SMITHSONIAN

CONTRIBUTIONS

to

ASTROPHYSICS

Smithsonian Institution

Astrophysical Observatory

Volume 8, Number 3

Type IV Solar Radio

Bursts, Geomagnetic

Storms, and Polar Cap

Absorption (PCA) Events

by Barbara Bell

Washington, D.C.

Smithsonian

Contributions to Astrophysics

VOLUME 8, NUMBER 3

TYPE IV SOLAR RADIO BURSTS,

GEOMAGNETIC STORMS, AND

POLAR CAP ABSORPTION (PCA) EVENTS

by BARBARA BELL

SMITHSONIAN INSTITUTION

Washington, D.C.

1963
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Type IV Solar Radio Bursts, Geomagnetic Storms, and

Polar Cap Absorption (PCA) Events

Barbara Bell1

This paper compares solar radio bursts of

type IV identified by means of spectrum observations in the meter bands with those identified from discrete frequency observations

particularly in the centimeter bands. The

comparison considers certain associated activesun phenomena and properties of the bursts

themselves as well as geomagnetic storm and

solar proton (PCA) effects. Solar activity preceding geomagnetic storms is discussed, with

sporadic and recurrent storms treated separately. In the final section the disk position

of the source is studied as a function of storm

intensity.

It has been well established that type IV

bursts have a high probability of being followed

by geomagnetic disturbance and by ionospheric

absorption over the polar cap (PCA). Among

the various studies, those by McLean (1959),

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

Bell (1963, hereafter called Paper I) are based

exclusively on radio spectrum observations.

The others (see Paper I for additional references) have been based wholly or in part upon

observations at discrete frequencies.

Paper I contains a detailed examination of

the relationship of solar radio bursts of spectral

type IV (and also type II) with flares, the

Mount Wilson magnetic class of the associated

sunspot, the north-south asymmetry, geomagnetic storms, and PCA effects. The study included a total of 96 type IV (and 197 type II)

bursts observed in the four years 1957 through

1960 with the dynamic spectrum analyzers at

the Harvard Radio Astronomy Station, Fort

1

 Harvard College Observatory, Cambridge, Mass.

680449—63

Davis, Texas, and at the Radiophysics Laboratory of the Commonwealth Scientific and Industrial Research Organization, Sydney, Australia.

These instruments record over the frequency

range 580 to 25, and 240 to 40 Mc/s, respectively. These same spectral data are used in

the present study.

The only published comprehensive list of

type IV bursts derived from discrete frequency

observations has been compiled by Pick-Gutmann (1961). These bursts are identified primarily by unusually intense (^lO"20

 W/m2/Hz)

and long-lived (>10 minutes) radiation in the

centimeter region of the solar spectrum (2800

and 9400 Mc/s). The requirement of intense

centimeter radiation makes Pick-Gutmann's

bursts of especial interest for comparison with

the spectrum bursts recorded at longer wavelengths.

Pick-Gutmann lists 145 type IV bursts in

the interval from January 1957 through March

1960. Over the same interval 70 type IV

events were recorded at Fort Davis and/or

Sydney. (Maxwell, Thompson, and Garmire

(1959) estimate that the Harvard and Sydney

stations together cover an average of 14 hours

per day.) Comparing the two lists we obtain

four categories of bursts:

H/S,P: observed by Harvard or Sydney, and listed

by Pick-Gutmann;

H/S.-P: observed by Harvard or Sydney, but not

listed by Pick-Gutmann;

P,-H/S: listed by Pick-Gutmann, but not by Harvard

or Sydney;

P,n.o.: listed by Pick-Gutmann, no observations by

Harvard or Sydney.

119
120 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

TABLE 1.—Properties of type IV bursts recorded by spectrum analyzer

VOL. I

Burst

category

H/S.P

H/S,-P

A11H/S

Number

46

24

70

Percent with duration

<15-

9

38

19

16-59"

22

42

29

l*-3h59m

41

12

31

> 4 h

28

8

21

Percent with max. freq.

>580

67

37

57

<580

13

42

23

Unknown

20

21

20

The (H/S,P) bursts radiate strongly on both

centimeter and meter wavelengths; the (H/S,-P)

bursts are strong in meter bands but fall below

Pick-Gutmann's limits in the centimeter region; and the (P,-H/S) are strong in the centimeter bands but weak or absent, or not of

spectral type IV, on meter wavelengths. About

one-third of the (P,-H/S) bursts have an

associated type II burst, and about two-fifths

have short-lived «5 m) continuum emission.

Table 1 compares the first two categories of

bursts with respect to duration and maximum

observed spectral frequency. The latter quantity was available to me only for Harvard

observations; the "unknown" are from Sydney.

