Smithsonian

Contributions to Astrophysics

VOLUME 2, NUMBER 5

GEOMAGNETISM AND THE EMISSION-LINE

CORONA, 1950-1953

By BARBARA BELL and HAROLD GLAZER

SMITHSONIAN INSnTUTION

Washington, D. C.

1957

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Publications of the Astrophysical Observatory

This series, Smithsonian Contributions to Astrophysics, was inaugurated

in 1956 to provide a proper communication for the results of research conducted at the Astrophysical Observatory of the Smithsonian Institution.

Its purpose is the "increase and diffusion of knowledge" in the field of astrophysics, with particular emphasis on problems of the sun, the earth, and

the solar system. Its pages are open to a limited number of papers by other

investigators with whom we have common interests.

Another series is Annals of the Astrophysical Observatory. It was started

in 1900 by the Observatory's first director, Samuel P. Langley, and has been

published about every 10 years since that date. These quarto volumes, some

of which are still available, record the history of the Observatory's researches

and activities.

Many technical papers and volumes emanating from the Astrophysica

Observatory have appeared in the Smithsonian Miscellaneous Collections.

Among these are Smithsonian Physical Tables, Smithsonian Meteorological

Tables, and World Weather Records.

Additional information concerning these publications may be secured

from the Editorial and Publications Division, Smithsonian Institution

Washington, D. C.

FRED L. WHIPPLB, Director,

Astrophysical Observatory,

Smithsonian Institution.

Cambridge, Mass.
Contents

Page

Introduction 51

Part I. Single intensity index for each coronal line 57

Green line, X5303 58

Correlation coefficient . 62

Red line, X6374 62

Red and green lines, simultaneously 63

Yellow line, X5694 64

Green line intensity around zero-days defined by large Kp . . 65

Heliographic latitude of the earth 66

Active centers 66

M-regions 67

Summary 69

Part II. Separate intensity indices for northern and for southern

solar hemispheres 69

Green line 70

Hemisphere versus angular distance 79

Red line 88

Red and green lines, simultaneously 91

Yellow line 91

Part III. Inverse solutions for hemisphere effects 91

M-regions 93

Other periods: 1942-44, 1944-49 97

Yellow line 102

Summary and interpretation 103

Acknowledgments 105

References 106

Abstract 107

m

Geomagnetism and the Emission-Line Corona,

1950-1953

By Barbara Bell1 and Harold Glazer

I ntroduction

In this paper we investigate the relation between the intensity of the coronal emissionlines, X5303 and X6374, and the geomagnetic

character figure, Kp, generally interpreted as

a measure of the solar corpuscular radiation.

However, a summary of previous research on

the general problem of solar-geomagnetic relations may be useful. We are limiting this

summary to those effects generally thought to

arise from solar corpuscular radiation; and we

are not considering geomagnetic phenomena

believed to arise from solar ultraviolet radiation.

The displaced and greatly broadened Balmer

lines observed independently by Meinel (1951)

and Gartlein (1950, 1951) in spectra of aurorae

comprise the only direct evidence for streams

of corpuscles striking the earth. Spectra obtained by Meinel during the intense aurora of

Aug. 18 and 19, 1950, indicated a velocity of

impact above 3000 km/sec for the protons. If

the corpuscles come directly from the sun, a

particle velocity of 3000 km/sec would correspond to a travel time of about 14 hours.

There can be little doubt that disturbances

of the earth's magnetic field (geomagnetic

storms), as well as aurorae, are caused by

streams of corpuscles emitted by the sun, because: (1) many geomagnetic storms are worldwide, affecting the dark and sunlit sides of the

earth equally; (2) storms are more pronounced

in higher latitudes; (3) storms often follow

flares and/or central meridian passage (CMP)

of large sunspots with a time lag of 1-4 days;

and (4) storms show a 27-day recurrence tendency corresponding very closely to the synodic

rotation period of equatorial regions of the sun.

The 27-day recurrence tendency was demonstrated by Maunder (1904) and confirmed by

Chree and Stagg (1927). Greaves and Newton

(1929), from analysis of Greenwich magnetic

records 1874-1927, found that the recurrence

tendency was most pronounced for storms of

moderate intensity, decreased with increasing

storm intensity, and vanished in the case of

great storms. Newton (1949) found that

superposed epoch analysis of 1907-44 data also

indicated this nonrecurrence of great magnetic

storms.

On the other hand, Benkova (1942) has

studied the 27-day recurrence tendency for

storms of different intensities from data recorded at the Sloutzk Observatory, 1878-1939.

She found that the extra-great magnetic storms

showed a pronounced tendency to occur in a

sequence, while the great storms showed less

such tendency than either moderate or extrai Harvard College Observatory. Cambridge, Mass.

* Formerly at Harvard College Observatory, Cambridge, Mass.; now at Sylvanla Waltbam Laboratories, Waltbam, Mass.

51
52 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL. 2

great storms, results partly contrary to those

of Greaves and Newton. All categories, however, showed an appreciable recurrence tendency. Extra-great storms, Sloutzk data indicated, occurred often as members of the longer

sequences and tended to be the second member

of the sequence.

Chernosky (1951) has found evidence of a

systematic variation in the length of the recurrence interval with phase of the spot cycle.

Early in the cycle he finds a period of 28%

days, which decreases to 26% days near the

end of the cycle. Recurrent magnetic storms

are most common on the declining branch of

the sunspot cycle and scarcest on the rising

branch and around maximum. Isolated storms

appear to parallel the general solar cycle of

activity more closely.

Terrestrial magnetic activity shows a 6-

month periodicity (Bartels, 1932) with maximal

disturbance in March and October. According

to Bartels, the maxima are closer to the equinoxes than to the times of maximal inclination

of the solar axis (March 5 and September 7);

and they are present about equally in years of

high, moderate, and low sunspot activity. The

cause of the equinoctial maxima in geomagnetism, which appear also in aurorae, has long

been a mystery. Sunspot numbers show no

significant seasonal variation.

Bhargava and Naqvi (1954) however have

pointed out two very long sequences of recurrent geomagnetic activity in the period 1950-53,

each of which appears to show an annual variation in storm intensity. The longest sequence,

beginning July 11, 1950, shows

activity around September and a

around March. The second sequence, beginning Nov. 21, 1951, has its maximum in March

and a minimum around September. These

authors accordingly suggest that the apparent

semiannual period may actually be an annual

period, caused by the inclination of the solar

axis to the plane of the ecliptic (see also Naqvi

and Bhargava, 1954).

Bartels (1932) first investigated the relation

between magnetic storms and visible features

of the solar disc. Using monthly means,

1928-30, he found no significant correlation of

Bunspots, bright or dark flocculi, or faculae

with the terrestrial magnetic activity figure u.

The correlations of character figure C with

solar disc features were negative, because during the period covered by his study general

solar activity declined while geomagnetic activity increased. (Large magnetic character figures, whether C or K, correspond to disturbed

conditions, small numbers to quiet conditions.)

Bartels found the various solar indices to be

highly correlated and concluded that one could

not therefore be expected to give significantly

better results than another. The hypothetical

sources on the sun of the corpuscular streams

producing recurrent magnetic storms he named

M-regions. M-regions, most numerous and

conspicuous in their effects one or two years

before sunspot minimum, are longer lived

phenomena than sunspots; some M-regions have

survived for more than a year. Bartels noted

that M-regions often occurred when the visible

solar hemisphere was bare of spots, and they

tended to be absent when spots were near the

central meridian of the sun. He also called

attention to long sequences of quiet conditions.

Waldmeier (1946, 1950) has suggested that

M-regions may develop in areas empty of spots

but where a large spot group recently disappeared.

Allen (1944) has suggested that one should

distinguish two fundamentally different types

of magnetic storms. He listed the following

distinguishing characteristics, but emphasized

that exceptions to all of the characteristics

listed often make it difficult to classify an

individual storm. Storms of the first type are

intense, are associated with sunspots, follow

flares by about one day, have world-wide sudden

commencement (sc) and show no recurrence

after 27 days. Storms of the second type are

of more moderate intensity, are independent

of sunspots, flares and other visible disc features,

begin gradually, and show a strong tendency

to recur with a period of 27 days. Newton and

Milsom (1954) emphasize the "sc" as the distinguishing feature and find that storms of the

"sc" type appear to follow the sunspot cycle in

frequency of occurrence, whereas storms of the

second type reach their maximum development

one or two years before sunspot minimum.

A statistical study by E. and O. Thellier

(1948) of 688 storms since 1884 supports the

validity of the distinction between storms of
GEOMAGNETISM AND EMISSION-LINE CORONA 53

sudden and gradual [commencement with respect to the recurrence tendency. By the superposed epoch method, with the character figure

C, they found that storms of sudden commencement showed no recurrence tendency, while

those of gradual appearance showed a marked

27-day period. With one exception the 37

most intense storms commenced suddenly, a

fact which may help to explain the results of

Greaves and Newton. (Benkova did not use

the superposed epoch method.)

Allen distinguished between recurrent and

isolated storms in his study of the relation of

storms to sunspots and flares. Isolated storms

showed a moderate rise in frequency after CMP

of large (i. e., >400 millionths of visible solar

hemisphere) sunspot groups, with a maximum

on day +4. M-type storms, on the other

hand, showed a marked decrease in frequency

of occurrence after CMP of large sunspots,

with a minimum on day +3; maxima occurred

on days —1 and +8. These maxima, together

with the decrease in frequency of M-type storms

after CMP of large spots, led Allen to conclude

that sunspots tend to interfere with M-regions

within about 40° of themselves and to increase

the frequency of those just outside this area.

The development of a spot in a previously spotfree M-region may end the storm sequence; in

other cases the sequence merely may be interrupted, to resume after the disappearance of

the spots. While M-regions apparently show

a significant tendency to avoid spot areas, this

avoidance does not seem to be universal.

Allen suggested that the white light coronal

streamers were a probable source of M-region

activity. Since white light coronal streamers

have not been observed outside of solar eclipse,

data to test this hypothesis are inadequate.

However, von Kliiber (1952) has called attention to a possible connection between an intense

streamer seen at the eclipse of Feb. 25, 1952, at

10° N, and the large M-type storm beginning on

March 3 (the CMP date of the streamer).

Newton (1949) has used Greenwich data,

1914-44, to study the relation between sunspots and geomagnetic storms. He grouped

the spots according to area. A marked increase

in geomagnetic activity, centered at about day

+2, followed CMP of spots larger than 1500

millionths of a solar hemisphere. This relationship diminished rapidly with decreasing spot

size and disappeared below 1000 millionths.

Sunspot grouping based on flare incidence

greatly improved the relation in the expected

sense that spots with frequent flares were more

likely to produce storms than were spots deficient in flares.

Newton (1943) also investigated the relation

between solar flares and geomagnetic storms.

From a study of class 3+ flares, 1859-1942, he

found that magnetic storms began within 2

days in 27 out of 37 cases. Eighty percent of

the 3+ flares that occurred within 45° of the

CM were followed by storms. Two-thirds of

the storms associated with flares in this central

zone were great storms, whereas no great

storms followed flares outside this zone. Some

3+ flares more than 45° from the CM did produce moderate storms. The most common

travel time of the corpuscles was 20-25 hours.

Extending his investigation to flares of class

3 and 2, Newton (1944) found that flares of intensity 3 in the central zone were followed by a

small maximum in geomagnetic activity on

day +2. Flares of intensity 2 produced no

significant geomagnetic effect.

Kiepenheuer (1947) found that filament area

along the central meridian showed a maximum

3-5 days before geomagnetically disturbed days

in the years of low solar activity 1922-24. The

increase in filament area appeared in both low

«30°) and high (>30°) latitude filaments.

To state the relation conversely, the magnetic

character figure C showed a (small) maximum 4

days after the CMP of regions with large filament areas. No systematic variation in filament areas appeared around magnetically quiet

days. We note that Kiepenheuer's filament

areas showed a minimum one day before geomagnetically disturbed days, as conspicuous as

the maximum 4 days before the disturbance.

Hence it is not clear whether either the maximum, the minimum, neither, or both are significant in the relation between filament areas

and geomagnetism. Neither of these phenomena appeared to continue during high solar

activity, 1926-28. (For figures illustrating

some of the above mentioned results of Bartels,

Allen, Newton, and Kiepenheuer, see Kiepenheuer, 1953, pp. 443-448.)

A recent study by Roberts and Trotter
54 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL.2

(1955), covering 16 months between May 1951

and June 1954, suggests that the minimum may

be the significant feature in Kiepenheuer's

curves. They calculated the average prominence area, from limb observations, around

zero days defined by the 5 most geomagnetically

disturbed and by the 5 least geomagnetically

disturbed days in each of 16 months. A minimum in area of prominences between the poles

and 20° and between the poles and 30° solar

latitude (thus the low latitude prominences are

omitted) appears in their curves 0-3 days west

of central meridian at the time of the geomagnetically disturbed days. The mean lines

shown in their published curves are individual

averages of the points plotted in each curve.

If Roberts and Trotter had instead drawn for

comparison an overall average prominence area

for the 16 months, the minimum in prominence

areas would appear considerably more impressive; in addition the curves would then suggest

that quiet days might be associated with aboveaverage prominence areas around the central

meridian.

Becker (1953) has made a superposed epoch

analysis for 1946-50 sunspot data. He defined zero days by the presence on the central

meridian of two sunspot groups of comparable

degrees of evolution and symmetrically located

with respect to the solar equator. He found

that the magnetic character figure possessed

a minimum 2 days after CMP of the pairs of

spot groups.

One of the most promising lines of investigation has made use of intensity measures of

radio noise in conjunction with visible disc

features. Measures of 158 Mc/s radiation have

permitted Denisse (1952, 1953) and coworkers

(Denisse et al., 1951) to classify the visible

active centers occurring in 1948-50 into two

groups: type B, producing an increase in 158

Me radiation; and type Q, producing no change

or a slight decrease in 158 Me radiation. They

used the superposed epoch method and defined

zero days by CMP of R- and Q-type spots.

Type R spots appeared to produce a rise in C,

with the maximum 1-2 days after CMP,

whereas the type Q spots were followed by a

decline in C, with a minimum 2-3 days after

CMP. Denisse noted that the magnetic activity associated with type B spots is of the

sudden commencement and nonrecurrent type

rather than the M-region type. He concluded

that noisy (R) spots appear to be the source

of relatively dense corpuscular streams, and

that quiet (Q) spots tend to inhibit the normal

evaporation of corpuscles from the corona.

He attributes the moderate level of geomagnetic

activity present outside of storms to this evaporation. Denisse further pointed out that the

existence of noisy and quiet spots, having

directly opposite geomagnetic effects, would

explain the commonly poor "spot versus C"

correlations obtained when the two groups are

not in some way separated.

Becker and Denisse (1954) subsequently

combined their methods of denning zero days

to study the period 1948-50. They found that

CMP of a pair of quiet spots, one north and

one south, tends to be followed by a decline

in geomagnetic activity—with a minimum on

day 3—greater than that following either MPC

of a single Q-spot or CMP of a pair of spots

without regard to their radio properties.

Further, the apparent storm-producing effect

of a noisy spot tends to be diminished if it is

paired with a quiet spot in the other solar

hemisphere.

We come now to the specific problem of the

present paper, namely the relation, if any,

between coronal emission-line intensity and

geomagnetism. So far we have seen that

isolated storms tend to be associated with

flares and with CMP of some but not all large

sunspots, while M-regions tend to avoid such

centers. No one-to-one relation has been

found between geomagnetism and any visible

solar feature. A mere tendency, while significant, is not adequate either for reliable prediction or for understanding the solar mechanism

of storm production.

The intensity of the coronal emission line

X5303 follows the same 11-year cycle, both in

intensity and in latitude distribution, as do

other manifestations of solar activity. Since

there exists, furthermore, a general tendency

to association among the various active features, a one-to-one relation between 5303

intensity and Kp is unlikely. On the other

hand, different active features do not always

show disturbances in the same area or to the

same degree, so that the relation of Kp to
GEOMAGNETISM AND EMISSION-LINE CORONA 55

5303 intensity could be closer than its relation

to other phenomena.

Waldmeier (1939, 1942) was the first to

study this problem, shortly after beginning

5303 observations at Arosa. Using necessarily fragmentary 5303 data for 1939-40, he

found that the appearance of regions of unusual brightness of the line 5303, which he

called C-regions, was correlated with the

occurrence of geomagnetic disturbances about

7.4 days later (E limb) and 6.2 days earlier

(W limb). Most subsequent studies have not

confirmed this result, perhaps because they

pertain to a later stage of the solar cycle.

Shapley and Roberts (1946), by the superposed epoch method, explored the relation

between the magnetic character figure CA, and

the Climax 5303 intensities for the period Aug.

1942-Aug. 1944. Since sunspot minimum occurred early in 1944, their investigation covers

a period of more M-type activity and fewer

isolated storms than Waldmeier's period. They

took 5303 intensity > 10 on the east limb to

define zero days and calculated the average

value of 20CA from days —4 through +14.

They calculated also the frequency of days

having 20CA >9, and of days having 20CA

> 15; the average value of 20CA for the period

was 8.6. Each of these three calculations indicated that the probability of a high CA was

markedly greater when bright 5303 regions

were on the eastern half of the sun. As their

figures 3-4 show, CA had a maximum on day

east limb passage +4 for Aug. 1942-July 1943,

and a broader maximum on ELP +2 through

+8 for Aug. 1943-July 1944. By the time

the active region passed CM the probability

of large CA was declining and was below average

on days 9-12 (i. e., 2-5 days after CMP of the

active regions).

Noting that C-regions tend to persist for a

number of rotations of the sun, Shapley and

Roberts adopted the hypothesis that C-regions

and M-regions were coincident. With this

hypothesis they then concluded that C-regions

are most effective in producing geomagnetic

storms when located about 40° east of the solar

central meridian.

Shapley (1946) has studied the relation

between Can plages and 20CA for 1942-44.

