geomagnetic “crochet†- geophysical supplements to mnras
TRANSCRIPT
GEOMAGNETIC “ CROCHET ” OCCURRENCE AT ABINGER, 1936-46, AND ALLIED SOLAR AND RADIO DATA
H. W. Newton
(Communicated by the Astronomer Royal)
(Received 1947 April 2)
Summary The rather infrequent occurrence at Abinger of geomagnetic ‘‘ crochets ”
(ultra-violet solar radiation effect) has been studied over I I years, in con- junction with data of solar flares and radio fade-outs. The trace movements in H, D and V of 23 examples are reproduced, in addition to tabulated data for these and for 32 other cases of less certain identification. The general synchronization of flare, fade-out and crochet (when present) is noteworthy. Comparison is made of the amplitude AH and direction AD of the crochets with the normal diurnal inequality of H and D. A striking feature is the change in direction from East to West in D at about 1oh.7 apparent solar time, in conformity with the diurnal inequality in D. The correlation with the diurnal inequality in H is less obvious, but AH is of the correct sign (neg- ative) over the range of daylight hours in which crochets occurred. There are indications of other diurnal and seasonal factors (other than a simple dependence upon the Sun’s zenith distance) in the occurrence of crochets at Abinger. Some crochets in 1946 have synchronized with reported bursts of “ solar noise ”.
INTRODUCTION The first individual geomagnetic impulse to be noticed in its possible relation
to a specific solar outburst was that included in Carrington’s classical account * of “ A Singular Appearance seen on the Sun on September I, 1859 ”. Observa- tions of the last twenty years have established these synchronously observed phenomena as the prototypes of geomagnetic crochets and of intense solar flares respectively. Moreover there has been added a third observable pheno- menon, namely, the sudden “ fade-out ” of short-wave radio transmission.
A fourth simultaneous phenonemon, difficult to test by direct observation t is highly probable-the onset of ejection of a solar stream of electrically charged particles responsible for a geomagnetic storm with its characteristic “ sudden commencement”. Yet a fifth related phenomenon has now been detected. Sudden bursts of solar radiation observable on radio wave-lengths of about 5 metres were recorded 1 at the time of peak intensity of certain solar flares (with their accompanying radio fade-outs) that occurred in the region of the giant sunspots of 1946 February and July.
* The sudden pulse on the Kew magnetograms at I I ~ 15*n U.T. on 1859 September I, as well as the complex storm that began 174 hours later, were discussed at the time by Balfour Stewart in Phil. Trans., 151, 423, 1861. See also J.B.A.A., 53, 244, 1943 for notes on the Greenwich magnetic record.
1. Oburrw?torj*, 66, 95, 275, 1945-46. 1 Appleton and Hey, Phil. Mag., 37, 73, 1946; Ellison, Observatwy, 66, 358, 1946.
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Geomagnetic " Crochet ') occurrence at Abinger, 1936-46 201
This paper is confined to the observational data of crochets at the Greenwich magnetic station at Abinger over one solar cycle. Crochets, like short-wave radio fade-outs (but unlike " sudden commencements ") occur only on the daylight hemisphere of the Earth but appear of far more restricted occurrence than do fade-outs for which there is a very close synchronous relation with the more intense solar flares.* It is accepted that the fade-outs themselves are due to abnormal ionization of the ionosphere at about the 70 km. level (D layer) caused by a sudden increase in ultra-violet radiation from the solar flare.
According to McNish t no measurable effects of the crochet phenomenon are recorded beyond 70" from the sub-solar point at the time of the related solar flare. For a complete geomagnetic study of crochets, data are wanted from a number of widely separated stations, as investigated by McNish for three crochets in 1936. Something can be learned, however, at this stage of scanty knowledge. of the phenomena involved, from records at a single station carried over several years. The recent detection of bursts of " solar noise '' referred to above has also stimu- lated interest in the crochet phenomenon and the need for systematic data, in addition to more extended solar observations. After Carrington's observation in 1859, no other synchronous cases of geomagnetic impulses and solar outbursts seem to have been observed until 1872. On August 3 and 5 of that yearf, C. A. Young observed spectroscopically in H a what were undoubted solar flares of high intensity. He drew attention to the apparent simultaneity between the chromospheric outbursts and impulses shown on the Greenwich magnetic traces. Re-examination of the original traces at Greenwich and comparison with the typical forms of crochets illustrated in the present paper, leaves considerable doubt as to the crochet nature of the impulses that occurred in some general disturbance. This does not necessarily imply that a typical crochet impulse might not have been recordable nearer the sub-solar point.
On 1909 September 28, a typical crochet was registered at Kodaikanal, S. India, at 4 9 U.T. or 10h.0 local solar time during a solar flare of first magnitude. A copy of the H trace is given by Michie Smith.$ No vestige of this crochet (AH= +154y in 20 minutes) appears on the magnetic traces at Greenwich, not then within the daylight hemisphere.