On the left, table 1 shows that the (H/S,P)

bursts are of substantially longer duration on

the average than the (H/S,-P) bursts. Only

20 percent of the latter, but 69 percent of the

former lasted more than one hour. From the

values in table 1 it can be found that only 16

percent of the spectral bursts lasting more

than an hour lacked type IV centimeter radiation. On the other hand, about 70 percent

of the spectral bursts lasting no more than 15

minutes, which in Paper I were shown to be

significantly less geoactive than longer lasting

bursts, lack type IV centimeter radiation. A

check of the radio data in the IAU Quarterly

Bulletin of Solar Activity revealed that about

57 percent of all (H/S,-P) bursts were too weak

at around 10 cm for inclusion in PickGutmann's list; 39 percent had no recorded

centimeter radiation; and 17 percent were of

too short duration. Among the (H/S,-P)

bursts of spectrum duration <15m, 30 percent

were apparently (-P) because of insufficient

duration but only 10 percent for this reason

alone.

On the right, table 1 shows that many more

of the (H/S,P) than of the (H/S,-P) bursts are

observed at the maximum frequency recorded

by the Harvard spectrum analyzer. A substantial percentage of the bursts without

sufficient type IV centimeter radiation for

inclusion in Pick-Gutmann's list had their

maximum observed spectral frequency well

below 580 Mc/s.

Table 2 compares the various categories of

type IV bursts with respect to association with

a major flare (rated importance 3 or 3 + by at

least one observatory, or 2+ by at least two

observatories), association with a sunspot of

complex magnetic class, location in the northern

solar hemisphere, and distance from the solar

central meridian (CM). Paper I gives comparative data for other active-sun phenomena

such as flares, sunspots, and type II bursts, as

well as a subdivision of the spectral type IV

bursts by duration. Bursts observed over the

full frequency-range (H/S,P) show the strongest

preference for sunspots of the complex classes,

and also the highest percentage of association

with a major flare. The lowest association

with major flares as well as with complex

sunspot groups occurs for the bursts that are

observed on meter but not on centimeter

wavelengths (H/S,-P).

Evidence was presented in Paper I that in

recent cycles the concentration to the northern

hemisphere tends to increase with the increasing

intensity in several manifestations of solar

activity. The percentages in the fourth column

of table 2 are in accord with this tendency,

although taken alone the differences between

the various categories are of limited significance.

The last three columns of table 2 show that

the spectral (H/S) type IV bursts are more
TYPE IV SOLAR RADIO BURSTS—BELL 121

TABLE 2.—Association of type IV bursts with major flares, complex (/3y or y) sunspots, and

position on the solar disk

Burst category

H/S.P

H/S.-P

All H/S

P,n.o.

P.-H/S

AU P

Percent of bursts associated with

Major

flare

68

21

51

61

38

55

Complex

sunspot

64

42

57

56

50

57

Northern

hemisphere

71

60

67

67

58

67

Meridian distance

<30°

50

58

52

33

34

39

30°-59°

36

42

38

35

33

35

>60°

14

0

10

32

32

26

concentrated toward the solar central meridian

than are the centimeter (P) bursts. The

(P,n.o.) and (P,-H/S) bursts indeed show a

quite uniform distribution with meridian distance. Pick-Gutmann (1961) found that bursts

with the centimeter and the meter radiation of

about equal duration were most numerous

from beyond 45° from the CM, while most

bursts with conspicuously prolonged meter

radiation arose from within 45° of the CM.

She presented evidence that the latter bursts

have a "second phase" which is confined to

meter and dekameter wavelengths and which

is more directional than the first phase.

Because of this directional effect, similar to

that observed for ordinary noise storms, the

second phase is not often observed in bursts

generated in regions remote from the CM.

A subdivision of the spectrum bursts by

duration, in table 3, reveals a systematic increase in concentration to the central regions

of the solar disk with increasing duration from

16m upward. The trend is in the direction to

be expected if Pick-Gutmann's concept of a

directive second phase is correct. On the other

hand, it is known that low-frequency waves are

more strongly refracted in the corona than are

high-frequency waves, and hence meter waves

will have more difficulty in reaching the earth

from oblique angles than do centimeter waves.

Refraction and the associated phenomena of

scattering and absorption may account for most

TABLE 3.—Relation between duration of spectral type IV bursts and distance from the solar

central meridian

Burst category

H/S.P

All H/S

Duration

<15">

16-59"

lb-3h59»

>4h

<15m

16-59TM

Ik_3h59m

>4h

Number

3

9

19

13

10

32

31

17

Percent with meridian distance

<30°

67

22

47

69

70

25

58

71

30°-59°

33

56

32

31

30

47

26

29

>60°

0

22

21

0

0

28

16

0
122

SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

or all of the relation between meridian distance

and duration of the meter wave bursts. As the

disturbance weakens in intensity the meter

waves can no longer reach the earth in

detectable amounts from highly oblique angles.