He defined zero days by CMP of large, long422953—67 2

lived plages. We include this plage study here

because the results are so closely similar to the

coronal results. The plage study indicated a

tendency for high CA when a large plage was on

the eastern half of the sun, and a marked minimum 3-4 days after CMP of the plage. Wolf

and Nicholson (1948, 1950) also found that

M-regions tended to occur, in 1942-44, when

plages were on the eastern half of the sun.

Kiepenheuer (1947) has made a superposed

epoch study using Wendelstein 5303 intensities,

1943-45. He defined zero days by "strong"

5303 corona at east and west limbs, and computed mean Potsdam magnetic indices, K, for

days ELP +4 to +14 and WLP - 6 to +4.

The most conspicuous feature is a minimum of

K at ELP +10 and WLP —3 in 1943. Perhaps there is also a maximum on ELP +4 but

Kiepenheuer's curves do not extend far enough

before CMP to establish this with certainty.

The agreement between the observational

results of Kiepenheuer and of Shapley and

Roberts appears quite satisfactory. Kiepenheuer, however, assumed that the corpuscular

radiation must be emitted from the region of

the CM and therefore concluded that strong

coronal intensities correspond to low geomagnetic character figures. Bright 5303 regions

tended to coincide with spots and plages and to

have a similar weakening effect on .K-indices.

The relation apparent between 5303 and K in

1943 virtually disappeared in 1944 and 1945,

with the start of the new cycle of solar activity.

Kiepenheuer made a separate calculation for

cases of "very strong" 5303 corona, 1943-45.

Here he found maxima in K on days ELP +7,

and WLP —7, in agreement with the results of

Waldmeier.

A recent study by Denisse and Simon (1954)

covering the period 1946-52 indicates that CMP

of active centers showing emission in the X5694

coronal line tends to be associated with disturbed geomagnetic conditions; whereas CMP

of centers where 5694 emission is clearly absent

tends to be associated with diminished values

of Kp. The study also points out a tendency

for enhanced 158 Me radio noise and 5694

emission to be present or absent together.

Lincoln and Shapley (1948) extended the

superposed epoch analysis through 1944-46,

using data from Climax, Pic du Midi, and
56 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL.!

Wendelstein combined. The authors stated

that "in general one suspects that the variations

shown in a superposed epoch diagram are

real if (1) the same relationship, or time lag,

is observed in each of two or more arbitrarily

chosen samples of the data, and (2) the variation

curve is more or less smooth . . . The evidence is not conclusive. The relationships for

1942 (second half), 1943, and 1944 are similar,

and differ from those for both 1945 and 1946.

The curves for 1942, 1943 and 1944 are relatively smooth, and the likelihood of a real

relationship thereby enhanced." If the variations from year to year are real, "the time lag

between east-limb passage of the selected

coronal region and the related magnetic activity varies with the stage of the sunspot

cycle." To settle this question, data from at

least a complete spot cycle would be needed.

The only feature the years 1944-46 have in

common is a below-average probability of

geomagnetic storm on day ELP +10, a property found also for 1942-44.

A similar analysis for large sunspots, 1890-

1936, divided into four groups by phase of the

sunspot cycle, showed no resemblance to

the coronal results. Lincoln and Shapley

concluded, in part from detailed examination

of 1944-46 data, that corona and sunspots

were more or less independent manifestations

of solar activity. "Of the 32 largest sunspots

with area greater than ten square degrees,

only three were within seven degrees of longitude of the center of a bright coronal region

while 14 were within 25°. Observations were

incomplete in some of the remaining cases;

in others, the largest sunspot was probably

associated with a coronal region of minor

importance according to the criteria used in

this paper. Apparently the size of a sunspot

bears little relation to the intensity of the

corona above it."

Note that, as we should expect if 5303 intensity measures are reasonably reliable, all

studies of the relation between geomagnetic

and visible solar features covering the same

time interval agree in results if not in interpretation. In 1942-44, magnetic storms tended to

occur when active regions were on the eastern

hemisphere of the sun; after CMP of the active

region the magnetic character figure tended to

show a minimum. Since both M-regions and

C-regions often persist for several months,

and since each has a recurrence period of 27

days, the existence of an apparent relation

between the phenomena is inevitable. The

appearance of a relationship therefore does not

necessarily establish any significant physical

connection between the two phenomena. The

existence of a physical relationship, of course,

will become increasingly probable the greater

the number of independent cases in which

the same relation appears.

Smyth (1952) has used the average intensity

±30°, from Climax data, to investigate the

period June 1950-Jan. 1951 by the superposed

epoch method. He found a tendency for a

lower-than-average geomagnetic index 1-5 days

after CMP of bright corona. The relation

appeared for both 5303 and 6374. The eastern

hemisphere effect of Shapley and Roberts

was not confirmed. Smyth has also calculated

the intensity of 5303 and 6374 around the time

of the principle M-region. The intensity of

5303 showed a pronounced minimum on the

east limb 8 days before the onset of the Mregion.

Bruzek (1952) has studied from Wendelstein

data the 5303 intensity around three Mregions—the above mentioned M-region of

1950 and the two prominent M-regions in

1942-44. He found that each of these Mregions appeared to coincide with the presence

on the central meridian of a region of unusually

weak 5303 corona.

On the other hand, Trotter and Roberts

(1952) noted that the eastern hemisphere effect,

found by Shapley and Roberts in 1942-44,

appeared to be returning in 1952 after being

largely absent during maximum solar activity.

Muller (1953) found similar results for Feb.-

Aug. 1952 from Weldenstein 5303 data. He

has suggested that the discordant results of

various workers may arise primarily from the

different phases of the sunspot cycle they have

studied. (This 1952 M-region is the one that

von Kliiber (1952) has attributed to CMP of the

white light coronal streamer seen at the

February 1952 eclipse.)
GEOMAGNETISM AND EMISSION-LINE CORONA 57

Part I. Single intensity index for each coronal

line

The present investigation covers an interval

of 45 months, from Feb. 1, 1950—the date

when regular observations were begun at

Sacramento Peak—through mid-October 1953.

This period, well on the descending branch of

the recent sunspot cycle, is much richer in

M-region activity than was the corresponding

phase of the preceding cycle.

As a measure of the solar corpuscular radiation, or geomagnetic activity, we used the daily

sums of the 3-hour Kp values. The scale for

the 3-hour Kp values is 0 (very quiet) to 9 (very

disturbed), giving a range of 0-72 for the daily

sums. This planetary index, derived from

data of 11 observatories between geomagnetic

latitudes 47 and 63 degrees, was designed by

Bartels to measure solar particle-radiation by

its magnetic effects.

In the first part of this investigation, we use

as a measure of the intensity of the green

coronal line, X5303, the daily mean of its five

highest east-limb intensities corrected to a

plate threshold of five. This quantity we hereafter designate simply as the daily "5303

intensity"; east-limb values are implied unless

we specifically say west-limb 5303 intensity.

We obtained both the coronal and the geomagnetic data used in this study from the

monthly Central Radio Propagation Laboratories (CRPL) Bulletins.

We corrected the 5303 intensities to a plate

threshold of five by the following equations

derived from a least squares analysis (Glazer

and Bell, 1954):

//=J.+0.93 (T.-5) for Sac Peak

//=/c+0.61 (re-5) for Climax,

(1)

where /, and Ie are the respective observed

intensities, 7/ and / / are the respective corrected intensities, and T, and Te are the plate

threshold values.

The daily 5303 intensities were obtained from the

Sac Peak observations. These were supplemented,

however, for days when bad weather prevailed at Sac

Peak, by /„' values adjusted to the Sac Peak scale by

the approximate relation

(2)

For days when neither station observed, we derived

interpolated intensities, primarily by reference to westlimb intensities 14 days before and after and secondarily by reference to east-limb intensities 27 days

before and after the day in question. Such interpolations seemed not unreasonable because the intensity

of the 5303 line generally changes only slowly with

time. In cases where the interpolations were particularly uncertain, we assigned intermediate values to

the interpolated intensities; consequently we made little

subsequent use of them.

For a few weeks in the spring of 1952, the Sac Peak

coronagraph was out of adjustment; and from March

14 through April 28 the Sac Peak 5303 intensities were

regularly no stronger than the corresponding Climax

observations. Therefore, we increased all Sac Peak

intensities in this period to bring them into their usual

relation with those of Climax, believing these corrected

values to reflect more accurately the 5303 activity on

the Sac Peak scale.

As a measure of the daily intensity of the

red coronal line, X6374, in Part I we use the

mean of the three highest east-limb intensities.

This quantity we define as the daily 6374

intensity. We did not introduce any threshold

correction for 6374 because at this stage of the

investigation it had not been adequately determined. The 6374 intensities are known

to change more rapidly than those of 5303.

Therefore we did not consider it proper to

make interpolations and have used simply the

observed 6374 intensities from both Sac Peak

and Climax.

We used the superposed epoch method of

analysis (Chree and Stagg, 1927). We were

fortunate in having access to IBM equipment

for the analysis, and accordingly punched the

data on standard 80-column IBM cards. The

use of IBM equipment permitted us to investigate a wider variety of possible relationships

than would otherwise have been practicable.

Because of the substantial decline in the 5303

intensity over the 46 months under consideration, we divided the data into four time periods:

(1) 1 Feb. 1950 to 31 Oct. 1950; (2) 1 Nov. 1950

to 31 Oct. 1951; (3) 1 Nov. 1951 to 31 Oct. 1952;

(4) 1 Nov. 1952 to 15 Oct. 1953. Relative intensities within each period, rather than absolute values, define the zero-days of strong and

weak coronal intensity. In selecting zero-days

for superposed epoch analysis we found it desirable to divide the first period into two equal

parts; the decrease in 5303 intensity was sufficiently great during 1950 that we could not use
58 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL.!

the same criteria for "strong" and "weak"

coronal intensity throughout the period without

causing most of the "strong" intensities to fall

in the early months and most of the "weak"

intensities in the later months of the period.

To be consistent we divided the 6374 intensities

into the same time intervals.

The green line, \5S08, [Fexiv].—We determined mean Kp values for days —4 through

+ 16 around zero days defined by the ELP of

the 10 percent, 20-10 percent, and 30-20 percent of the strongest and the weakest 5303 intensities within each of the four time periods.

Since we did not wish to include any given intensity in two different categories, the percent

limits are necessarily only approximate.

To obtain an over-all result for the 45 months

we combined the separate results of the four

periods, thus taking account of the decreasing

general level of 5303 activity during these years.

The geomagnetic activity, Kp, increased from

1950 to the spring of 1952 and decreased thereafter. We shall examine the 45-month result

first, and then compare the results of the individual periods.

Figure la,b shows the average values of Kp

for the 45-month period. The sample size for

each of these curves is approximately 125. The

average value of Kp for the period is 22.65, as

indicated on each curve.

In the period studied the standard deviation

of a single daily Kpsxxm is 9.87, so that the ordinary standard error of the mean would be

<r=9.87/ViV, where Nis the size of the sample.

To take account of the positive autocorrelation

known to exist between the Kp's for lags of

1, 2, 27, and 54 days, however, the ordinary

standard error of the mean must be increased

by factors involving the autocorrelation coefficients.

The mathematical definition of the variance (standard deviation squared) of a variable x, in terms of "expected values," is

**=E[x-E(x)]*. (3)

E(x) is the expected value of x, or the mean TO. We call

o*Jthe second moment about the mean. To get the variance of x=ZxilN, we replace x by * in equation (3).

Whence

(4)

When we assume independence between x, and

the second term becomes zero and we have

where <rz is the ordinary standard error of the mean.

When Xi is not independent of xit we define

E (xt—TO) (xt—TO) = pjo1

where pj=the autocorrelation coefficient of lag \j—i\.

Shapley (1947), working with the autocorrelation of the magnetic character figure C,

found correlations of about 0.5 for 1-day lag,

0.2 for 2-day lag, and negligible correlation for

a lag of 3 days. He also found strong autocorrelation in the 27-day recurrence tendency,

so we consider both the 27-day and the 52-

day lag correlations. Accepting his figures as

adequately representing the autocorrelations

existing in our sample, we have used 0.5 and

0.25 as the correlations for lags of 27 and 54

days respectively. We then listed all values

from which we calculated our 10 percent

maximum and 10 percent minimum, and determined the frequencies of lag 1, 2, 27, and 54

days in our sample. With this information,

we may now proceed to use equation (4).

Let

ptf=the autocorrelation coefficient of lag d,

and

nd=the frequency of occurrence in sample

of days with lag d.

Then

=E[JJ S(*<-TO) J

(5)

From this equation we obtain

a' (maximum 10%) = 1.28

a' (minimum 10%) = 1.25.

These values may be compared with 0.88, the

ordinary standard error derived under the assumption of independence of the Kp's from

one another.

As a guide to the significance of the variations

in the curves in figure la,b, where relevant,

we have drawn dotted lines at a distance of

two standard errors (adjusted for autocorrelation, <r') from the mean of 22.65. Although
GEOMAGNETISM AND EMISSION-LINE CORONA

12 16

r r

59

12 16 DAVS

FIGURE 1.—Mean Kp for

each day following ELP

of various coronal conditions, a, Kp after

ELP of the 10 percent

strongest (a), the 20—

10 percent strong (b),

and the 30—20 percent

strong (c) intensities of

5303, 1950-53. b, Kp

after ELP of the 10

percent weakest (a), the

20—10 percent weak

(b), and the 30—20

percent weak (c) intensities of 5305,1950-53.

c, 10 percent brightest

5303 intensities for the

individual years (3-day

running means), d, 10

percent faintest intensities for the individual

years (3-day running

means). Number of

cases shown in parentheses.
60 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

this criterion of significance is not exact when

applied to the results obtained by the superposed epoch method of analysis, it should

provide a fairly reliable guide for judging the

results.

Figure la shows that strong 5303 corona on

the east limb is followed on da}-s 8-12 by a Kp

below the mean, although only Kp(-\-lQ) falls

outside the 2-standard-error band. This minimum in Kp after CMP of strong 5303 corona

agrees with the result of Kiepenheuer (1947)

and appears also in the curves of Shapley and

Roberts (1946) for the previous cycle. On the

other hand, we find no indication of the maximum around day ELP+4 that showed so

prominently in the curves obtained by Shapley

and Roberts in the previous cycle. The

patterns of Kp'a after ELP of strong, very

strong, and moderately strong 5303 intensities

do not differ significantly from one another.

Even the 5 percent strongest intensities failed

to produce any significant rise in Kp around

days +7 to 8, such as that found by Waldmeier (1939, 1942) and by Kiepenheuer (1947)

for "very strong" corona.

From line (a) of figure 16 we see that weak

5303 corona on the east limb of the sun is

followed on days 7-12 by Kp significantly above

the mean, with a maximum on day +9 . Kp

(+9) is very close to four standard errors (<rf)

above the mean, while Kp (+8) and (+10) are

almost three standard errors above the mean, so

that this result appears highly significant.

The most probable time for a geomagnetic

disturbance, in this phase of the solar cycle, is

two days after CMP of a weak coronal region, a

conclusion in general agreement with the results

of Smyth (1952) and of Bruzek (1952). The

5 percent weakest 5303 intensities give a curve

very similar to the 10 percent curve shown.

The effect, however, rapidly disappears with

increasing 5303 intensity, being very slight for

the weak 20-10 percent group and disappearing

entirely for the weak 30-20 percent group of

intensities. Thus the moderately weak corona

shows no relation to Kp (line (c) of figure 16);

while the moderately strong corona shows a

relation similar to that of the strong corona.

Comparison of results from two or more

independent samples of data provides an important test for the reality of the results of any

superposed epoch analysis. With this in mind

we examined the results of the four time

periods (see p. 57) separately. Because of the

smaller samples, these curves are less smooth

than the 45-month curves. In order to emphasize the principal trends and minimize the

irrelevant minor fluctuations we_plottcd these

results as 3-day running means, Kp. Figure lc

shows the values of Kp following ELP of

strong (10 percent) 5303 coronal regions for the

four periods, and figure Id shows the values of

Kp following ELP of weak (10 percent) 5303

coronal regions. Mean Kp's are shown for

each period.

Looking at figure lc we see that the curves of

Kp following ELP of strong 5303 intensitydiffer considerably from year to year. The

Kp's for the first and fourth periods are conspicuously below the means for these periods.

The second period curve rises more or less

continuously after ELP of the strong corona.

The curve for the third period resembles rather

closely the results found by Shapley and

Roberts in the preceding cycle, and shows a

strong maximum in Kp on day 3. Period

three covers the time for which Trotter and

Roberts (1952) and Muller (1953) noted a

return of the eastern-hemisphere effect. Because this effect does not appear in our other

three periods—and for reasons to be seen in

Part II—we are inclined to doubt the existence

of any physical connection between the 5303

corona on the eastern hemisphere of the sun

and the M-region responsible for the rise in Kp.

One feature common to all four curves in figure

lc is a tendency for a dip in the Kp values 9-12

days after ELP of the bright coronal regions.

While the tendency is present in each curve, in

some cases it is very slight and not particularly

convincing.

In figure Id the curves of Kp following ELP

of weak 5303 intensity are very similar in shape

for all four periods, particularly after day 5.

A conspicuous maximum occurs in each period

9-11 days after ELP of the faint corona.

The relationship between 5303 intensity and geomagnetism can be approached, alternatively, through a

study of the frequency distributions of Kp following

ELP of various coronal conditions. This approach, as

Shapley and Roberts (1946) have pointed out, serves to

reduce the possibility that one or two very intense
GEOMAGNETISM AND EMISSION-LINE CORONA 61

storms might dominate the shape of the curve, particularly when the sample size is small.

