Electron precipitation equatorward of the auroral oval and the mantle aurora in the midday sector

In the midday sector, the hard electron precipitation and the associated patchy aurora at geomagnetic latitude ～65° are the only auroral features (? 20 keV) located equatorward of the dayside auroral oval during intense and moderately disturbed geomagnetic conditions. We identify the patchy luminosity in the midday and late morning sectors as the active mantle aurora. The mantle aurora was found by Sanford (1964) using the IGY-IGC auroral patrol spectrographs and which was thought to be non-visual. The precipitating electrons reside mostly at energies greater than several keV with an energy flux of ? 0.1 erg cm^?2 s^?1 sr^?1 during geomagnetic active periods. This hard precipitation occurs in a region which is asymmetric in L.T. with respect to the noon meridian. The region extends from the morning sector to only early afternoon (13–14 M.L.T.) along the geomagnetic latitude circle of about 65–70°. The model calculation indicates that the mantle aurora is produced by the precipitation of the energetic electrons which drift azimuthally from the plasma sheet at the midnight sector to the dayside magnetopause during magnetospheric substorms.     

Electron precipitation and ionospheric radio absorption in the auroral zones

Auroral absorption, measured with riometers at various stations, has been compared with the fluxes of energetic particles detected simultaneously by a satellite passing overhead. The absorption and the particle flux are found to be correlated, the relation depending on the time of day. The observed relationship is discussed in the light of computations of the absorption, based on the rates of ion production previously given by Rees and on reasonable atmospheric models. A case is presented for more prolonged comparisons of this kind, the combination of a polar orbit with ground-based observations at the South Pole station being particularly fruitful in this type of experiment.     

Fluctuations of electron precipitation to the dayside auroral zone modulated by compression and expansion of the magnetosphere

Concurrent variations of CNA fluctuations and geomagnetic fluctuations are classified into Type 1 (substorm-type), Type 2 (Pc5-type), and Type 3 which is the object of the present study. Type 3 apparently has peculiar characteristics in that CNA fluctuations at a certain auroral-zone station show a pronounced positive correlation with magnetic fluctuations at distant low-latitude stations. The magnetic fluctuations of this type are identified to generalized Si which tends to take place much more frequently than the so-called Si. The CNA fluctuations of this type are found to take place on the dayside auroral zone only when preceded by a magnetic substorm on the night side. Considering the change of the growth rate of electron cyclotron instability and enhancement of pitch angle diffusion due to the change of magnetic field intensity, the following model is proposed to explain the occurrence mechanism of Type 3 concurrent variations. The CNA fluctuations take place only when two conditions are satisfied; generation of the accelerated electrons in association with substorm onset and modulation of the precipitation of the electrons by compression and expansion of the magnetosphere, in other words, by generalized Si.     

The auroral oval during the substorm development

The direction of motion of the auroral forms in several sectors of the auroral oval during substorms is studied. The creation phase is characterized by the equatorward displacement of the luminous region in evening (15–21 LT) and in day (09–15 LT) hours, while individual forms in the luminous region drift mainly poleward with a mean velocity of 230 m/sec in day hours and equatorward with the mean velocity of 230 m/sec in evening hours. The equatorial shift of the luminous region correlates well with the B_Z-component of the interplanetary magnetic field. The onset of the displacement coincides with the southward B_Z-rotation and is accompanied by auroral intensity increase for about 10–20 min.During the expansive and recovery phases the day auroras drift poleward with mean velocities of 330 and 300 m/sec, respectively. In the evening sector the individual auroral forms drift both poleward and equatorward during the expansive phase and drift mainly towards the pole during the recovery phase with a mean velocity of 200 m/sec. In the morning sector characteristics of the motion of the individual auroral forms are more complicated than in the other sectors. The well defined shifts of the luminous region are not discovered. The possible relation between the motions of individual auroral forms with the magnetosphere convection is discussed.     

Field-aligned current reversals and fine structure in a dayside auroral arc

A serendipitous event is reported in which the MAGSAT satellite intercepted an auroral arc over Svalbard, Norway where an all-sky television camera, a photographic camera and a meridian scanning photometer were making continuous measurements. The high time resolution of the optical measurements and the high spatial resolution of the magnetometer data are combined to investigate the relationship between the fine structure in the field-aligned current reversals and the temporal and spatial morphology of the auroral structure. Meridian scans of several optical emissions in the auroral arc, which had its upper portion in sunlight, are utilized to derive the total energy input and the intensity of the precipitating energetic electrons. The MAGSAT satellite apparently intercepted a fold within an extended intense upward current sheet. The current carried by the energetic electrons responsible for the optical aurora is found to be smaller than the field-aligned current derived from the magnetic perturbations, implying that there may be a large flux of low energy particles in this arc. Within the spatial-temporal constraints of this event there is a suggestion that the rayed structure is related to the field-aligned current reversals.     