Only the stronger bursts, during their period of

greatest intensity, can be observed from large

CM distances at meter wavelengths. The

strong concentration to the CM of bursts

lasting less than 15m suggests that these bursts

come from rather moderate disturbances. The

(H/S,-P) bursts are too few in number to

establish any systematic trends with duration,

but as table 2 shows, none of them originated

more than 60° from the CM. (The lower

section of table 3 includes the spectral bursts

from the last nine months of 1960 in addition

to the interval covered by Pick-Gutmann's

list.)

The concentration to the central zone of

bursts lasting more than 4h is no doubt related

to their outstanding success in producing great

geomagnetic storms, found in Paper I. As Newton

(1943) first showed, and Bell (1961) and others

have confirmed, most great storms follow flares

that occur within 45° of the CM. (See also

the concluding section of this paper.)

Geomagnetic and PCA effects

Each burst was classified according to its geomagnetic success as measured by the planetary

magnetic indices, Kp and Ap, 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. Table 4

gives data on the geomagnetic success of

each category of type IV burst. The last line

of the table shows the level of success to be

expected from a random sample of days and

was derived from the number of storms of

each intensity within the four years 1957

through 1960. To facilitate comparison, table

5 summarizes the geomagnetic success of radio

bursts of spectral types IV and II from Paper I.

Table 4 shows that type IV bursts that are

strong over the full frequency range (H/S,P)

have a high probability of being followed

within three days by a great storm. In striking

contrast, bursts of type IV that are identified

only on centimeter wavelengths (P,-H/S) or

only on meter wavelengths (H/S,-P) are followed by a great storm slightly more often

than might be expected to occur by chance.

Moreover, in most of the cases where such a

limited-frequency burst was followed by a

great or moderate storm, there was also another

burst of full frequency range preceding the

same storm. Type IV bursts of limited

frequency range have no higher level of geomagnetic success than do type II bursts without

type IV (see table 5 and Paper I). At most

they are significantly associated with small

storms.

In the second section of table 4 the bursts

of spectral type IV are divided according to

their maximum and their minimum recorded

spectral frequency. The frequency range over

which the burst was intense enough to be

recorded, in the case of the Harvard data,

proves to be one of the most useful properties

of type IV bursts for forecasting the probability of a great or moderate storm. A type

IV burst recorded over the entire range from

580 to 100 Mc/s or below has a high probability

of being followed by a great or at least a moderate geomagnetic storm. This probability is

not significantly above the random for Harvard

bursts observed over less than the complete

range of the equipment, whether terminating

above 100 Mc/s or starting below 580 Mc/s.

The latter bursts do, however, show some

association with small storms.

Other attributes found useful in Paper I for

forecasting the success of a type IV burst are

the duration of the burst, and the importance

and the location (north or south of the solar

equator) of the associated flare. The third

section of table 4 shows that the optical importance of the associated flare remains of value

in forecasting even when only full-frequencyrange bursts are considered. Only when accompanied by a major flare is even a (H/S,P)

burst likely to be followed by a great magnetic

storm and/or a PCA event. The full-frequencyrange bursts with minor flares show an unusually high probability of being followed by a

small storm, which suggests some relation between storm intensity and flare importance.
TYPE IV SOLAR RADIO BURSTS—BELL 123

TABLE 4.—Percentage of type IV radio bursts followed by a PCA, and 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 by bold

face type, respectively

Properties of bursts

H/S.P

H/S.-P

P.n.o.

P.-H/S

All H/S

AUP

Spectral freq. range

> 580 to <100 Mc/s

>580to>100

<580

Unknown (S)

H/S,P bursts

Mir

Random sample

Number

46

24

54

45

70

145

37

19

20

20

30

14

21

9

1460

Percent followed by

Great

storm

33

(8)

13

(9)

24

18

32

(11)

(10)

30

40

(14)

57

(0)

5

Moderate

storm

(20)

(8)

(17)

(11)

(16)

(16)

27

(21)

(10)

(15)

(20)

(14)

(24)

(11)

11

Small

storm

(35)

42

(33)

(31)

37

35

(24)

(37)

50

(30)

(27)

64

(19)

(44)

23

PCA

30

(4)

13

(7)

23

17

41

(11)

(5)

30

43

(7)

48

33

3

TABLE 5.—Percentage of radio bursts of spectral types II and IV followed by a PCA, and by

various levels of geomagnetic activity (see table 4)