In 1926 Hale produced his standard model of a spectrohelioscope, thus greatly enlarging the observational field of solar flares, and reviving interest in inter- related geomagnetic effects, especially in the greater magnetic storms that may follow about a day after intense flares.11 Ten years later came evidence for the close relationship between flares and short-wave radio fade-outs. La Cour was probably the first to connect a crochet with a radio fade-out, but Dellinger had priority in publishing T[ comparative ionospheric data at the time of solar flares and crochets. The first investigation of crochets by McNish in 1937 has already been quoted. He concluded that : " . . . . the change in the magnetic field caused by the chrornospheric eruption consisted of an augmentation of the diurnal-variation departure obtaining at that time in each element ".
* -V*N., W, 5948 1937. -f Terr. Magn. ntmos. Elect., 41, 109, 1937. 1 C. A. Young, The Sun, 3rd edition, pp. 157-8, 1888. I M.N., 70, 24, 1909. II M.N., 103, 253, 1943. 'I[ Convenient references are given in Chapman and Bartels' Geomagnetism, Vol. I, p. 341, 1940.
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202 H. W. Newton, Geomagnetic " Crochet " occurvence
-
tef. VO .
- I
2
3
4
5
6
7
8
9
I 0
I 1
I 2
I3
I4
15
16
17
18
I9
20
21
22
23 -
Date
(1)
1936 Apr. 8
May 28
Aug. 28
Nov. 26
I937 July 27
Sept. 30
1938 Mar. 31
I939 APr. 29
July I I
July 17
Sept. 8
I940 Mar. 23
1941 Feb. 28
June 8
Sept. 17
1942 Feb. 21
Feb. 28
Mar. 31
Apr. 13
Nov. 4
1946 Feb. 13
Mar. 5
Aug. 23
TABLE I Crochets Recorded at Abinger, 1936-1946, List A,
Begins U.T.
(2)
16 49 h m
07 29
09 24
08 58
06 02
I 0 20
I 0 31
07 41
09 29.8
07 51.9
I I 36.2
1 1 07
09 28.9
09 03
08 24.8
I3 30'3
I 2 0 1
I I 24
1 1 07
1 1 33
nII 35) L 11 45
I I 27.8
09 31.5
Impulse (in H)
Duration
(3)
u1 2
6
6
6
6
I 0
9
9
I -6
1 '5
9.6
5
I 'I
9
9 '4
18.0
6
5
7
5 (15)
5
2'2
2.8
Recovery o Normal
(4)
tl m 16 59
07 43
09 45
09 18
06 20
I 0 52
I 1 00
08 12
09 43
08 04
I2 00
I 2 20
I 0 00
I 0 00
08 42
15 - I3 0 5
I I 42
11 45
11 44
I 2 I 0
I2 25
09 45
h g e of Crochet
AH
(5)
/
-. 9
I 0 -
I 0 -
- '4
9
- 28
- I9
- 16
- '5
- 51
- 27
-
- 64
- 28
- I9 - 29
- 40
- 69
- I8
- 18
- I3
- 14
- 81
- 13
AD
(6)
0 '0
E. 1.9
0 '0
E. 2.8
E. 2.9
E. 2'0
E. 1.6
E. 5.6
E. 3-7
E. 5 . 1
w. 3'7 0'0
E. 1.4
E. 4'7
E. 6.0
W. 8.4
w. 3'7 E. 0.8
0'0
w. 2'0 w. 5'5
w. 1'0
E. 2.1
AV
(7)
Y 0
--_ . 3
0
0
0
- 6
- 8
- 8
- 7
- 14
- 6
0
- 8
- 18
- I3
- 7
- 30
- 4
0
- 6
0
- I7
0
Sun's Zenith
Distance
(8)
(i
73
60
53
81
76
57
5 1
64
42
58
45
52
69
44
67
65
60
48
44
67
65
58
5 0
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at Abinger, 1936-46, and Allied Solar and Radio Data Tmm I (cont.)
Associated Fade-outs, Solar Flares, Sunspots and Magnetic Stornts -
Ref. N O .
- I
2
3
4
5
6
7
8
9
I 0
I 1
I 2
13
14
'5
16
17
I8
19
20
21
22
23 --
Fade-out
Begins U.T. (9)
h m 16 49
07 29
09 30
09 00
06 03
I 0 22
1 0 35
07 46
09 32
07 54
11 40
I1 I 0
09 30
09 05
08 30
I3 30
12 00
11 30
I1 I 0
1 1 34
11 35
I I 30
09 35
Ends U.T. (10)
h m 17 15
07 47
I 0 02
09 28
06 30
I 1 00
I I 05
09 40
09 42
08 44
I 2 00
I 2 20
10 30
I 0 10
09 05
15 00
'5 30
13 30
1 2 00
I 2 00
12 30
1 2 07
09 45 -
Begins U.T. (11)
I6 47 h m
07 I5
09 20
...