We were particularly interested, of course, in the

extremes of 5303 and in the high values of Kp. Because

of the tendency to weak Kp on day 10 after ELP of

strong corona, the distribution of weak values of Kp is

also of interest. We determined, for each time period,

the fraction of strong corona on the east limb that is

followed by (a) Kp> 30 and by (b) Kp< 12, and the

fraction of weak 5303 corona that is followed by (c)

K»3 0 and by (d) Kp<\2. For comparison we

computed the fraction of strong and of weak intensities

of 5303 which should be followed by strong or weak Kp,

if there were no connection between 5303 intensity and

Kp. This "expected fraction" is essentially the

number of days with Kp~>30 (or of days with Kp< 12)

in the period divided by the total number of days in

the period.

The results agree well with those found by the previous method. Result (a) is quite similar to figure lc.

The four periods do not agree with one another, except

that the observed fraction tends to lie below the expected value (or in period 3 to decline toward it) after

CMP of strong corona. Group (b) shows, in each of

the four periods, a fairly clear tendency for the observed

fraction of weak Kp to rise above the expected value

0-4 days after CMP of strong corona. Result (c) is

substantially in agreement with figure Id. The observed fraction lies significantly above the expected

fraction 1-4 days after CMP of weak corona in each

of the four periods; that is, strong Kp follows CMP of

weak corona significantly more frequently than would

be expected in the absence of any actual relation between these phenomena. Category (d) shows a systematic tendency in each period for the observed fraction of weak Kp to fall below the expected value 0-5

days after CMP of weak corona.

To investigate the effect of the threshold corrections and the interpolations on our results,

we calculated mean Kp'a for the 10 percent

maximum and minimum 5303, using three

different sets of data: (a) the Sac Peak observed intensities, uncorrected for plate threshhold, (b) the Sac Peak observed intensities,

corrected to plate threshold five, and (c) Sac

Peak corrected intensities plus intensities derived

from Climax observations and interpolated intensities. Figure 2 shows the results for the

10 percent maximum and 10 percent minimum

5303 intensities. For comparison, the graph

includes similar calculations made for the quantity used by Shapley and Roberts (1946),

namely the (d) single maximum intensity of

5303 observed at Sac Peak and corrected to

plate threshold five.

While the four curves differ in detail and in

smoothness, each indicates the same general

results, a minimum in Kp 10 days after ELP

of strong 5303 and a maximum in Kp 9 days

after ELP of weak 5303 regions. We found no

statistically significant differences between the

four curves for either the strong or the weak

5303 cases.

After ELP of strong 5303 intensities, curves

(a), (b), and (d) show a secondary minimum on

day 5, followed by a slight rise in Kp around

days 7-8. This rise may be a trace of the effect

observed by Waldmeier (1939,1942). We recall

Kp

24

22

20

-4 C

_ 1

(A)

—

—

1

)

\

\

4

1

"~"\ \ /

|

8 12

1 1

\ 1 •/

i i

p7

16

1

.-(b)

/(d)

/(ai

DAVS

(O

(d)

(b)

-4

1

(B)

(a)

A A m

1

c

J

•

4

1

|

h

I

v /

8

1

Wo

/ \

1

12

1

\

i

16

1 _

—

V ~\ •

\

—

1

- 2 4

— 22

— 20

FIGURE 2.—Mean Kp following ELP of strong (A) and weak (B) 5303 intensities. The curves show the results from coronal intensities uncorrected for threshold (a), corrected to plate threshold five (b), corrected and interpolated (c), and the tingle

maximum observed intensity (d).
62 SMITHSONIAN CONTMBUTIONS TO ASTROPHYSICS

that Denisse (1952, 1953) found evidence, from

radio-noise observations, for two types of solar

active centers, those (R) that produce geomagnetic storms and those (Q) that reduce

corpuscular radiation. The curves of Kp

following ELP of strong corona are therefore

probably a composite of these two types of

active centers. From the behavior of the composite, we infer that o-type centers were probably more numerous than R-type centers during

the interval covered by our study. Note that

this rise of Kp around CMP is present also in

figure la and more or less in the four curves

of figure lc.

Correlation coefficient.—To compute the coefficient of correlation between Kp and coronal

intensity we divided the data into our usual

four time periods. For each period we sorted

the cards by 5303 intensity into ten equal

groups, and assigned intensities on a new arbitrary scale of 1 to 10. This change in the 5303

scale was made in order to remove the effect of

the progressive decline in coronal intensity over

the 45 months. In the usual way we then computed the correlation between the 5303 intensities on the 1-10 scale and Kp (—2) through

Kp (+15). Figure 3 shows the correlogram of

the average correlation coefficients for the four

periods. The correlations are very close to

zero except on days 7-13 when they become

negative. The largest correlation, —0.18,

occurs for Kp (10), indicating once again a

•10

DAYS

time lag of about 3 days between CMP of a

strong or weak coronal region and its maximum geomagnetic effects.

The red line, X6374, [Fe x].—We determined

mean values of Kp for days —4 through +16,

as shown in figure 4, around zero days defined

by ELP of the strongest 10 percent and of the

weakest 10 percent of the 6374 values; the data

from Sacramento Peak and those from Climax

were used separately. As with the green line,

the percent limits with the red line are necessarily only approximate. To obtain the over-all

result shown in the figure for the 45 months,

the separate results of the four periods were

combined.

Figure 4 indicates, with a general similarity

between the Climax and Sac Peak curves, that

ELP of strong (10 percent) 6374 corona tends

to be followed on days 8-9 by a Kp significantly

below the mean. Examination of the years

separately supports this conclusion. On the

other hand, following ELP of weak 6374 the

Kp curves are very irregular and lack any

significant features; the strong rise in Kp

following CMP of weak 5303 intensities is

entirely absent for 6374. Outside of active

-4

FIGURE 3.—Coefficients of correlation between 5303 intensity

and Kp, with lags in Kp of —2 to + IS days after coronal

ELP.

20

FIGURE 4.—Mean Kp following ELP of strong (a) and weak

(b) 6374 intensities, from Sacramento Peak (SP) and

Climax (Cx) data.
GEOMAGNETISM AND EMISSION-LINE CORONA 63

regions the intensity of the 6374 line appears

to be more uniformly distributed than that of

5303. Therefore minima are perhaps less

precisely defined for 6374 and no relation with

Kp should be expected.

In general we conclude that Kp shows a

substantially closer relation to the intensity

of the 5303 line than to the intensity of the

6374 line.

Green line and red line, simultaneously.—

In investigating the effects of the two lines

simultaneously, we used only Sacramento

Peak red line intensities and the green line

intensities corrected to standard threshold.

We determined mean values of Kp for days

—4 through +16 around zero days defined by

22

20 -

various combinations of the two lines, with

the results shown in figure 5a, b.

The top curve of figure 5a shows that when both

coronal lines are strong, Kp declines quite smoothly

(heavy line) from day 3 to a minimum on day 10 which

is deeper than any of the minima obtained by using

only one coronal line. Days 8-10 have Kp two or

more standard errors (cr') below the mean value of

22.65. The asymmetry of the curve about its minimum

probably reflects the tendency of Kp to rise more

rapidly than it declines. To facilitate comparison

we include the mean Kp following ELP of strong

5303 corona and of strong 6374 corona, each computed

as before without regard to the behavior of the other

line. We suggest also comparison with figures la,

2, and 4. Most noteworthy is the disappearance of

the small rise in Kp around days 6-8 when both lines

are used; this rise we attribute to storm-producing

active centers as distinguished from the more common

22 -

20 -

a

FIGURE 5.—Mean Kp following ELP of various combinations of red and green line intensities, a, Kp after ELP of regions withthe two lines in agreement: line (a), strong 5303 intensities (20 percent) and strong 6374 intensities (30 percent), 108 cases?

line (b), weak 5303 intensities (20 percent) and weak 6374 intensities (30 percent), 83 cases; results for each line alone included

for comparison, b, Kp after ELP of regions having one line strong, the other not strong: curve (c), strong 5303 (20 percent)

and weak 6374 (30 percent) intensities, 11 cases; curve (d), strong 6374 (30 percent) and weak 5303 (20 percent) intensities,.

23 cases; curve (e), strong 5303 (20 percent) and not-strong 6374 (lower 70 percent) intensities, 66 cases; curve (0, strong

6374 (30 percent) and not-strong 5303 (lower 80 percent) intensities, 111 cases. (Curve (a) is reproduced from figure 5a for

comparison.)
64 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL. *

(in this period) storm-inhibiting active centers. ThuB

the curves of figure 5a may have special interest.

The top curves of figure 56 show the mean Kp'a

following ELP of coronal centers in which one line

is strong and the other weak. The curve when both

lines are strong is included for comparison. The results

are surprising. When the green line is strong and the

red line weak, Kp is above the mean on days 3-11;

when the red line is strong and the green line weak,

Kp is above the mean on days 2-10; whereas when

both lines are strong Kp is below the mean on days

4-12. While these differences appear to be statistically

significant, we should keep in mind that the sample

sizes are small. The bottom curves of figure 5b show

the mean Kp'a following ELP of coronal centers in

which one line is strong and the other line is not strong.

While neither curve (e) nor (f) deviates significantly

from the mean Kp, each appears to differ significantly

from the case of both lines strong.

The bottom curve of figure 5o shows that no improvement over green line alone is obtained for the

case of both lines weak. This result is perhaps to be

expected since the weak red line alone shows no relation to Kp.

The interpretation of the results shown in

figure 5a, and b, is not at present clear. However, the curves do indicate that the red as

well as the green line should be considered in

future studies of the relations between geomagnetism and coronal intensity, and that

some improvement in predicting the geomagnetic effects of an active center might result

from attention to the relative intensities of

the two lines.

The yellow line, X5694.—The rare appearance

of the yellow line in the coronal spectrum

appears to indicate a center of unusual activity.

Waldmeier (1945), Roberts (1952), and Dolder

(1952) have pointed out the frequent association between 5694 emission and small, highly

active and sharply curved prominences of the

sunspot type. They find evidence also for a

close association between 5694 emission and

solar flares (especially Dolder, 1952).

A search of the CRPL Bulletins revealed

that the yellow line was observed at Sacramento Peak and/or Climax on 22 occasions

between February 1950 and December 1952,

12 times on the east limb and 10 times on the

west limb. We omitted three cases occurring

in relatively high latitudes in 1953. Figure

6 shows the mean Kp's associated with regions

of yellow line emission. The mean Kp for

the period covered by the 22 appearances of

the yellow line is 22.65. Because of the small

samples the curves are very irregular and the

standard errors are large. Most of the fluctuations are not common to the east and the

west limb curves and therefore should not be

considered significant. However, each curve

does show a maximum in Kp around CMP

FIGURE 6.—Mean Kp following ELP of regions of yellow-line

emission. The solid curve shows combined results for east

and west limb observations; the broken curves show the

results for east and west limb appearances separately.

of the region emitting the 5694 radiation. The

individual maxima occur on days ELP +8

and WLP —5, while the maximum of the

composite curve occurs on day 7. This maximum does not attain two standard errors from

the mean.

To investigate further the relation between Kp and

5694, we determined the frequencies of Kp >30 , 20-30,

and <2 0 on days —2 through + 4 around CMP of

yellow-line regions, shown in table 1.
GEOMAGNETISM AND EMISSION-LINE CORONA 65

TABLE 1.—Frequencies of Kp around CMP of yellowline regions

Kp\CMP - 2 - 1 0 +1 +2 +3 +4

Kp>30

20>Kp>20

Kp<20

3

8

11

3

9

10

10

5

7

10

8

4

6

11

5

7

5

10

6

7

9

%Kp>30 14 14 45 45 27 32 27

A chi-square test indicates that the deviations of these

frequencies from the expected values are not quite

statistically significant; that is, they provide insufficient

evidence to reject a hypothesis of independence between

Kp and 5694. The last row of table 1 shows the

percent of 2Cp>30; the individual values may be

compared with a mean frequency of 24 percent for the

three years in which the yellow line emissions occur.

While none of the yellow-line results can be

declared statistically significant, the above

tests do not take account of the location of the

deviations from the mean or expected values,

i. e., of the fact that the rise in frequency of

strong Kp occurs when the yellow line region

is on the central meridian. Moreover, yellow

line emission appears to be relatively shortlived. Only two of the regions in our sample

showed 5694 on both ELP and WLP; (in

seven cases coronal observations were obtained

on only one limb). Therefore, probably not all

of the regions were emitting 5694 radiation when

they crossed the central meridian. Certainly

the changes in the frequency distribution of

strong Kp are in the direction and the position

to be expected if yellow line regions tend to

produce geomagnetic storms. The results are

also in general accord with recent findings by

Denisse and Simon (1954) for yellow line

regions in the period 1946-52. From the

present small sample, however, we can draw

no definite conclusions.

Green line intensity around zero-days defined

by large Kp.—In order to obtain a picture of the

average distribution of 5303 intensity as a

function of longitude on the face of the sun at

the time of geomagnetic storms, we prepared

an inverse deck of IBM cards containing east

limb intensities for days CMP —7 through +9

(ELP —14 through +2), and west limb intensities for CMP —9 through +7 (WLP —2

through +14). Using these cards we first

calculated the average intensity of 5303 around

zero days defined by the maximum 20 percent

of the Kp's. Examination of the results for

the 45 months revealed that the 5303 intensity

tended to have a minimum on the central

meridian of the sun 2-4 days before the occurrence of geomagnetic storms. This minimum

appeared in each of the four time periods.

To determine whether sporadic and recurrent geomagnetic storms appeared to be associated with different patterns of coronal intensity we selected, from the lists published

regularly in the Journal of Geophysical Research, those storms observed to have a sudden

commencement (sc) at ten or more observatories. We omitted six storms that appeared

clearly to be members of prominent Msequences. For the remaining 38 storms we

computed the pattern of 5303 intensity, shown

by line (a) in figure 7. The average intensity

was determined from six-month averages

weighted by the number of sc-storms occurring

in that six-month period. As figure 7 shows,

nonrecurrent sc-storms apparently tend to be

associated with above-average intensity of

5303 emission around the central meridian of

the sun, but no clear time lag can be determined from our graph. The width of the

maximum may perhaps arise from the tendency

of sporadic storms to be associated with

flares; the flares presumably occur in C-regions

which need not be at the central meridian of

the sun.

We also selected nine of the more prominent

recurrence sequences, which gave 58 M-storm

epochs, with the zero day defined by the onset

+8

16 —

FIGURE 7.—Mean pattern of S3O3 intensity at various positions on the solar disk: at times of sporadic sc-storms, line

(a); of M-sequence storms, line (b).
66 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

of the storm. Figure 7, line (b), shows the

resulting average distribution of 5303 intensity

on the sun. M-type storms tend to be preceded

by a minimum in 5303 intensity on the central

meridian of the sun 1-2 days before the onset

of the storm. The intensity of 5303 crossing

the central meridian is below average from 3

days before to 5 days after the onset of the

storm. Since each storm is represented only

once, the time lag here should be rather precisely defined. The intensity is above its

mean 4-9 days before the onset of the storms

and drops quite sharply, a drop that corresponds

to the relatively sharp if not "sudden" onset of

the storm. Indeed the shape of the curve,

inverted, resembles an M-storm profile.

We conclude that in the period covered by

this investigation, nonrecurrent sc-storms tend

to be associated with bright or at least moderately bright 5303 emission and not with weak

5303 areas. However, more C-regions appear

to inhibit storms than to produce them. On

the other hand, M-regions seem to be associated with regions of weak 5303 corona, with

a lag of 1-2 days between CMP of the weak

coronal region and the onset of the storm.

Hdiographic latitude of the earth.—In the

introduction, we mentioned the well-known

tendency of aurorae and geomagnetic storms,

especially the recurrent storms, to be most

intense around March and September. No

satisfactory explanation of these seasonal effects

has been given, but most attempts associate

the maxima in geomagnetic disturbance either

with the equinoxes (i. e., with the geographic

latitude of the sun), or with the heliographic

latitude of the earth. Following the terminology of Bartels (1932) we may call these explanations equinoctial and axial, respectively.

In this paper we consider primarily the axial

hypothesis, since it is the more susceptible to

investigation with our data and, we believe, is

the more plausible.

Because the plane of the earth's orbit is inclined 7.2° to the solar equator, the heliographic

latitude of the earth varies from 7.2° north of

the solar equator about September 7, to 7.2°

south of the solar equator about March 5.

Thus we see a maximum area of the sun's

northern hemisphere in September and a maximum area of the southern hemisphere in March,

at approximately the times of greatest geomagnetic activity. The geomagnetic activity tends

to be at a minimum around the times when the

earth is crossing the heliographic equator.

Apparently solar corpuscles intercept the earth

more readily when the earth is farthest removed

from the equatorial plane of the sun. On the

other hand, the geomagnetic maxima and

minima appear to lag behind and to fall closer

to the equinoxes and solstices than to the axial

parameters (Bartels, 1932).

The usual axial explanation is based on the

fact that spots occur most frequently in heliographic latitudes 10-15° while the equatorial

belt contains few spots. If the corpuscular

streams start from the spot belts and leave

the solar surface in a radial direction, corpuscles

from the northern and southern belts are most

likely to intercept the earth in September and

March respectively. However, as we shall

discuss later, a relation with the latitude of

maximum spottedness is not the only possibility

for an axial explanation.

The March and September maxima in geomagnetic activity have not been adequately

investigated. Bartels (1932) studied the relation between facular area in the northern and

southern hemispheres and geomagnetism. His

results offered no support to an axial interpretation. Since he used only monthly means of

facular areas and of the geomagnetic index,

however, his results are not necessarily significant. The effect of the heliographic latitude

of the earth in relation to active centers on the

sun has not, so far as we are aware, been

studied on a daily basis. We made an exploratory study of the problem from two directions.

Active centers or C-regions.—We had recorded,

on the original IBM cards, the latitude of the

brightest single 5303 intensity for the day.

The value of this latitude we took to indicate

whether the associated C-region was located

in the northern or in the southern solar hemisphere. When two C-regions occurred simultaneously, one on either side of the equator, we

disregarded the region with the weaker maximum; days with maxima of equal intensity in

north and south were omitted.