Events

Type IV

Type II

II and IV

II, no IV

IV, no II

Random sample

Number

96

197

51

146

35

1460

Percent followed by

Great

storm

23

12

24

(8)

14

5

Moderate

storm

19

(11)

(17)

(8)

(20)

11

Small

storm

34

33

(32)

34

40

23

PCA

24

9

26

(3)

20

3
124 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

A striking contrast appears in table 4 between northern and southern full-frequency

bursts with a major flare. In agreement with

the results of Paper I, only a northern burst

has a high probability of being followed by a

great geomagnetic storm; a southern burst is

not likely to be followed by more than a small

storm in spite of possessing the other auspicious properties of a major flare and full

frequency range. On the other hand, the

difference in PCA success between northern and

southern bursts is less and not statistically

significant with the present sample size.

Bursts from complex (£7 and 7) spot groups

have a somewhat greater probability of success

than do bursts from /3 and a groups, but the

difference is not statistically significant. Present results confirm the conclusion of Paper I

that the magnetic class of the sunspot has its

greatest value in longer-range forecasting, in

advance of the solar flare and radio burst.

The last column of table 4 shows that the

frequency range covered by a type IV burst is

also relevant to the occurrence of PCA events.

Only bursts of full frequency range are strongly

associated with PCA. This conclusion is in

accord with the finding by Warwick and

Haurwitz (1962) that most PCA's are preceded

by a radio burst of unusual intensity and duration at wavelengths from the decimetric to the

dekametric. It would appear that most of the

Sydney bursts are strong over a wide band of

frequencies since their geomagnetic and PCA

success is not greatly inferior to that of the

580 to <100 Mc/s Harvard bursts. The last

column of table 5 shows that PCA events are

not significantly associated with type II bursts

that lack type IV emission.

The greater success of full-frequency-range

bursts can be qualitatively understood under

the widely accepted hypothesis that type IV

bursts are synchrotron emission from highenergy (relativistic) electrons spiralling in a

magnetic field. The electrons radiate over a

more or less wide band of frequencies. The

maximum frequency is proportional to the

strength of the magnetic field and to the square

of the energy of the electrons. The minimum

freqency required for waves to escape from a

given depth in the corona is proportional to the

square root of the electron density. Thus

weakness of type IV radiation at high frequencies (H/S,-P) implies weakness of the

magnetic field and/or relatively moderate electron energies. Weakness at low frequencies

(P,-H/S) implies that the disturbance never

rose high enough in the corona for the meter

waves to escape. If the disturbance cannot

reach the outer corona, it would seem to have

small chance to reach the vicinity of the earth.

Without both highly energetic electrons and a

disturbance extending into the outer corona,

the probability is small that a major corpuscular

cloud was ejected.

The spectral meter burst usually starts a few

minutes after the 10-cm burst. In my sample

the delay time, II/S minus P, varies from —14*

to + 31m, with a mean value of -f 8m for the

Harvard bursts (580 Mc/s) and +21 n for the

Sydney bursts (240 Mc/s). These mean values

are compatible with the concept of a source of

excitation moving outward at around 1000

km/sec. However, the wide dispersion in delay times argues against any simple interpretation of type IV bursts in terms of an outward

moving source of excitation directly associated

with the ejection of a cloud of storm corpuscles.

Solar activity preceding geomagnetic

storms

In the previous section we considered geomagnetic activity following type IV bursts.

Here, as in the concluding section of Paper I,

we begin with a list of geomagnetic storms,

subdivided by intensity, and determine the

percentage preceded by a major flare and by a

type IV burst of each category. When studying this inverse problem earlier we found that a

more refined classification of storm intensity

was useful. The two additional measures

introduced in Paper I were the maximum 3-hr

Kp attained during the storm, and] 2a,,, the

sum of the four largest consecutive 3-hr ap

values. Here we shall use mainly the latter

index, and consider only storms with a value of

"Lap >160. This limit excludes 19 of the weakest "small storms".

In Paper I we also found that nonrecurrent

storms showed a much stronger association
TYPE IV SOLAR RADIO BURSTS—BELL 125

with major flares and type IV radio bursts than

did recurrent storms. This result supported

the hypothesis first proposed 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 sporadic storms

showed a positive association with the central

meridian passage of large spot groups, but recurrent storms showed a negative association

or a tendency to avoid large spot groups. A

similar result was obtained by Saemundsson

(1962) from a study of the relation between

plages and geomagnetic storms. Bell and

Glazer (1956, 1957) obtained a negative relation

between recurrent storms and the intensity of

the green (X5303) emission-line corona. Newton and Milsom (1954) showed that SC (sudden

commencement) and non-SC storms are differently related to the sunspot cycle.