... I 0 I 0
10 17
... 09 26
... 11 271
I I 30
... 08 57
08 38
'3 40
12 42
IZ 20
11 07
...
...
...
...
Ends U.T. (12)
h m 18 30
08 00
I I 30
...
... 11 30
13 00
... 10 25
... 12 30
12 30
... 10 03
09 59
15 09
15 22
14 10
II 30
...
...
...
...
Solar Flare -- Int. (13)
3
2
3
...
... 3
2 -1-
... 3 .. ... 3
3
... 3
3-1-
3
3
3
2-3
...
...
...
... -
Dist. from C.M.
(14)
gCW.
82 E.
32 E.
...
... 54 E.
82 E.
... 33 w. ...
25 E.
37 E.
... 10 E.
9 w. 90 E.
5 E.
49 w. 59 w.
...
...
...
...
- rlR
ti51 -
0'44
0.98
0.6 I
...
... 0.8c
0.98
...
0.57
...
0.53
0.65
... 0.30
0.16
I '00
0'22
0.77
0.85
...
...
...
... -
203
Time Interval Crochet to
Storm (if any)
(17)
* Mean of two observing stations ; the time of maximum is given Q( I Ih 4 1 ~ and I 111 43m respectively.
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204
Ref. No.
- I 2
3
4
5 6 7 8 9 I0
rr
I3 I4 I5 16
I7 I8
I9
I2
20 21
22 23 24 25
26 27
28
29 30 31 32 _.
Date
(1)
1936 Apr. 6
Sept. 5 June 4
1937 June 24
1938 June 30 July 28 Sept. 21 Oct. 6 Nov. I I Dec. I I
I939 May 4 May 9 July 16 Sept. 2 Sept. 9 Sept. 29
1940 Jan. 6 Nov. 9
1941 Jan. 2 Feb. 28 July 8
1942 Feb. 24 Mar. 3 Mar. 24 June 16
1943 Feb. 10 Apr. 22
1944 Dec. 10
1946 Feb. 11
Feb. 12 Aug. 12 Nov. 21
H . W. Newton, Geomagnetic " Crochet " occurrence
TABLE I1
Crochets Recorded at Abinger, 1936-1946, List B,
Impulse H, D or V'
Begins U.T.
(2)
h m I3 54 I1 52 09 01
13 26
15 03 12 39 I 0 24 I I 09 09 35 I 0 29'9
09 54 09 25 06 25 I 1 30'4 I 0 2 I ' O 07 56.8
13 21
13 25
11 42 IS 24'4
I4 I4 30 23> I4 57 I I 26 11 32 I7 27
09 44 I4 23
I 1 52
10 37 I I 43 14 02.0 16 29
Duration
(3)
m I
4 3
2
4
3
3
2
I1
I 0
I5 5 ... 3.6 4 '2 2 *6
15 3
2 2.6
3 3> 2 2 6 4
2
4
2
2
5 I I
Recovery to Normal
(4)
h m 14 08 12 06 09 I4
13 37
15 I2 ?
10 40
09 44 I2 00
I 1 00
1 0 2 5 f 09 57f
I I 36 10 28 08 I 0
14 20
I3 55
11 47 15 30
...
14 I4 40 30} 15 20 I I 42 I I 48 I7 50
09 5' 14 33
I 2 I 0
10 42 I 1 52 14 12
16 35
Range of Crochet
AH
(5 )
Y - 5 - 13 - 8
- 14
- 4 0
-11
- 28 -11
- 22
- 23 - I4 -18 - 13 - 15 - 5
-41 - I3
- 5 - I3
- 15 - 16 + 20
- 19 - 28
- 12 - I 0
-11
- 7 - 6
0
0
_11
t
W. 1.3
E. 2.0
0'0
2 ' 0
w. 1.8 W. 2.4 W. 1-9 E. 2-2 E. 1-3 E. 1.4
E. 1.6 E. 0.7 E. 3'9 w. 2'2 E. 1-4 E. 1.6
W .3*8 w. 2'1
w. 2'1 W. 1.4
E. 2.1
0'0
w. 3'9 E. 3.8
... 0 '0
W. 1.6
W. 0.9
0'0
W. 0.9 W. 1.6
0 '0 -
AV
(7)
Y 0
- 5 0
- 5
0
0
0
- 2?