We designated as "favorable" (f) those Cregions on the same side of the solar equator

as the earth, and as "unfavorable" (u) those
GEOMAGNETISM AND EMISSION-LINE CORONA 67

C-regions on the opposite side of the solar

equator from the earth. Using approximately

the 20 percent highest intensities in each time

interval to indicate C-regions, we subdivided

the data into the following groups:

If. (South) 1 Feb. 1950- 6 Jan. 1950,

8 Dec. 1950- 6 Jun. 1951.

lu. (North) 1 Feb. 1950- 6 Jun. 1950,

8 Dec. 1950- 6 Jun. 1951.

2f. (North) 7 Jun. 1950- 7 Dec. 1950,

7 Jun. 1951- 7 Dec. 1951.

2u. (South) 7 Jun. 1950- 7 Dec. 1950,

7 Jun. 1951- 7 Dec. 1951.

3f. (South) 8 Dec. 1951- 6 Jun. 1952,

8 Dec. 1952- 6 Jun. 1953.

3u. (North) 8 Dec. 1951- 6 Jun. 1952,

8 Dec. 1952- 6 Jun. 1953.

4f. (North) 7 Jun. 1952- 7 Dec. 1952,

7 Jun. 1953-15 Oct. 1953.

4u. (South) 7 Jun. 1952- 7 Dec. 1952,

7 Jun. 1953-15 Oct. 1953.

For each group we computed the Kp'a for

days —4 through +16 around ELP of the Cregions. We computed also the over-all

Kp'a for the favorable C-regions (lf+2f+3f

+4f, 72 cases) and for the unfavorable Cregions (lu-f-2u+3u+4u, 118 cases). Figure

8 shows the results and reveals a conspicuous

difference between the Kp curves following

ELP of favorably and of unfavorably located

C-regions. The unfavorable curve shows only

irregular fluctuations without any significant

deviations from the mean, but the favorable

curve shows a clear and fairly symmetric

minimum on day 10. This minimum is similar

.8 .16 WfS

20 —

FIGURE 8.—Mean Kp following ELP of favorably (solid line)

and unfavorably (broken line) located regions of bright

green-line emission.

to others previously shown except that it is

deeper than any other, including that shown

when both green and red lines were strong.

The Kp values on days 8 through 11 are two

or more standard errors below the mean.

We suggest comparison with figures la, 2,

4, and 5a.

Figure 9a shows the four separate curves,

as 3-day running means, for the four groups

of favorable C-regions; similarly, figure 96

shows the curves for the four groups of unfavorable C-regions. These groups of curves

may be compared with figures lc and Id with

respect to homogeneity, but it should be remembered that the 45 months have been subdivided

differently in the two cases so that individual

curves are not directly comparable.

In each of the four curves for the favorable

C-regions (figure 9a) Kp is below the mean of

its respective period on days 9-11. Figure 9a

shows a greater homogeneity for the four

periods after CMP of the C-regions, and declines in Kp of greater magnitude than appear

in figure lc where heliographic latitude is not

considered. Since figures 9a and 96 do not

involve any interpolated values of the intensity of 5303, they should be less influenced

by autocorrelation; therefore, a variation in

Kp in figure 9a should possess greater significance than a variation of equal amplitude in

the curves of figure lc. The four curves for

the unfavorably located C-regions show very

little homogeneity.

We conclude that, at least in the period

covered by this investigation, C-regions on

the same side of the solar equator as the earth

have a greater influence on Kp than do Cregions on the opposite side of the solar equator.

ELP of such favorably located C-regions tends

to be followed by a decline in Kp with a minimum on day 10, or three days after CMP;

whereas ELP of the unfavorable C-regions

shows much less, if any, relation to subsequent

Kp values. These relationships tend rather

strongly to support an axial explanation of the

seasonal variation in geomagnetic activity.

Certainly such results would appear inexplicable

by any equinoctial hypothesis.

M-regions.—From the evidence presented

earlier we tentatively concluded that M-regions

are associated with CMP of areas in the active
68

-4 0

SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

8 12 16 -A 0 4

22| -

20| -

261—

24

22

22

22

20

OS)

(23) i

26

24( -

(33)

(24)

-(Nu)

-(5U)

-(NU)

•-BU)

a b

FIGURE 9.—Mean Kp following ELP, for individual time periods, of: a, favorable C-regions; b, unfavorable C-regions.

zones where the 5303 emission is unusually

weak. The data recorded on our IBM cards

were not the most suitable for fully exploring

this hypothesis if an axial explanation of the

semiannual period in geomagnetism is correct.

Instead we should need separate intensities for

the northern and southern hemispheres. Therefore, to investigate in a preliminary way the

effect of heliographic latitude of the earth on

M-storms, we calculated the average intensity

of 5303 crossing the central meridian, for

5-25°N and for 5-25°S separately, for five days

before and three days after the onset of each of

the 58 M-storms comprising our nine M-se-
GEOMAGNETISM AND EMISSION-LINE CORONA 69

quences. Both east and west limb observations of the 5303 intensity were used. For each

M-region we determined the average intensity

distribution for each of the nine days, in the

favorable and the unfavorable solar hemisphere

separately. Inspection of this data (see Bell

and Glazer, 1954a, for details) showed that the

onset of most of the larger recurrent storms

appeared to be associated with a region of weak

5303 intensity in the favorable hemisphere 1-3

days west of the central meridian.

This evidence appeared to indicate that the

heliographic latitude of the earth possessed an

importance to geomagnetic conditions not previously realized, and suggested that further exploration of the problem by use of separate intensity indices for the northern and the southern

solar hemispheres might be fruitful. Such an

approach would permit a more adequate study

of the effect of the relative heliographic latitude

of earth and of strong and/or weak coronal regions than had been possible with our single

intensity index. The results are presented in

Part II of this paper.

Summary of Part I.—The principal results

and conclusions so far reached from a study of

1950-53 data are, in order of their apparent

significance, as follows:

CMP of regions of weak 5303 corona tends

to be followed 1-3 days later by above-average

Kp index or by disturbed geomagnetic conditions. This tendency appears clearly in each

of the four time periods and is the best established and most clearly significant of our conclusions. Conversely, the onset of magnetic

storms of the recurrent type tends to be associated with regions of weak 5303 corona that

have passed the central meridian 1-2 days previously. Investigation of individual M-sequences suggests that M-storms may be associated with weak 5303 intensities located on the

same side of the solar equator as the earth, and

that the semiannual period in geomagnetic activity can best be explained by some sort of

axial hypothesis.

CMP of regions of strong 5303 corona tends

to be followed 1—4 days later by diminished Kp,

although the four time-period curves less

clearly support this conclusion. The weakening

in Kp (9-11) appears to be more pronounced if

both coronal lines (5303 and 6374) are strong

than if only one is strong. The weakening is

also greater and more consistently present following CMP of C-regions in the favorable solar

hemisphere than after CMP of C-regions in the

unfavorable hemisphere. This result again

suggests that the heliographic latitude of the

earth may be a significant factor in geomagnetic

conditions.

CMP of some C-regions appears to be associated with sporadic geomagnetic storms, particularly those C-regions having yellow line

emission, and perhaps those having one coronal

line strong and the other weak. These conclusions are least well established from our work.

These results suggest that geomagnetic conditions are influencd by corpuscles from two

entirely different solar sources. The apparent

association between some C-regions and some

magnetic storms may be explained most obviously by assuming that some active solar

centers, characterized by frequent flares, enhanced radio noise, and 5694 emission, eject

corpuscles at high velocities and produce storms

directly. These corpuscles we shall hereafter

refer to as "active-center corpuscles." The apparent association between M-storms and the

weakest 5303 regions, on the other hand, seems

to imply some general source of corpuscles,

perhaps arising, as Denisse has suggested, by

evaporation from the corona over wide areas.

The escape of these, which we shall hereafter

call "the general solar corpuscles," is apparently

inhibited by the presence of active centers.

Further discussion will be postponed until

after the presentation of the evidence in the

following sections.

Part II. Separate intensity indices for

northern and for southern solar hemispheres

The investigation reported in this section covers

a period of 47 months, from Jan. 1, 1950,

through Dec. 7, 1953. As in Part I, the daily

sums of Kp are used as the measure of geomagnetic activity. As measures of the 5303 intensity, however, separate daily indices are employed for the northern and the southern solar

hemispheres. For the northern index, we have

taken the mean of the three highest east-limb

intensities north of the equator, while for the

southern index we took the mean of the three
70 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL. I

highest values south of the equator. Equatorial

intensities themselves did not enter into either

index. These intensity indices were corrected

for plate threshold by the method described

previously (Glazer and Bell, 1954). By interpolating between observations we obtained an

east-limb index for the north and one for the

south for each day; similarly, we obtained a

pair of west-limb indices.

For the 6374 line we used as daily indices the mean of

the two highest intensities for each solar quadrant.

These red line indices were corrected to plate threshold

five by the formulae

/.'=/.+0.48 (T.-5) (6)

J.'=/.+ 0.43 (7V-5),

and slight correction was made of the Climax (/«)

values to the Sac Peak (/,) scale. We attempted no

interpolations for 6374, but did use both Climax and

Sac Peak intensities. The intensities of both coronal

lines, and values of Kp for days ELP—4 through +19

were punched on standard IBM cards.

We divided the data into eight time periods,

four with the northern solar hemisphere favorable and four with the southern solar hemisphere favorable. Each period began and

ended with the earth's crossing of the solar

equator. The periods are as follows:

Spring: Sf, Nu

1 Jan. 1950 - 6 June 1950

8 Dec. 1950 - 6 June 1951

8 Dec. 1951 - 6 June 1952

8 Dec. 1952 - 6 June 1953

Fall: Nf, Su

7 June 1950 - 7 Dec. 1950

7 June 1951 - 7 Dec. 1951

7 June 1952 - 7 Dec. 1952

7 June 1953 - 7 Dec. 1953

Since for each period there was a favorable (f)

and an unfavorable (u) hemisphere index, we

had 16 groups of data to work with. As

before, relative intensities within each group

rather than absolute values define the zero

days of strong and weak coronal intensity.

The green line.—We computed mean Kp

values for days —4 through +19 around zero

days defined by ELP of the 10 percent, 20-10

percent, and 30-20 percent of the strongest

(H) and of the weakest (L) 5303 intensities

within each of the eight favorable and the

eight unfavorable groups. We first combined

these 16 results into four groups, Sf (south

favorable), Su, Nf, Nu, and then further combined them into the two groups: f=Nf+Sf,

u=Nu+Su . Using west-limb indices we made

similar calculations for days —18 through +5

around zero days defined by WLP of strong

and of weak corona.

Figure 10a shows the composite results for

bright corona for the entire 47 months. In

figure 10a and in all subsequent figures, favorable (f) hemisphere curves appear as solid lines,

while the unfavorable (u) hemisphere results

appear as broken lines. The average value

of Kp for the period is 22.4. The dotted lines

indicate a distance of two standard errors

from the mean. The standard errors (o7) have

been increased by about 35 percent over ordinary standard errors to take account of the

autocorrelation among the Kp's as discussed

in Part I.

Line (a) in figure 10a shows that bright corona

(10H) on the east limb in the favorable solar

hemisphere is followed on days 8-11 by a

Kp>2a' below the mean. Bright (10H) corona on the east limb in the unfavorable hemisphere, on the other hand, is followed by no

significant deviations from the mean Kp.

The minimum in the favorable curve occurs

for Kp (10), and lies about 3.5 standard

errors {a') below the mean. This minimum

in Kp is substantially deeper and of greater

statistical significance than that in figure la

where we took no account of the relative heliographic latitude of earth and bright coronal

center. Note also the small rise at Kp (6-7)

and a moderate maximum at Kp (14).

Corresponding to figure 10a, figure 106

shows the Kp's associated with bright 5303

for the spring half-years (Sf, Nu), while

figure 10c shows those for the autumn halfyears (Nf, Su). Figure lla-c shows the

average values of Kp preceding WLP of

bright 5303 intensities in the favorable and

the unfavorable hemispheres.

Since the corona can be observed only at the

limbs of the sun, seven days before or seven

days after CMP, the intensity of the corona at

central meridian passage is necessarily somewhat uncertain. In an effort to minimize the

uncertainty we have made a further set of calculations based on simultaneous consideration

of the east-limb index and the west-limb index

14 days later. Figure 12a-c exhibits the aver-
GEOMAGNETISM AND EMISSION-LINE CORONA 71

L A /*•"•

(133) \

I I I I I

« K 16

FIGURE 10.—Mean Kp following ELP of strong 5303 intensities in the favorable (solid line) and unfavorable (broken line) hemispheres; line (a) represents the 10 percent strongest intensities (10H), line (b) the 20H—10H, and line (c) the 30H—20H. a,

Composite results for the 4-year period; b, spring half-years; c, autumn half-years.

WLP -(6

1

—(137)

V--V

_IW8l/\

113?) \

V \

-

— V

I

1

\

\

.-- •

/

-9

"I

/ \

\ /

1 i 1

-4

1

\ ,<7

y

"^ A'"'

i

1

S \ /(•!

v'

-

—TC* (H

ZOHOH~

.-' 3o-aw_

-

1 I I II I I I ]_ l II I I I

FIGURE 11.—Mean Kp preceding WLP of strong 5303 intensities in the favorable (solid line) and unfavorable (broken line) hemispheres; line (a) represents the 10 percent strongest intensities (10H), line (b) the 20H—10H, and line (c) the 30H—20H.

a, Composite results for the 4-year period; b, spring half-years; c, autumn half-years.
72 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL.*

FIGURE 12.—Mean Kp associated with CMP of regions bright in 5303 intensity at both limb passages, in favorable (solid line) and

unfavorable (broken line) hemispheres; line (a) represents regions with the 10 percent strongest intensities (10H) at ELP and

at WLP; line (b) represents 20H (E and W) minus 10H (E and W). a, Composite results for the 4-year period; b, spring halfyears; e, autumn half-years.

age values of Kp associated with CMP of

regions that appeared bright in 5303 at ELP

and bright also at WLP 14 days later. We can

feel more confident that these regions were

actually strong at CMP than we can of the

regions used in figures 10 and 11. Note that

the minima in the favorable curve around Kp

(10) are deeper; because of the reduced sample

size, however, they have no greater statistical

significance than those shown in figures 10

and 11.

Since the regular appearance of any effect in

independent samples forms an important test

of the significance of any superposed epoch

result, in figure 13a, b, we give the 10H

results for the individual half-years. A greater

number of days are plotted in connection with

east-limb corona because only east-limb observations can be of practical use to forecasters.

The others are of interest, however, in any

attempt to understand the physical significance

of the results.

Inspection of figures 10-13 indicates that

CMP of bright corona located in the favorable

solar hemisphere appears to be associated with

the following Kp tendencies, listed in order of

apparent significance:

The minimum in Kp around three days after

CMP of Hf corona is the most conspicuous

effect and occurs with convincing uniformity

and regularity. The minimum exceeds 2<r' on

all nine of the lOHf curves of figures 10-12, and

occurs in 30 out of 32 of the curves in figure

13a, 6. The improvement in regularity compared with that in figure lc is striking.

Kp tends to be somewhat above the mean

around day 14 in most but not all of the Hf

curves. Whether this reflects a tendency of

corpuscles to concentrate at the edge of a region

of deficiency, or a tendency of the magnetic

field of the earth to react more strongly to an

average supply of corpuscles after a dearth,

cannot be decided without more understanding

of the mechanism of geomagnetic storms.

However, because of the general tendency of

Kp to rise rapidly and decline more slowly, we

incline to the second explanation.

A small rise in Kp is often but not always

present around CMP of bright regions, days

6-8. As stated in Part I, we ascribe this rise

to the storm-producing active centers, which

in this phase of the solar cycle are substantially

fewer than the number of inhibiting centers.

In the unfavorable hemisphere curves we see
GEOMAGNETISM AND EMISSION-LINE CORONA 73

no systematic trends. In the spring (Nu) Kp

tends to be below the mean after CMP, whereas

in the fall (Su) Kp tends to be above the mean

after CMP of bright corona. Because of this

inconsistency we conclude that our data do not

indicate any systematic relation between Hu

5303 and Kp.

It is possible that such minima as do occasionally

appear may result from the chance occurrence of days

common to both the favorable and the unfavorable

samples. This possibility is especially likely in the

spring of 1953. The Nu curve shows a marked minimum in Kp around CMP; in this period, however, the

only major active centers, N and S, lie within a few

degrees of longitude of one another, so that bright Nu

corona would inevitably appear to "produce" a minimum in Kp even though the Sf bright regions were the

actual cause of the minimum. Investigating this

possibility further, we found that for ELP of 10H,

22 (31 percent) Nu were included in Sf samples while

only 8 (11 percent) Su were included in Nf samples.

Of the eight periods, only two, 1950 (spring half) with

nine cases and 1953 (spring half) with seven cases, had

more than four days common to both favorable and

unfavorable samples. In view of Becker's (1953) findings with paired sunspots, it is of interest to note that

the depth of minima in Kp following CMP of strong

favorable corona shows no apparent relation to the

number of paired coronal maxima entering the sample.

Kp conditions that appear to be associated

with CMP of faint 5303 corona are exhibited in

detail in figures 14-17. Figure 14a-c shows the

average values of Kp following ELP of faint

(L) 5303 intensities in the favorable and unfavorable solar hemispheres. The top curve of

figure 14a shows that faint corona on the east

limb in the favorable solar hemisphere is

followed on days 8-12 by a Kp>2a' above the

mean. The maximum occurs for Kp (9),

which lies about 4.5 standard errors (<r') above

the mean. This maximum in Kp is only

slightly greater than that in figure 16, where we

took no account of favorable and unfavorable

hemispheres. Although the improvement is

only minor it appears entirely compatible with

the existence of a hemisphere effect when we

recall that a faint 5303 intensity defined by the

index used in Part I necessarily indicated faint

5303 corona in both the favorable and unfavorable solar hemispheres. Moreover only a slight

rise in Kp appears to follow CMP of faint 5303

regions located in the unfavorable solar hemisphere.

