The 27-Day Period (Interval) in Terrestrial Magnetic Disturbance

The 27-Day Period (Interval) in Terrestrial Magnetic DisturbanceAuthor(s): A. L. CortieSource: Proceedings of the Royal Society of London. Series A, Containing Papers of aMathematical and Physical Character, Vol. 106, No. 735 (Jul. 1, 1924), pp. 19-32Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/94232 .

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The 27-day Period in Terrestrial Magnetic Disturbance. 19

it is shown that the formulce based on the, assumption of mono-electronic quantum 8cattering, lead to lower values for the total scattering absorption, and much lower values for the relative scattering, than those observed.

We would like to express our thanks to Professor Sir Ernest Rutherford for his interest in the work, and for helpful discussion on points raised in the present paper.

The 27-day Period (Interval) in Terrestrial Magnettc Disturbance.

By the Rev. A. L. CORTIE, S.J., D.Sc., F.Inst.P., Director, Stonyhurst College Observatory.

(Communicated by Dr. C. Chree, F.R.S.-Received April 1, 1924.)

(PLATE 1.)

OBJECT OF THE RESEARCH.

?1. The object of the present research is to show that the existence of a 27-day period or interval, in a series of magnetic disturbances, is connected with a restricted area of solar disturbance, and not with a periodic disturbed condition of the whole solar surface. This question is raised in Dr. Chree's exhaustive study of the 27-day period in Terrestrial Magnetism ('Roy. Soc. Proc.,' A, vol. 101, p. 390, 1922). Dr. Chree likewise notes " that low sun-spot frequency and a low latitude of spots both conduce to the development of the 27-day period " (loc. cit., p. 382). This conclusion is entirely in accordance with my own results for the last three solar cycles, as set forth in former papers (e.g., Monthly Notices, R.A.S. 83, p. 214, 1923). Accordingly the years 1921, 1922 and 1923 to July have been selected for study, in which years both the conditions mentioned by Dr. Chree have been fulfilled. Two long series of magnetic disturbances mark this period of comparative solar calm, which extend from 1921 (May) to 1923 (July), in which latter year the sun was almost entirely free from solar disturbance. The details are set forth in the following Tables. The first concerns a series of magnetic disturbances, at 27-day intervals, which covered the period 1921 (May 12) to 1923 (April 13), and the second deals with a similar series, extending from 1921 (October 27) to 1923 (July 11).

c 2



20 A. L. Cortie.

DESCRIPTION OF THE TABLES.

?2. In these Tables, the first column gives the serial numbers of the magnetic disturbances, the second the dates of their occuLrrence, the third the intervals in days between successive disturbances, and the fourth and the fifth the extreme ranges of declination angle in minutes of arc, and of horizontal force- in units y, where ly 10-5 C.G.S. Units. The next six columns, from the seventh to the eleventh, are concerned with the magnetic character of the days of disturbance, and since disturbed conditions may not unfrequently persist for three or four, or even rnore days, also with the magnetic character of the two days immediately preceding, and of the two days immediately following that actual day of disturbance, which forms a member of the series. For the series days the magnetic character is given in two forms, the international figure and the Stonyhurst letter, according to the method which has been adopted in our Annual Reports since the year 1889. In the international scheme of character figures, 0 means a quiet day, 1 a moderately disturbed day, and 2 a highly disturbed day. In the Stonyhurst nomenclature, c means a calm day, s a day of small magnetic disturbance, m of moderate disturbance, g of great disturbance, and v.g. of very great disturbance. From a comparison of these character letters with the figures published from the central international station at De Bilt for the years 1921, 1922, the mean values of the letters in international figures are c =0 * 2, s.- 6, m = O * 9, g = 1 * 3 and v.g. = 1I 5. These values have been employed in the Tables, with slight modifications in particular cases, depending on the character of the disturbance, for the year 1923, for which the international figures from De Bilt were not available at the time of writing.

The twelfth column gives the heliographic longitude of the sun's central meridian for each successive day of the series. The remaining columns set out the statistics of the associated solar phenomena, which occurred somewhere near these longitudes on the days in question. For these longitudes are a guide to the approximate heliographic longitudes of such solar phenomena as might have been connected with the magnetic disturbances, although it is not a condition of the association of a solar outburst with a magnetic storm, that it should be on or very near the sun's central meridian. These columns also show the number of the spot-group in the Stonyhurst series, its disc-area, in units, 1/5000th of the visible disc, on the date of disturbance, the mean heliographic latitude and longitude of the spot-group, and the same mean co-ordinates of the accompanying faculh, or calcium flocculi associated with the spots, or in evidence when the spots had disappeared. For the calcium flocculi the figures have



(Jortie.

