The Earth's ultraviolet airglow contains fundamental diagnostic information about the state of its upper atmosphere and ionosphere. Our understanding of the excitation and emission processes which are responsible for the airglow has undergone dramatic evolution from the earliest days of space research through the past several years during which a wealth of new information has been published from high-resolution spectroscopy and imaging experiments. This review of the field begins with an overview of the phenomenology: how the Earth looks in the ultraviolet. Next the basic processes leading to excitation of atomic and molecular energy states are discussed. These concepts are developed from first principles and applied to selected examples of day and night airglow; a detailed review of radiation transport theory is included. This is followed by a comprehensive examination of the current status of knowledge of individual emission features seen in the airglow, in which atomic physics issues as well as relevant atmospheric observations of major and minor neutral and ionic constituents are addressed. The use of airglow features as remote sensing observables is then examined for the purpose of selecting those species most useful as diagnostics of the state of the thermosphere and ionosphere. Imaging of the plasmasphere and magnetosphere is also briefly considered. A summary of upcoming UV remote sensing missions is provided.

Ultraviolet spectroscopy and remote sensing of the upper atmosphere

The detailed analysis of an isolated dispersionless substorm is performed on the basis of field and particle data collected in situ by the geostationary satellite GEOS 2 and of data from ground-based instruments installed close to the GEOS 2 magnetic footprint. These data give evidence for (1) quasi-periodic variations of the magnetic field configuration, which is alternatively taillike and dipolelike, (2) in-phase oscillations of the flux of energetic electrons, which is high when the configuration is dipolelike and vice versa, (3) a gradient in the flux of energetic ions, which is, on the average, earthward but undergoes large fluctuations around this average direction, and (4) large transient fluctuations of the quasi-dc electric field, which reverses its direction from eastward to westward. It is shown that these results are consistent with the development of an instability which leads to a westward propagating “wave”. The source of the instability is the differential drift of energetic electrons and ions in a highly stressed magnetic field configuration (in a high β plasma). Evidence is given for a system of localized field-aligned currents flowing alternately earthward and equatorward at the leading and trailing edges of the westward propagating wave. This current system resulting from the temporal development of the instability produces the so-called Pi 2 pulsations, at the ionospheric level. The closure of this current system in the equatorial region leads to a current antiparallel to the tail current, and therefore to its reduction or cancellation. This reduction/cancellation of the tail current restores the dipole magnetic field (dipolarization) and generates a large westward directed induced electric field (injection). Hence, dipolarization and injection are the consequences of the instability. Finally, it is suggested that the westward traveling surge observed simultaneously by all-sky cameras, close to the magnetic field of GEOS 2, is the image of the instability in the equatorial region transmitted to the upper atmosphere by precipitating electrons.

Plasma sheet instability related to the westward traveling surge

The optical spectrograph and infrared imager system (OSIRIS) on board the Odin spacecraft is designed to retrieve altitude profiles of terrestrial atmospheric minor species by observing limb-radiance profiles. The grating optical spectrograph (OS) obtains spectra of scattered sunlight over the range 280-800 nm with a spectral resolution of approximately 1 nm. The Odin spacecraft performs a repetitive vertical limb scan to sweep the OS 1 km vertical field of view over selected altitude ranges from approximately 10 to 100 km. The terrestrial absorption features that are superimposed on the scattered solar spectrum are monitored to derive the minor species altitude profiles. The spectrograph also detects the airglow, which can be used to study the mesosphere and lower thermosphere. The other part of OSIRIS is a three-channel infrared imager (IRI) that uses linear array detectors to image the vertical limb radiance over an altitude range of approximately 100 km. The IRI observes both scattered sunlight and the airglow emissions from the oxygen infrared atmospheric band at 1.27 mum and the OH (3-1) Meinel band at 1.53 mum. A tomographic inversion technique is used with a series of these vertical images to derive the two-dimensional distribution of the emissions within the orbit plane.

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The OSIRIS instrument on the Odin spacecraft

Three decades of research in magnetospheric substorms has not led to a general consensus view of the substorm process. Several substorm models, mostly phenomenological, are presently under consideration. These competing models, each being justifiable on the basis of certain features of a substorm, have major differences as well as similarities among them. A synthesis substorm model is desirable, as first suggested by Siscoe (1986). In this paper we construct a coherent description of substorm development by extracting some important components from these existing models. The scenario of the synthesis model includes the ionospheric influence on substorm expansion onset, current disruptions leading to convection surges and tailward propagating rarefaction waves, wave-induced precipitation, local time expansion of the disturbance region via velocity-shear-related instabilities, plasma sheet heating by resonant absorption of hydromagnetic waves, and the formation of magnetic reconnection domains. This synthesis represents one possible way to integrate the different existing models coherently.