- 4 - 8
0
0
- 9
- 9 0
0
0
- 9
0
0
- I 0
0
+ 13
- 9 0
0
- 6
0
0
- 6
- 7 0
Sun's Zenith
Distance
(8)
0
50 29 58
32
44 33 54 57 74 76
46 43
70 47 50 73
75 71
75
40 ::} 71 60 5' 67
74 49
75
69 66 43 94 ---
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at Abinger, 1936-46, and Allied Solar and Radio Data 205
TABLE 11. (coni.)
Assot iateJ Fade-outs, Solar Flares, Sunspots and Magnetic Storms -
Lef. JO.
- I 2
3
4
5 6 7 8 9 I0
11 12
I 3 I4 I 5 16
17 I 8
I 9 20
21
22
23 24 25
26 27
28
29 30 31
Fade-out
Begins U.T: (9) -
h m I3 57 11 54 09 04
13 28
I5 07 12 45 I 0 22
o;*& 10 30
09 5 0 09 30
11 33 I 0 25 08 00
I3 24
...
...
... I5 27 I4 35
I5 00
11 28
I7 27 ...
w 46 I4 25
I1 5 0
10 35 11 47 I3 55 16 30 -
Ends U.T. (10)
h m I4 25 12 06 09 25
I3 5 0
15 I2
13 - I0 50 ... I0 00 I2 I5
I 1 00 I0 20 ... 11 40 10 40 08 10
I3 45 ... ...
I5 40 I4 5 0
19 00
11 45
1;'46
I0 05 I4 40
I2 I0
10 5 5
14 10
17 10
I2 00
h m ... ... ... ...
15 03 ... I 0 25
09 40 I2 I8
09 32 09 37
...
...
...
... 08 02
... I3 33
11 33
I4 39 ...
... I 1 20 ... ... ... ... ...
10 40 ... ... ...
h m ... ... ... ...
15 06
I3 00 ... ...
I0 XI ...
11 5 0 10 30 ... .., ...
08 1 3
... I4 30
...
... 16 05
... 12 00
...
...
...
...
... I1 15 ... ... ...
Solar Flare
Int.
(13) - ... ... ... ... I
... 1-2 ... 2+ I
3 I
...
...
... I
... 2-3
2
... 3
... 3 ... ... ... ... ... 3 ... ... ... -
Dist. rom C.M.
(14)
0 ... ... ... ...
E. 15
E. 87
w. 12
W. 62
E. I I
w. 53
...
...
...
...
... E. 65
E. 38
E. 7
W. 70
...
...
... W. 42
...
...
... ...
... W. 64 ...
...
...
- ,IR
15) - ... ... ... ... 1-48
"99
B'22
...
... ,a88
v36 v82 ... ... ...
> -90
... 3.61
2'15
3'93 ...
... 9-69 ... ... ... ... ...
0.94 ... ... ... -
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206 H. W. Newton, Geomagnetic " Crochet " Occurrence
In 1936, Jouaust and his French collaborators were using the term " crochet magnetique " which with its abbreviation to " crochet " seems convenient to adopt.
DATA Previous experience from sample cases of the apparent inter-relation of solar
flares, short-wave radio fade-outs and crochets, suggested that, in addition to a general search for crochets on each day's magnetogram, a critical examination of the traces should be made at the following times :-
(u) Radio fade-outs recorded in this country. (b ) Solar flares of highest intensity ( = 3 on the I.A.U. scale). (c) Solar flares of medium intensity ( = 2).
Data for (a) were kindly provided by Cable and Wireless Ltd. for fade-outs recorded at Brentwood (Essex) on all their channels and covering a range of wave-lengths of approximately 1.5 to 40 metres according to the station involved. These data have the advantage over (b) and (c) of coming from a nearly continuous record unimpeded by weather conditions. The correct type of ionospheric irruption producing the fade-out (often called a Dellinger fade-out) has, of course, to be distinguished by the radio engineer from another type of fading (usually not so complete) that accompanies marked geomagnetic disturbance. *
After the comparison of data (a) with the Abinger magnetograms had been made it was satisfactory to find that a similar comparison with data (b) and (c ) yielded no other certain crochet for List A, but two small probable crochets were found for List B. It is believed therefore that no crochet of consequence has been missed on the Abinger magnetograms. Table I (List A) contains 23 crochets for which little or no doubt is felt as to their correct identification. Each is reproduced in Fig. I. .
List B contains generally the much smaller impulses whose typical features as shown by crochets A are more or less difficult to recognize. Their acceptance as probable crochets, and the rejection of suspected cases, depend mainly upon a time- interval criterion derived from crochets of List A with respect to fade-outs as described later. Consideration in the selection was also given to the general character of the traces (H in particular) during the 12 hours before and after the suspected crochet-whether a unique impulse (as in some cases) or whether there were others approximately similar. As will be seen later from crochets A, the AD movement, in its change of sign during the day, provides a further criterion that would suggest exclusion of No. 21 of List B, in spite of its correct minus sign for AH that characterizes the typical crochet at Abinger. Nos. 8,13 and 24 of List B were included because of their crochet-like appearance ; they were not synchronous with recorded fade-outs or with observed solar flares. -t
It is known from sample years of Lerwick magnetograms, the loan of which was kindly arranged by the Director of the Meteorological Office, London, that several probable crochets of List B are closely reproduced at Lerwick. This rules out the possibility that these impulses are purely local phenomena at Abinger $ but does not necessarily establish their nature as crochets.
