Radio wave emissions in the v.l.f.-band observed near the auroral zone—I occurrence of emissions during disturbances

Abstract

Abstract Very low frequency emissions in the 8 kc/s-band have been recorded at the Auroral Observatory, Tromso and bursts of enhanced emissions have been compared with geomagnetic records, absorption effects recorded on a riometer and appearance of aurorae. It is shown that bursts of enhanced radiations in the 8 kc/s-band appear regularly during disturbed conditions near the auroral zone, the energy is of the order 1–10 × 10 −16 W / m 2 c . The occurrence of the bursts exhibits a pronounced diurnal variation, their occurrence is limited to the time interval 1800–2400 hours M.E.T. The average duration of the bursts may vary from one minute to several hours, with a maximum occurrence of 10–12 min duration. The v.l.f.-emissions show a complicated dependence on geomagnetic disturbances and absorption within the ionosphere. Small dips of absorption in the riometer curves of the order 0·3–0·9 dB may be accompanied by v.l.f.-emissions; there is a positive correlation. Great values of absorption of the order 5–9 dB are accompanied by a complete cessation of v.l.f.-emissions; there is a negative correlation. These two effects indicate conclusively that the v.l.f.-emissions are strongly absorbed in a disturbed ionosphere. The moderate positive geomagnetic disturbances in H appearing early in the night (which is often followed by quiet auroral forms, arcs and bands on the Northern sky) are very often accompanied by strong v.l.f.-emissions. Sudden and strong disturbances in the geomagnetic field when these appear as a sudden commencement, are usually followed by a pronounced v.l.f.-emission. When the disturbance develops with (usually negative) peaks in H the v.l.f.-emissions are completely absorbed. It is pointed out that the close connection between the v.l.f.-emissions and the geomagnetic disturbances is limited to appear only in the time interval 1800–2400 hours M.E.T. Outside this 6-hr interval v.l.f.-emissions do not appear independently of the degree of geomagnetic disturbances, auroral luminosities or level of ionospheric absorption. The point of view that the v.l.f.-emissions are generated in the near and local part of the ionosphere by an impact of charged particles (for instance as Cerenkov-radiation) which simultaneously are producing auroral luminosities, must therefore be regarded as less probable. If this were the case one would expect an appearance of v.l.f.-emissions throughout the whole night. Records demonstrating the parallel amplitude variations in v.l.f.-emissions and 5577 A-photometer recordings are shown. Parallel records of v.l.f.-emissions have been made at Tromso and Kiruna, the latter lying 210 km to the South of Tromso. In some cases the Kiruna-records show similar changes in amplitudes for great bursts as the Tromso-records. Three cases of “noise storms” have been recorded during which the enhanced v.l.f.-radiations appeared during the whole night, the “noise storms” coinciding with large geomagnetic disturbances. The amplitude of the “storm” declined and disappeared when the ionosphere was illuminated by the sun's rays in the morning and when the normal day-time absorption was formed.—In an Appendix several cases of enhanced radiation are presented on the riometer recording frequency of 28 Mc/s. Records are shown which demonstrate coincidence between v.l.f.-bursts on 8 kc/s and sudden increases in the normal galactic noise level received on 28 Mc/s. These common bursts appear only when a sudden deflection in the geomagnetic curves appears, which may often have the character of a sudden commencement. The 28 Mc/s bursts are rare events, but it is concluded that the bursts are not produced by man-made interference.