From these findings it is clear that any future

study of solar activity preceding geomagnetic

storms should consider separately the sporadic

and the recurrent storms. I therefore subdivided the storms according to whether they

were recurrent, as in Paper I, and in addition

here according to whether they had an SC.

Recurrent, as defined in Paper I, means a member of a sequence with at least three recurrences

at an interval of 26 to 28 days. A storm was

classified as SC only if the SC was observed by

10 or more stations, according to the list of

SC's published quarterly in the Journal of

Geophysical Research.

This subdivision yielded two "pure" and two

"hybrid" categories of storm. The pure categories are, of course, the sporadic SC storms,

and the recurrent non-SC storms, and together

comprise just over two-thirds of the storms in

my sample (January 1957 to December 1960).

Table 6 shows the population of each category

as a function of storm intensity, and demonstrates that the presence of an SC is strongly

related to storm intensity. With very few exceptions storms with maximum Kp>7° and/

or 2a,,>400 have an SC observed at ten or

more stations, even when they are members of

recurrent sequences. The relation between

storm intensity and recurrence is much weaker.

Among the very great storms, however, only

that of 6 October 1960 is recurrent as here defined. Since it appears to lack any explanation

in terms of solar activity, and also lacks a PCA,

it is probably a genuine M-region storm in spite

of its high intensity.

Table 7 shows the percentage of geomagnetic

storms preceded within three days (with a minimum delay time of 10 hours) by a major flare,

by a type IV burst, and by a PCA. The data

confirm the findings of the previous section that

type IV bursts of categories (H/S,-P) and

(P,-H/S) are of little geomagnetic significance.

Type IV bursts of the full frequency range

(H/S,P)—including (P,n.o.), and for April to

December 1960 also (H/S,n.o.)—show a very

strong association with sporadic SC storms

with 2ap>200. The association increases

with increasing storm intensity to 100 percent

for storms with 2ap>1000. The percentage

preceded by a PCA falls off more rapidly with

decreasing storm intensity and is significant

only for storms with 2aP>500.

The recurrent storms without SC show a

strong negative relation with both major flares

and with (H/S,P) type IV radio bursts, a relation significantly below the association to be

expected to occur by chance in each case. All

intensities were combined because there was

no significant variation with storm intensity.

This next-to-last line in table 7 provides additional evidence that M-region storms avoid

active solar centers, and thus supports Allen's

hypothesis that sporadic and M-region storms

have a different mechanism of origin.

The hybrid categories of geomagnetic storm

(sporadic without SC, and recurrent with SC)

show only a random association with type IV

radio bursts, and a negligible association with

PCA. The stronger recurrent SC storms show

a statistically significant association with major

flares, but not with type IV emission. However, the number of such storms is small. The

hybrid categories probably represent a mixture

of active-center and M-region storms, which

tend to cancel each other out in the statistics.

Table 7 may be compared with tables 11 and

12 of Paper I. In particular, we may recall
126

SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

TABLE 6.—Numbers of sporadic sudden commencement (SC) and recurrent geomagnetic storms,

as related to storm intensity

Storm intensity

Great

Moderate

Small

Max Kp

>9~

8+ to 8-

7+, 7°

7-, 6+

6°, 6-

5+, 5°

4

>1000

500-999

400-499

300-399

200-299

160-199

Total

Totfil

number

26

57

87

15

24

24

36

54

17

9

25

19

25

56

30

170

Number

Sporadic

SC

20

31

28

13

14

18

19

13

2

8

16

12

18

18

7

79

NonSC

1

4

25

1

0

2

4

16

7

0

1

1

3

10

15

30

Df storms

Recurrent

SC

5

9

9

1

8

3

5

5

1

1

6

5

1

7

3

23

NonSC

0

13

25

0

2

1

8

20

7

0

2

1

3

21

11

38

Percent

sporadic

81

61

61

93

58

83

64

54

53

89

68

68

84

50

61

64

Percent

SC

96

70

42

93

92

88

67

33

18

100

88

89

76

45

28

60

that the percentage of storms preceded by a

type IV burst dropped abruptly for 2ap below

900. The reality of this drop was uncertain

because an unusually high percentage of storms

with 2ap=300 to 900 were preceded by flares

that occurred outside the Harvard and Sydney

observing hours. Inclusion of the (P,n.o.)

bursts substantially raises the percentage of

moderate storms preceded by a type IV burst.

However, there remains in the data of table 7

some evidence for an abrupt drop in the percentage of storms preceded by typo IV emission

at around the 2ap=1000 level of intensity.

With respect to association with type IV radiation, the intensity range from 999 down to 300,

perhaps even to 200, appears quite homogeneous.

