Emission of VLF during the great disturbance of 25–26 May 1967

Abstract

During 25–26 May 1967 a violent disturbance appeared associated with great solar events of an unusual nature. A N.-S. chain of VLF recording stations (on 8 kc/e) was in operation covering geomagnetic latitudes Φ = 75°–50°. During the night 25–26 aurorae appearing over middle Europe started at about 23 UT At the same time strong VLF-emissions appeared at low-latidude stations Φ = 62°–50° with the greatest intensity at the most southern station. The appearance of VLF-emissions coincides with the start of a great Forbush-decrease of 11 per cent in the cosmic-ray intensity. The association of VLF-emissions, Forbush-decrease and SC of geomagnetic H with a solar flare of importance 3B is discussed and speeds of clouds from the Sun of the order of 850–1000 km/sec are deduced. It is concluded that the VLF-emissions are being produced through interaction with the magnetospheric plasma in outerspace of the charged particles producing the Forbush-decrease. The amount of absorption suffered by the VLF-waves when passing the disturbed ionosphere is discussed and it is concluded that the initial energy of the emissions must have been above 10 −11W/m 2c. It is pointed out that great Forbush-decreases are not always followed by VLF-emissions recorded.