History of Geophysical Research in The Netherlands ... - DWC - KNAW

on the formation of the ionized layers, on the propagation of radio waves in
the ionosphere, and on prediction and selection of the most suitable frequencies
for covering a certain distance by radio. In a further paper (Groenewold,
1949) he discussed temporary variations in the ionization and the influence on
the ionosphere of outbursts on the sun. He published also a theoretical study
on the triple magnetic splitting of radio waves reflected in the F2-1ayer (Groenewold,1950).
Veldkamp and Scholte (1954a) also studied the magnetic double refraction of
electromagnetic waves in the ionosphere. They investigated the equatorial
electrojet in the ionosphere, as revealed by the analysis of solar flare effects ,
and explained some properties of the electrojet by the non-coincidence of the
geomagnetic and geographic equators (Veldkamp and Scholte, 1954). H.J.A.
Vesseur (1970) succeeded in inferring from the fading pattern of radio waves
reflected in the ionosphere the rhythmic tidal movements of air in the ionosphere,
which are responsible for the diurnal geomagnetic variation. The wind
movements in the E-layer are recorded almost continuously by an automatic
apparatus developed by Vesseur. He could determine the diurnal and the semidiurnal
period of the tidal movements in the E-layer above the Netherlands
over a period of years (Vesseur, 1972). A similar apparatus was set up near
Paramaribo (Suriname); it was in operation from January 1971 until September
1972.
H. Kelder (1975) investigated the influence of acoustic gravity waves on ion
density in the E-layer. The tidal movements are in fact strongly disturbed by
acoustic waves with a very long wavelength , which travel through the ionosphere
and the thermosphere (Kelder, 1976).
Temporary disturbances in the geomagnetic field, which are caused by "disturbed"
solar radiation - corpuscular as weIl as electromagnetic - penetrating
the outer layers of the atmosphere, are important for changing the reflecting
power of the ionosphere. For this reason study of the ionosphere is of practical
importance for radio propagation through the ionosphere (Vesseur, 1974).
The results of this research are published each month by the KNMI in an "Ionospheric
Bulletin". In this monthly publication are given, not only heights and
plasma freq uencies of the ionospheric layers, but also factors for calculating the
radio frequencies for covering a certain distance and further the ionospheric
and geomagnetic disturbance-indices, disturbances in the intensity of cosmic
radiation , intensity of radio radiation from the sun and summaries of special
phenomena of the solar radiation (KNMI, 1949 up to the present).
The monthly publication of the "Ionospheric Bulletin" by the KNMI gives
evidence of the close relation existing between the ionosphere, the earth's
magnetic field, cosmic radiation and phenomena on the sun.
1.5. GEOMAGNETISM AND COSMIC RAYS
About 1930 J. Clay (professor at the Technical University of Bandoeng from
1920 to 1929, and professor at the Municipal University of Amsterdam from 1929
to 1952) discovered that the geomagnetic field exerts an influence on cosmic
rays. During his voyages bet ween the Netherlands and the Netherlands East
Indies, Clay studied the intensity of cosmic radiation (at that time called ultraradiation)
. lt appeared that this very penetrating radiation depended on the
geomagnetic latitude. The intensity was minimal in the Gulf of Aden, which is
in the neighbourhood of the geomagnetic equator (Clay, 1932). From this the
corpuscular character of cosmic rays became evident; only electrically charged
particles can be influenced by the magnetic field of the earth. The latitude
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