A study of auroral displays photographed from the DMSP-2 satellite and from the Alaska meridian chain of stations

In the first part of the paper, a brief historical review of the progress of our knowledge on morphological aspects of the aurora is given. A particular emphasis is made in describing technical developments in auroral observations. In the second part, a large number of DMSP-2 photographs are examined in detail; in particular, substorm features, such as the initial brightening, the poleward expansion, westward traveling surges, eastward drifting patches, omega bands, torch-like structures, polar cap auroras, are illustrated. Whenever available, simultaneous photographs from the Alaska meridian chain of stations are used to describe in detail time variations of auroral displays before, during and after the passage of the satellite. In the last part, recent progress in understanding auroral phenomena is briefly reviewed from the point of view of magnetospheric physics.     

Electron precipitations and polar auroras

In the first part (Sections I–III) a brief historical review of the progress of our knowledge of the precipitation of auroral electrons is given. Observations by different techniques, in terms of detectors aboard balloons, sounding rockets, and polar-orbiting satellites, are reviewed (Sections I). The precipitation morphology is examined in terms of synoptic statistical results (Section II) and of latitudinal survey along individual satellite passes (Section III). In the second part (Section IV), a large number of simultaneous observations of auroras and precipitating auroral electrons by DMSP satellites are examined in detail, and it is shown that precipitation characteristics of auroral electrons are distinctly different for the discrete aurora and the diffuse aurora. In the third part (Section V), the source region of auroral electrons is discussed by comparing the auroral electron precipitation at low altitudes observed by DMSP satellites with the simultaneous ATS-6 observations near the magnetospheric equatorial plane approximately along the same geomagnetic field line. It is shown that the diffuse aurora is caused by direct dumping of the plasma sheet electrons from the equatorial region, whereas discrete auroras require acceleration of electrons between the plasma sheet and the polar atmosphere. The parallel electric field along the geomagnetic field line above the ionosphere is a likely candidate for the acceleration mechanism.Applied Physics Laboratory, The Johns Hopkins University, Laurel, Maryland 20810, U.S.A.     

Electron precipitation in the auroral zone

The present experimental knowledge of electron precipitation in the auroral zone is summarized, using observations primarily from riometer, balloon, rocket and satellite studies. This includes the association of electron precipitation events with aurora, magnetic disturbances as well as temporal and spatial aspects of the particle bombardment.     

Neutral Atmospheres

This paper summarizes the understanding of aeronomy of neutral atmospheres in the solar system, discussing most planets as well as Saturn’s moon Titan and comets. The thermal structure and energy balance is compared, highlighting the principal reasons for discrepancies amongst the atmospheres, a combination of atmospheric composition, heliocentric distance and other external energy sources not common to all. The composition of atmospheres is discussed in terms of vertical structure, chemistry and evolution. The final section compares dynamics in the upper atmospheres of most planets and highlights the importance of vertical dynamical coupling as well as magnetospheric forcing in auroral regions, where present. It is shown that a first order understanding of neutral atmospheres has emerged over the past decades, thanks to the combined effects of spacecraft and Earth-based observations as well as advances in theoretical modeling capabilities. Key gaps in our understanding are highlighted which ultimately call for a more comprehensive programme of observation and laboratory measurements.     

The Freja Hot Plasma Experiment—Instrument and first results

The Hot Plasma Experiment, F3H, on boardFreja is designed to measure auroral particle distribution functions with very high temporal and spatial resolution. The experiment consists of three different units; an electron spectrometer that measures angular and energy distributions simultaneously, a positive ion spectrometer that is using the spacecraft spin for three-dimensional measurements, and a data processing unit. The main scientific objective is to study positive ion heating perpendicular to the magnetic field lines in the auroral region. The high resolution measurements of different positive ion species and electrons have already provided important information on this process as well as on other processes at high latitudes. This includes for example high resolution observations of auroral particle precipitation features and source regions of positive ions during magnetic disturbances. TheFreja orbit with an inclination of 63° allows us to make detailed measurements in the nightside auroral oval during all disturbance levels. In the dayside, the cusp region is covered during magnetic disturbances. We will here present the instrument in some detail and some outstanding features in the particle data obtained during the first months of operation at altitudes around 1700 km in the northern hemisphere auroral region.     

