This book is a collection of papers presented at a symposium held in 1964 January, and sponsored by the Lockheed Missiles and Space Company. It is an excellent source of much recent information concerning the aurora, and is well pravided with references. It suffers, however, from one regrettable defect-many of the papers seem to have been set down in print in much the same form as they were spoken. This fact, coupled with a Transatlantic fondness for elaborate terms-'a telemetry failure prevented data acquisition from other instruments'-makes for somewhat heavy reading.The material in the book is highly condensed, and the first two papers which deal respectively with the morphology of the aurora and with its optical measurements are very terse, dealing with only the facts and with very little of the discussion which might be expected. These two papers would make the subject seem very tedious to a newcomer. This is a great pity, because the book is only 170 pages long, and would have been improved by expansion.The third paper is a short account of the interaction of energetic charged particles with the atmosphere, and is followed by a series of longer (and therefore more interesting) papers on the more modem methods of studying the aurora and its associated phenomena. These include balloon measurements of X-rays, the precipitation of energetic particles, photoelectric and radar observations and finally an account of elaborate, and surely expensive, observations made simultaneously by satellite and aircraft.The book closes with an account of the present state of auroral theory and a short summing up. Despite the subtitle, it deals mainly with the observational aspects of the subject. There are a couple of instances where 'academic inexactitudes' crop up. On p. 120 the phase 'derivative ... as a function of' is used instead of 'with respect to'; this is momentarily confusing. Also, on p. 94, we are told that 'the scatter at given values of L is real, not just due to statistical fluctuations in counting rates'; but is not all scatter in physical measurements a statistical fluctuation?Despite the present reviewer's carpings, the book is well worth reading; it is a pity it is not longer.
Some of the spectral forms (e.g. ‘inverted’ hook) of discrete VLF emissions are not explained satisfactorily by present theories of generation based simply on gyroresonance between energetic streaming electrons and whistler‐mode waves traveling in the opposite direction. An extension of the gyroresonance idea is proposed in which the spatial variations of the electron gyrofrequency and the Doppler‐shifted wave frequency are matched. The coupling time between a resonant electron and the wave is then maximized, and hence the output wave intensity is maximized. Application of this condition leads directly to an expression for the time rate of change of emission frequency in terms of the location of the interaction region. An approximate analysis of the postulated interaction process leads to a theorem that states: The magnetic field intensity is limited to a value less than that at which the bunching time approximately equals the resonance time. When the input particle flux exceeds the value required to account for this limiting value of wave intensity, the interaction region drifts downstream. If the interaction begins on the falling‐tone or ‘upstream’ side of the equator, positive drift carries the interaction across the equator into the rising‐tone region, giving rise to the well known ‘hook’ shape. Reversal of the drift, resulting from wave damping or other factors, carries the interaction back across the equator, giving rise to the inverted hook, a shape not explained by previous theories. Combinations of positive and negative drifts can explain the principal emission forms. The triggering delay and offset frequency of artificially triggered discrete VLF emissions can be explained by the theory.
We report the results of measurements of low frequency magnetic noise by two independent monitoring systems prior to the occurrence of the MS 7.1 Loma Prieta earthquake of 17 October 1989. Our measurements cover 25 narrow frequency bands in the more than six‐decade frequency range 0.01 Hz–32 kHz, with a time resolution varying from a half hour in the ULF range (0.01–10 Hz) to one second in the ELF/VLF range (10 Hz–32 kHz). The ULF system is located near Corralitos, about 7 km from the epicenter. The ELF/VLF system is located on the Stanford campus, about 52 km from the epicenter. Analysis of the ELF/VLF data has revealed no precursor activity that we can identify at this time. However, the ULF data have some distinctive and anomalous features. First, a narrow‐band signal appeared in the range 0.05–0.2. Hz around September 12 and persisted until the appearance of the second anomalous feature, which consisted of a substantial increase in the noise background starting on 5 October and covering almost the entire frequency range of the ULF system. Third, there was an anomalous dip in the noise background in the range 0.2–5 Hz, starting one day ahead of the earthquake. Finally, and perhaps most compelling, there was an increase to an exceptionally high level of activity in the range 0.01–0.5 Hz starting approximately three hours before the earthquake. There do not appear to have been any magnetic field fluctuations originating in the upper atmosphere that can account for this increase. Further, while our systems are sensitive to motion, seismic measurements indicate that there were no significant shocks preceding the quake. Thus, the various anomalous features in our data, and in particular the large‐amplitude increase in activity starting three hours before the quake, may have been magnetic precursors.
Satellites OGO 1 and OGO 3 observe VLF discrete emissions in the magnetosphere primarily in a single, variable frequency band. The frequency ƒ of this ‘banded chorus’ depends on the equatorial electron gyrofrequency ƒHO for the field line passing through the satellite, typical ratios of ƒ/ƒHO being 0.2–0.5. Evidently the emissions are produced near the equator at a fraction of the electron gyrofrequency, as predicted by electron cyclotron resonance generation mechanisms. A secondary dependence of the banded chorus frequency on dipole latitude, such that the lower ratios of ƒ/ƒHO are found at higher latitudes, is interpreted to mean that the emissions are generated at about half the electron gyrofrequency, but deviate inward from the field line to lower L values as they propagate earthward. Theoretical support is given by ray tracings showing the inward deviation of nonducted whistler‐mode radiation due to the curvature of the magnetic field. Banded chorus has been observed at all local times, but is most common in the morning magnetosphere, outside the plasmapause.
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