs. M. W. Haurwitz for providing me with a copy of their

list of PCA events in advance of publication.

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Abstract

Solar radio bursts of spectral types II and IV, observed in the years 1957-1960, are discussed in relation to flares,

the magnetic class of the associated sunspot, the north-south asymmetry, and to geomagnetic and solar proton (PCA)

effects. A total of 96 type IV and 197 type II bursts were studied.

About 50 percent of the type IV and 28 percent of the type II bursts were associated with a major (>2+) flare.

Sunspot groups of complex (fiy and y) magnetic class produced 61 percent of the type IV but only 36 percent

of the type II bursts. Comparative data are given for sunspots, flares, and geomagnetic storm and PCA sources.

The north-south asymmetry appears to increase with increasing intensity in various types of solar activity.

Long-lived bursts of each type are more concentrated to the north than are short-lived bursts. But there is no

significant difference between type II and type IV bursts in north-south distribution.

The data indicate a very strong association between type IV bursts and great geomagnetic storms. The increase

in probability of a great storm within three days after a type II burst is also significant but less striking. However

those type II bursts that were not accompanied by type IV appear to be significantly associated only with small

storms.

The bursts were subdivided by various properties. A major flare or a long duration of the burst each increases

the probability of geomagnetic success. Type IV bursts lasting less than 15 minutes have little geomagnetic importance. Given a major flare with type IV emission, the magnetic class of the sunspot is of minor significance.

However, a py or y sunspot is about five times more likely to produce a major flare with type IV emission than is

a P or a spot group of comparable size.

The inverse problem, of solar activity preceding geomagnetic storms, was analyzed as a function of storm

intensity. A significant difference between recurrent and nonrecurrent storms was found, in that recurrent storms

tend to avoid centers of major flare and radio burst activity.

A strong association is found between type IV radio events and PCA events, with 23 of the type IV bursts followed within a few hours by a PCA. The duration of the burst is positively related to the intensity of the PCA.

A high percentage of proton events arise from complex sunspot groups.

I), •OTIIIKIT PIIITIM •mct.no