Flow Bursts in the Plasma Sheet and Auroral Substorm Onset: Observational Constraints on Connection Between Midtail and Near-earth Substorm Processes

Causality between near-Earth and midtail substorm processes is one of the most controversial issues about the substorm trigger mechanism. The currently most popular model, the outside-in model, assumes that near-Earth reconnection is initiated in the midtail region before substorm onset and that the associated flow burst causes tail current disruption in the near-Earth region. However, there remain some outstanding issues that may serve as critical tests of this model. The present article reviews recent satellite and ground observations addressing three such critical issues with a focus on substorm-related auroral features. First, near-Earth reconnection, even if it reaches the lobe magnetic field, does not necessarily trigger a global substorm, but it is often related to a pseudobreakup. This fact suggests that there is an additional or alternative condition for substorm development. Secondly, although there appears to be one-to-one correspondence between flow bursts in the plasma sheet and equatorward-moving auroral structures (auroral streamers), no such auroral feature that can be associated with the fast plasma flow can be identified prior to auroral breakups. On the other hand, the flow burst is widely regarded as a manifestation of reconnection and therefore, according to the outside-in model, should be created in the near-Earth plasma sheet before substorm onset. Finally, auroral arcs poleward of a breakup arc are not affected until the front of auroral intensification reaches those arcs. The last two points suggest that if substorm is triggered as the outside-in model describes, the ionosphere is electromagnetically detached from the magnetosphere, which, however, has not been addressed theoretically. Thus, it should be crucial for a better understanding of the substorm trigger process to implement the magnetosphere-ionosphere coupling in future modeling efforts and to address those basic issues as a guide for critically evaluating each model.     

Alfvén Waves and Their Roles in the Dynamics of the Earth’s Magnetotail: A Review

The Earth’s magnetotail is an extremely complex system which—energized by the solar wind—displays many phenomena, and Alfvén waves are essential to its dynamics. While Alfvén waves were first predicted in the early 1940’s and ample observations were later made with rockets and low-altitude satellites, observational evidence of Alfvén waves in different regions of the extended magnetotail has been sparse until the beginning of the new millennium. Here I provide a phenomenological overview of Alfvén waves in the magnetotail organized by region—plasmasphere, central plasma sheet, plasma sheet boundary layer, tail lobes, and reconnection region—with an emphasis on spacecraft observations reported in the new millennium that have advanced our understanding concerning the roles of Alfvén waves in the dynamics of the magnetotail. A brief discussion of the coupling of magnetotail Alfvén waves and the low-altitude auroral zone is also included.     

Observations of inverted-V electron precipitation

The characteristics of inverted-V electron precipitation fluxes deduced predominantly from observations by the Atmosphere Explorer satellites are reviewed. The energy and pitch angle distributions are presented and shown to be generally in agreement with acceleration by a parallel electrostatic potential. Characteristics of secondary electrons are examined, and effects of beam plasma instabilities on these electrons are discussed. The properties of the monoenergetic component are compared with theoretical models of creating parallel DC electric fields, and found to favor the anomalous resistivity model. The article also discusses relations of inverted-V events with other auroral phenomena including auroras, electrostatic shocks, convective electric field reversals, field-aligned currents and wave emissions. The principal conclusions are: (1) plasma sheet electrons are continuously accelerated to form inverted-V structures in the pre-midnight hemisphere independent of substorm phase, (2) the acceleration processes are probably related to large scale electrostatic wave turbulence observed at altitudes of a few thousand kilometers, (3) narrow bursts of intense electron precipitation fluxes are found to be imbedded within some inverted-V's. It is argued that the narrow bursts of intense electron precipitation have the proper characteristics to cause discrete auroral arcs in the atmosphere. We suggest that these narrow bursts are accelerated by an electrostatic shock at higher altitude and capable of producing discrete auroral arcs below the observing satellite.     

Auroral Radio Emissions, 1. Hisses, Roars, and Bursts

The Earth's auroral electrons produce copious non-thermal radio emissions of various types, including auroral kilometric radiation (AKR), whistler mode auroral hiss, mode conversion radiation such as auroral roar and MF-burst, and possibly HF/VHF emissions. In some cases, mechanisms have been identified and quantitatively described, whereby the energy of the auroral electrons is converted into electromagnetic radiation. In many other cases, the radiation mechanism, or the relative significance of several possible mechanisms, remains uncertain. This review covers fairly comprehensively experimental and theoretical research on types of auroral radiation other than AKR, concentrating on emissions with frequency higher than about 1kHz and treating only emissions which are unique to the auroral zone. The review covers both ground-based and in-situ observations. It covers a wide range of theoretical approaches, emphasizing those which at present appear most important for producing non-AKR auroral radiations.     

