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ANl«njAL REPORT SMITHSONIAN INSTITUTION,, 1942 



slow recovery. The activity is smallest at the earth's magnetic 

 equator and increases in a regular manner to its greatest value in the 

 auroral zone. This daily figure of activity, known as the u measure, 

 is roughly defined as the average change from day to day, regardless 



SOLAR HOURS 



LUNAR HOURS 



Figure 5. — (A) solar and lunar diurnal variation in summer at Greenwich; 

 {B) magnetic records on quiet day, August 8 and 9, 1929, Huancayo Magnetic 

 Observatory, Peru. 



of sign, of the magnetic force near the equator. It is not useful as 

 a measure for a single day, but its monthly and annual values express 

 quite definitely the average amount of magnetic disturbance. 



A certain lag of u behind the sunspots supports the theory that 

 when sunspots are nearer the sun's equator there is greater probability 

 that the corpuscles, emitted more or less radially from the sun, are 

 more likely to sweep across the earth. 



Developments in the science of geomagnetism and correlative ef- 

 fects with other geophysical phenomena, especially those concerned 

 with radio communication, have made desirable a quantitative and 

 more detailed measure of geomagnetic activity. This measure is 

 the 3-hour range index, K^ which evaluates geomagnetic activity 