Figure 15a-c shows the average values of

Kp preceding WLP of weak 5303 intensities

from the favorable and the unfavorable westlimb indices. Here also we have attempted to

minimize the uncertainty regarding the intensity of the 5303 line at central meridian by

making a set of calculations based on simultaneous consideration of the east-limb index

and the west-limb index 14 days later. Figure

16a-c shows the average values of Kp associated with CMP of regions that appeared

faint in 5303 at ELP and faint also at WLP 14

days later. These are the regions that we can

feel most certainly were actually faint at CMP.

As in the case of the bright 5303 corona, the

principal effect in Kp—but not its statistical

significance—is enhanced by considering E and

W indices simultaneously. The rise in Kp

following CMP of unfavorable faint 5303 is

also increased but continues to be less than for

the favorable corona. Moreover, the twostandard-error bands in figure 16a and 16c

apply only to the favorable curves; because the

unfavorable samples are smaller, their standard

errors would be larger.

Figure 17a, 6, exhibits the 10L results for

the individual half-years. The rise in Kp

following CMP of faint 5303 corona in the

favorable solar hemisphere appears with convincing regularity in the curves of figures

14-17. The one exception occurs in the spring

of 1950 (top curves, figure 17a); this period is

also distinguished from the other seven by an

absence of any clear recurrent-storm sequences.

Therefore the failure of the relation in the

spring of 1950 can be readily understood in the

light of our previous finding (figure 7) that

only recurrent or M-type storms are associated

with regions of faint 5303 corona. For this

reason it might appear justifiable to remove

1950 Sf from the composite spring curves;

when this is done the maximum in figure 166

rises to 30.6 (at day 10) with a mean of 23.7,

as contrasted with the values shown of 27.6

and 22.8. Other curves would be similarly

improved and the apparent inferiority of the

Sf curves relative to Nf would be largely

eliminated. If this explanation of the failure

in 1950 Sf is correct, we should expect the

relation of figures 14-17 to hold only at times

of M-region activity; that is, primarily on the

declining side of the solar activity cycle.
74 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL. 2

ri f T"

V \

J_L

FIGURE 13.—Mean ATp associated with various circumstances of bright 5303 intensities for individual half-years. Columns from

left to right are 3-day running means of Kp associated with: ELP of the 10 percent strongest intensities in the favorable hemisphere (10Hf); WLP of lOHf; lOHf at ELP or WLP, with 20Hf at the other limb; lOHf at both ELP and WLP; ELP of the

10 percent strongest in the unfavorable hemisphere (lOHu); and lOHu at ELP or WLP, with 20Hu at the other limb, a, Spring

half-years; b (on facing page), autumn half-years.

While the unfavorable curves do show some

tendency to a rise in Kp after CMP of faint

5303, this rise is always less than in the corresponding favorable curve; moreover the

tendency is only irregularly present in the

eight half-years. We therefore conclude that

there is no clear relation between Kp and weak

5303 corona in the unfavorable solar hemisphere.

Table 2 shows the number of cases common

to the favorable and the unfavorable ELP

samples in figures 13 and 17.

TABLE 2.—Number of cases common to f and u samples.

Figure

13a

136

17a

176

1950

9

2

2

6

1951

4

2

9

2

1952

2

4

3

2

1953

7

0

1

9

The possible effect on Kp of similar coronal

conditions in both favorable and unfavorable

solar hemispheres has a special interest because

of the findings of Becker (1953) and of Becker

and Denisse (1954). We have already mentioned that we computed Kp following ELP of
GEOMAGNETISM AND EMISSION-LINE CORONA 75

0 A 8

/ \

\

-\J

/ y

A /

I |_L

9ZKJ.

/

• \

\ /

FIGURE 134.—See explanation on facing page.

regions having lOHf and lOHu simultaneously;

this did not notably deepen the minimum in

Kp, and because of the small sample sizes the

curves were conspicuously irregular. To work

with somewhat larger samples, Sf cards were

divided into three groups: 30H, 40M, and

20L (note: 30-20L omitted). The Kp's computed from these groups form the top row of

curves in figure 18a. Each of these three

groups was divided into three parts (Nu 30H,

40M, 20L) and the associated Kp's were

computed for each combination of Sf-Nu,

as shown in figure 18a. The first column shows

the three Nu-groups independently. A similar

treatment of Nf, Su indices produced the results

shown in figure 186, which indicate that, insofar

as Kp depends on 5303 intensity during the

four autumn seasons covered by our study,

Kp depended mainly on the Nf intensity and

not perceptibly on the Su. No statistically

significant differences appear among the curves

in any given column; and the curves change

along the rows in the manner to be expected

under conditions of Nf dominance. In figure

18a, on the other hand, the situation is less

clear, and there does appear some indication

that Sf is more effective in giving the "expected" Kp results when paired with a similar

Nu. The data appear to suggest that the

northern solar hemisphere may have a more

powerful "control" over Kp conditions than

the southern; but in view of the short time
76 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

i r

J ^ _«,• -

i i n i

FIGURE 14.—Mean Kp following ELP of weak 5303 intensities in the favorable (solid line) and unfavorable (broken line) hemispheres; line (a) represents the 10 percent weakest intensities (10L); line (b) the 20L— 10L. a, Composite results for the

4-year period; b, spring half-years; c, autumn half-years.

-M

1

OS) ,'

182)

<S2>

-12 -6

i i 1

£^ _

\

i / \

v ) " \

1 1 1

-4

1

~T\

\

V

__/

i

0

1

1,-

\

> \

V

i

•*i

s

-

• ^ ^

(U

-

-16

1

/

(63) r

- /"

1

12 -«

1 1

^ -

\

/ -

1 1

-4 0

1 I

A

/ ^-'\ \

•4

1

—2<r

r

-

-

(o)

(H

15.—Mean Kp preceding WLP of weak 5303 intensities in the favorable (solid line) and unfavorable (broken line) hemispheres; line (a) represents the 10 percent weakest intensities (10L), and line (b) the 20L— 10L. a, Composite results for the

4-year period; b, spring half-years; e, autumn half-years.

covered, we would not propose that such an

inherently unlikely suggestion be taken too

seriously. Nor indeed should the apparent

effect be regarded as real without further

observational study.

As in Part I, we computed the frequency distribution

of Kp associated with CMP of various conditions in

the 5303 corona. Here we used the sum of the east-limb

and west-limb indices as the measure of 5303 intensity.

We omitted the spring of 1950 from these calculations

in order to obtain samples more homogeneous with

respect to M-region activity, as mentioned above.

Figure 19a, 6 shows the fractions of strong corona

and the fractions of weak corona in the favorable and

unfavorable solar hemispheres separately, each associated with K»3 0 and with Kp< 12. The broken
GEOMAGNETISM AND EMISSION-LINE CORONA 77

Oil/

i r

j I J I

-

(0)

on

its

1

A

f

"7

f

i

/

\

1

A '

iH \

AA

A\

i

\ \

\ \

\

\

i

\v

-

(o)

/ .

-

|

FIGURE 16.—Mean AT/> associated with CMP of regions weak in 5303 at both limb passages, in favorable (solid line) and unfavorable (broken line) hemispheres; line (a) represents regions with the 10 percent weakest intensities (10L) at ELP and at WLP;

line (b) represents 20L (E and W) minus 10L (E and W). a, Composite results for the 4-year period; b, spring half-years;

c, autumn half-years.

line indicates the "expected fraction" of strong and

weak intensities of 5303 that should be followed by

high (or low) Kp if the 5303 intensity and Kp were

completely independent. The results agree well with

those found by the previous method, and serve further

to assure us that they do not arise from domination of

the curves by a small number of extreme values.

The coefficient of correlation between Kp

and east-limb 5303 intensity in the favorable

and the unfavorable solar hemispheres was

computed by the procedure described in Part I.

Figure 20 shows the resulting correlograms and

may be compared with figure 3. Around Kp

(10) we have indicated the correlations resulting

when 1950-Sf is removed. The largest correlations occur in the favorable curves, on day 10,

and amount to —0.26 for Nf, —0.20 for

Sf (1951-53), and —0.23 for the composite f

(omitting 1950-Sf), with about 720, 550, and

1270 cases, respectively. Very similar curves

are obtained if we correlate Kp with the sum

of the east and west indices; each coefficient

at Kp (10) is increased by about 0.01 over

the value obtained from east-limb intensities

alone.

Figure 21 illustrates the behavior of mean

Kp's associated with the entire range of

intensity percentiles for the more significant

groups of days. We calculated the average Kp

associated with the percentiles 0-10 (Low),

10-20, 20-50, 50-70, 70-85, and 85-100 (high)

of 5303 intensities using days (4, 5, 6), (9, 10,

11), and (14, 15, 16).

The graphs for days (4, 5, 6) reveal no

notable trends, as one would expect from the

low correlations shown in figure 20; those for

days (14, 15, 16) show a slight rise in mean Kp

with increasing intensity of 5303. For days

(9, 10, 11) the averages of Kp might appear

to be more closely related to the intensity in the

favorable hemisphere than the correlations of

only about —0.2 would suggest. However

one should bear in mind two facts. First, the

scatter of the points around the respective

mean values is not shown; and the standard

errors of the groups of Kp's, while almost equal

for each intensity interval (homoscedastic),

are not very much less than the standard deviation of all Kp's regardless of 5303 intensity.

Also if one examines the Kp's associated with

the low intensities, the sharp drop (nonlinear)
78 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL.!

0 4 4

n r i"1

I '' I I I I I

FIGURE 17.—Mean Kp associated with various circumstances of faint 5303 intensities for individual half-years. The columns

from left to right are 3-day running means of Kp associated with: ELP of the 10 percent weakest intensities in the favorable

hemisphere (10LQ; WLP of lOLf; lOLf at ELP or WLP, with 20Lf at the other limb; lOLf at both ELP and WLP; ELP

of the 10 percent weakest in the unfavorable hemisphere (lOLu); and lOLu at ELP or at WLP, with 20Lu at the other limb.

a, Spring half-years; b (on facing page), autumn half-years.

in mean Kp from the 10L to the next decile

would tend to decrease the coefficient of linear

correlation. In any case the results for Kp

(9,10,11) agree with those previously obtained:

Kp extremes are inversely associated with

coronal extremes, whereas intermediate intensities are at most very slightly related to

Kp.

These results are purely statistical in nature

and no physical connection can be established

between two phenomena by statistical means

alone. On the other hand, figures 10-17 do,

we believe, show several trends which might be

expected if a physical connection did exist

between Kp and the intensity of the 5303

corona in the favorable solar hemisphere:

1. Minima and maxima in Kp are, in every

sample, more pronounced following CMP of

favorable regions than of unfavorable regions.

2. In each of the figures 10-12, the 10 percent

highest indices show the greatest negative Kp

effect; these minima at Kp(10) are always

deeper than those of 20H-10H, the second

decile. Similarly, in each of the figures 14-16,
GEOMAGNETISM AND EMISSION-LINE CORONA 79

\ \

\ v

\ A

/

\,

/ \ .

FIGURE \7b.—See explanation on facing page.

the 10 percent faintest 5303 show the greatest

positive effect; these maxima at .ftp(9-10) are

always higher than those of the second decile.

3. The deepest Kp minima result when we

employ both east and west indices to select

coronal regions most certainly bright at central

meridian. Similarly, the strongest Kp maxima

result when we use both indices to select regions

most certainly weak in 5303 at central meridian.

4. The principal Kp variations occur at a

reasonable time interval, 2-4 days, after central meridian passage of the apparently associated coronal region.

Hemisphere versus angular distance.—The

above results lead to a further important question: Is the contrast between favorable and

422953—57 3

unfavorable solar hemispheres, in regard to the

apparent effect of 5303 intensity on Kp, merely

a hemisphere effect in which the solar equator

acts as some sort of barrier to the movement of

solar corpuscles? Or is the angular distance

between the earth and the bright coronal region

significant?

To explore this question we used the latitude

of the maximum intensity as an indicator of

the position of the solar center. (Dr. Yii has

informed us that Sacramento Peak line intensities

contain a zero point error of about 5° in position angle; in determining latitudes of intensity

maxima for Sacramento Peak 5303 we have

therefore assumed the solar equator to lie at

the solar position angles published as 95° and
80 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

\ .

*-« . S f

-

j

/ \ ' • r

_ (25!)

(158)

/

,-ClI—'. 1^_

'"i 1 i i i

,

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(58;

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-A

(461

e

I

1

„

\

\

\

V

—/sj

/ \

/

j

|

/

lfc

;

30 H

/ \

/

/

/

/

/

/

/

/

1 1

C85.

16:;

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i,

r*

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i i

1 I

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/

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1

(143) r~

<y

\ /

V134)

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155)

(54) /""

\ /

|

1 1

\

\

\

\

/

/

/

/ V

/

/

1 1

w

i

20L

•A

\

V

_

\

-

-\

\

\

\ -

1

4 e tz 4 6 12 It

I I I 30 H

Nf

40 M

J \

(27S1

ISC!

V.

V"-..--.-''

175) /

V

\ /

J LL

\l \l

J L

(73) ,-

r J L

FIGURE 18.—Mean Kp following ELP of various combinations of 5303 intensities in the favorable and unfavorable hemispheres;

curves in top row are for favorable hemisphere alone; those in left column are for unfavorable alone, a, Spring half-years;

b, autumn half-years.
GEOMAGNETISM AND EMISSION-LINE CORONA

> 12. 16 0 4 8

81

L_ 15H

Kp>30

1 I 1 L

60

20

40

20

a

u>\-

FIGURE 19.—Fractions of strong and weak 5303 intensities in the favorable and unfavorable hemispheres, associated with high

and low Kp't. Curves at left show fractions of strong corona (15H) associated with Kp~>30, top line, and Kp<12, bottom line.

Curves at right show fractions of weak corona (10L) associated with Kp^>30, top line, and Kp<\2, bottom line, a, Spring halfyears; b, autumn half-years.
82 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS TOUJ

FIGURE 20.—Coefficients of correlation between S3O3 intensity

and Kp, with lags in Kp of —4 to +19 days after coronal

ELP in favorable (solid line) and unfavorable (broken line)

hemispheres, for the 4-year period (top), the spring halfyears (middle), and the autumn half-years (bottom).

275°.) The days of 5303 intensity 20H,

favorable and unfavorable separately, were

divided into groups according to the location

of the maximal intensity at 5°, 10°, 15°, 20°,

>25°, from the solar equator. Those that had

no maximum on the relevant side of the equator

formed a further group, "O." For each latitude

group the associated Kp values were computed

in the usual way. Since we did not interpolate

latitudes of maximal intensity, only days with

an actual observation at Sac Peak or Climax

are included. Therefore the east-limb and

west-limb results here are based on completely

independent observations, as is not altogether

true when interpolated values are used.

Figure 22a-c shows the results from eastlimb indices, while those from west-limb

indices appear in figure 23a-c. The curves are

the average Kp's associated with 20H indices

centered at 5° intervals of solar latitude. Some

of the 25-degree curves are lacking, due to the

very small size of the sample. The mean line

for each curve has been computed from the mean

Kp's for the 8 half-years, weighted by the

number of cases occurring in each half-year.

In these figures the average latitude of the

earth may be taken as 3-4° f. Accordingly, if

the /^-minima around day 10 depend on the

angular distance between the earth and coronal

center, we should expect the largest dip in Kp

at 5° f, with the effect diminishing about

equally at increasing angular distances above

and below this position. On the other hand,

if the hemisphere effect arises from some sort

of equatorial barrier, we should expect no

systematic trends within the f-group, nor

within the u-group, but only a difference

between the two groups. The evidence appears

to favor the second alternative.

More exactly, of course, the position of the

earth varies by about 7° over a half-year.

The latitude range covered by the solar centers

(5-25°) is much greater than 7°, however, so

that unless this procedure were to give evidence

in favor of the angular-distance hypothesis,

no more detailed attention to the position of

the earth would appear worthwhile. Actually

the figures provide no indication that the

angular distance between the earth and the

maximum 5303 intensity plays a significant

role in the magnitude of the associated Kp-dip.

Coronal maxima at 5°f appear no more favorable than those at 20° or 25°f; and maxima

at 5°u appear, if anything, more unfavorable

than those at 20° or 25°u. Analogous calculations for 10H and for 30H gave results very

similar to those shown for 20H. It is relevant

further to note that one of the first really

bright 5303 centers of the new cycle, located

at 35°Nf, appeared to be associated with the

very low Kp observed on Oct. 12, 1954. We

conclude that the relative location of the solar

equator, rather than the angular distance

between solar region and earth, is the significant factor in the hemisphere effect.
GEOMAGNETISM AND EMISSION-LINE CORONA 83

£0 70 85 ICO XI

28

2«

t*

22

20

-

-

—I

1

-

-

i i

DAYS (9-tO-ll)

70 85 100 %1

FIGURE 21.—Average Kp associated with various percentiles of 5303 intensity for selected groups of days after ELP, for favorable

(solid line) and unfavorable (broken line) hemispheres, a, Spring half-years, 1951-53; b, autumn half-years, 1950-53.

Those days possessing indices in the 20H group, but

lacking any maximum in the hemisphere under consideration, remain for comment. In defining the "O"

group, we did not distinguish between days having

the maximum located at the equator and those having

it in the other hemisphere, but the latter condition is

the more common. The Kp's associated with these

"O" groups tended to have a minimum around days

9-12, with no significant difference appearing between

the favorable and the unfavorable groups. If the unfavorable index is high but has no maximum, we may

reasonably infer an even higher index in the favorable

hemisphere. Therefore a Kp minimum apparently

associated with an Hu "O" index is easily reconciled

with the hemisphere effect.