Auroral flares and solar flares

The morphology of development of auroral flares (magnetospheric substorms) for both electron and proton auroras is summarized, based on ground-based as well as rocket-borne and satellite-borne data with specific reference to the morphology of solar flares.The growth phase of an auroral flare is produced by the inflow of the solar wind energy into the magnetosphere by the reconnection mechanism between the solar wind field and the geomagnetic field, thus the neutral and plasma sheets in the magnetotail attaining their minimum thickness with a great stretch of the geomagnetic fluxes into the tail.The onset of the expansion phase of an auroral flare is represented by the break-up of electron and proton auroras, which is associated with strong auroral electrojets, a sudden increase in CNA, VLF hiss emissions and characteristic ULF emissions. The auroral break-up is triggered by the relaxation of stretched magnetic fluxes caused by cutting off of the tail fluxes at successively formed X-type neutral lines in the magnetotail.The resultant field-aligned currents flowing between the tailward magnetosphere and the polar ionosphere produce the field-aligned anomalous resistivity owing to the electrostatic ion-cyclotron waves; the electrical potential drop thus increased further accelerates precipitating charged particles with a result of the intensification of both the field-aligned currents and the auroral electrojet. It seems that the rapid building-up of this positive feedback system for precipitating charged particles is responsible for the break-up of an auroral flare.     

The application of image orthicon techniques to auroral observation

The relatively high quantum efficiency of the photocathodes now used in photoelectric devices is an essential feature which makes these devices attractive for use in astronomical and auroral observations. In addition to possessing high sensitivity by virtue of the photocathode, certain types of photoelectric devices allow an image to be accumulated over some increment of time and then be read out in the form of an electrical signal which can be processed directly or used to produce a 2-dimensional image. One of these devices, the image orthicon television tube, has been found to be quite useful for obtaining auroral photographs and auroral and airglow spectra of very short exposure.     

Finite-Difference Modeling of Acoustic and Gravity Wave Propagation in Mars Atmosphere: Application to Infrasounds Emitted by Meteor Impacts

The Jovian Auroral Distributions Experiment (JADE) on Juno provides the critical in situ measurements of electrons and ions needed to understand the plasma energy particles and processes that fill the Jovian magnetosphere and ultimately produce its strong aurora. JADE is an instrument suite that includes three essentially identical electron sensors (JADE-Es), a single ion sensor (JADE-I), and a highly capable Electronics Box (EBox) that resides in the Juno Radiation Vault and provides all necessary control, low and high voltages, and computing support for the four sensors. The three JADE-Es are arrayed 120^° apart around the Juno spacecraft to measure complete electron distributions from ～0.1 to 100 keV and provide detailed electron pitch-angle distributions at a 1 s cadence, independent of spacecraft spin phase. JADE-I measures ions from ～5 eV to ～50 keV over an instantaneous field of view of 270^°×90^° in 4 s and makes observations over all directions in space each 30 s rotation of the Juno spacecraft. JADE-I also provides ion composition measurements from 1 to 50 amu with m/Δm～2.5, which is sufficient to separate the heavy and light ions, as well as O+ vs S+, in the Jovian magnetosphere. All four sensors were extensively tested and calibrated in specialized facilities, ensuring excellent on-orbit observations at Jupiter. This paper documents the JADE design, construction, calibration, and planned science operations, data processing, and data products. Finally, the Appendix describes the Southwest Research Institute [SwRI] electron calibration facility, which was developed and used for all JADE-E calibrations. Collectively, JADE provides remarkably broad and detailed measurements of the Jovian auroral region and magnetospheric plasmas, which will surely revolutionize our understanding of these important and complex regions.     

Observations of Magnetospheric Waves and their Relation to Precipitation

Magnetospheric wave observations are discussed from the viewpoint of their potential importance for precipitation of charged particles into the auroral zones. While wave processes are a fundamental part of magnetospheric plasma physics, occurring most of the time in most of the magnetospheric regions, their direct role in and relative importance for auroral precipitation are not easy to assess. The role of the waves varies from one spatial region to another and is very different for electrons and ions. Furthermore, the distinction between wave processes and other precipitation mechanisms is not at all straightforward. This review focuses on four main topics: The problem of diffuse electron precipitation, the recent surprise on the detailed structure of broad-banded electrostatic noise in the plasma sheet boundary layer, ion precipitation through electromagnetic ion cyclotron waves, and the role of low-altitude waves in precipitation. It is concluded that, while the observational status of high-altitude ion cyclotron waves is reasonably good, in most areas more thorough studies of existing data as well as refined observations are very much needed. Successful observational studies are to be carried out jointly with theoretical work as well as with studies on the large-scale context of the often localized wave processes. This is especially important when interests are moving toward more nonlinear phenomena, such as shocks, double layers, or strong quasi-static gradients, where a strict adherence to classical wave concepts is becoming more and more diffuse and less motivated.     