The westward thermospheric jet-stream of the evening auroral oval

One of the most consistent and often dramatic interactions between the high latitude ionosphere and the thermosphere occurs in the vicinity of the auroral oval in the afternoon and evening period. Ionospheric ions, convected sunward by the influence of the magnetospheric electric field, create a sunward jet-stream in the thermosphere, where wind speeds of up to 1 km s^?1 can occur. This jet-stream is nearly always present in the middle and upper thermosphere (above 200 km altitude), even during periods of very low geomagnetic activity. However, the magnitude of the winds in the jet-stream, as well as its location and range in latitude, each depend on geomagnetic activity. On two occasions, jet-streams of extreme magnitude have been studied using simultaneous ground-based and satellite observations, probing both the latitudinal structure and the local time dependence. The observations have then been evaluated with the aid of simulations using a global, three-dimensional, time-dependent model of thermospheric dynamics including the effects of magnetospheric convection and particle precipitation. The extreme events, where sunward winds of above 800 ms^?1 are generated at relatively low geomagnetic latitudes (60–70°) require a greatly expanded auroral oval and large cross-polar cap electric field ( ~ 150 kV). These in turn are generated by a persistent strong Interplanetary Magnetic Field, with a large southward component. Global indices such as K_p are a relatively poor indicator of the magnitude and extent of the jet-stream winds.     

Some auroral properties from far ultraviolet observations

Recent far ultraviolet spectra of four nightside auroras observed with the extreme ultraviolet (e.u.v.) spectrometer abroad the STP 78?1 satellite (Bowyer et al., 1981) are discussed in terms of two secondary electron flux distribution models. One of these is based on calculations published by Rees and Maeda (1973) and the other on electron flux observations. Using the 1084 Å NII line as the norm, the relative intensities of several lines in the range 900–1400 Å could be fitted equally well by either model, indicating that some of these emission ratios are somewhat independent of the energy dependence of the secondary electron flux. The ratio of the NI 1243 Å to 1084 Å emission is seen to yield an effective correlation for the other observed emissions and a reasonably consistent estimate of the penetration depth of the aurora.     

A spherical harmonic analysis of the austral auroral oval

Computerized spherical harmonic analysis is applied to the morphology of the southern auroral oval. Records from 23 All Sky Camera stations together with visual observation reports for the period 1957–1959 constitute the raw data set. The mode of the derived auroral occurrence distribution function F(K_p, θ, φ) is regarded as the maximum probability contour and yields a set of auroral ovals. These 10 contours, one for each K_p level, are expressed in the invariant magnetic co-ordinates of Bond (1968).     

The continuity of the auroral oval in the afternoon sector

United States Air Force DMSP satellite auroral images and South Pole auroral all-sky camera data are presented to provide new direct evidence that the afternoon sector auroras lie within a continuous oval belt. An example is also presented to illustrate that afternoon substorm auroras occur within the auroral oval configuration.     

Configuration of the auroral oval and the interplanetary magnetic field

The poleward boundary of the auroral oval, whose footline forms the periphery of the polar cap, is calculated, based on a model in which the geomagnetic field is interpermeated with the interplanetary field. It is shown that the calculated auroral oval size varies with the strength and direction of the interplanetary magnetic field, in agreement with recent observations of the location of large-scale nightside auroras.     

Night-time location of the auroral oval and mid-latitude thermospheric temperature

The thermospheric response to the energy input into the polar regions can be studied by monitoring the thermospheric temperature at mid-latitudes using optical techniques. During the course of thermospheric temperature measurements at Albany, New York it was noticed that an increase in the auroral oval radius is followed by an increase in the thennospheric temperature.In order to obtain a possible relationship between the mid-latitude thennospheric temperature and the location of the auroral oval (that is in turn related to the energy input into the polar region), a cross-correlation analysis of temperature measurements and the oval radius was done for time lags from 0 to 75 min, at 15 min intervals. The results show that the correlation coefficient ranged between 0.38 and 0.79 for different time lags, with the best coefficient at a time lag of about 45 min, and that for every 1° change in the radius of the oval, the temperature varies by about 137°.     

