IGY IN RETROSPECT—ROBERTS 273 
dence and oblique radio probes or soundings from the ground to deter- 
mine ionospheric conditions by echo recordings. They observed radio- 
wave propagation characteristics under all manner of conditions, and 
they sought to find the extent, magnitude, and location of electric 
currents comprising streams of the ionized particles. 
The ionospheric electric currents are particularly intense and com- 
plex in the auroral zones, but they exist everywhere in some form. 
Interesting features of local ionization and a strong electric current 
were found in the close vicinity of the magnetic equator. Much was 
learned about the neutral and ionized gases of the ionospheric re- 
gions—helpful information in the problem of determining the best 
working radiofrequencies to suit conditions. Scatter propagation of 
radio waves, often useful despite disturbed conditions, was studied 
intensively. Among the techniques responsible for such findings were 
geomagnetic studies and the analysis of radio signals from earth 
satellites. 
Geomagnetism.—The geomagnetic program of the IGY was di- 
rected mainly toward the investigation of magnetic storms and other 
transient phenomena related to the ionospheric electric currents. 
Special arrays of recording stations were used to discover the dimen- 
sions of the magnetic fields in question, and thus to describe the cur- 
rent streams and systems. Among such arrays were ingenious “dif- 
ferential magnetographs” designed to record the field gradients across 
station arrays continuously as they fiuctuate. 
Stations in several places close to the magnetic Equator strongly 
indicated the intensity and extent of the previously mentioned equa- 
torial electrojet—a powerful concentrated stream of current almost 
precisely along this zero-dip line. It exists chiefly on the sunlit side 
of the earth, and it must result from ionization by ultraviolet light 
or X-rays, rather than by energetic particles, which presumably can- 
not cross the magnetic field lines to that region. Electron and ion 
counts were made in the body of the current by rocket firings. The 
circuit return mechanism has not been determined. 
The large number of special IGY magnetic observatories, partic- 
ularly in Antarctica and other new places, provided working material 
for extensive investigations into many aspects of natural electric and 
magnetic phenomena and their relation to solar events. For the first 
time they furnished evidence of essentially simultaneous worldwide 
magnetic effects, and served to round out our global concepts. Tel- 
luric currents (in the earth’s crust), the result of induction from the 
ionospheric currents, were found in unexpectedly great intensity and 
wide distribution, finely dissected by local ground conditions. The 
combined effects of ionospheric and telluric currents posed complex 
new problems impeding analysis attempts. 