We also subdivided the 10L deciles of favorable

and of unfavorable indices into two groups according

to whether the hemisphere in question possessed a maximum in 5303. In the 10L group of course any such

maximum would be very small, only 1-2 units above

the surrounding intensities. Between these two groups

of the 10L indices we found no significant difference in

associated Kp's. Both favorable groups clearly

showed the expected maxima at Kp (9-10), in contrast

to small or negligible maxima for the unfavorable

groups.

Although the relative location of the solar

equator rather than angular distance between

earth and solar centers appears to be the significant factor in the hemisphere effect, inspection

of a number of individual recurrence sequences

reveals most often a gradual weakening of

storm intensity rather than a sudden termination of the series at the time the earth crosses

the solar equator. We shall therefore consider

next not the angular distance between earth

and solar centers but rather the angular

distance of the earth from the solar equator.

To investigate the effect of the angular distance

between the earth and solar equator we divided

each half-year into three parts: (a) the middle

group of about 75 days when the heliographic

latitude was greater than ~5.8°; (b) a group

covering approximately the second solar rotation from the crossing of the equator, when

the latitude of the earth was between 3.0 and

5.8°; and (c) the 27 days at the ends of the

period when the latitude of the earth was less

than ~3.0°. The periods were extended

to overlap one another by 2 days on either end.

Obviously no more detailed subdivision of the

earth's heliographic latitude is practical since
84 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS voi-i

1 1

-(in __.--''

OS) ', /

(34)

J17)\ / v / \

(26) / ^

(35)

~-A

1 1

,''

r\

/

v/

A / ^_

7

 A» :

/ \ *•

/

v/

FIGURE 22.—Mean Kp following ELP of bright 5303 with maximal intensity occurring at various favorable (solid line) and unfavorable (broken line) heliographic latitudes, at 5° intervals from 25°u to 25°f; for samples less than 20 the curves are 3-day running

means, a, Composite for the 4-year period; b, spring half-years; c, autumn half-years.
GEOMAGNETISM AND EMISSION-LINE CORONA 85

FIGURE 23.—Mean Kp preceding WLP of bright 5303 with maximal intensity occurring at various favorable (solid line) and unfavorable (broken line) heliographic latitudes, a. Composite results for the 4-year period; b, spring half-years; c, autumn halfyears.
86 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL.!

FIGURE 24.—Effect of angular distance of earth from solar equator; mean Kp following ELP of bright 5303 corona when the heliographic latitude of the earth is: >5.8°, line (a);<"»'3.0 to 5.8°, line (b); and <3.0°, line (c). a, Composite for the 4-year period;

b, spring half-years; c, autumn half-years.

each period should cover at least a full solar

rotation. We must give every M-region a

chance to appear in each curve if we are to

gain an accurate impression of their waxing

and waning with the change in the earth's

heliographic latitude. Group (a) may be

designated "most favorable" or "most unfavorable," with (b) "moderately" and (c)

"slightly" favorable or unfavorable.

From each of the above groups we selected

the 10H and 10L 5303 intensities, for zero days.

The associated average Kp's appear in figure

24a-c for the 10H, and in figure 25a-c for the

10L groups. To trace a sequence for the

southern hemisphere from most favorable to

most unfavorable, one proceeds in order from

top to bottom in the figure for the spring halfyears; then from bottom to top in the figure

for the autumn half-years; and conversely for

the northern hemisphere. As we should expect, from the well-known March and September maxima in geomagnetic and auroral

activity, the means for the (a) curves are

higher than the corresponding (b) means, and

the (c) means are lowest of all. Also the

favorable versus unfavorable differences are

much more pronounced in the (a) curves than

in those of (b) or (c); and the expected maxima

and minima in Kp are largest in the favorable

(a) curves.

From figure 24a-c we see that no hemisphere

effect for the bright 5303 corona appears to

exist while the earth is within 3° of the solar

equator; that the effect begins to appear

between 3° and 5.8°, and is conspicuous above

5.8°. The corpuscle-inhibiting power of an

active center would thus appear to extend

about 4-6° into the other solar hemisphere.

Figure 25a-c, for the faint corona, is especially interesting, and shows clearly what
NO. 5 GEOMAGNETISM AND EMISSION-LINE CORONA 87

a b e

FIGURE 25.—Effect of angular distance of earth from solar equator; mean Kp following ELP of faint 5303 corona when the heliographic latitude of the earth is: >5.8°, line (a); < 3.0 to 5.8°, line (b); and <3.0°, line (c). a, Composite for the 4-year period;

b, spring half-years; c, autumn half-years.

appears to be a waxing and waning of the

geomagnetically-disturbing power of M-regions

as the heliographic latitude of the earth varies

from most favorable through the equator to

most unfavorable. Within 3° of the solar

equator the two hemispheres appear to act

equally; around days 9-11 the favorable and

unfavorable (c) curves are closely parallel and

of equal height. In the interval of the second

rotation, 3-5.8°, however, the predominance

of the favorable hemisphere becomes clearly

visible in both spring and fall. The most

unfavorable curves (a) tend to lie below the

means for days 8-12, while the most favorable

ones are showing the largest peaks. We suggest that the progressive nature of the variations in Kp, especially in the case of the weak

422953—57 4

corona, provides particularly convincing support for the reality of a hemisphere effect and

an axial explanation of the seasonal variation

in Kp.

We shall defer comment until Part III on

the Kp maxima that do appear, well away

from CMP, in the most unfavorable curves.

However, we note here that they appear to

arise from the relative spacing in longitude of

the principal northern and southern solar

M-regions.

It should be emphasized that the hemisphere

effects in the relation between Kp and the

intensity of 5303 coronal emission, discussed

so far in this paper, arise from the apparent

behavior of the general solar corpuscles. Our

results suggest that the general corpuscles,
88 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL.2

i n r r

CMP -g -4.

I I I I '5 t S 8 ?'

1 1

(50) / \ A

144) s

1

 v s

1 1

1 1

FIGURE 26.—Mean Kp associated with strong 6374 intensities in favorable (solid line) and unfavorable (broken line) hemispheres.

a-c: Mean Kp following ELP of the 10 percent strongest (10H), line (a); of the 20H — 10H, line (b). a, Composite for the

4-year period; b, spring half-years; c, autumn half-years, d-f: Mean Kp preceding WLP of bright 6374 corona (10H), line (a);

and 20H — 10H, line (b). d, Composite for 4-year period; e, spring half-years; /, autumn half-years, g-i: Mean Kp associated with CMP of regions of bright 6374 corona (20H) at ELP and at WLP 14 days later, g, Composite for 4-year period;

A, spring half-years;», autumn half-years.

whatever their exact source, encounter at the

solar equator some sort of (presumably magnetic) barrier to their free movement. Of

those originating in one solar hemisphere, only

a minor portion appear able to cross more than

about 3° beyond the solar equator and to affect

the earth on the other side. Further, a bright

coronal center apparently inhibits the escape

of general corpuscles only 3-5° beyond its own

hemisphere. However, we have no evidence

as yet bearing on the presence or absence of a

hemisphere effect in the behavior of the corpuscles that appear to be emitted from the

relatively rare (in the 1950-53 period) stormproducing active centers.

The red line.—Using the indices of red line
KO. 5 GEOMAGNETISM AND EMISSION-LINE CORONA 89

1—Tin i

WLP

—

(Ml

- /

1112)

-16

1

/

j

V

1

-K -«

1 I

vs/ A\ •

i i

-4

1

\ A i

V x ^

i

i

\ 20-10L1

FIGURE 27.—Mean Kp associated with weak 6374 intensities in favorable (solid line) and unfavorable (broken line) hemispheres.

a-c: Mean Kp following ELP of the 10 percent weakest (10L), line (a); of the 20L — 10L, line (b). a, Composite for the

4-year period; b, spring half-years; c, autumn half-years, d-f: Mean Kp preceding WLP of weak 6374 corona (10L), line (a);

of 20L — 10L, line (b). d, Composite for the 4-year period; e, spring half-years;/, autumn half-years, g-i: Mean Kp associated with CMP of regions of weak 6374 corona (20L) at ELP and at WLP 14 days later, g, Composite for 4-year period;

k, spring half-years; i, autumn half-years.

intensity, we computed mean Kp values

associated with ELP of the 10 percent, 20-10

percent and 30-20 percent of the brightest

and of the faintest 6374 intensities within each

of the favorable and unfavorable groups. We

made similar calculations from the WLP indices

and computed the mean Kp's associated

with CMP of regions having the red index 20H,

and 20L, at both ELP and WLP 14 days later.

The principal red-line results are shown in

figure 26a--£ and in figure 27a-i.

These figures show immediately that Kp

appears to be much less closely related to red

line intensity than to green line intensity.

Indeed, if we had no preconception derived

from the green line results it is doubtful that
90 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS

2 4 -

FIGURE 28.—Mean Kp associated with ELP of various combinations of 5303 and 6374 intensities in favorable (solid line) and unfavorable (broken line) hemispheres. Bright 5303 (10H) taken independently of 6374, curve (a); both lines bright, 5303 (10H)

and 6374 (30H), curve (b); 5303 bright (30H) and 6374 faint (30L), curve (c); faint 5303 (10L) taken independently of 6374,

curve (d); 5303 faint (30L) and 6374 bright (30H), curve (e); both lines faint, 5303 (10L) and 6374 (30L), curve (f).

we should see any relation between Kp and

6374 intensity. Certainly there are no clearly

regular and systematic effects present in the

various samples, either the favorable or the unfavorable. Some of the samples do show

effects similar to but less conspicuous than the

green line results, but these are not present in

all cases. No hemisphere effect is distinguishable. But since any 6374-2£p relation is

slight, this particular absence need not overly

concern us.

This contrast between red line and green

line results may have special interest in the

light of certain results obtained by Athay and

Roberts (1955) from observations of the 1952

eclipse. They measured the intensities of the

5303 (Fexiv), 6374 (Fex) and 7892 (Fe xi)

lines, and of the continuum at 15 points around

the limb. They found that the continuum

intensity showed a substantially higher correlation with the intensity of 5303 (r=0.87)

than with those of 6374 (0.41) or 7892 (0.54).

This close relation between 5303 and continuum

occurs primarily in the low and middle solar

latitudes and appears to break down at high

latitudes. As Athay and Roberts point out, the

continuum, arising mainly from electron scattering, is very insensitive to changes in temperature as long as the temperature is high

enough to completely ionize hydrogen. At

coronal temperatures, therefore, changes in

the brightness of the continuum should arise

primarily from changes in the electron density.

Their results indicate the brightness of 5303

to be substantially better than 6374 as an

indicator of the coronal electron density, at

least in lower and middle solar latitudes (presumably because the corona there is near the

optimum temperature for Fe xiv).

We accordingly suggest that the difference

between the red and the green line results may

indicate that Kp, insofar as its variations arise

from the behavior of general solar corpuscles, is

closely related to the particle density in the

corona of the favorable solar hemisphere at

CMP. A high particle density in the CM-f
GEOMAGNETISM AND EMISSION-LINE CORONA 91

corona would appear to be associated with a

reduced density of general solar corpuscles in

the vicinity of the earth; whereas a low particle

density in the CM-f corona appears to be

associated with an above-average density of

general solar corpuscles in the vicinity of the

earth.

Red and green lines, simultaneously.—We

have made a number of calculations of mean

Kp's associated with various combinations of

red and green line indices, in the favorable and

unfavorable solar hemispheres. Figure 28

shows an example of the results. Curves (a)

and (d) have no interpolated values of the

5303 index and may be compared with those of

figures 12 and 16 where interpolated values are

used.

In figure 28 comparison of curve (a) with (b)

and curve (d) with (f) indicates that the 5303

index alone gives results equally as good as

those obtained when both indices are high (b)

or both are low (f). When the two indices

differ, one being high, the other low, the Kp's

tend to behave as previously found for the 5303

index alone. These results do not altogether

agree with those shown in Part I, figure 5a, b, and

may therefore be regarded with some reservations. However, using independent west-limb

indices, we obtained results very similar to

those shown in figure 28. We tend to feel,

therefore, that figure 28 gives the more accurate

picture and that some of the results of figure

5a, b may have arisen from neglect of the hemisphere separation.

The yellow line.—Because of the small

sample size, we have made no hemisphere

calculations for the yellow line cases. Of the

22 occurrences observed during the period

under study, 5 appeared in the favorable

hemisphere and 17 in the unfavorable. Therefore the Kp rise shown in figure 6 must be

associated in large part with unfavorable 5694

regions, an inference confirmed by inspection

of the individual cases. Our sample provides

no evidence for any hemisphere effect in the

Kp's associated with yellow line regions.

We therefore tentatively conclude that the

motions of those storm-producing corpuscles

apparently emitted by active centers are not

significantly restricted by the equatorial barrier, which appears to act on the general solar

corpuscles.

Part III. Inverse solutions for hemisphere

effects

In order to study the average distribution of

5303 intensity as a function of distance from

the solar central meridian at the time of geomagnetic storms, we prepared an inverse deck

of IBM cards which contain Kp(0), and the

sum of the east-limb 5303 index and the westlimb index 14 days later, for days CMP —8

through + 9 (ELP —1 through +16), for the

northern and for the southern solar hemispheres

separately. We had two objectives: to investigate the coronal conditions associated with

nonrecurrent sc-storms and with recurrent

storms from the point of view of favorable and

unfavorable solar hemispheres; and to study

further the apparently distinctive coronal

intensity pattern characterizing M-regions.

For Part I we compiled a list of 38 geomagnetic storms observed to have a sudden

commencement (sc) at ten or more observatories. We excluded six storms with the

requisite "sc" that were members of prominent

recurrence sequences. The group of 38 does,

however, contain several sc-storms that appear

to recur although they are not members of the

most striking sequences. Figure 29a, b shows

the average favorable and unfavorable intensity patterns associated with onset of these

38 sc-storms, in spring and fall respectively.

Day zero is the day of the "sc."

We also removed from the deck all high

Kp's that appeared to be part of one of the 38

sc-storms. The remaining high Kp's thus belonged to storms without a widely observed

"sc"; they may perhaps be considered to be

of the M-region and "general corpuscle" type

even if not all of them are clearly recurrent.

The resulting average 5303 intensity distributions associated with zero day defined by

Kp > 32 (except for the 38 sc-storms) are shown

in figures 29c, d. Because the means of the

north and of the south intensities are often

quite different we did not superpose the

favorable and unfavorable curves, but the

scales were shifted vertically to bring the means

of the two curves for a half-year on more or

less the same line.
92 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL. X

Figure 29a-d indicates a marked difference

in the coronal patterns associated with the

nonrecurrent and with the recurrent type of

storms, particularly in the favorable solar

hemisphere. The occurrence of nonrecurrent

sc-storms appears to be associated with the

presence of above-average coronal intensity

around the central meridian of the sun although

as shown in figure 7, the time lag does not

appear to be precisely defined. The curves

give no clear evidence for the presence or absence

of a hemisphere effect in the case of sc-storms.

Such limited evidence as we possess would appear to contradict rather than to favor the

1

M

It

IC

. 1

> A

i

1 I

i ' i

1

\ / \/

i

02) /

1

\ t \

1

1

1

€ d

FIGURE 29.—a, b: Mean pattern of S3O3 intensity in favorable (solid line) and unfavorable (broken line) hemispheres, associated

with onset of geomagnetic storms of sc-type. a, Spring half-years; b, autumn half-years, c, d: Mean pattern of 5303 intensity

in favorable and unfavorable hemispheres, when Kp is greater than 32 and not part of sc-storm; curves at top are composites

of the 4 half-year curves below, c, Spring; d, autumn.
GEOMAGNETISM AND EMISSION-LINE CORONA 93

hypothesis of a hemisphere effect in such

storms.

In the spring (figure 29a) both the favorable and

unfavorable solar hemispheres show above-average

intensity west of the central meiidian at the time

of the "sc," but the rise in the unfavorable curve occurs

nearer to the expected time. In the fall only the favorable intensity is above average to the west of the central

meridian but both are high on the eastern hemisphere

at the time of the "sc." In this connection we note

that during 1950-53 the north solar hemisphere may

have had more highly active centers than the south;

for example, of the 22 yellow line cases, 15 appeared

in the north and only 7 in the south. Because of the

small number of sc-storms during the years 1950-53,

it is not practical to have more than two independent

samples, and, since these do not agree, our results

must remain inconclusive. Like the yellow line studies,

however, figure 29a, b, provides no clear evidence for

a hemisphere influence in the behavior of active center

corpuscles.

Looking next at figure 29c,d we see that, as

expected, the occurrence of M-region storms

tends to be associated with the presence 2-4

days west of the central meridian of belowaverage 5303 intensity in the favorable solar

hemisphere. This coronal minimum in the

favorable hemisphere appears in seven of the

eight periods; it is absent only in the spring of

1950, a period lacking clear recurrent sequences,

as previously noted. Since all cases of

Kp>32 (except the 38 sc-storms) were used

to define zero days in figure 29c,d, the time lag

is not as precisely defined as when we use only

the day of storm beginning. The 5303 intensities of the unfavorable solar hemisphere show

no consistently recurring pattern.

M-regions.—The identification of Bartels'

M-regions, the hypothetical solar sources of

the corpuscles producing the long series of

recurrent geomagnetic storms, has long been

a perplexing problem. The results from Parts

I and II indicate that, during the years 1950-53,

M-regions were characterized by unusually

weak emission in the 5303 coronal line in the

favorable solar hemisphere. The weakness of

5303 appears to indicate a region of lowerthan-average density in the corona. With

figures 30-32 we may explore further some of

the apparent properties of M-regions as indicated by the intensity of the 5303 emission.

First, we have calculated the favorable and

unfavorable intensity patterns associated with

nine of the more prominent recurrence sequences

in the 1950-53 period, with zero day defined by

the onset of the storm. When originally

selecting these sequences, we divided them

into Class I and Class II, according to their

impressiveness and the degree of certainty we

felt in identifying them. Figure SOa-d shows

the patterns of 5303 intensity associated with

the nine sequences identified as follows:

Class I (figure 30a, 6):

(a) 11 July, 7 Aug., 3 Sept., 1 Oct., 28 Oct., 25 Nov.,

1950, Nf.