Relationship Between Substorm Auroras and Processes in the Near-Earth Magnetotail

Although the auroral substorm has been long regarded as a manifestation of the magnetospheric substorm, a direct relation of active auroras to certain magnetospheric processes is still debatable. To investigate the relationship, we combine the data of the UV imager onboard the Polar satellite with plasma and magnetic field measurements by the Geotail spacecraft. The poleward edge of the auroral bulge, as determined from the images obtained at the LHBL passband, is found to be conjugated with the region where the oppositely directed fast plasma flows observed in the near-Earth plasma sheet during substorms are generated. We conclude that the auroras forming the bulge are due to the near-Earth reconnection process. This implies that the magnetic flux through the auroral bulge is equal to the flux dissipated in the magnetotail during the substorm. Comparison of the magnetic flux through the auroral bulge with the magnetic flux accumulated in the tail lobe during the growth phase shows that these parameters have the comparable values. This is a clear evidence of the loading–unloading scheme of substorm development. It is shown that the area of the auroral bulge developing during substorm is proportional to the total (magnetic plus plasma) pressure decrease in the magnetotail. These findings stress the importance of auroral bulge observations for monitoring of substorm intensity in terms of the magnetic flux and energy dissipation.     

Auroral electrons

Satellite and rocket measurements of auroral electrons (which have been made since Brown's (1966) and Pfister's (1967) papers have appeared) are reviewed, and the salient characteristics of auroral electrons which emerge from all types of measurements are summarized. Effects of the atmosphere on the energy distribution of electron fluxes are discussed. Ionization rates associated with typical fluxes are derived. Observable effects produced in the atmosphere and the fate of auroral electrons are briefly described.This paper does not discuss the role of auroral protons (or particles). A recent review on the subject has been given by Eather (1967).     

Mechanisms for the discrete aurora — A review

The V-shock is identified as the primary mechanism for the acceleration of electrons responsible for the discrete aurora. A brief review of the evidence supporting the V-shock model is given, including the dynamics of auroral striations, anomalous motion of barium plasma at high altitudes and in-situ observations of large electric fields. The V-shock is a nonlinear, n = 0 ion cyclotron mode soliton, Doppler shifted to zero frequency. The V-shock is also shown to be a generalization of the one-dimensional double layer model, which is an ion acoustic soliton Doppler shifted to zero frequency. The essential difference between the double layer theory and the theory for the oblique, current-driven, laminar electrostatic shock is that the plasma dielectric constant in directions perpendicular to the magnetic field is c ²/V _a ^/2 , where V _a is the Alfvén velocity; but the plasma dielectric constant parallel to the magnetic field is unity. Otherwise, in the limit that the shock thickness perpendicular to the magnetic field is much larger than an ion gyroradius, the equations describing the double layer and the oblique shock are the same. The V-shock, while accounting for the acceleration of auroral electrons, requires an energy source and mechanism for generating large potential differences perpendicular to the magnetic field. An energy source is the earthward streaming protons coming from the distant magnetospheric tail. It is shown how these protons can be energized by the cross-tail electric field, which is the tailward extension of the polar cap dawn-to-dusk electric field. The local, large cross-field potential differences associated with the V-shock are seen to be the result of a non-linear, E × B drift turbulent cascade which transfers energy from small- to large-scale sizes. Energy at the smallest scale sizes comes from the kinetic energy in the ion cyclotron motion of the earthward streaming protons, which are unstable against the zero-frequency flute-mode instability. The review points out the gaps in our understanding of the mechanism of the diffuse aurora and the mechanism of the auroral substorm.     

Satellite measurements and theories of low altitude auroral particle acceleration

Several previous and new S3-3 satellite results on DC electric fields, field-aligned currents, and waves are described, interpreted theoretically, and applied to the understanding of auroral particle acceleration at altitudes below 8000 km. These results include the existence of two spatial scale sizes (less than 0.1 degree and a few degrees invariant latitude) in both the perpendicular and parallel electric fields; the predominance of S-shaped rather than V-shaped equipotential contours on both spatial saales; the correlated presence of field-aligned currents, low frequency wave turbulence, coherent ion cyclotron wave emissions and accelerated upmoving ions and downgoing electrons; intense waves inside electrostatic shocks and important wave-particle interactions therein; correlations of field-aligned currents with magnetospheric boundaries that are determined by convection electric field measurements; electron acceleration producing discrete auroral arcs in the smaller scale fields and producing inverted-V events in the larger scale fields; ion and electron acceleration due to both wave-particle interactions and the parallel electric fields. Further analyses of acceleration mechanisms and energetics are presented.Also Department of Physics.