+250O

+20'0

S.5.o

+1501

+10-0 0@ - , S l0* * ' 0

* . . *

0~~~~~~~ i - ** .

D

O0 30 --600 90o_ 120?_ _ 00

<~~~~~~~~~~~~~~~L I Lo 1 0

0501, * *0

0 . 0 0

* 0

-*10.00

-i5.0

-{50~~~~~~~~~~~~~~~~~~~~~~~~~~



SUNSPOT POSITIONS 192 1 dJANUARY -1923 ~JULY.

.

*~~~~~~~~~~~~

*~~~~~~~~~~~~~~~ *

* * S

*~ 0

* 5

-9Q0 12 5 1800-- 2100 - 2400-- 2

* v

*0 0

* 0

0 0X .~~~

0 0



Roy. Soc. Proc., A, vol. 106, Pl. 1.

;UNSPOT POSITIONS 192 1 dJANUARY -1923 ~JULY.

* 0 e

*~~~~~~~~ ..

S 0 0~~~~~~~~~~~~~~~~~~~~~~~~~~~~.

S~~ ~ ~ ~ 4 . .~~~~~~~~~~~~~~~~~~~~~~~~~~~~

2100 2400 2700 3000 330? 3600

* ~ ~ ~ *~~ X

* S

.~~~~~

. ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ . ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

0~~~~~~~~~~~



The 27-day Period in Terr estrial Magnetic Disturbance. 21

been derived from the monthly bulletins of the Observatory of the Ebro, and for the faculhe from the Stonyhurst drawings.

THE DIAGRAM OF SUN-SPOTS.

?3. A diagram (Plate 1) of sun-spot positions has been prepared from the Stonyhurst records covering the period 1921 (January) to 1923 (July), the spot- groups being represented in their heliographic latitudes and longitudes, according to their disc-area, in a similar manner to star magnitudes on a star map. The largest spots, and there are only three of these, had an area greater than 10 units, the next magnitude an area between 10 and 5 units, the third between 5 and 1, the fourth less than 1, while the mere dots represent an area of 0.1 unit. Of the three largest groups, that at mean latitude + 1.8? and mnean longitude 2 8? was the greatest disturbance of the year 1921, crossing the sun's disc, May 8 to May 21, when it attained a maximum area of 16.5 units. Its passage was synchronous with a very disturbed state of the magnets, from May 12 to May 20, culminating in the violent storm of May 13 to May 15. In the Stonyhurst records we have to go back to 1882 (November) to find such another continuous set of magnetic disturbances. The particular disturbance of May 12 heads the list, as the first of one of the long sequences to be discussed immediately. The second of these first magnitude sun-spot groups, in time, was that which crossed the sun's disc 1922 February 27 to March 8, attaining a maximum area of 21 5 units. The various spots constituting this group extended over some 4? in latitude, from + 7 0 to + 11.0, and over no less than 310 in longitude, about a mean position of 120.00. The spot-group recurred in March, and again as a very small spot in June. The third first magnitude spot-group crossed the sun's disc 1922 December 22 to 1923 January 4, attaining a maximum area of 13 0 units. There were intermittent recurrences of this group, in the form of spots of very small area in 1923 (January, Maarch, July). Between these three groups in the Northern Hemisphere of the sun will be seen several other groups, with a mean latitude about + 9 00, of the second and of the third magnitude, but constituting with them a well-marked patch of sun-spot disturbance. In the Southern Hemisphere, on the contrary, about these same longitudes, it will be seen from the diagram that there are only two isolated spots of any consequience, of the second magnitude. These appeared in 1921 (February and June). But there is no Well-marked area of sun-spot disturbance in the Southern Hemisphere about these longitudes.

The diagram shows a great gap between longitudes 120? and 2800, in which



22 A. L. Cortie.

the sun-spots were few and small. But, from longitude 2800 to longitude 360?, there is another cluster of sun-spot disturbances, principally about longitude 330?, and now more marked in the Southern than in the Northern Hemisphere. We may neglect the two recurrent disturbances, about latitude + 9. 4 and longitude 283 00, which appeared in 1921 (March and April). They have but a very remote connection with the cluster of sun-spots about longitude 330 . 0?. The only spot of any consequence in the gap is one at latitude + 8 00 and longitude 220.00, which appeared 1923 June 25 to July 2, and attained a maximum disc-area of 4-9 units. Therefore, to sum up this discussion of the sun-spot groups, between 1921 (May 12) and 1923 (June 25), with the exception of a few small spots, all the sun-spots were practically confined to about one-half of the sun, or more precisely to the longitudes 280? to 360?, and 3600 to 1200. Moreover, the chief sun-spot disturbances were between longitudes 0? to 60? and 900 to 1200 in the Northern Hemisphere and between 3200 to 360? mainly in the Southern Hemisphere, and these associated and recurrent disturbances formed two well-defined clusters or patches.