(b) 15 Dec, 10 Jan., 6 Feb., 3 Mar., 30 Mar., 28

Apr., 26 May, 1952, Sf.

(c) 1 July, 28 July, 24 Aug., 20 Sept., 17 Oct.,

12 Nov., 8 Dec, 1951, Nf.

(d) 24 Feb., 23 Mar., 19 Apr., 15 May, 1953, Sf.

(e) 29 June, 26 July, 23 Aug., 19 Sept., 15 Oct.,

12 Nov., 1953, Nf.

Class II (figure 30c, d):

(f) 7 Sept., 3 Oct., 30 Oct., 26 Nov., 1952, Nf.

(g) 12 JaD., 8 Feb., 6 Mar., 2 Apr., 1 May, 1951, Sf.

(h) 5 Jan., 1 Feb., 27 Feb., 24 Mar., 21 Apr., 18

May, 1952, Sf.

(i) 24 July, 19 Aug., 16 Sep., 14 Oct., 10 Nov.,

6 Dec. 1950, Nf.

The dates listed were used in our calculations

as "date of onset of storm" or zero days in

figures 30a-d. They include the most prominent storms of each sequence, and may, but

do not necessarily, include all of the lesser

storms that appear to belong to some of the

sequences. Also, the zero days used for each

sequence in these calculations were confined

to a 6-month interval within which a given

solar hemisphere was favorable.

Each of the five recurrence sequences of

Class I (figure 30a, b) shows a weak area in the

5303 corona in the favorable solar hemisphere

0-3 days west of central meridian at the time

of storm onset. No pattern appears systematically in the unfavorable hemisphere corona.

For Class II sequences the picture is less clear.

Only sequence (f) shows an entirely satisfactory

coronal pattern. Sequence (h) has a conspicuously weak 5303 region in the unfavorable

solar hemisphere west of CM but no minimum

at the expected position in the favorable

hemisphere. We feel some doubt whether

sequence (i) is altogether a true M-region

sequence, arising from general corpuscles. It

contains 3 sc-storms (included among the 38),

and one of the few "great" storms (19 Aug.)

of the four years under study.
94 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL.2

-onACLASS I Sf

—

/ \

~/ f V - //

V y

CM

' \

\

\

\

A •

A

J

1

j

M*_ a.

I • • »

7 ^~

,*.

/\

j

1

A

A/ v

I

•

(4) •

U

1 1

•

\

\

\

CM 1L

V

I

\

|

V

- \

I

\ —

\

_

, —

-

I \

! \

\

5

\

\

\

\

~

a b

FIGURE 30.—Mean pattern of 5303 intensity in favorable (solid line) and unfavorable (broken line) hemispheres, associated with

onset of storms in selected recurrence sequences; curves at top are composites of the individual M-regions below, a, Major

recurrence sequences in spring half-years; b, in autumn half-years; c, recurrences of intermediate importance in spring halfyears; d, in autumn half-years.

Sequences (a) and (b) have been discussed by

others. Smyth (1952) found from Climax data

that the intensity of 5303 showed a minimum

on the east limb about eight days before the

onset of the storms of sequence (a). Bruzek

(1952) found a similar result from Wendelstein

data, for this sequence as well as for two sequences in 1942-44. Both of these results are

in good agreement with our findings. (Maxwell (1952) also has discussed this sequence.)

Trotter and Roberts (1952) and Muller

(1953), discussing sequence (b), found evidence

for the return of the eastern hemisphere effect

of Shapley and Roberts. This sequence is

responsible for the apparent eastern hemisphere

effect showing in the 1952 unfavorable curve of

figure 13a; the corresponding coronal peak is

conspicuous in figure 30a (unfavorable curve).

Von Kluber (1952) has suggested a possible

connection between sequence (b)—in particular
HO. 5 GEOMAGNETISM AND EMISSION-LINE CORONA 95

FIGURE 30C, d.—See explanation on facing page.

the storm commencing 3 March—and the CMP

of the prominent white light coronal streamer

observed at the 25 Feb. 1952 solar eclipse.

More generally, Allen (1944), von Kliiber

(1952), and others have suggested that the white

light streamers may be a manifestation of the

corpuscular streams responsible for recurrent

magnetic storms, and hence a property of Mregions. Since the white light corona has been

observed to date only during total solar eclipses,

there has been little opportunity to test this

hypothesis.

Although its exact longitude is uncertain, the

streamer of 25 Feb. 1952 could have passed the

central meridian at a time appropriate for the

storm of 3 March. However, the streamer is

centered at about 20° N, whereas the southern

solar hemisphere is favorable for this sequence

of storms and contains a region of weak corona

at the proper location. Inspection of eclipse

photographs (see Biesbroeck, 1953, figs. 5, 6)

indicates that the streamer, whatever its nature,

was pointing in a direction very unfavorable

for intercepting the earth in the spring. At

most, the earth might pass near its edge, and

even this looks doubtful. On the other hand,

the storm sequence in question was one of the

most impressive of the four years (see fig. 31a).

We consider it improbable that one of the most

impressive sequences of this cycle should arise

from so unfavorably directed a streamer.

What could we then expect from a favorably
96 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL.*

located streamer? Therefore, and because of

our other results, we doubt that any physical

connection exists between the white light

streamer of 25 Feb. 1952 and sequence (b). Also,

since there is no streamer in a favorable location, we are inclined to reject Allen's original

hypothesis that white light coronal streamers

are a characteristic of M-regions.

On the other hand, according to the hypothesis that seems to be indicated by the relation of

recurrent storms to green line weakness, we

should expect a region of relatively low coronal

density to exist somewhere on the SE limb at

the time of the February 1952 eclipse. The

results of Hepburn (1955) on the variation of

electron density with position angle at this

eclipse, appearing in the interval between the

completion and the publication of our paper,

now make possible a check on this expectation.

It can be seen from her figure 10 that the electron density is indeed relatively low along the

entire SE limb, and especially in the region

15-30° S. Because of our other results, we

therefore suggest that this area of relatively low

electron density, rather than the streamer at

20° N, may be the M-region associated with

the geomagnetic storm of 3 March and with

sequence (b).

Sequence (e) is the one the Babcocks (1955)

have suggested may arise from a magnetic unipolar region that passes the solar central

meridian 1-4 days before the onset of the storms

of this sequence. As they point out, their observations cover too short a period to permit

any definite conclusions from the observed coincidence. We do note, however, that the

unipole was in the northern or favorable solar

hemisphere. Also a second, less conspicuous,

unipole, not followed by any geomagnetic

storms, would appear to be a southern one,

which could account for the apparent failure

of this unipole as an M-region.

Figure 31a,6 shows half-year averages of Kp

over 27-day periods, with day zero corresponding to the zero days of Bartels' magnetic rotations (5 Jan. 1950, etc.). Note that the Kp

scale is half that used in Parts I and II. The

nine individual recurrence sequences stand out

more or less clearly. Spring and fall are

plotted separately to emphasize a characteristic

of the present cycle first noticed by Bhargava

and Naqvi (1954). (See also Naqvi and

Bhargava, 1954.) They pointed out the two

very long sequences of geomagnetic activity

that appear clearly in our figure 31a,6. Their

longest sequence, which maximizes in September, comprises our peaks labeled (a,c,f, e), connected by broken line (A) ; their second sequence,

maximizing in March, shows as our peaks (b,d),

connected by broken line (B). From the displacement of the successive annual peaks we

estimate periods of 27.08 days for sequence A

and of 27.1 days for B. Bhargava and Naqvi

found the mean period between individual recurrences for each sequence to be 27.05 days.

Corresponding half-year averages of the

coronal index appear in figure 32a for the

southern hemisphere and figure 326 for the

northern hemisphere. The time scale is displaced relative to that of figure 31a,6 to correspond to a 3-day lag between coronal CMP

and the expected associated Kp.

Several points of interest may be noted in

figures 31 and 32. A region of weak 5303

corona (A) in the north (figure 326) appears

to extend, in approximately the same position,

from mid-1950 through 1953. Only when the

north solar hemisphere is favorable, however,

does storm sequence A appear in a conspicuous

manner. Similarly, a weak coronal region (B)

in the south appears in approximately the same

position from mid-1951 through 1953; but only

in the spring when south is favorable does the

magnetic-storm sequence B appear prominently.

Note particularly that the areas of weak corona

persist without significant change through their

unfavorable periods, but that in each case the

magnetic storm sequences weaken markedly or

even disappear when the apparently associated

solar hemisphere becomes unfavorable. The

solar M-regions appear to persist substantially

unchanged in coronal pattern and thus, by

inference, in intrinsic character and influence

on solar corpuscles; whereas the position of

the earth varies between favorable and unfavorable for the interception of their corpuscles.

The behavior of these sequences appears to

demand an axial explanation for the March

and September maxima in geomagnetic activity.

Moreover, as Bhargava and Naqvi first suggested, rather than a semiannual period in geomagnetism, we should properly speak of two
GEOMAGNETISM AND EMISSION-LINE CORONA 97

annual periods overlapping and out of phase

with each other by six months.

To compare the curves in figures 30a1 and 316

showing sequence (f), we have labeled two

other Kp peaks, (y) and (z), which fit nicely

into the coronal minima to either side of the

(f) minimum. Magnetic peak (y) has a coronal

minimum in the unfavorable hemisphere also,

corresponding to M-region B ; plots of the daily

Kp-sum against time indicate that sequence

(y) is stronger in January and May and weakest

around March, so that it appears to be a part

of sequence B. Similarly magnetic peaks (x)

and (w) appear to be parts of sequence A,

weakened when the earth passes into regions

relatively unfavorable for the interception of

corpuscles from M-region A.

With the two major M-regions, A and n, in

mind, we suggest the reader look again at figures

14-16 and 25 in Part II. The locations of the

Kp maxima associated with ELP of unfavorable

weak corona can now be readily accounted for

without postulating any actual relation between Kp and the very unfavorable weak

corona. The unfavorable maxima in Kp occur

in approximately the positions we should expect as a consequence of the relative locations

of coronal regions A and B. Similarly the

coronal curves (figure 32) for the first half of

1953 reveal why we should expect a minimum

in Kp in figure 13a for 1953 unfavorable as

well as favorable, and why the former should

be less sharp than the latter.

In concluding this section let us return for a

moment to examine quantitatively the principal characteristics of figures 24 and 25. For

each of the curves we have computed Kp

(8-12); from this quantity we subtracted the

mean plotted Kp. We thus define AKp=

Kp (8-12)-Kp. In figures 33a and 336 we

have plotted AKp and Kp (8-12), respectively,

against (10 sin <£)3, where <f> is the average

heliographic latitude of the earth, taken as

positive when favorable and negative when

unfavorable. The quantity AKp shows a close

linear relation with (10 sin </>)3 and yields the

regression equations and correlations

AKp(H) = —1.98-1.40 (10 sin «^)3,r2=0.94,

(7)

AKp(L) =2.08+2.22 (10 sin «)3,r2=0.91.

A sin2 <t> relation gives about equally good correlation but requires an artificial introduction

of negative sin2 <£'s for the unfavorable latitudes.

We suggest that the good fit of the above

equations provides further significant support

for the hemisphere effect in the relation between

Kp and the intensity of 5303 emission, and for

the hypothesis of a substantial axial contribution to the semiannual variation in Kp and

in the aurorae.

The possibility of some additional contribution from equinoctial conditions in the earth's

atmosphere is, however, suggested by the plot

of Kp (8-12) against (10 sin</>)3 (figure 336).

The linear relationship with sin3<£ shown by

AKp is here distorted by the end points, all of

which appear to lie too high. If we subtract

the constant amount of 2.5 from each of the

four end points, we then obtain the relations

Kp (8-12) (H) = 19.2-1.37 (10 sin <£)3

7^=0.95, (8)

Kp (8-12) (L) = 23.2+2.23 (10 sin <f>)3

r*=0.94.

These slopes are very close to those in the AK^

equations. Note that in figure 336 the end

points are plotted in then* observed positions

so that they deviate conspicuously from the

regression lines.

We hope to explore these relations between

Kp and sin<£ more fully in a later paper.

However, we tentatively suggest that equations

(7,8) may represent the axial contribution

while the 2.5 subtracted from each of the end

points before we solved equation (8) may

represent the equinoctial contribution to the

semiannual variation in Kp.

Other periods: 1H2-U, 19U~49— While we

feel that the results for 1950-53 are quite convincing, a word of caution is in order at this

point. It should not be forgotten that, in any

analysis which covers only four years, the possibility of coincidences between two series of

phenomena, each with a 27-day recurrence

period, can not be altogether ruled out. However, geomagnetic quiet following CMP of

bright corona has now been found by a number

of investigators covering the period 1942-46

and 1950-53. An apparent association of Mstorms with coronal minima has also been
98 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL.J

'950 , 0 _

_ (953 a _

FIGURE 31.—Half-year averages of Kp over 27-day periods, showing major long-lived recurrence sequences, (A) and (B), with annual

periods of intensity variation; small letters identify individual half-year sequences with those in figures 30a-d and 32a, b. a,

Spring half-years; b, autumn half-years.
GEOMAGNETISM AND EMISSION-LINE

FIGURE 32.—^Half-year averages of 5303 intensity over 27-day periods, showing long-lived regions of weak corona, (A) and (B),

which correspond to storm sequences (A) and (B) in figure 31. The coronal curves are represented by a solid line when in the

favorable, and by a broken line when in the unfavorable solar hemisphere. Small letters identify half-year coronal features

associated with the Kp recurrence sequences in figure 31; the time-scale is displaced by 3 days relative to that of figure 31.

a, Spring half-years; b, autumn half-years.
100 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS vout

FIGURE 33.—Seasonal variations in Kp. a, Deviation from mean of Kp for days 8-12, after ELP of bright (x) and faint (•) 5303

corona, plotted against heliographic latitude of earth; favorable hemisphere taken positive, unfavorable taken negative, b,

Kp for days 8-12, plotted in the same way.

found for 1943, by Bruzek (1952). On the

other hand, the hemisphere effect has not been

previously noted, and if we examine Bruzek's

three M-regions we find that two of them

(1943/b and 1950) have the coronal minimum

in the favorable solar hemisphere, while for

the third (1943/a) the principal part of the

5303 minimum lies in the unfavorable hemisphere but extends also about 20° into the

favorable hemisphere.

To investigate this earlier period with our

hemispheric index, we computed the indices

for 5303 from Climax data from August 1942

(when Climax began observing) through 1949,

for the northern and southern solar hemispheres separately. These indices were corrected for plate threshold. To date no interpolations have been attempted except for

1942—44. This earnest period has special

interest here because of its correspondence, in

phase of the solar cycle, to our 1950-53 period.

For these two years, we therefore tried to fill

in some of the gaps in Climax indices, at least

approximately by reference to data from Arosa,

Pic du Midi, and Wendelstein. Thus in the

1942-44 period the locations of the principal

bright coronal regions seem reasonably well

defined. The coronal minima, on the other

hand, are less satisfactorily located. On a

substantial number of days the 5303 corona

was below the threshold of visibility at Climax,

and in general, as Kiepenheuer (1947) has

remarked regarding faint Wendelstein values,

the weak values are more uncertain because of

scattered light.

We divided the coronal data into three

periods according to phase of the solar cycle^—

Aug. 1942-6 June, 1944 (declining phase); 7

June, 1944-7 Dec, 1946 (rising phase); 8 Dec,

1946-8 Dec, 1949 (maximum phase)—and

performed the ordinary superposed epoch

analyses, based on relative intensities within

each half year. The results for the period from

mid-1942 to mid-1944 are shown in figure

34a-/.

In figures 34a-c the "eastern hemisphere

effect" found by Shapley and Roberts for this

period appears conspicuously in each of the

curves, favorable and unfavorable, which are

remarkably parallel. The expected Kp minima

around 9-11 days after ELP also appear, with

no significant difference between favorable and

unfavorable bright corona.

In figures 34d-/, CMP of faint corona is

followed by above-average values of Kp,

again without significant difference between

favorable and unfavorable weak corona. In

the autumn curves the unfavorable weak

corona actually appears to give the better

results. The curves of figure 34/ are determined more by the autumn of 1943 than of

1942, and appear to be in accord with the

results of Bruzek. The sequence he calls

1943/b appears as the small rise after CMP in
GEOMAGNETISM AND EMISSION-LINE CORONA 101

the favorable curve; while his sequence 1943/a,

the principal sequence of the two years, appears as the rise in the favorable curve with its

peak on day —2 and in the unfavorable curve

with a peak on day 11. Bruzek found his

sequence 1943/b associated with a weak coronal

area which appears to lie primarily in the

northern solar hemisphere; while the storms of

his 1943/a sequence appeared to be associated

with a southern weak region which, however,

extended about 20° into the northern solar

hemisphere.

Thus, while the results from the period

1942-44 are not clearly incompatible with a

hemisphere effect, they provide no evidence in

favor of such an effect. The final answer to

this question must therefore wait, probably

until the decline of the next solar cycle and the

appearance of a new series of M-regions. For

the present, the best evidence in support of a

hemisphere effect is, we suggest, the waxing

and waning with annual periods of the two

long-lived M-sequences, A and B (figures 25,

31 and 32), while their respective coronal

minima remain more or less constantly present

throughout both favorable and unfavorable

periods. It may be worth noting that 1950-53

was a period of more numerous M-sequences

and higher average Kp than the corresponding

phase of the previous cycle.

Our study of the rising and maximal phases of the

past solar cycle, 1944—49, is only preliminary and we

shall defer any detailed discussion. It seems fairly

clear however, that the association between regions of

weak corona and high Kp does not appear in any

sytematic and convincing way in years of high solar

activity. In these years even the "weak" corona is

rather bright. Perhaps this relation appears only when

the intensity of the "weak" corona falls below some

particular level on an absolute scale.