Note that in tables 4 and 5 (and tables 5 and

6 of Paper I) the apparent statistical significance

of the association between small storms and

type IV bursts is weakened by the inclusion of

the M-region storms in the determination of the

random sample. If we excluded all the recurrent non-SC storms—shown in table 7 to have

no connection with major flares or type IV

emission—then the probability that any given

day will be followed within three days by a

moderate storm would drop from 11 to 9 percent, and the probability for a small storm

would drop from 23 to 14 percent. The percentage of full-frequency-range bursts (H/S,P)

followed by a moderate storm would then exceed

the corrected random probability by more than

2<r. The statistical significance of the smallstorm percentages in table 4 would be more

strikingly increased, leaving only the (>580 to

<100 Mc/s) value below 2<r, and raising several

values above 4a.
TYPE IV SOLAR RADIO BURSTS—BELL 127

TABLE 7.—Percentage of geomagnetic storms preceded within three days by a major flare, by

a type IV radio burst, and by a PC A, with storms subdivided by intensity and type. Values

deviating less than 2a and more than J,<r from the random sample are designated by parentheses

and bold face type, respectively

Storm type and intensity

(O

Sporadic, SC

>1000

500-999

300-499

200-299

160-199

Sporadic, non-SC

>200

160-199

Recurrent,'SC

>500

160-499

Recurrent, non-SC

>160

Random sample

Number

8

16

30

18

7

15

15

7

16

38

1460

Percent preceded by

Major

flare

100

88

80

72

(43)

(53)

13

86

(38)

8

41

Type IV

burst *

100

75

77

72

(43)

(34)

(13)

(43)

(25)

11

32

Type IV burst

H/S, P

P, n.o.

H/S, n.o.

100

69

70

61

(43)

(27)

(13)

(29)

(19)

3

20

H/S, -P

P, -H/S

(25)

(19)

(13)

(22)

(0)

(27)

(0)

(14)

(6)

(8)

12

PCA

100

56

(20)

(6)

(14)

(0)

(0)

(14)

(0)

(3)

9

• The percentages in this column are often less than the sum of those in the next two columns because some storms

are preceded by both a full-frequency- and a limited-frequency-range burst. The difference between this column and the

next would give the percentage preceded only by a limited-frequency burst.

Disk position of geomagnetic storm sources

Table 8 provides a convenient summary of data

on the sunspot magnetic class and the position

on the solar disk of the sources of geomagnetic

storms, and of PCA and ground-level cosmicray (GCR) events. Section (a) contains storms

preceded by a flare of any importance (mostly

major, a few minor) that was accompanied by

any category of type IV emission. Section (b)

contains storms preceded by a major flare

without listed type IV radio emission. Each

storm was assigned a single most probable

cause, and a flare with a full-frequency burst

always ranked ahead of a flare with a limitedfrequency burst, which in turn ranked above a

flare without any type IV burst.

Section (c) contains the storms of the first

two sections together with all storms preceded

by a major flare (Bell 1961) in the years 1937-

1956, and 1961. The disk position of each of

these storm sources is plotted in figure 1. The

linkage between a particular flare and storm

can, of course, be accepted with most confidence in section (a).

The PCA events in section (e) are divided

into large (L, VL), moderate (M), and small

(S, VS). The intensities are from Bailey (1962),

and the flare assignments are those of Warwick

and Haurwitz (1962).

The magnetic class is of interest because

major flares from sunspots with complex (0y

or y) magnetic field patterns are much more
128

TABLE S.-

SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

-Sunspot magnetic class and solar disk position of the sources of geomagnetic storms