* For the physical aspects, see Appleton, Proc. Roy. Sor. A, 162, 475, 1937. t A sunspot unusually active in solar flares was, however, on the disk at the time of No. 8, and
$ See Addendum to my paper on " Sudden Commencements ", loc. n't. likewise another spot above average activity at the time of No. 24.
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at Abinger, 1936-46, and Allied Solar and Radio Data
R p p t ~ ~ . Scolc . cl936-37)
100‘3 in Hor
23!3 in D, 95 ‘5 in V. - H and V dscreosinq ; 0 increainq West. - 3-
Clpprox. Scale ( I 937-46)
1007 in H o r -1 20!7 in 0, _1cLc__ 3 5 7 in V.
FIG. Ia.-Typical Crochets at Abinger, 1936.1946.
G 16
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208
H
H. W. Newton, Geomagnetic " Crochet " occurrence
zov
v
Tni, Scale Ilhrur( FIG. i~.-Typical Crochets at Abinger, 1936-1946.
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at Abinger, 1936-46, and Allied Solar and Radio Duta 209 The following notes and reference to Fig. IA and B will explain the tabulated
data of Tables I and 11. (a) Table I, Cols. z and 3, cases for which the time is given to the tenth of a
minute have been recorded on the quick-run magnetograms giving 12 times the amplification of the time-scale of the normal traces.
(b) In Col. 6, E denotes increasing East and W increasing West declination of the crochet impulse.
(c ) Col. 8, Sun’s zenith distance (or distance of Abinger from the sub-solar point) at commencement of crochet.
(d) Cols. 11 and 12, italics signify that the origin or cessation of the solar flares was actually observed. (I.A.U.) data.
(e) Col. 16 gives the average area of the sunspot during its disk passage, corrected for foreshortening and expressed in millionths of the Sun’s visible hemisphere. In cases where no solar flare was observed, the area in brackets is that of the spot of largest mean area on the disk at the time.
(f) The occurrence of a geomagnetic storm within 72 hours after the crochet refers to Abinger. A great storm (G) has minimum ranges of 30oy in H or in V, or 60’ in D.
Cases in which there was a sudden commencement, not followed by distur- bances of storm strength, are indicated by sc. The above remarks apply to the corresponding data of Table 11.
-Measures of the crochets on the magnetograms and the tracings for Fig. I were made by Miss Chapman of the Solar Department.
Later values are approximate only.
Small storms (S) have half these ranges as a minimum.
DISCUSSIOX General.-In the following table are given some comparative data of sunspots,
crochets (Abinger), radio fade-outs (Cable and Wireless Ltd.) and solar flares (Greenwich) for the 11-year epoch 1936-46. Figures for 1935 are added to give an overlap from the minimum solar conditions of 1934.
TABLE 111 Frequencies of Crochets, Radio Fade-outs arid Solar Flares, 193 5-46
Sunspot Crochets Radio Fade-outs Solar Flares Number A B All 260mins . All Int. = 3 Year
I935
37
39 40 41 42 43 44 45
36
38
46
36 80
1 1 4 I 1 0
89 68 47 31 16
33 91
I 0
0
4 2 I
4
3 5
I
0
0
0
3
0 8 3 36 I 76 6 99 6 55 2 31 3 35 4 26 2 9 I 6 0 4 4 84
0
2 6 4 7
7 7 3
I
I
0
5
25 66 88
114
33 13
24 8 3
39
69
I 1
0
6 5 7 5 I
2 2 0
0
I
5
We first note that crochets are not frequent at Abinger at any time, being com- parable in number with the long duration fade-outs and the more intense solar flares. Secondly, they conform only approximately in distribution to the sunspot
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210 H. W. Newton, Geomagnetic “ Crochet ” occurrence
curve; they are absent at solar minimum but occur in years other than those of highest spot-frequency. Intense chromospheric activity of individual spots seems more important in this respect than the general level of activity for the year. For instance, in 1942, more than four years after sunspot maximum, no fewer than three crochets of List A and two (if not three) of List B were associated with one particular spot (C.M.P. February 28) and its return. This region with its associated solar flares was the first to provide definite evidence of enhanced “ solar noise ” observable on radio wave-lengths of about 5 metres.+
Bearing in mind the weather difficulties for solar observations, the number of flares of highest intensity (themselves not a frequent phenomenon) observed with crochets, especially in List A, is most informative. The frequent occurrence of big sunspots somewhere on the disk at the time of crochets is also noteworthy (see Col. 16 of Tables I and 11).