THE MAGNETIC SEQUENCES.

?4. Concurrently with these two patches of disturbance on the sun, there were two long sequences of magnetic disturbances, at approximately 27-day intervals, which were synchronous with the intermittent appearances of the sun-spots in these regions, and which persisted when the spots had disappeared. It may be remarked incidentally that it is much easier to unravel any syn- chronisms between sun-spots and magnetic disturbances in the periods approaching sun-spot minimum, as in the present instance, when the sun-spots are relatively few, than it is at sun-spot maximum, when the sun-spot disturbances are practically continuous.

The problem is not, given the patches of sun-spot disturbance, find the corresponding sequences of magnetic disturbances, but the opposite, given the sequences of magnetic disturbances, find, if any, the corresponding sun-spot disturbances. In the case of both sequences here discussed, the occurrence of active magnetic disturbances at a time of solar calm, led to the formation of lists of such disturbances at 27-day intervals, and to the procedure, detailed below, of finding a corresponding area of intermittent, or of past solar disturbance.

In the case of SequLence I a series of notable magnetic disturbances, at intervals of approximately 27 days, occ-urred in 1922 between the dates July 16 and October 6, extending over four synodic rotations of the sun, at dates and at



The 27-day Period in Tcrrestricdl Miagnetic Disturbance. 23

periods when the sun was entirely free from spots. It was noticed that this series was in continuation of three former disturbances, at intervals of 27 days, the first of which, on March 3, had been synchronous with the passage across the sun's disc of the greatest sun-spot of the year. Using the same mean interval the sequence was carried back to 1921 (May 12), which was the first of a set of violent magnetic storms on several successive days. It was preceded by a period of magnetically quiet conditions. Nor could the sequence be extended further, the dates at 27-day intervals being all quiet days, as far as 1921 (January 24). It was natural, therefore, to adopt the disturbance of 1921 (May 12) as the first of the sequence. Similarly, by working in the other direction, without any arbitrary selection, the sequence could be extended to 1923 (April 13). As a verification of the correctness of the sequence, the magnetic character figures of the two days preceding, and of the two days following each number of the sequence, were set down in parallel columns, and the rmean character number of each of the five columns was obtained. That of the selected sequence, beginning 1921 (May 12), was markedly in excess of the other four mean character numbers. It is nevertheless true, that in several cases the individual disturbances in these associated columns were greater than that in the main sequence. But, had all the greatest disturbances been linked together, the 27-day period, which is well established and is taken as granted, would not have been maintained.

The same procedure was adopted with regard to Sequence II. It was based in a similar manner on a considerable magnetic storm, which took place on 1923 (March 24-25), which, with a predecessor at a 27-day interval on February 25-28, had been accompanied by auroral displays and well-defined electrical earth-currents. These two storms occurred at a period of very marked solar calm, the sun having been practically spotless for two months. Tracing the numbers of the sequence backwards they terminated on 1921 (October 29). But this date was not that of the beginning of a magnetic disturbance. It began on October 27, and was preceded by a period of magnetic calm. To be con- sistent, 1921 (October 27) -must be taken as the first day of disturbance, and consequently two days must be added to the sum of the intervals between the successive numbers of the sequence, giving, as it happens, a mean interval of exactly 27 days. But even if October 29 is taken as the beginning of the sequence, it makes but little diflerence in the computation, the mean synodic period, 26-91 days, corresponding to a mean sidereal period of 25 06 days, and a daily rotation of the sun in arc of 14 . 360, in place of 14 - 32? , if 27 days be taken as the mean synodic period. Here again the correctness of the sequence is



24 A. L. Cortie.

verified by the excess of its mean character numnber over those of the columns of associated disturbances. We now proceed to discuss th-e two long~ sequences of magnetic disturbances in detail, as well as the cor responding disturbed solar areas.

? 5. Terrestrial Mag

Extreme ~~Magnetic Character.

