Rapid auroral fluctuations are categorized into several groups according to their deformation modes, and a composite distribution of these modes is given with respect to the auroral oval. The distribution could impose some restrictions on the nature of plasma in the magnetosphere involved in the auroral precipitations.

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In the middle of the 18th century Celsius observed that there was a correspondence between a great aurora in Europe and the extreme motion of his observed compass needle. By the nineteenth century it was well established that the geomagnetic field always fluctuated violently at the height of an auroral display, that the high latitude zones of peak field disturbance and luminosity had similar locations, and that there were concurrent solar cycle changes in activity levels for the two phenomena. After the International Geophysical Year of 1957 to 1959 the correspondence of the ionospheric electrojet currents and auroral forms became a focus of observational programs. In these recent times the studies of a relationship between short-period pulsations of the geomagnetic field and luminosity pulsations (or the pulsations of bombarding, low energy electrons causing the auroral intensity changes) appeared prominently in the literature. A summary of the scientific progress in the study of these phenomena is presented in this paper.

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Auroral zone microburst electron precipitation is reviewed. It can be shown from existing data that microbursts represent a major precipitation form on the dayside magnetosphere. Balloon X-ray data indicate that microbursts are periodic and they are small-scaled. Rocket and sattellite data indicate that microbursts may consist of substructures and that microburst precipitation is fractionally more enhanced along the magnetic field direction. The source of electrons appears to be nearby, _??_4R_e from the surface of the earth. Microbursts are correlated to VLF and magnetic micropulsations. Study of microburst phenomena should lead to better understanding of wave-particle interaction mechanisms responsible for electron precipitation.

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Discrete auroras are primarily composed of linear elements, whereas diffuse auroras appear in a variety of shapes. Dynamical variations within discrete auroras are characterized by shear, evidently caused by field-aligned currents and charge excesses. In contrast, diffuse, pulsating aurora exhibits little shear, indicating lack of large currents and electric fields associated with this type of aurora.

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A short history of studies of Birkeland currents and a summary of measurement techniques is provided. The observations made to date show that discrete auroral arcs and upward flowing Birkeland currents are usually associated. There is experimental uncertainty about whether these upward currents are strongest just above the arcs or are concentrated at the edges. A consistent pattern relating the location of the downward return current, the electrojet, and the electric field has not been found.

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The present state of knowledge on the relationships between high-latitude particle precipitation and the aurora is reviewed. Attention is focused on the large-scale relationships between auroral forms and magnetospheric particle populations, on the relationships between satellite and sounding-rocket measurements, and on the interaction of auroral electrons with the atmosphere. While significant progress is being made in relating the large-scale features of the aurora to magnetospheric plasma domains, and in understanding the way in which auroral electrons deposit their energy in the atmosphere, only slight progress has been made in relating satellite data to the small-scale phenomena associated with auroral arcs.

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The theoretical basis and the experimental evidence for the interpretation of photometric data in terms of precipitating particle morphology will be reviewed. Particular emphasis will be laid on the interpretation of auroral morphology in terms of the spectroscopic signature of the precipitating particles. The meridian scanning photometric data will be discussed briefly to emphasize the auroral morphology related findings. The new type of monochromatic TV system which was specially developed for two-dimensional spectrophotometric auroral imaging will be described. The auroral morphology related findings will be discussed according to the local time sector of the observations. The early evening auroras are characterized by soft electron precipitation on the equatorward edge of the auroral oval. This precipitation is not a steady diffuse region but it is structured and the structures show coherent convective motion often in the westward direction. Harder precipitating features superimpose on this soft precipitation. These harder features (electrons 2keV or larger) are observable by all-sky cameras and they participate in the substorm auroral dynamics. Following substorm onsets and northward expansions the electron auroras harden considerably and the protons expand poleward. During early morning the diffuse soft regions occur poleward of the hard auroral features. These early morning hard auroral features often show very hard pulsating patches.

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Magnetic-field-aligned electric fields in the magnetosphere can be generated in several different ways.
Current driven wave instabilities can lead to either anomalous resistivity or electric double layers. Both can support potential drops of many kilovolts. In the former case this requires wave turbulence extended over large distances, because observed wave amplitudes allow only moderate field strength. In the latter case the voltage drop is concentrated to one or more (perhaps numerous) thin regions, sometimes referred to as electrostatic shocks, each of the order of several tens of Debye lengths thick. Once established, such layers may exist with or without substantial wave turbulence. It has also been proposed that wave-particle interaction may establish a collisionless version of the thermoelectric effect. In the hot magnetospheric plasma potential drops can be created as a consequence of the magnetic mirror force. In a magnetically trapped plasma this, in principle, requires no current, although some leakage current is expected. The mirror effect has its most dramatic impact in regions of upward Birkeland currents, where it limits the current density to moderate levels unless large voltages are applied (many times the voltage-equivalent of the thermal energy of the magnetospheric plasma). The distribution of this voltage depends critically on the distribution functions of the plasma particles involved, and it may well degenerate into electric double layers.
Observational data now available indicate that more than one of the mechanisms mentioned are operative in the magnetosphere but it is not yet possible to evaluate their relative importance.

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Discrete auroral arcs involve both magnetic-field-aligned electric fields and currents. Theories agree that the currents close below the arcs through the E region ionosphere. Postulated closures above the arcs include: (1) currents due to pressure gradients; (2) polarization currents; (3) currents in the conjugate ionosphere; (4) currents on the magnetopause. The energization of particles to several keV by parallel electric fields requires an energy input and hence a force doing work in the outer magnetosphere. For the above, this force is identified as: (1) diamagnetic repulsion of the plasma by the earth's field; (2) inertial forces as a flux tube gains and loses momentum due to interaction with the ionosphere; (3) magnetic stresses due to conjugate points not being those required for a minimum energy magnetosphere; (4) discontinuities in the magnetic stresses at the magnetopause arising from either the different histories of flux tubes or topological features such as the cleft. Any of the above result in a shear in the magnetic stresses from one side of the current sheet to the other consistent with the existence of the current sheet. Thus, if the magnetospheric convective flow is decoupled from the ionosphere by a non-zero resistivity, rapid convection occurs in thin sheets on each side of the arc producing: (1) a potential drop along field lines; (2) a net downward component of the Poynting vector to the region of particle acceleration. It is concluded that the process is one of energy transfer from magnetic fields to plasma and the possibility arises that this could occur at any tangential discontinuity in space where the electric current is subject to a non-zero resistivity. The process is compared with merging and possible flows are presented.

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Anomalous resistivity and laminar shock mechanisms for the acceleration of electrons responsible for the discrete auroral arcs are reviewed. Ion acoustic or Buneman mode instabilities are shown unlikely to play any significant role in magnetospheric field line resistivity or electron acceleration. Although ion cyclotron turbulence is commonly observed, it is concluded that it is also unlikely to contribute to auroral acceleration. The double layer, oblique and V laminar shock models are reviewed. The relationship among these shock models and the relationship of the shock to the anomalous resistivity models is described. It is pointed out that the double layer model is inconsistent with the highly structured auroral forms. The V-shock model fits the observations so well that it appears that a fundamental understanding of the acceleration process for the discrete aurora has been achieved.

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We report the results of two on-going laboratory experiments studying the formation and steady-state characteristics of double-layers.

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