Table IV shows the distribution of fade-out durations, and the ratio of the number of crochets to that of fade-outs in each duration group.
TABLE IV Duration of radio fade-outs and relative crochet frequency
Percentage Ratio, Crochets/Fade-outs Number List A List B
Total =48 I Total = 23 Total = 3 2
Duration in minutes
I - I 5 35 0’01 16 - 30 38 0 -04 31 -45 I4 0.06 46 - 60 6 0’12
>60 7’ 0.18
0 -06 0.07
0 -04 0.09
0’01
This table shows that the longest duration fade-outs (only 7 per cent. of the total number) are the most closely associated of any duration group with crochets, as would be expected from the flare and fade-out relationship.
Synchronization of Crochets and Fade-outs.-There is systematic agreement in the commencement times of both phenomena, but not, apparently, exact simul- taneity. The mean difference, fade-out nzinus crochet, is 2m.5 0.3, as derived from crochets A ; the p.e.’ of a single observation is k 1”-4.
No systematic error as large as I minute is likely in the time measurement of the crochets on the normal time-scale magnetograms or of om.l for the quick-run traces. Although the timing of the onset of a fade-out is not of this accuracy, it seems unlikely from the information given by Cable and Wireless Ltd. that a systematic error as large as zQ minutes exists in the radio data. Whether intrinsic or not, this consistent time-difference between crochet and fade-out has been used as an empirical criterion in preparing List B. With one exception, no trace-movement near the time of a fade-out has been accepted if the time difference (fade-out begins minus crochet begins) exceeded +zrn.5 i- 7.0 (7”.0=5 x p.e. of 1”-4). Cases of 24 possible crochet-movements were rejected from an original list, on account of this criterion or because of the trace character suggesting a purely accidental impulse. It may be noted that the mean time-difference of fade-out and crochet from List B is 1.”*9 f 0.4, the residuals being quite small as indicated by the p.e. of om.4.
* Nature, 157, 47, 1946.
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at Abingev, 1936-46, and Allied Solar and Radio Data 21r
In several cases it will be seen that the duration of crochets A approximates to that of the respective fade-out and solar flare. In seven cases for which the origin of the flare was actually observed (times given in italics) the mean time-difference crochet minus solar flare is 5 minutes. Five to ten minutes is the right order of magnitude for a solar flare to rise to its maximum intensity or, if it is a very brilliant one, to reach a central intensity equal to that of the adjacent continuous spectrum beyond the absorption wings of the normal spectrum line (Ha).
The Crochet impulse in H and V.-From List A and Fig. I, AH is seen to be invariably negative at Abinger. These signs are opposite to those of the normal sudden commencement (solar particle stream effect). It has been shown *, however, that a small class of inverted SCs ( -AH, -Air, + AD at Abinger) have a tendency to occur around 8" when the normal SC is least frequent. There are the following points of difference between crochets and inverted SCs to assist correct identification :-
(I) The crochet impulse is usually the less rapid: 7 minutes (for the 23 crochets in List A) as compared with about 3 minutes for the average SC.
(2) The crochet has no small preliminary "kick" in the opposite direction to the main impulse as do more than half the SCs (loc. cit.).
(3) The behaviour of the traces after the main impulse is usually different for the two phenomena, even when the SC is not immediately followed by the onset of rapid oscillations of a storm.
(4) Apart from the above differences, a complete discrimination appears to be given by radio fade-outs. During the epoch 1936-46 there were 84 sudden commencements in Greenwich daylight h0urs.t Not one of the 481 fade-outs of the same epoch was synchronous within a few minutes with a SC. The smallest time-difference (on one of the 15 days on which a fade-out and an SC occurred together) was 48 minutes, the average difference being 4h 2om without respect to sign-a result very different from the mean time-difference of 2m.5 for crochet and fade-out.
Fig. 2 shows AH for each crochet A plotted against the Sun's zenith distance ( = distance of Abinger from the sub-solar point) at their respective commence- ments. The symbols indicate the epoch of the year, equinox, summer and winter.
AV, when present, is also negative.
100 i
i I ! a
?.
C I .-
-a 0
-7- - ---- - 7- ~ - - - - ~ 40" 60" 80"
Sun's zenith distance FIG. z.-Crochet amplitude (AH) and the Sun's zenith distance.
Equinox: 8 Summer: A Winter.
* Newton, M.N., Geophys. Suppl., 5, 159, 1946; Observatory, 66, 371, 1946. t 100 SCs occurred in the night hours.