Series Date. Inter- Raniges. Series Date. ~~Val Associated disturbance (days). N O. ~ ~~~Day s. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

____________ _ - ______ - D H Itn. St. -2. -. + 1. +2.

1921. I- _ _ _ _

I May 12. . - 21 150. 1.4 q 0 7 0-4 1.9 2-0 2 June 8.... .. 27 18 150 1-2 g 11 0.8 1.0 0.9 3 J-uly 4. ... . 26 16 75 0.7 n 0.1, 02 0.1 047

4 J-uly 30 .... 26 13 70 0 7 s 0.6 0-7 0-2 0.0 5 August 26 ... 27 16 88 1.1 n 0-2 0.0 0-9 0.2 6 September 23 ... 28 18 75 1.1 in 0.5 0-2 0-0 0.0 7 October 21 ... 28 19 57 1.1 in 01 0-4 0-6 0-4 8 November 17 ... 27 12 84 1-7 in 0-4 1-6 1.2 1-0 9 December 13 ... 26 27 128 1.6 v.g. 0-8 I-4 0-8 0-6

1922. 10 January 8 ... 26 31 114 1.5 v.g. 0-7 1.1 1-3 Li1 11 February.4 ... 27 17 84 1.1 in 0-6 1-4 0-8 0-9 12 March 3 ... 27 19 84 1-1 vs 1-2 1-0 0-7 1-5

13 March 30 27 25 97 1-4 g 0-4 0-8 1-1 1-0 14 April 26. ... 27 21 101 1-4 m 1-3 1-1 1-1 0-6 15a May 23. . 27 11 79 0-8 s 1.2 0-9 0-8 0.8 16 June 19 ... 27 10 70 0-6 s 1.1 0-9 0-4 0-5 17 July 16... .. 27 - 132 1-2 q 1-0 1-0 0-8 0.8 18 August 13 ... 28 26 141 1-3 v.a. 1-6 1-4 1-3 0-9 19 September 8 ... 26 39 163 1 - 5 v.a. 0.7 1-4 1-2 1.1 20 October 6 ... 28 26 123 1-3 v.a. 0-9 1-9 1.2 1-1 21 November 2 ... 27 - 197 1-2 a 1-6 1-2 1-2 047 22 November 29 .... 27 22 79 1-3 a 1-0 1-0 0-9 0-8 23 December 26 ... 27 22 79 1-3 q 0-4 0-8 0-3 0-3

I1923. 24 January 22 ... 27 12 35 -0-9 n 0.9 0-9 0-9 0-6 25 February 17 .... 26 16 62 0-9 in 0-2 0-6 0-9 0-6 26 March 16 ... 27 13 110 0.9 Mn 0-9 - 0-6 0-9 27 April 13 ... ..28 22 70 0.9 in 0-6 0-5 0-5 0-4

Means .26-96 20.1 96.2 1-2 - 0-8 0-9, 0-8 0-8

THE, CORRESPONDING DISTURBED SOLAR AREA.

? 6. The series is c'ontinuous at intervals of 26, 27, 28 days, if- the distiurbances miarked " small " in the Stonyhurst lists for 1921 (July 30) and 1922 (May 23, June 19) be included. These disturbances have the mean international character



The 27-day Period in Terrestrial Magnetic D.isturbance. 25

numbers 0 -7, 0-8, 0-6 respectively, indicating a certain amaount of maovement of the needles beyond the calm stage. The whole series includes 26 synodic rotations of the sun, extending over 701 days, the 26 intervals between the successive magnetic disturbances having a mean period of 26-96 days. The

netic Sequence 1.

Longi- Sun-Spots. Flocculi. tude Sun's Central Mean Mean i ean Mean Meridi- o Dise- Longi- Lati- Longi. Lati- a,n No. Area. tude. tude. tude. tude.

32-2 40 14-9 6-9 + 1-8 23-0 +10.0 Diffusedflocculi. 34.9 46 3-2 15-5 + 1-7 27-9 .+11*0 Diffused flocculi. 60-7 58 11-2 21-5 + 4-0 23-7 + 7-0 Flocculiabout spot 66-8 67 4-8 62-5 - 8-5 34-0 +12-0 Diffused flocculi, 69 9 73 6 9 53-6 +11 6 50-9 +11-0 Flocculi about spot. 60-1 80 3 0 54-7 + 9-2 1 63-1 +13-0 Diffused flocculi. 50-7 83 1-4 61-3 + 79 58-7 +10-0 Flocculi about spot. 64-7 88 3-5 44-0 + 6-8 40-9 + 8-0 Flocculi about spot. 72-1 92 4-6 46- 6 + 7-6 47-9 + 9-0 Flocculi about spot.

