The Ionosphere in Polar Latitudes

Abstract

AT the Friday evening discourse delivered at the Royal Institution on October 27, Prof. E. V. Appleton described aspects of the work of the second International Polar Year. It is now known that there is an intimate connexion between magnetic storms, displays of the northern lights, and electrical currents in the upper atmosphere. Radio methods of sounding the electrified regions of the atmosphere have been developed in Great Britain, and the Polar Year Committee, besides sending out an expedition to Fort Rae in Canada, recommended that a radio expedition should also be sent to Tromso in northern Norway. The work of this party showed the marked difference between wireless transmission conditions in latitude 70 ° and in south-east England (see NATURE of September 2, p. 340). During the powerful magnetic disturbances which take place in Tromso, it was found that the ionised layer of the upper atmosphere ceased to reflect, thus making long-distance communication by short waves difficult, if not impossible. Such blank periods have a tendency to recur after 27 days, which is the time required for the sun to make one revolution. This result makes clear some of the anomalies which have been noticed in polar exploration. It will be remembered that in the case of General Nobile's polar flight in the airship Italia, there were two periods separated by about 27 days when wireless communication between the airship and the outside world was interrupted. It is clear from the records that these were both periods of intense magnetic activity during which we now know that radio waves are absorbed and not reflected by the upper atmosphere. The results of the British radio expedition therefore clearly indicate that if radio is to be used in polar exploration, note must be taken of this monthly sequence of unfavourable conditions. It would also be advisable to choose a year of sunspot minimum, and not a year of sunspot maximum as in the case of the ill-fated Italia expedition in 1928, since magnetic and wireless disturbances are relatively less marked when the spots on the sun are few.