ON THE EXISTENCE OF A REGION OF ENHANCED AURORAL ACTIVITY TO THE SOUTH OF THE ZONE OF MAXIMUM AURORAL FREQUENCY During the International Geophysical Year visual observations of polar aurorae were carried out by a large network of meteorological stations in the Soviet Union. According to the international agreement, the data had to be reduced to tables of the southern limit of aurorae seen near the zenith. The southern limit of visibility of polar aurorae is one of the most objective characteristics of auroral activity.
The statistical treatment of the initial material for the period from September 1957 to December 1958 presented to us by N.V. Pushkov (IZMIRAN) showed that the behavior of the southern limit of polar aurorae (aurora borealis) near the zenith has the following remarkable property: 1. In the first half of the day (00-12 hrs universal time) the mean position of the southern limit of zenith auroral visibility has a distinct stable maximum at geomagnetic latitude cp= 60-62°.
2. In the second half of the day (12-24 hrs universal time) the mean position of the southern limit of zenith auroral visibility, besides the maximum at cp = 60-62°, develops a second distinct maximum at cp = 57-58°.
This pronounced regularity is illustrated in Figure 1, which gives the mean distribution of the number of occurrences of southernmost zenith aurorae as a function of the geomagnetic latitude in the first and the second half of the day for separate geomagnetic longitudinal belts. Figure 2 gives the results of an analysis of the behavior of these maxima for various geomagnetic activities (K-index)*. The figures lead to the following conclusions: 1. Aurorae clearly prevail in the second half of the day; their number increases with increasing K-index.
2. In the first half of the day (00-12 hrs U.T.) the mean position of the southern limit of auroral visibility shifts southward by a mere 2° with increasing geomagnetic activity; with K= 1 the maximum is located at cp= 62°, and with K>5 at cp= 60°.
3. In the second half of the day (12-24 hrs U. T.) the position of the maximum is more stable; it is located on the average 1° to the south of the maximum observed in the first half of the day. As the K-index increases,
the maximum almost does not change its latitude.
4. Although retaining its mean position despite variation of the K-index, the second maximum (at cp= 57°) sharply changes in magnitude. A weak maximum appears at K= 2; at K= 3 it is already well pronounced; at K= 4 the maximum is more than twice as large as the first maximum (at cp= 60-62°).
• K-index according to the data of Soviet stations was taken from "Kosmicheskie dannye" published by IZMIRAN. To ensure higher statistical reliability the data for longitudinal belts 145-190' and 190-235' have been combined.
275 If radiation escapes from the outer belt along the geomagnetic force lines, it should reach the Earth's atmosphere at cp= 52-62°, creating X-rays there.
It is assumed that this radiation gives rise to X-rays of a special kind of polar aurora observed to the south of the main auroral zone and accompanied by magnetic disturbances with Indeed, on 16-17 August a bright polar aurora of ray structure was observed at a latitude of 57°.
Our conclusions statistically confirm the possibility of the existence of an enhanced auroral zone in middle latitudes during geomagnetic storms; the results of studies of these aurorae can be successfully used to elucidate the behavior of the outer radiation belt, which is related to the solar cor puscular radiation /2, 3, 4/.
Barbier and Glaume /8/ observed in winter in Tamanrasset (22°47'N) arcs which moved from the north to the south toward the end of the night. In these arcs the 6300A radiation reached an intensity of 557 R and the 5577 A radiation had an intensity of 500 R. At the same time in Haute Pro vence (43°56'N) the intensity of 6300 A radiation was — 200R. In summer the intensity of 6300 settled to the value common for night-sky glow. The authors maintain that the arcs in Tamanrasset were due to the existence, in low latitudes, of auroral zones related to electrons of the inner radiation belt. The mechanism of polar aurorae in middle and low latitudes during geomagnetic storms can thus be of the same origin, being directly connected with radiation belts of the Earth.