NO. 8 BAROMETRIC PRESSURE — DUEI.L AND DUELL 3 



regions is situated in the vicinity of that point where the hne con- 

 necting tlie center of the sun with the center of the earth crosses the 

 sun's surface. The radiation emitted from these regions, causes a 

 cliaracteristic brightening of certain coronal emission Hues, especially 

 5303 and 5694 A. The intensity of these lines can be measured with 

 the coronograph if the M-regions are situated at the cast or west 

 limb of the sun. With the assistance of such intensity measurements 

 of the mentioned corona lines, it will not be impossible to forecast 

 particle in\asions and accordingly also ionospheric storms, for the life 

 duration of these M-regions amounts to several weeks, and sometimes 

 even months (J. P.artcls (6), M. W'aldmcier (7)), the velocity of the 

 solar jiarticlcs can be determined with the aid of special methods, 

 and the rotation velocity for the respective solar zones is known. 



An ionospheric storm is particularly characterized by a considerable 

 decrease of the equivalent electron density in the so-called F-layer. 

 Frequently the behavior of the F-layer during an ionospheric storm 

 has been compared with an expansion, because at the same time there 

 can be observed an essential increase in height of the layer, and tem- 

 porarily there appear diffuse reflections from heights which exceed 

 the normal height by several hundred percent. In fact, very often 

 the "fixed-frequency" height records give the impression that the 

 whole F-layer is blown asunder. Frequently the occurrence of iono- 

 spheric storms is connected with the appearance of irregular geo- 

 magnetic disturbances or "magnetic storms" at approximately the 

 same time. 



B. Statistical Results 



As direct observations of the frequency and intensity of ionospheric 

 storms were not available for the period 1906- 1937, covered by this 

 part of our investigation, we had to use instead of these missing 

 data the rather reliable figures of the geomagnetic activity. Fitted 

 for our purpose were the international character figures for terrestrial 

 magnetic conditions which are averaged from the observations of 

 about 50 observatories and regularly published by the International 

 Union of Geodesy and Geophysics, Section of Terrestrial Magnetism 

 and Electricity. The use of 5 particularly disturbed and 5 particularly 

 quiet days in each month, selected by the Royal Meteorological Insti- 

 tute of the Netherlands (in De Bilt), can be justified by the advantage 

 that, after dividing the whole statistics arbitrarily into periods of 

 equal length, each part contains an equal number of key days. Such 

 a subdivision has been accomplished by considering separately : years 

 with high sun activity, where tlie annual mean of the relative sun- 