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212 H . W. Newton, Geomagnetic “ Crochet “ occurrence
LMcNish’s conclusion that no measurable effects are shown beyond 70‘ from the sub-solar point has to be extended to include three cases of greater distances in Fig. 2, viz. 73”, 76” and 81”. In List B there is a possible crochet (in V only) for which the Sun’s zenith distance was 94” (see infra : note on No. 32.)
As against this occasional extension of crochet effects there are some notable cases of absence of crochets at Abinger that might well have been expected from the joint solar and geophysical conditions. A recent case in point is the great solar flare of 1946 July 25 when the distance of Abinger from the sub-solar point was barely 57”. Yet neither at 16” 10’” U.T. when the flare effectively began, nor at 1 6 ~ 27”l i- I”’ when its intense maximum was recorded by Ellison was there any movement on the Abinger traces that could be identified as a crochet.* This absence is the more remarkable because there was a synchronous burst of solar noise that may signify especially intense solar disturbance, as indeed the flare spectrum indicated.
Inspection of Col. 2 of List A shows in point of fact a marked absence of crochets in the afternoon hours; only three out of 23 occurred after lzh U.T. This afternoon hiatus is especially striking in the case of the five summer crochets A which all occurred before 9h 30” apparent solar time.
The distribution of the fade-outs which formed the basic comparison shows no such disparity between a.m. and p.m. hours. Out of the 481 fade-outs recorded from 1936 to 1946,50*3 per cent. came before 12” and 49.7 per cent. after 12” U.T. (There is however, in point of fact, some asymmetry in their distribution with respect to noon, their maximum frequency being around 10” U.T.) Even if we restrict ourselves to the long duration fade-outs ( b60*”), which correlate best with crochets, we still find no preference for the morning hours. The numbers are : 21 before noon; 22 after.
So far as can be decided, the major solar flares also show equal distribution. Of 37 great flares observed at various observatories from 1859-1942 1,16 flares occurred from 0’’ to 12” and 21 flares from 12’~ to 24’’ U.T. Twelve out of 29 of thege flares within Greenwich daylight hours were synchronous with crochets or suspected crochets at Greenwich, seven being before 12”, one at 12” precisely and four (of which two are now rendered doubtful by the present results) after 12”. As against this, crochets B, with their secondary weight, do not show this avoidance feature ; whether real or spurious, further data should decide.
It is worth noting from Fig. 2 that the seasonal distribution in 3-monthly epochs (centred at the respective equinox and solstice) are for crochets A:- equinox ( z epochs) 17 : summer 5 and winter I. Moreover, out of the nine largest crochets (AH) eight occur within the equinoctial epochs, the average interval from the equinox itself being 15 days. It does seem therefore that the Sun’s zenith distance is not the only controlling geophysical factor in the occurrence of crochets at a particular station.
AH and A D Impulse and the Diurnal Inequality.-In Fig. 3 are plotted the AH amplitudes of crochets A against apparent solar time of their occurrence. Sym- bols indicate the seasonal epochs. For comparison are shown on a smaller scale
The associated radio fade-out was of long duration.
* Information has now come through the Carnegie Institution of Washington that a crochet
t Loc. cit., Obsematory, p. 358. was recorded at both Huancayo and Honolulu. (1947 October 27).
% M.N., 103, 246, 1943.
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at Abinger, 1936-46, and Allied Solar and Radio Data 213
three seasonal curves representing the mean diurnal inequality ( S , field of Chap- man’s nomenclature) of H at Abinger derived from all days, 1936-46.
Any correlation there may be between the AH crochet amplitude and the diurnal inequality of H appears at first to be only general. In favour of a corre- lation, it will be seen from Fig. 3 that :
(I) Within the limited hours (daylight) in which crochets occur, the respective diurnal inequality curve for the season is also wholly negative or very nearly so ;
(2) The few largest crochets occur when the diurnal inequality has its greatest negative value ;
(3) If it be accepted that the small crochets observable between 8*’ and 12’’ are due to intrinsically weak primary causes and would have been missed altogether if they had come at other hours, then the apparent correlation for the remaining crochets is much stronger, especially when the respective seasonal curves * are separated out as in Fig. 3.
Y
0 - I .. Y 0 - r $ 4 0 - v -
80 -
I20 - I I
0 t 8 I2 , I 6 20 24
FIG. 3.-Crochet mnplitudes (AH) and the diurnal inequality ( 3 seasonal epochs). AH Crochets DimZ umiation (H.F.)
...... . ........... * ......... . ___---------- Winter A
A relationship between the crochet impulse and the diurnal inequality is, however, much more clearly shown by the AD movement, as plotted in Fig. 4, together with the average diurnal inequality for D (all seasons combined). In particular, a decisive change of sign of AD from East to West occurs at least an hour before apparent noon. Fig. 5, plotted on an enlarged time scale, shows that the average time when AD becomes zero is 1oh-7 apparent solar time.
