SUN AND THE EARTH'S MAGNETIC FIELD^ — FLEMING 187 



for each of the eight 3-hour periods of the Greenwich day. The 

 activity in excess of the regular daily variation is measured for each 

 3-hour period and an index from to 9 is assigned in accordance 

 with the activity "0" for very quiet and "9" for extremely disturbed. 

 Thus, by means of eight indices, the geomagnetic activity for a day 

 is abstracted from the continuous magnetic records at each observa- 

 tory. The progress of a magnetic storm is readily followed and the 

 geomagnetic activity at one observatory may be compared with that 

 at another observatory in a different geomagnetic latitude; for ex- 

 ample, the K indices for the period 09"^ to 12^ February 5, 1942, at 

 observatories arranged in order of increasing north geomagnetic 

 latitude $ were 3, 3, 5, 5, 6, and 7 at Honolulu ($ = 21°), San Juan 

 ($=30°), Tucson (*=40°), Cheltenham ($ = 50°), Sitka (* = 60°), 

 and College ($ = G4°). For the same period the index at Watheroo 

 at 42° south geomagnetic latitude was 4. 



Violent magnetic storms with 3-hour-range index of 9 occur only 

 a few times near a sunspot maximum, but it is equally rare that any 

 full 3-hour interval is perfectly free from disturbance. This means 

 that the earth is almost constantly, even near sunspot minimum, 

 under the influence of particles (presumably solar), weak as this 

 influence may be at times. 



The intensity of the ionizing solar wave radiation absorbed in the 

 ionosphere on the daylight hemisphere can likewise be measured 

 geomagnetically in the amplitudes of the solar daily magnetic varia- 

 tion. Analysis of 18 years' records (1922-39) of horizontal inten- 

 sity at the Huancayo Magnetic Observatory, in which the magnitude 

 of the solar daily magnetic variation is exceptional, gave a measure 

 for the ionizing solar wave radiation comparable with the relative 

 sunspot numbers as the only available complete series of daily meas- 

 ures of solar activity. The correlation coefficient between them is 

 -f 0.92 for monthly means and -1-0.984 for annual means. These are 

 the closest relations so far established between phenomena on the 

 sun and the earth. 



From the indices supplied by individual observatories, a mean 

 index may be derived which will represent world-wide conditions. 

 The mean indices computed from the data supplied by seven Amer- 

 ican-operated observatories during the year 1941 show three out- 

 standing storms in 1941 on March 1, July 5, and September 18 and 

 19. A remarkable recurrence of quiet conditions also occurred dur- 

 ing the year. January 5 was the first quiet day of this series fol- 

 lowed by 13 quiet-day recurrences at exactly 27-day intervals. 



Magnetic disturbances may be classified as (1) world-wide mag- 

 netic storms, simultaneous over the earth and primarily due to 

 changes in the electric conditions outside the earth, and (2) smaller 

 disturbances restricted to parts of the globe and with center of field 



