1. Weekes, K., Polar Atmosphere Symposium, Part 2, 12 (Pergamon Press, 1957).
2. Maeda, H., Rep. Ionos. Space Res., Japan, 13, 79 (1959).
3. Martyn, D. F., The Physics of the Ionosphere, 163 (The Physical Society, 1955).
4. Dougherty, J. P., J. Atmosph. Terr. Phys., 20, 167 (1961).
5. Chapman, S., Nuovo Cimento, 4 (10), 1385 (1956).