89-6 99 0-5 101-0 +10-3 103-4 +12-0 Flocculi about spot. 94-1 - 92-3 + 8-0 Diffused flocculi. 98 5 106 26-1 119-0 + 8-9 115-5 +11-0 Greatest spot-group of year. Ex-

tended 31? in longitude. 102-6 115 2 1 120 1 + 8-6 121-6 +10-0 Semi-diffused flocculi. 106-1 118 0-6 105-5 + 5-7 106-1 + 9-0 109-1 - 0-0 - 92-1 + 9-0 Diffused flocculi. 111-7 0-0 - - No spots or flocculi in this position. 114-5 - 0-0 - 124-5 +10-0 Condensed flocculi. 104-1 - 00 Solar calm, but the central meridian 120-6 - 0-0 - longitudes are those of the March .111-0 - 0-0 - spot which extended from 106? to 114-9 - 0-2 - - - 1370 inlongitude. 119-0 - - - 123-2 151 11-3 89-6 + 6-2 89-2 + 6-0 Condensed flocculi.

127-7 - 0-0 - - 98-0 + 6-0 Diffusedflocculi; andfacule. 145-4 - 0-0 - 1 112-0 + 7-0 Semi-diffused flocculi; andfacule. 149-7 157 0-3 87-9 + 6-3 122-0 + 9-0 Faint and diffused faculae. 140-3 162 ! 0-1 115-8 + 4-6 125-0 + 8-0 Faint and diffused faculao.

95-3 - 5-6 69-1 + 6-8 79-6 + 9-3

corresponding sidereal period is 25 17 days, or a mean daily rotation period of the sun of 14 - 34?. By Carrington's formula, this is the rotation period for solar latitude 7. 8 and by Spoerer's formula for 6- 80. We may take 7 30 for the mean. The mean value of the successive longitudes of the sun's central



26 A. L. Cortie.

meridian on the days of magnetic disturbance was 95* 3?. If there was a disturbed area on the sun, even though it were intermittently active, which was connected with this long series of magnetic movements, it should be centred somewhere in the neighbourhood of the region having these heliographic co-ordinates, which have been determined solely from the magnetic disturbances. From the Stonyhurst records of sun-spots, and from the Ebro lists of flocculi, we find that there was such an area of intermittent solar activity in the required position. We compare the results as follows

From Magnetic Disturbances. Number of recurrences, 26. MIean synodical period, 26 96 days. Mean sidereal period, 25 4 17 days. Mean daily arc of solar rotation, 14* 34'. Mean corresponding latitude 7 3'. Mean longitude of central meridian, 95 30

From Solar Spots and Floceuli. (a) Spots mean latitude, + 6 8?

Spots mean longitude, 69 10, (b) Flocculi mean latitude, + 9 3.

Flocculi mean longitude, 79 60.

The calculated and the observed latitudes are very close for the spots, and, considering the diffuse nature of the phenomena, quite close for the faculae and flocculi. In longitude, the difference between that of the mean of the positions of the cenitral meridian corresponding to the magnetic disturbances and of the mean longitude of the series of sun-spots is 26. 2, and for the flocculi 15 70. This is a sufficiently close agreement, when we remember that sun-spots are not by any means most active, as regards their effect on magnetic disturbance, when they are on the suns central meridian. Mloreover, the biggest sun-spot group connected presumably with this series of magnetic movements, which was also the greatest sun-spot group of the year 1922, extended over an arc of as much as 31? in longitude. This concordance of calculated and observed positions indicates an intimate connection between a particular area of solar activity, and a long series of magnetic disturbances.

DIsCUsSION OF SEQUENCE I.

?7. We may note the following facts from the details set forth in the Table. (1) The most active disturbance of the sequence, that numbered 19, with