A synthesis of magnetospheric substorm models

The average global characteristics of precipitating auroral ions were determined using the data from the SSJ/4 detectors on the F6 and F7 satellites of the Defense Meteorological Satellite Program (DMSP). For this study the high-latitude region was divided into spatial elements in magnetic local time (MLT) and corrected geomagnetic latitude (CGL). One such spatial matrix was created for each of seven levels of magnetic activity as defined by Kp. Approximately 26.5 million, individual, 1-s spectra were used to determine the average ion spectrum over the energy range from 30 eV to 30 keV in each spatial zone and at each level of activity. Where appropriate, the spectra were extrapolated to 100 keV to provide a more complete estimate of the total integral energy flux, number flux, and average energy. The global patterns of the integral energy flux, integral number flux, and average energy derived from the average spectra vary smoothly with latitude, MLT, and activity. For a given Kp value the higher levels of integral energy flux occur in C-shaped regions symmetric about a meridian running prenoon to premidnight. The maximum integral energy flux is found premidnight; the minimum peak integral energy flux in latitude is found prenoon. Except in the prenoon region, the overall level of integral energy flux increases with Kp with the premidnight maximum increasing by a factor of 6.1 from Kp = 0 to Kp ≥ 6−. The maximum integral number flux is centered at noon and is taken to be the center of the cusp. The maximum value does not vary significantly with activity, although its location moves to lower latitude with increasing Kp. A minimum in the average energy also occurs in the noon meridian, several degrees poleward of the integral number flux maximum. The separation in latitude between the integral flux maximum and the average energy minimum of the cusp increases with Kp. The maximum average energy occurs on the eveningside of the oval near the equatorward boundary of the ion precipitation and shifts toward noon with increasing activity. The average integral number flux for the precipitating auroral ions is 1-2 orders of magnitude less than that for the precipitating auroral electrons at all latitudes, MLTs, and activities. The integral energy flux of the ions, on the eveningside of the oval, can equal or exceed that for the electrons near the equatorward edge of the auroral region. Integrated globally, the ratio of the ion to electron number flux varies from 0.024 to 0.015 over the range of activity from Kp = 0 to Kp ≥ 6−. The ratio for the integral energy flux ranges from 0.11 to 0.17 with no trend in activity.

A statistical model of auroral ion precipitation

Proper interpretation ofin situ satellite data requires a knowledge of the global state of the magnetosphere-ionosphere system. CANOPUS is a large-scale array of remote sensing equipment monitoring the high latitude ionosphere from the north-central to the north-west portion of North America. The array comprises thirteen magnetometers and riometers four meridian scanning photometers, a digital allsky imager and an auroral radar linked by geostationary satellite to a central receiving node in Ottawa, where the data are archived and made available in near real time to participating scientists. This paper provides a technical description of the various instruments in the CANOPUS array, and contains a summary of the key parameters which will be provided to the Central Data Handling Facility (CDHF) located at NASA/Goddard Space Flight Center, for use by the ISTP/GGS community.

Canopus — A ground-based instrument array for remote sensing the high latitude ionosphere during the ISTP/GGS program

The altitude profile of the infrared atmospheric system of oxygen at 1.27 μ in the dayglow has been measured using a two-channel filter photometer flown to an altitude of 128 km over White Sands, New Mexico, at a solar elevation of 14.5° in October 1966. The emission peaks at 49.5 km and the maximum volume emission rate is 1030 kR/km. The total emission intensity of 21 MR is in agreement with that of balloon-borne observations. The main emission layer agrees well with that predicted from an excitation mechanism of the photolysis of ozone in the Hartley continuum. There is some excess emission above 80 km, and three possible explanations are considered: additional ozone, enforced fluorescence via the ¹Σ level, and recombination of atomic oxygen. A combination of these can explain the observations, but which ones are most important cannot be decided. The amounts of metastable O2 observed above 70 km are sufficient to provide an important source of ionization and of NO in the lower D region.

Altitude profile of the infrared atmospheric system of oxygen in the dayglow

An oxygen-hydrogen atmospheric model and its application to the OH emission problem

The explosive onset and development of a clearly defined auroral substorm has been analysed using data from the ultraviolet imager aboard the VIKING satellite. These data permit the evolution of the substorm to be traced with a time resolution as high as 40 seconds. The envelope of the disturbed region - the "eye" - expands both poleward and azimuthally to the east and west reaching an azimuthal extent of almost six hours of magnetic local time after about 15 minutes. The poleward expansion involves irregular poleward motion accompanying intensifications within the eye, with the maximum poleward displacement being at the meridian of the center of the eye. Within the "eye", discrete regions of enhanced luminosity develop with a tendency for each newly disturbed region to be to the west of the previous ones. It is, however, observed that a newly excited region can be to the east of the pre-existing activated region. Each new intensification is accompanied by a brief further poleward expansion of the high latitude edge of the "eye". Each region of enhanced luminosity within the "eye" remains relatively stationary and fluctuates in intensity over its lifetime. We identify these regions of enhanced luminosity with westward traveling surges as defined from ground observations in the earlier literature. Our UV imager data demonstrate that these surges do not necessarily propagate although the overall envelope of the substorm disturbed region expands both eastward and westward during the lifetime of the event.

The development of the substorm expansive phase: The “eye” of the substorm

A chain of sensitive meridian-scanning photometers and all-sky cameras was operated during two new moon periods in 1978 to observe auroral proton and electron precipitation patterns with high time resolution (30 s). The latitude range was from 59.5/sup 0/ to 73.3/sup 0/N (invariant) with Churchill, Manitoba, as the northernmost of three stations. Intensity plots on a latitude-time scale were prepared, and from these, 14 auroral substorms over a broad range of local times were selected for analysis. Data from a widely spaced array of auroral zone and low-latitude magnetometers were used to determine substorm onset times and longitude sectors. Representative meridian intensity profiles of H/sub ..beta../lambda4861 and OIlambda5577 were then assembled as a function of invariant and local magnetic time to obtain a synthetic model of a typical substorm. It is found that the data are better ordered when the local magnetic time scale for the individual substorms is shifted so as to place the substorm origins at local magnetic midnight. While the results are in general agreement with earlier observations, the quantitative nature of the present data base leads to a more objective proton substorm model.

A. Vallance Jones

Papers

Canopus — A ground-based instrument array for remote sensing the high latitude ionosphere during the ISTP/GGS program

Altitude profile of the infrared atmospheric system of oxygen in the dayglow

An oxygen-hydrogen atmospheric model and its application to the OH emission problem

The development of the substorm expansive phase: The “eye” of the substorm

Auroral studies with a chain of meridian‐scanning photometers: 1. Observations of proton and electron aurora in magnetospheric substorms