It will be noted from Fig. 4 that, with only one outstanding exception (No. 2Z), the smaller and less certain crochets of List B are in general accordance with this change of sign occurring at 10~l-7. At about this time, the D element of the Earth’s magnetic field is changing most rapidly from East to West of its mean daily value. It is also about then that the related point of inflexion occurs in the diurnal
The readiest data for these curves involve (less precisely) the division of the year into three equak epochs, the equinoctial period being taken as the months of March, April, September and October.
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214 H . W. Newton, Geomagnetic " Crochet " occurrence
6' - E 4 -
Y Y
5 2 w 4 -
- U
8 -
.* * . - 3-
0 n
................................................... 5 3 % -P
- ............... ..........
- 6
I I 1 I I I I
0 4 8 I2 16 20 24 Hours (apparent solar time)
FIG. q.-Crochet amplitudes (AD) and the diurnal inequality of D. Crochets A ; 0 Crochets B.
\ 6' -
E 3 - n a 0 - . " .................................
Y 0 c g w 3 - U
6 -
a ...... * ....................................... ,,-. - \ 0 I I I I I I
8 9 10 I I 12 13 14 Hours (apparent solar time)
FIG. 5.-Diu+nal change of sign of crochet amplitude (AD). Equinox; Summer; A Winter.
inequality of H which, having reached a minimum, begins to rise to a flattish maximum at about 19~.
The foregoing discussion has been carried as far as the present data would seem to allow. Clearly there is need of extended data for a detailed research into this interesting manifestation of a solar-terrestrial relationship.
There was a large crochet at Huancayo (Lat. 12' S.: Long. 5h-o W.), the AH impulse being +IO+ in 5 minutes commencing at 1 6 ~ 51" U.T.
No. 12. An example in which the durations of flare, crochet and fade-out approximate closely.
No. 16. Of special interest because of its association with anextensive solar flare presented uniquely in elevation right at the $un's east limb (L'Astronomie,
Notes on Special Crochets.-List A, No. I.
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at Abinger, 1936246, and Allied Solar and Radio Data 215
5 6 , ~ , 1942 : J.B.A.A., 57,54,1946). Flares in this very active solar region were also responsible for crochets A Nos. 17 and 18 : crochets B No. 23 and probably Nos. 22 and 24.
No. 17. One of the most distinctive crochets in H of the whole epoch, associated with an intense flare and a decisive fade-out. A great storm followed 198 hours later: Obserwatory, 64, 260, 1942; M.N., 103, 246, 1943. "Solar noise" was detected for the first time from February 26 to 28. At Hermanus (Lat. 34'04 S. : Long. 1 ~ . 3 E.) AH commencing 1 2 ~ 01"' U.T. was relatively small and oscillatory, but AD was conspicuous, 13'-9 westerly increase in four minutes (Ten. Magn. atmos. Elect., 47,181, 1942). A large crochet in H was recorded at Huancayo. At Lenvick H was -88y and in shape closely resembles that at Abinger .
No. 22. The largest crochet of the present data and of great interest because of the synchronous burst of " solar noise " (-160 fold that of the adjacent level) as observed by Mr J. S. Hey. Cloudy weather prevented the solar flare, that must have occurred, from being observed. The solar area of disturbance was most probably that of the great February sunspot making its second passage of the disk. No geomagnetic storm followed about a day later but the probable emitting region was only just within the general statistical limits of 45" or 50" from the centre of the disk for a storm to occur. The crochet impulse was unusually rapid, - 81y in 2.2 minutes, almost resembling an inverted SC.
A relatively small crochet but of significance in connection with a synchronous burst of solar noise, observed at about 0 9 ~ 35m by Mr M. Ryle at the Cavendish Laboratory, Cambridge. The giant July sunspot came to the central meridian again on this day (August 23).
Although there is a very satisfactory degree of synchroniz- ation with a solar flare and its accompanying fade-out, this trace.-movement occurred.during minor magnetic disturbance, and its identity as a crochet is therefore open to doubt. It is the only one listed with a positive movement in H. The solar flare was brief but, though not extensive, was exceptionally brilliant. (Obserwatory, 64, 263, 1942.)
No. 23.
List B, No. 23.
No. 31. No. 36.
The AD impulse is distinctively that of a crochet. The impulse is shown in V only, but there is good corroboration in
A burst of solar time with a marked fade-out ; the Sun's zenith distance was 94" noise was recorded in the U.S.A. (Nature, 158, 945, 1a46).
Acknowledgments are gratefully made to the Astronomer Royal for permission to carry out this piece of work additional to that on sudden commencements.
My thanks are due to Dr R. d'E. Atkinson, Chief Assistant, and also to Mr W. M. Witchell, who has charge of the Magnetic Department, for helpful discussion.
Royal Observatory, Greenwich :
1947 March 24.
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