The 27-day Period in Terrestrial Magnetic Disturbanee. 27

ranges 39' in D and 163y in H, occurred at a period of marked solar calm. The mean disc-area of all sun-spots observed in 1922 (September) was only 0 3. (2) This was not an isolated disturbance, but was one of a series of active disturbances, Nos. 17-22, which took place when the sun was entirely free from spots for long periods, and when such spots as did appear were of small size. The mean daily disc-area of all spots from July to November was only 0 - 6 unit. (3) On the contrary, when the greatest and most active spot-group of the year appeared, crossing the disc February 27 to March 8, the accompanying magnetic disturbance, of character number 1 1, was not so marked as those of the 27-day sequence which subsequently appeared, when the solar calm had ensued in the position of the disturbed area. At the synodic rotation succeeding that of February 27 to March 8, when the sun-spot group had diminished from 21 -5 to 2-8 units of area, the magnetic activity of March 30 had a character number higher than that of March 3. Two minor magnetic disturbances marlked the next two synodical rotations, the solar surface having now reached a state of calm. Then ensued, the solar calm still persisting, seven major magnetic disturbances, corresponding to each successive synodic rotation of the sun. That is, the synodic passages of the region of the greatest sun-spot group of 1922 were marked by magnetic disturbances of more notable activity and of greater persistency than when the spots were actually present. (4) These magnetic disturbances extended over several days, as is evident from the columns in the Table, in which are set out the " character " numbers of the associated disturbances. (5) With regard to the whole sequence, and this, too, is verified for the second long sequence, the mean character numbers for the main series and for the four associated series show that the main series has the greatest mean character number, and therefore is correct. And, although the mean character numbers of the four associated series are practically equal, nevertheless individual disturbances in these associated series, e.g., Nos. 18, 21, may be more marked than those of the main sequence. (6) Of the sun-spot groups which were coincident with many of the magnetic disturbances of the sequence, although they were associated as appearing in the same region of the sun, only three, numbered 99, 106, 115, that is from 1922 (January 10 to April 3), were recurrences of the same sun-spot disturbance. (7) The faculae and the flocculi surrounding the spots are frequently observed as persistent in the rotations when the spots have disappeared, and are an evidence of the maintenance of the activity of the region, especially if the facule and flocculi are in the semi-diflused stage of their life history.



28 A. L. Cortie.

______________ _ ___________ ___? 8. T errestrial M ag

Magn-etic Character. Extreme _____________________

Inter- Ranges. Series ~Date. val Associated disturbances (days).

N o. ~ ~~Days. _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

D H Itn. St. -2. -1. + 1. +2.

1,921. ' 1 October 29 .... 15 7'0 0-8 n 1-0 0.9 0-2 0.9 2 November 25 .... 27 12 57 0.4 s 1-1 0-5 0-2 0.1 3 December 23 .... 28 21 70 1.0 g 0-0 0-9 0-7 0. 1

1922. 4 January 18 .... 26 10 57 0.9 iv 1.2 141 0-8 0.7 5 Februaryl ... 28 21 110 1.4 g 0.8 1.0 I1.5 0-9 6 March 13 .... 26 36 132 1 -8 mg. 0-8 I1-1 -19 0-7 7 April 9 ... 27 .29 136 1 3 vug. 00o I- 1.1 I 0.8 8 May7 ....28 24 1'28 1 4' 0-7 0-7 1-3 1 .4

9 June 2 ... .. 26 18s 1101 1.1 m 0.0 0-2 11 0 10 June 29. ...27 26 167 1.-3 v.g. 0.7 0 9 1 14 1.1 11I July 26 ......27 31 97 1-5 a 0.8 021 1-5 1.5 12 August 23 ... 28 27 128 1-2 a 1 0.9 0-8 0-9 0-7 13 September 18 ... 26 7 44 0-3 c 0-4 - 06 0-2 1-4 14 October 14 2 26 1 9 53 0 7 m 0.4 0.5 04 0-1

1 5 November 10 .... 27 24 88 1 2 a 0.3 0.4 0.-2 0.1 16 December 6 ... 26 7 48 0.91 O.]Il 0-9 0-3 1 0.0

1923. 1 17 January 3 .... 28 7 18 j 0Sj1 0.2 0.21 06 0-6

18 January 30 .... 27 12 48 1.0 m 0.2 0 9 0 6 0-9

19 February 26 .. 27 23 110 1 15 v.a. 0 6 1-3 1P5 0.9 20 March 24 ... 26 66 238 Ilar v.a. 0 -6 0-2 1.5 0.9 21 April 2.1... 28 18 97 10-9 mT 0.6 0 6 0 9 0-6 22 May 17 ......26 24 128 1 3 a -06 0-2 0-9 0-9 23 June 13. ...27 25 114 1-3 a 016 0 6 0-9 0-6 24 July10 ......27 18 106 10 0.2 02 09 02

Means ... .. ...27 00 21 7 97-7 1 1 - 0.5 0-7 I 0 9 0.7j

CORRESPONDING DISTURBED SOLAR AREA.