The tendency to a Kp minimum following CMP of

bright corona continues to appear in the 1944-46

period. In the maximum period we find a tendency,

in the favorable curves at least, for the CMP of the

•4 C

1 1

~<56)

(63) V"-

1

\

1 1

/J

1

1

16

1

V-

-

1

FIGURE 34.—Mean Kp following ELP of selected coronal intensities in favorable (solid line) and unfavorable (broken line) hemispheres for the period 1942-44. a-c: For bright 5303 corona: a, mean for 2-year period; b, spring half-years; c, autumn halfyears, d-f: For faint 5303 corona: d, mean for 2-year period; e, spring half-years; /, autumn half-years.
102 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL.*

top 10 percent of 5303 to be followed by enhanced Kp,

while the 20-10H group shows a decrease in Kp. The

tendency, which ia merely suggestive and not present

in every half-year period, should not, however, be

surprising. Waldrneier (1939, 1942) found an association between bright 5303 corona and geomagnetic

storms in 1939, nearer to sunspot maximum than most

investigators have worked. Kiepenheuer (1947) found

that CMP of "very strong" corona tended to be

followed by high Kp, whereas moderately strong corona

was followed by diminished Kp. Also the work of

Denisse (1952, 1953) has made it clear that active

centers can act on geomagnetism in two ways: some

centers appear to inhibit geomagnetic activity while

others appear to produce storms. It seems probable

that in years of high solar activity we cannot hope to

attain particularly reliable results from a simple 5303

coronal index, and that radio noise data and yellow-line

observations will be important in separating the two

types of coronal centers.

The yellow line.—Our previous results (Bell

and Glazer, 1954a, and Part I of this paper)

and those of Denisse and Simon (1954) have

indicated that CMP of regions of yellow line

emission tends to be followed by enhanced

geomagnetic activity. To investigate further

CMP -4

the possibility of a hemisphere effect in activecenter corpuscles, we used the appearances of

the yellow line, observed at Climax and Sac

Peak, 1946-51, and listed by Dolder, Roberts,

and Billings (1954). Figure 35a-c shows the

average Kp's associated with CMP of these

yellow line regions. The tendency for aboveaverage Kp following CMP of yellow line

regions appears to be about the same whether

the region lies in the favorable or the unfavorable solar hemisphere. The peaks in the

favorable curves occur at CMP+2, while those

in the unfavorable curves appear at CMP +4,

a difference in time lag which may have some

significance since it appears in both spring and

fall samples independently. Since the samples

are small, however, any conclusions should be

regarded as tentative.

In table 3 are given the percents of Kp>3Q

and the percents of Kp>40 associated with

the favorable and with the unfavorable appearances of the yellow line.

a b e

FIGURE 35.—Mean Kp associated with CMP of regions in which the yellow line was observed, divided according to location in the

favorable (solid line) and unfavorable (broken line) hemisphere, a, For the period 1946-51; b, spring half-years; c, autumn

half-years.

TABLE 3.—Percents of high Kp associated with f and u yellow line.

Day - 6 -5 -4 - 3 - 2 - 1 0 +1 +2 +3 +4 +5 +6 +7 +8

%Kp>30 f 9 5 5 23 23 14 18 32 50 27 27 27 36 41 23

u 15 8 12 8 12 8 12 31 19 31 31 19 12 4 8

%Kp>4 0 f 40049094 9 1 4 1 8 99 4 9

u 440400048 4 1 2 40 0 0
GEOMAGNETISM AND EMISSION-LINE CORONA 103

For the entire period, 18 percent of 2if;p>30,

and 4.7 percent of i£p>40. For CMP (—5

through —1) the average percents of Kp>30

are 14 and 9 for (f) and for (u) respectively,

while for CMP (+1 through +5) we find 33

and 26 percent. The contrast between preand post-CMP days is nearly the same for each

hemisphere. The difference between favorable

and unfavorable results is not statistically significant and is actually less than similar numbers would show for the advantage of north

(14, 37 percent) over south (8, 17 percent).

See also figures 356 and 35c.

These yellow line results support a tentative

conclusion that active-center corpuscles show

no significant hemisphere effect.

Summary and interpretation

Since the present investigation, like others

in this field, is statistical in nature, we emphasize that no physical connection between any

solar and geomagnetic phenomena has been

demonstrated. We have examined the relation between two sets of phenomena, each of

which shows a recurrence period of 27 days.

Therefore, whether or not any causal connection

actually exists, an apparent relation might be

found, particularly for any short time interval

in which C-regions and M-regions are about

equal in number.

While no physical connections have been or

can be demonstrated by statistical procedures

alone, the available evidence does suggest certain connections. The principal effects occur

at a reasonable time lag after CMP of regions

of bright and faint coronal intensities. The

apparent Kp effects are greatest for the extreme

(10H, 10L) deciles of green line intensity; they

are enhanced when the coronal regions, bright

or weak, are located with the greatest certainty

by using east-limb and west-limb indices simutaneously. The homogeneity of the curves for

the various samples is an important indicator

of the likelihood that the relationship between

coronal and geomagnetic conditions is real.

Agreement of our results with those from other

solar cycles should also be considered. Our

principal conclusions, based primarily on study

of the 1950-53 period, are summarized below.

Solar M-regions appear to be distinguished by

unusual weakness of the green coronal-line

emission. In contrast to the previously noted

(Allen, 1944) characteristic absence of sunspots,

this weakness in the coronal line often permits a

fairly definite localization of the M-region in

solar longitude. Because of the high correlation between green line and white light coronal

intensities in spot-zone latitudes (Athay and

Roberts, 1955), we infer that M-regions may be

characterized by a relatively low coronal

density. This inference receives direct support

in the one instance where a test is available.

We believe that M-regions should not be

identified with white light coronal streamers.

They may possess weak unipolar magnetic

fields (Babcock and Babcock, 1955). We

suggest that an M-region permits more or less

continuous escape of general solar corpuscular

radiation along its meridian of longitude as long

as its green coronal emission remains weak.

The efficacy of an M-region in producing geomagnetic storms is greater, however, when the

earth is on the same (favorable) side of the

solar equator as the M-region. Other things

being equal, the efficacy of an M-region appears

greatest when the earth is around its maximum

favorable heliographic latitude, and least when

the earth is near its maximum unfavorable

heliographic latitude.

When the earth is within about 3° of the

solar equator, favorable and unfavorable Mregions seem to be about equally efficient, while

beyond this distance the storm-producing power

of an unfavorable region falls off markedly.

This hemispheric tendency would appear sufficient to account for most of the semi-annual

variation in Kp and in aurorae; however, the

possibility of some additional contribution from

equinoctial conditions in the earth's atmosphere

should not be excluded.

While unusual weakness in the green coronal

line indicates a condition peculiarly favorable

for the escape of general solar corpuscles, the

bright green line conversely appears to distinguish regions that inhibit the escape of

general corpuscles. The inhibiting power of a

bright C-region acts primarily on the corpuscles

of its own solar hemisphere but shows no

discernible relation to the heliograpliic latitude

of maximal intensity or to the angular distance

between earth and C-region. Relative to the

mean level of activity, the inhibiting power
104 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS TOUt

appears to vary systematically with the angular

distance of the earth from the solar equator,

being greatest when the earth is at maximum

favorable and least at the most unfavorable

heliographic latitude. This systematic variation arises as much from change in the general

level of geomagnetic activity as from apparent

variation in actual inhibiting power. Near the

equator favorable and unfavorable C-regions

inhibit about equally, while the inhibiting power

of an unfavorable C-region tends to disappear

as the earth moves more than about 5° beyond

the solar equator.

We suggest that every bright coronal center

tends to inhibit the escape of the general solar

corpuscles arising along its meridian of longitude

and on its side of the solar equator. However,

CMP of some C-regions appears to be associated with nonrecurrent geomagnetic storms.

These centers, by virtue of their more violent

activity, appear able to eject streams of what

we have called active-center corpuscles. These

centers tend to be distinguished by yellow lin

emission, large and/or frequent flares, and enhanced radio noise, and to be most important

in years of high sunspot activity. We find no

evidence for a hemisphere effect in this type of

storm, although because of the small sample

sizes this conclusion must be more tentative

than some of the others.

It seems clear that geomagnetic storms may

properly be divided into two categories, according to their origin from (1) the general solar

corpuscles from M-regions, or (2) the activecenter corpuscles from C-regions. This dichotomy based on source of corpuscles corresponds

at least roughly to previous dichotomies based

on the tendency to recurrence, sudden commencement, etc. However, we should be inclined to put into the "general corpuscle" category most, if not all, storms that cannot reasonably be associated with some solar feature or

event, regardless of recurrence or "sc" properties.

It is generally agreed that Kp is a measure of

the solar corpuscular radiation and that large

Kp indicates a relatively high density of solar

corpuscles in the vicinity of the earth. The

evidence presented in this paper suggests that

extremes of high or low coronal density tend to

be inversely associated with the density of general solar corpuscles in the vicinity of the earth.

CMP of a solar region of relatively high coronal

density tends to be associated with a relatively

low corpuscular density in the vicinity of earth;

and, conversely, CMP of a solar region of unusually low coronal density tends to be associated with a relatively high corpuscle density

around the earth.

These conditions may be most obviously interpreted by assuming that corpuscles arise—

perhaps from solar spicules—more or less uniformly and constantly over the entire solar surface. M-regions occur, when the local force

fields are favorable for the acceleration and escape of the corpuscles, as electrically neutral

streams of ions and electrons. When the local

force fields are not favorable, the corpuscles are

trapped and C-regions of increased density are

built up. The Babcocks (1955) have suggested

that these two types of local force fields may be

identified respectively with the weak unipolar

and the bipolar magnetic regions they have observed on the solar surface.

The Babcocks have also pointed out that if

unipolar regions in low latitudes are conducive

to the escape of corpuscular streams, then the

sun's polar regions, with their weak general

fields, may be even more copious sources. They

note that the lines of force of the general solar

magnetic field, although distorted by^ the moving ionized clouds, should tend "to guide diffuse

corpuscular streams ejected from the polar regions to the general vicinity of the equatorial

plane."

Following this line of thought, we should

like to suggest further that these high latitude

regions may be the major source of general

solar corpuscles that reach the vicinity of the

earth. We would postulate that corpuscles,

as electrically neutral streams of ionized gases,

are regularly ejected from the high latitude

regions of the sun. The paths taken in space

by these polar corpuscles are determined

primarily by the general and local magnetic

fields of the sun. If unperturbed by local solar

fields, the general field tends to direct the

corpuscular streams into, or near to, the plane

of the equator where they often are able to

strike the earth. Local magnetic fields, in

the spot zones, influence the paths of corpuscles

along and near their meridian of longitude.
GEOMAGNETISM AND EMISSION-LINE CORONA 105

These magnetic centers tend to deflect the

corpuscles from their normal paths, perhaps

guiding a substantial portion into their own

vicinity where these corpuscles add to the

coronal density and become visible as enhanced brightness of the 5303 line and as Hafilaments. For the most part, active centers

guide into their own neighborhood corpuscles

ejected from the high latitudes of their own

hemisphere, although very large and powerful

centers conceivably may influence the paths

of corpuscles from the other pole. The density

and energy of the ejected high-latitude corpuscles, as well as the pattern of the controlling

magnetic field, probably varies during the 11-

year cycle of solar activity.

This theory is strictly phenomenological,

developed to explain our observational results.

It does not specify any particular mechanism

for ejecting the particles or accelerating them

to the required velocities of 500-5000 km/sec.

However, let us now consider more specifically

how this "polar source" idea might apply to the

various observational results.

M-storms, we postulate, occur when the

polar corpuscles are deflected only by the

general magnetic field of the sun. The center

of the corpuscle stream from a given pole is not

deflected as far as the equator. The earth

therefore intercepts a greater density of corpuscles when it is near its maximal heliographic

latitude and intercepts fewer corpuscles when

it is near the plane of the solar equator. The

27-day recurrence in geomagnetic activity

results simply from the recurrence in the spot

zone of undisturbed regions, which are necessary to permit the corpuscle streams to escape

to the vicinity of the earth. These undisturbed

regions are thus characterized by weak 5303

emission precisely because they are unable to

trap any high-latitude corpuscles with which

to increase their density above that provided

by their underlying surface.

A C-region, on the other hand, exists largely

because its magnetic field has concentrated

the corpuscles ejected on and near its meridian

of longitude. The corpuscles, thus concentrated, appear as regions of higher density;

they cannot escape and intercept the earth and

Kp is diminished. The "polar source" theory

seems to us to explain—more easily than a

direct spot-zone theory—the apparent independence of inhibiting power and angular

distance between earth and bright center.

We further suggest that this picture of Cregions is compatible with photographs by

Lyot of the edge of the sun in the light of 5303

and 6374 (see Kiepenheuer, 1953, p. 426).

These photographs show that the emission-line

corona possesses a marked filamentary structure, not radically different perhaps from

that of the Ha prominences. General corpuscles from the unfavorable hemisphere do not

normally cross the solar equator more than

about 3-5° and are thus less significantly

related to geomagnetic conditions.

Statistical procedures alone, of course, can

merely suggest relationships between Kp and

solar phenomena. The most accurate predictions of geomagnetic conditions cannot be

obtained without theoretical understanding of

the causal physical connection between solar

and geomagnetic phenomena. At present, since

no adequate theory exists to describe the

dynamical behavior of an ionized gas stream

in a magnetic field, we cannot definitely accept

or reject either of the two possibilities discussed

in this paper.

Acknowledgments

The work reported herein has been accomplished in part under the sponsorship of the

Air Force Cambridge Research Center, Geophysics Research Directorate, through Contract AF19(6O4)-146 with Harvard University,

and in part with the support of the Central

Radio Propagation Laboratory (CRPL) of the

National Bureau of Standards during a 3-

month visit as a consultant by one of the

authors (Bell).

A detailed account of the results of Part I

previously appeared in Harvard Scientific

Report No. 17 (Bell and Glazer, 1954a) and in

brief summary elsewhere (1954b). For a summary of Parts II and III see Bell and Glazer

(1956).

We wish to thank Hope I. Leighton, Rhoda

McDuffie, Marion B. Wood, and William L.

Taylor of CRPL for assistance in various

phases of the numerical calculations. We are

indebted to Dr. Walter O. Roberts, Director of

the High Altitude Observatory of the University
106 SMITHSONIAN CONTRIBUTIONS TO ASTROPHYSICS VOL.J

of Colorado, for a number of stimulating and

helpful discussions of solar-geomagnetic problems during our stay with CRPL.

Also, special thanks go to John E. Alman,

Director, and Lloyd C. Peterson, Assistant

Director, of the Office of Statistical and Research Services of Boston University, and to

James E. Starkey, Director, and James C.

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Abstract

This paper analyzes the relations between the intensity of the coronal emission lines and the geomagnetic activity

index, Kp. The study emphasizes the years 1950-53, with brief reference to 1942-49, and begins with a general

discussion of the problems of solar-geomagnetic relationships.

In Part I, the analyses employ a single daily index of intensity for each coronal line (X5303 and X6374). Solutions

are carried out by the superposed epoch method for the average Kp around zero-days defined by east limb passage

(ELP) of regions of strong and of weak coronal emission. Inverse solutions are made for the average 5303 intensity

around zero-days defined by specified geomagnetic conditions. It is found important to distinguish between sporadic

sc-type geomagnetic storms and recurrent storms, which appear to be associated with different types of 5303 intensity

distribution. The concluding sections explore the possible influence of the heliographic latitude of the earth on the

March and September maxima in geomagnetic activity. The concept of favorable and unfavorable solar hemispheres

is introduced, and possible hemisphere effects for active centers and for M-regions are considered.

In Part II, the analyses employ, for each coronal line, separate intensity indices for the northern and for the southern solar hemispheres, and the hemisphere effects for the years 1950-53 are explored more fully. In solutions for Kp

from specified coronal conditions, the effect of angular distance of the bright coronal region from the sun's equator

and of the heliographic latitude of the earth on geomagnetic conditions is investigated.

Part III presents inverse solutions for the patterns of coronal conditions, in the favorable and in the unfavorable

solar hemispheres, which are associated with various geomagnetic conditions. The distinction between recurrent and

sc-type storms with respect to coronal intensity patterns is explored further, and the distribution of 5303 intensity

associated with several individual recurrence sequences is studied. Two long-lived M-regions are found, each characterized by unusual weakness in the 5303 corona in one solar hemisphere and by an annual period of waxing and

waning in its geomagnetic effectiveness. The relations between coronal and geomagnetic conditions for the years

1942-44 and 1944-49 are briefly discussed.

The principal conclusions are as follows:

The green line (X5303) shows a closer relation to geomagnetic conditions than does the red line (X6374).

ELP of regions of bright 5303 emission tends to be followed by a decrease in geomagnetic activity, with a minimum on day +10.

ELP of regions of unusually faint 5303 emission tends to be followed by geomagnetic disturbance, of the recurrent type, with a maximum on day +9.

Both the above tendencies are enhanced markedly if only coronal intensities in the favorable solar hemisphere

are considered, i. e., intensities on the same side of the solar equator as the earth.

Bartels' M-regions are identified with areas of unusually weak emission in the 5303 corona in the favorable

solar hemisphere.

An axial explanation is indicated for the major part of the March and September maxima in geomagnetic activity

in years dominated by recurrent storms.

The relative location of the solar equator, rather than the angular distance between coronal region and earth,

appears to be the major factor in the hemisphere effects on geomagnetic activity, which are negligible when the

heliographic latitude of the earth is less than about 3°.

Finally, the hemisphere effect manifests itself in the recurrent or M-region storms and in the inhibiting power

of C-regions but not in the sporadic sc-type of magnetic storms. The distinction between these two types of storms

appears to depend on a difference in origin—the recurrent storms arising from general solar corpuscles, the sporadic

from active-center corpuscles.