The ELF emissions were detected in the midnight sector of the magnetosphere in conjunction with magnetospheric substorms. The emissions were observed at local midnight and early morning hours and are accordingly called 'post-midnight chorus.' The characteristics of these emissions such as their frequency time structure, emission frequency with respect to the local equatorial electron gyrofrequency, intensity-time variation, and the average intensity were investigated. The occurrence of the chorus in the nightside magnetosphere was investigated as a function of local time, L shell, magnetic latitude, and substorm activity, and the results of this analysis are presented. Specific features of postmidnight chorus are discussed in the context of possible wave-particle interactions occurring during magnetospheric substorms.

Postmidnight chorus: A substorm phenomenon

Starting with post World War II studies of fading of radio star sources and continuing with fading of satellite signals of Sputnik, vast quantities of data have built up on the effect of ionospheric irregularities on signals from beyond the F layer. The review attempts to organize the available amplitude and phase scintillation data into equatorial, middle-, and high-latitude morphologies. The effect of magnetic activity, solar sunspot cycle, and time of day is shown for each of these three latitudinal sectors. The effect of the very high levels of solar flux during the past sunspot maximum of 1979-1981 is stressed. During these years unusually high levels of scintillation were noted near the peak of the Appleton equatorial anomaly (∼ ±15° away from the magnetic equator) as well as over polar latitudes. New data on phase fluctuations are summarized for the auroral zone with its sheet-like irregularity structure. One model is now available which will yield amplitude and phase predictions for varying sites and solar conditions. Other models, more limited in their output and use, are also available. The models are outlined with their limitations and data bases noted. New advances in morphology and in understanding the physics of irregularity development in the equatorial and auroral regions have taken place. Questions and unknowns in morphology and in the physics of irregularity development remain. These include the origin of the seeding sources of equatorial irregularities, the physics of development of auroral irregularity patches, and the morphology of F-layer irregularities at middle latitudes.

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Global morphology of ionospheric scintillations

The distribution of extremely low frequency (10-1500 Hz) magnetospheric chorus to all local times and latitudes is investigated in order to determine dependence on substorms, and to evaluate the conditions under which chorus is generated. The analysis carefully separates space and time effects by an investigation of data obtained by the OGO 5 search coil magnetometer. A study of spatial dependencies shows that chorus occurs in two magnetic regions: equatorial chorus is located near the equator, and high-latitude chorus is located above 15 degrees. An analysis of chorus in each of the regions illustrates that equatorial chorus is definitely related to substorm, whereas high-latitude chorus often occurs within magnetically quiet intervals.

Two types of magnetospheric ELF chorus and their substorm dependences

Fast magnetic field-line merging at the magnetospheric bow and in the tail are examined to determine their implications in regard to the concept of bulk motion of the magnetosphere (convection) and the associated electric field. Upper atmosphere geophysical measurements are surveyed to determine to what extent the available data support this concept. The dawn-dusk asymmetry in energetic particle fluxes, the movement of auroral ionization, ionospheric currents, and the location of the whistler knee (or ‘plasmapause’) are all consistent with this concept, the latter three all giving estimates of the electrostatic potential difference across the magnetosphere in the dawn-dusk meridian of a few tens of kilovolts. A ‘present best estimate’ of the flow pattern in the magnetosphere is derived, based primarily on the diurnal variation in the location of the whistler knee. The convective flow in from the tail appears to be stronger before midnight than after, which is consistent with the location of the maximum nighttime precipitation of energetic electrons. The derived flow is also qualitatively in agreement with that deduced from field-line merging. However, the Axford-Hines types of viscous drag will also give rise to convective flow, and the currently available data do not allow a definitive determination of whether field-line merging or viscous interaction is the principal driving force.

N.M. Brice

Papers

The general pattern of auroral particle precipitation