?9. After a period of comparative magnetic calm, there were three sucicessive days of moderate magnetic -disturbance, commencing on 1921 (October 27). Of these dist-urbances, that of October 29 forms the first of a series, at intervals of 26, 27, 28 days, which termin-a ted with two moderate distuLrbances on 1923 (July 10, 11). The whole sequence of 23 sy-nodic rotationis of the sun covers a period of 621 days, with a mean interval between the successive numiibers of the series of exactly 27 days. In this series the disturbance N ,o. 2, 1921 (Novem- ber 25), was small, as also was No. 17, 1923 (January 3). There w as only one



The 27-day Period in Terrestrial Magnetic Distur)bance. 29

netic Sequence If

Longi- SuIn-Spots. Flocculi. tude. Sun's s- _

Central Mean MIean Mean Mean Meridi- Group Disc- L ongi- Lati- Longi- Lati- an. No . Area . tude. tude. tude. tude.

305-2 85 12-4 326.30 L~ 3.40 326-0 + 4-0 Condensed flocculi N.E. of spot. 309 3 90 6-2 319 3 5 59 316-8 - 6-0 Compact flocculi about spot. 300.4 93 3 2 325-2 - 5-4 323-2 - 6-0 Compact flocculi S.E. of spot.

317-9 100 1-1 329-4 - 6-1 326 9 - 8.0 309 2 103 7.5 337.1 - 6-6 338-1 - 7.0 Spot a.rea for February 14. 326 8 109 6-8 345-2 4-6 347 8 v 4-0 330-6 - 0-0 - - 351-0 3-0 320 7 120 1-5 14-2 + 8-5 350-7 - 5-0 Spot-group 120 not connected with

disturbed area. 336-8 - 00 - - 369-4 - 6-0 339.4 - 00 - - -

342 - 1 132 2-6 357-4 - 6-6 356-1 6-0 Renewal of spot-activity. 331-9 135. 0-1 340-7 -12-6 342.7 -11-0 Same region. Maximum area, 3 3. 348-5 138 1.4 344-6 -11-4 343-5 -11-0

5-5 144 2-0 352-7 - 5-6 351-7 8-0 Spots seen October 19-21. Flocculi October 18.

9.4 146 3-0 0-8 - 6-0 358-6 6-0 26-7 150 0-4 4-0 -5 -4 361-7 6 6 0 Diffuse flocculi.

17-9 - - - - 16-8 - 9-0 Diffuse faculhe and flocculli. No spots near this longitude.

22-4 - _ - 30 -0 - 3-0 Diffuse faculae. Mean position. No spot near this longitude.

26-8 - 0-0 - - - -

34-2 - 0-0 - 41-0 - 7-0 Faint diffuse faculse. Mean position. 34-6 163 0-8 3-2 - 6-1 - - Faculse surrounding spot. 51.0 - 0.0 - 15-0 - 6-0 Faintdiffusefacule. Meanposition. 53.7 - 0.0 - - 51-0 - 2-0 Faintdiffusefaculon. Meanposition. 56-3 - 0-0 - - 51-0 - 3-0 Faculhe and veiled spots. Mean

position.

354.9 2-8 3435 -5-0 358-0 - 5-7

day of magnetic calm, No. 13, 1922 (September 18). With this exception all the other numbers of the sequence were days of magnetic disturbance, ranging from 0 7 to I-5 on the international character scale. Even with regard to the day of magnetic calm, one of the associated disturbances, that on September 17, had a character number 0 - 6, and that on September 20 a character number 1-4. The synodic period, 27 days, corresponds to a solar sidereal period, 25 14 days, on a daily rotation in arc of 14- 320. This is the solar rotation .period of latitude 8 .70, according to Carrington's formula, and of 7.20 by Spoerer's formula, giving a mean latitude + 8-0g. The mean longitude



30 A. L. Cortie.

of the sun's central meridian on the successive days of the magnetic sequence was 354 9?. The result is

From Magnetic Disturbances.

Number of recurrences, 23. Mean synodical period, 27 0O days. Mfean sidereal period, 25d 14 days. Mean daily arc of solar rotation, 14 320. Mean corresponding latitude, ? 8 00. Mean longitude of central meridian, 354 *9?.

From Solar Spots and Flocculi.

(a) Spots: Mean latitude, - -5 0?. Mean longitude, 343 5?.

(b) Flocculi: Mean latitude, -5. 7?. Mean longitude, 358 00.

We may therefore reasonably conclude that it was this patch on the sun of intermittent disturbance by spots, faculae and flocculi, which was connected with the long sequence of magnetic disturbances.

THE SUB-SERIES OF SEQUENCE II.