and of solar proton (PCA) and ground level cosmic ray {GCR) events

VOL. •

Years

(a) 1957-60

(b) 1957-60

(c) 1937-61

(d) 1937-59

(e) 1956-60

(f) 1942-60

Storm

source

Flare &

type

IV

Major

flare,

no IV

Major

flare

and/or

type

IV

Storm

intensity

2ap>1000

500-999

300-499

200-299

160-199

300-999

160-299

Sap>1200

1000-1199

700-999

500-699

300-499

200-299

160-199

All major flares

All PCA flares

L, VL

M

s, vs

GCR flares

Number

8

16

24

22

6

9

10

8

8

28

31

48

54

30

580

43

9

12

22

12

Percent

complex

75

62

62

52

67

33

30

75

75

59

66

50

43

39

41

70

78

67

59

92

Percent

north

88

88

71

55

17

67

70

100

88

79

84

69

57

37

62

70

89

67

64

92

Percent with

<30°

88

47

42

46

50

39

10

88

88

54

52

50

41

40

45

45

33

67

41

33

distance

30°-59°

12

31

37

27

50

17

50

12

12

32

32

33

31

40

33

28

45

25

18

17

>60°

0

22

21

27

0

44

40

0

0

14

16

17

28

20

22

27

22

8

41

50

likely to be followed within three days by a

geomagnetic storm than are major flares from a

simple bipolar (0) or unipolar (a) spot group

(Bell, 1961). Table 8 shows that the GCR and

PCA events and the greatest geomagnetic

storms tend particularly to come from complex

sunspot groups. It was found in Paper I that

the geomagnetic success of a major flare with a

spectral type IV burst is independent of the

magnetic class of the sunspot. However, a

j8y or 7 sunspot is about five times more likely

than an a or 0 group of comparable area to

produce a major flare with an associated radio

burst of spectral type IV.

Bell (1961) found that a major flare in the

northern solar hemisphere had a much higher

probability of being followed by a great geomagnetic storm than did a major southern

flare. The relation of this north-south asymmetry to storm intensity is shown in greater

detail in sections (a) and (c) of table 8, where

we can see that the percentage of storms coming

from northern flares increases systematically

with increasing storm intensity. The PCA

events show a similar trend.

We should not however expect that the northern hemisphere will continue indefinitely to

produce most of the great geomagnetic storms

and cosmic-ray events. There is evidence

(Bell, 1962; Bell and Wolbach, 1962) that the

northern hemisphere of the sun has been the

preferred source of great storms for most of
TYPE IV SOLAR RADIO BURSTS—BELL

40° N 40°

129

©

©

©

W +H—I 1-

\ \ \ \ \ \ VS

o °o .

© • oo

• •©

o

O • <D

o o

+4

o © •

FIGURE 1.—Plot of the location on the solar disk of probable sources of geomagnetic storms occurring in

the years 1937 through 1961. The symbols indicate the intensity of the storm, as measured by the sum

A.

of the four largest consecutive values of the ap index,

O, 500-699; and ., 300-499.

this century, while in the latter half of the

nineteenth century the southern hemisphere

was the more active in production of great

storms. It is impossible at present to predict

when the southern may again replace the

northern as the more geo-active solar hemisphere.

The tendency for the very great (2Jap>1000)

storms to come from small meridian distances

appears clearly in figure 1 and in table 8. All

of the very great storms of the present sunspot

cycle arose from flares located no more than 30°

from the solar CM (one at 30°). Over the past

three cycles (section (c) and figure 1) one very

great storm came from 37 °E. About 80 percent of the very great storms arose from within

20° of the CM and the remainder from the

20-39° zone. All but one arose from the northern hemisphere.

4

At no intensity level below 2ap=1000 does

the distribution of storm sources with CM distance differ significantly from the distribution

of major flares (cf. section (d) from Bell, 1961).

There is however a tendency for more of the

>, >1200; Q, 1000-1199; <D, 700-999;

smallest storms to come from the larger CM

distances. The distribution of storm sources

suggests that the directivity of type IV emission (see table 2) and of low energy corpuscular

emission are similar. A parallel can be seen

between the sharpening of directivity with increasing duration of the burst (table 3) and

with the intensity of the geomagnetic storm.

Among PCA sources there is no clear concentration of the largest events to the CM corresponding to that of very great storms. The

distribution of all PCA sources is similar to that

of all major flares and all storm sources, except

for a slight excess of PCA from meridian distances beyond 60°. The strong concentration

of GrCR sources beyond 60° (west) has been

pointed out by several authors, and is generally

attributed to propagation conditions in the

interplanetary medium rather than to solar

directivity in emission.

The directivity effects in the emission of geomagnetic-storm corpuscles suggest that major

corpuscular clouds may have a relatively dense

core that is highly directional, with a beam

width rarely exceeding 40° in longitude. Such
130 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

a core is required to produce a very great

storm. Surrounding the core, and in lesser

clouds existing alone, is a less dense cloud

emitted over a wide angle and capable of producing all but the very great geomagnetic

storms from any position on the disk. The

directivity in emission of high-energy (PCA)

protons appears to be low.

Occasionally a great storm, in the range

2ap=700 to 1000, arises from a flare near the

limb. There is at present no basis for deducing

whether such storms arise from intense corpuscular emission over a very broad beam, or

from nonradial ejection of a core.

Summary of conclusions

The data presented in this paper suggest that

radio bursts commonly classified as type IV are

a rather heterogeneous collection of events

which, at least for purposes of forecasting geomagnetic and PCA activity, should be subdivided into two categories: those observed

over a full range of frequencies from >2800

Mc/s down to < 100 Mc/s; and those occurring

over a limited range of two or three octaves.

