Abstract
By means of a systematic spectral analysis of the irregular pulsations of the Earth's magnetic field that were recorded at two geomagnetically conjugate stations, we were able to classify them in two categories already introduced by Troitskaya (1961): the irregular pulsations of diminishing periods (IPDP) and the short irregular pulsations (SIP); this study deals only with the former category. An IPDP consists of a broad-band noise (Δƒ ∼ 0·3−0·6 c/s) , the mean frequency of which suddenly increases (from ∼0·5 to ∼1.· c/s in approximately 30 mn). Upon this noise, some stronger oscillations, with a rapidly increasing frequency, are super-imposed. It is shown that these oscillations appear simultaneously at the conjugate stations, and that they have an opposite polarization. The IPDP occur principally between 1700 and 0100 local time, at geomagnetic latitudes between ∼55° and ∼65°, and during periods of magnetic activity ( Σ K p ∼ 25). They are related to the aurorae and to the apparition of an auroral sporadic- E layer. The comparison with satellite measurements made at the same time shows that the IPDP occur when there is: (a) a variation of the flux of low energy (∼050–150 keV) trapped electrons, (b) a diminution of the size of the outer Van Allen belt, (c) an increase of the magnetic field in the tail of the magnetosphere. A discussion of the experimental results leads to the following hypothesis: the IPDP are generated by a wave-particle interaction with low energy protons (∼10–100 keV) taking place at ∼5–6 Earth's radii. During the deconnexion of the geomagnetic lines offeree which follows a sudden commencement, the interaction region moves towards the Earth, thus leading to an increase of the emitted frequency. It is inferred that the deconnexion of the magnetic field lines is to take place at first on the evening part of the magnetosphere.
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