?10. This sequence may be conveniently divided into two sub-series by the days of magnetic calm, 1922 (September 16-19), which intervened. The sub- series (a) extended from 1921 (October 27) to 1922 (September 17), and covered twelve solar synodic rotations, the mean interval being 27*10 days. This results in a solution approximately the same as that given above, for the long sequence taken as a whole. During this period the sun was very much more spotted near the mean longitude of the central meridian, at the successive synodical rotations, than it was during the second sub-series (b), which extended from 1922 (September 18) to 1923 (July 11). During this second sub-series, with the exception of the 1922 (October and November) rotations, the sun was practically spotless near the critical meridian, and was altogether spotless for five of the ten synodical recurrences of magnetic disturbance. But there was a certain amount of diffuse faculhe and flocculi at each of these recurrences, which extended over large areas. And the solution for sub-series (b) gives a much closer approximation for the flocculi than for such spots as were visible. The mean latitude corresponding to the synodic rotation period of the magnetic



The 27-day Period in Terrestrial Magnetic Disturbance. 31

disturbances was ? 8 00, and the mean longitude of the central meridian at the successive recurrences was 33 5?. In the few spots visible, comprising four groups, the mean observed latitude was -5 8? and the mean longitude 360 20. But for the faculae and flocculi comprising nine groups, the observed mean latitude was -5. 6?, and the mean longitude 21U90, which is a much closer agreement.

DISCUSSION OF SEQUENCE II.

?11. The series of magnetic disturbances 1922 (January 18), No. 4, to 1922 (December 6), No. 16, were synchronous with the intermittent appearance of a group of spots, which crossed the disc at its ftrst appearance 1922 (January 12 to 22). It is group 100 of the Tatle, and its successive appearances as groups 103, 109, 132, 144, 146, 150, 163 mark the same disturbed area of intermittent recurrent activity. It is most probable that this is the area of sun-spot disturbance, which is most closely connected with the long sequence of magnetic disturbances. The groups 1921, Nos. 90, 93, also mark the same active area, which was first in evidence by the appearance of a very small spot on 1921 (June 13). The group 1921, No. 85, about the same mean longitude, was in north latitude near the equator, and the groups 1922, Nos. 135, 138, were similarly placed as regards longitude, but they were in lower south latitude. If we regard the recurrent magnetic disturbances as attached to a disturbed solar area centred on a mean longitude, these groups of spots must be taken into account, as concomitant to the predominant area of sun-spot activity. The greatest disturbance of the whole sequence, No. 20, 1923 (March 24), which was the most active that had occurred since the exceptionally violent storm of 1921 (May 13 to 15), and its predecessor too, of considerable activity, took place at a phase of marked solar calm, when the sun was entirely free from spots, and when the observed faculae were faint and dif[use. Similarly Nos. 7, 9, 10 occurred with a solar disc free from spots, as did also a later group of magnetic disturbances, Nos. 22, 23, 24. This same phenomenon was also apparent in Sequence I. Consequently we may conclude, that rmagnetic disturbances at a 27-day interval may persist, when the spots of the associated active solar area have disappeared, or are dormant. Also that faculhe and flocculi are the connecting links between solar and terrestrial magnetic activity, which occur in the region once marked by the appearance of recurrent and intermittent sun-spots.



32 The 27-day Period tn TerrestrialMHagnetic Disturbance.

SUMMARY.

?12.-I. A long period of coiparatively solar calm extended from 1921

(May) to 1923 (July). 2. Such spots as appeared were mainily confined to one-half of the sun's

disc, and constituted two well-marked patches of disturbance. 3. There were also two very long sequLences of magnetic disturbance at

approximately 27-day intervals. 4. From the magnetic sequence intervals a iean solar latitude, and from

the longitudes of the sun's central meridian on the days of magnetic disturbance, a mean solar longitude is derived.

5. The two com.puted positions from the magnetic phenomena fit the two regions of observed solar activity, one in the Northern and the other in the Southern Hemisphere.

6. In each of these long magnetic sequences the most active disturbance took place at a period of solar calm, and there is no parity between the intensity of sun-spot or of floccular and of magnetic activity.

7. Magnetic disturbances with a 27-day interval persist, when the sun-spots have disappeared.

8. The flocculi and the faculse which remain, when the accompanying sun-spots have disappeared, are an evidence of the maintenance of the activity of the region.

9. The second sub-series of the second long sequence of magnetic disturbances corresponds more with the region of the flocculi, than with that of the spots which had disappeared.



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The 27-Day Period (Interval) in Terrestrial Magnetic Disturbance