All of the data presented here support such a

separation. The full-frequency-range bursts

have a longer duration, and a higher association

with major flares and with complex sunspots,

and they are strikingly more successful in the

production of PCA's and of great geomagnetic

storms. Although the two groups may not

differ in mode of origin, the frequency range

covered is a significant and useful index of the

importance of the burst.

The data strongly support Allen's (1944)

hypothesis that sporadic and recurrent storms

have a different mechanism of origin. Sporadic

storms with SC show a strong association with

major flares and with type IV radio bursts.

Recurrent storms without SC show a statistically significant avoidance of major flares and

type IV activity.

The percentage of storms arising from the

northern solar hemisphere increases systematically with storm intensity. Very great storms

arise from flares within 40°, and usually within

20°, of the solar CM. The distribution of the

remaining storms parallels that of major flares.

Ackno wledgmen ts

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)-1962.

For helpful discussions I am indebted to Dr.

Alan Maxwell of the Harvard Radio Astronomy

Station, Fort Davis, Texas.

References

ALLEN, C. W.

1944. Relation between magnetic storms and solar

activity. Monthly Notices Roy. Astron.

Soc, vol. 104, pp. 13-21.

BAILEY, D. K.

19G2. The detection and study of solar cosmic rays

by radio techniques. In Proceedings of

the International Conference on Cosmic

Rays and the Earth Storm, pt. 1, pp.

106-112. Journ. Phys. Soc. Japan, vol.

17, suppl. A-I.

BELL, B.

1961. Major flares and geomagnetic activity.

Smithsonian Contr. Astrophys., vol. 5,

no. 7, pp. 69-83.

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

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

Astrophys., vol. 5, no. 12, pp. 187-194.

1963. Solar radio bursts of spectral types II and IV:

Their relations to optical phenomena and to

geomagnetic activity. Smithsonian Contr.

Astrophys., vol. 5, no. 15, pp. 239-257.

BELL B., AND GLAZER, H.

1956. Geomagnetism and the emission-line corona.

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1957. Geomagnetism and the emission-line corona,

1950-1953. Smithsonian Contr. Astrophys., vol. 2, no. 5, pp. 51-107.

BELL, B., AND WOLBACH, J. G.

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

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Astrophys., vol. 5, no. 12, pp. 203-208.

MCLEAN, D. J.

1959. Solar radio emission of spectral type IV and

its association with geomagnetic storms.

Australian Journ. Phys., vol. 12, pp.

404-417.

MAXWELL, A.; THOMPSON, A. R.; AND GAKMIRE, G.

1959. The association of solar radio bursts with

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vol. 1, pp. 325-332.
TYPE IV SOLAR RADIO BURSTS—BELL 131

NEWTON, H. W.

1943. Solar flares and magnetic storms. Monthly

Notices Roy. Astron. Soc, vol. 103, pp.

244-257.

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

1954. The distribution of great and small geomagnetic storms in the sunspot cycle.

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PlCK-GuTMANN, M.

1961. Evolution des Emissions radioelectriques

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pp.183-210.

SAEMUNDSSON, TH.

1962. Statistics of geomagnetic storms and solar

activity. Monthly Notices Roy. Astron.

Soc, vol. 123, pp. 299-316.

THOMPSON, A. R., AND MAXWELL, A.

1960a. Solar radio bursts and low-energy cosmicrays. Nature, vol. 185, pp. 89-90.

1960b. Solar radio bursts and cosmic rays.

Planetary Space Sci., vol. 2, pp. 104-109.

WABWICK, C. S., AND HAUHWITZ, M. W.

1962. A study of solar activity associated with

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Abstract

This paper compares solar radio bursts of type IV identified by means of spectrum observations in the meter

bands with those identified from discrete frequency observations in the centimeter bands. The comparison discusses

associated active-sun phenomena and properties of the bursts themselves as well as geomagnetic storm and polar

cap absorption (PCA) effects. Type IV bursts that cover the full frequency range from >2800 Mc/s down to <100

Mc/s are found to be strikingly more successful in the production of great geomagnetic storms and PCA events than

are type IV bursts limited to two or three octaves of the spectrum.

Evidence is presented that strongly supports the hypothesis that sporadic and recurrent storms come from

different types of solar regions. Sporadic SC storms are, in a high percentage of cases, preceded within three days

by a major flare and by a type IV radio burst. Recurrent non-SC storms, by contrast, are preceded by major flares

and by type IV bursts significantly less often than would be expected by chance.

The disk position of the source is studied as a function of storm intensity. About 80 percent of the very great

storms are found to arise from flares located within 20° of the solar central meridian. The distribution of the sources

of lesser storms parallels that of major flares and type IV bursts.

D.S. GOVERNMENT PRINTING OFFICE: 1963