PRINCIPAL MAGNETIC STORMS 8•

at about 16 h, November 9, and was replaced by short-period, low-amplitude oscillations superposed on large bays. This condition continued until •bout 11 h 15 m, November 11, when again there occurred v]o!ent changes. Between 13 h 25 m and 14 h 15 m a K-index of 9 was recorded; however, after 15 h, November 11, there was a marked decrease in storminess when the character of the disturbance changed to rapid oscillations of low amplitude. The storm began to die out at 01 •, November 12, and it may be said to have ended at 11•,. November 12.

December 6--A moderately disturbed period began gradually at about 08 • GMT, December 6, and continued until about 21 h of the same date. The most disturbed portion of the period was between 12 h and 15 h, when H and Z both decreased to form rather large bays. A_ K-index of 7 resulted.

December/•--Magnetic conditions again becam• moderately disturbed at about 09 h GMT, December 9, when rather small bays began to form. The greatest disturbance occurred between 12 a and 15 •, when a K-index of 6 was recorded. This abnormal activity subsided at about 21 a, December 9; however, there was lesser disturbances during the dark. hours for several days following.

. Jo•. B. Cz•rr•LL, Observer-in-Cha•'ge

Oc•os• •o D•c•s•, 1947

(Latitude 5• ø •8'.8 N., longitude 6 ø ,•0'.1 or O h •6m.7 E. of Gr.)

October 1-3---This period of magnetic activity •ith only temporarily moderate disturbances had started suddenly at 18 • 09 m, September 30. Activity was strongest between• 14 • 50% October 2• .and 6 •, October 3. On al• three days a K-index of 5 was noted for the first 3-hour-interval, while between !2 • and 24 a, October 2, one K-index of 6 and three K-indices oœ 5 occurred.

Ociober 7--i bay-disturbance (mainl'5; in D) between 20 a 20 • and 22 a 15 m gave rise to two K-indices of 5. A weak*auroral arc was observed from about 20 a 40 • till 21 • 10 TM-

October 9-16--In this period magnetic activity was limited mainly to the hours between 15 a and midnight. The disturbances were nearly sll bays or had a bay-like character; in the evening hours on October 9-12 some of these bays were accompanied by weak aurorM forms (H.A. or 1•.). 17 K- indices of 5 and two of 6 were noted; the distribution of theirs occurrence over the eight 3-hours interv•s was' two indices of 5; none; one indei of 5.; none; none; four indices of 5; four indices of 5 and two oœ 6; six indices of 5.

October 18-œO--Again a period with' only temporarily moderate dis- turbance, mainly between 18 a and 24 •. Four K-indices of 5 were noted,

86 PRINCIPAL MAGNETIC STORMS [VOL. 53, No. 1]

while one K-index of 6 occurred in the seventh 3-hour-interval of October

19. in the latter case sqme auroral forms (tt.A. and 1%.) were observed. October 24--At 17 h 36 '• there was a weak abrupt commencement of an

insignificant disturbance. November 8-12--After slight activity during November 8 and the first

nine hours of November 9, a long 68-hour period of in general moderately disturbed conditions developed gradually after 8 h 57% November 9. The disturbance was strongest between 20 h, November 9, and 2 h, November 10; in that time-interval there was an auroral display behind the clouds and three K•-indices of 6 were noted. Magnetic activity increased again be!•ween 20 h, November !0, and 2 •, l•ovember 1! (three K•-indices of 5 occurred); and after an abrupt commencement at 6 n 54 m, November 11, activity gradually increased, giving rise to two/•-indices of 5 in the fifth and sixth 3-hour-interva!, November l!. At 5 •, November 12, the disturbance had nearly ended.

November œ3-œY•--A small disturbance (possibly a solar-flare-effect) was recorded from !2 a 14 • till 12 a 32 •, November 23; there was no confirmation as radio-informatiOn was incomplete owing to reduced service on Sunday (on 21 and 22 November there were radio reports about De!linger effects in the trai•c with South and Central America, respectively at about 11 a 50 m and 12 • 55•). At 17 • 56% November 24, a sudden commencement occurred. The disturbance that followed gave rise to one K-index of 6 for the seventh 3-hour interval. From 20 -• 30 •, November 24, activity vanished quickly.

November œ7--At 18 a 34 • there was an abrupt commencement of an insignificant disturbance.

December /--At 8 • 53 m an insignificant disturbance started with a sudden commencement.

December 4-7--A period of somewhat increased activity began at 6 a 15 m, December 4. On December 6, two K-indices of 5 were noted for the first and the last 3-hour interval. '

December œ3-œ•--A weak disturbance started suddenly at 11 • 26 m, December 23. After scarcely disturbed conditions during the first 21 hours on December 24 one K~index of 5 was noted for the last 3-hour interval

when a bay occurred. • H.P. TH. V• LOH•ZE•, 02•½er-in-Charge

CHELTENHAM MiGi•ETIC OBSERVATORY

OCTOBER TO DECEMBER, 1947

(Latitude 38 ø 44'.0 N.; Lor•gi•ude 76 ø 50'.5 or 5 • 07•.• W. of Gr.)

October 9-16--A prolonged period of moderate magnetic disturbance w• ushered in with long-period osc•lations, at 01 n GMT, October 9. Shor•