Photometric observations of the dayside cleft emissions from Cambridge Bay,December 1976

Dayside cleft emissions were observed from Cambridge Bay, N.W.T. during quiet, very quiet and disturbed periods from 10–17 December 1976. Good agreement is found in comparisons of the ground based photometric data with ISIS satellite data for the cleft region. A marked asymmetry around noon was found. Dynamical changes, observed especially in the poleward emission boundary, appear to have no counterpart in the nightside magnetometer data.     

High temporal and spatial resolution observations of low energy electrons by a mother-daughter rocket in the vicinity of two quiescent auroral arcs

Low energy precipitated electrons have been measured with high time resolution through an auroral display by a series of high geometrical factor particle counters on a ‘mother-daughter’ sounding rocket, launched during wintertime near 2100 LT from Andenes, Norway.The observations show that the 0·5–3 keV electron fluxes are anisotropically distributed, with a maximum in a direction parallel to the local geomagnetic field vector at all latitudes covered by the rocket, except within the visual auroral forms where the pitch-angle distributions are isotropic or slightly peaked in a direction normal to the geomagnetic field. The 1 and 3 keV electron fluxes are weakly anticorrelated in the vicinity of the arcs, where also the 3 keV electron flux displays a more structured variation than the 0.5 and 1 keV electron fluxes.     

Rocket-borne and groundbased observations of coincident field-aligned currents,electron beams,and plasma density enhancements in the afternoon auroral oval

Results are reported from a rocket experiment conducted at Søndre Strømfjord, Greenland, on 22 August 1976, at 16.00 M.L.T. A series of plasma, particles, and fields and wave experiments were carried on board the payload, and the venture was supported by data from the AE-C satellite and by groundbased ionosondes and magnetometers at the launch site and at Godhavn. Two regions of field-aligned electron precipitation, electron density and temperature enhancements, and field-aligned upflowing current sheets were intercepted by the rocket. The density enhancements were also observed by groundbased ionosondes. Significant discrepancies were found between the currents carried by the streaming electrons in the 0.15–10 keV range and the upflowing currents seen by the on board magnetometer, suggesting that the upflowing current could not be the primary driver of the electron acceleration mechanism. The E-region was unstable to the combined Gradient-Drift and Farley-Buneman instability, and plasma turbulence was observed in situ, but the absolute density fluctuations were too small to return detectable HF-radar power to the ground.     

Particle and optical measurements in the magnetic noon sector of the auroral oval

Simultaneous airborne photometric and satellite particle measurements in the mid-day sector of the auroral oval, around magnetic local noon, are presented. The two sets of measurements are employed independently to delineate various magnetospheric boundaries. The results derived from the particle measurements are compared with those from the photometric observations to assess the reliability of the photometric technique in identifying various magnetospheric regions.     

Field line topology in the dayside cusp region inferred from low altitude particle observations

Dayside low altitude satellite observations of the pitch angle and energy distribution of electrons and protons in the energy range 1 eV to 100 eV during quite geomagnetic conditions reveal that at times there is a clear latitudinal separation between the precipitating low energy (keV) electrons and protons, with the protons precipitating poleward of the electrons. The high energy (100 keV) proton precipitation overlaps both the low energy (keV) electron and proton precipitation. These observations are consistent with a model where magnetosheath particles stream in along the cusp field lines and are at the same time convected poleward by an electric field.The electrons with energies of a few keV move fast and give the “ionospheric footprint” of the distant cusp. The protons are partly convected poleward of the cusp and into the polar cap. Here the mirroring protons populate the plasma mantle. Equatorward of the cusp the pitch angle distribution of both electrons and protons with energies above a few keV is pancake shaped indicating closed geomagnetic field lines. The 1 keV electrons, penetrate, however, into this region of closed field line structure maintaining an isotropic pitch angle distribution. The intensity is, however, reduced with respect to what it was in the cusp region. It is suggested that these electrons, the lowest energies measured on the satellite, are associated with the entry layer.     

The aries auroral modelling campaign: characterization and modelling of an evening auroral arc observed from a rocket and a ground-based line of meridian scanners

An auroral arc system excited by soft electrons was studied with a combination of in situ rocket measurements and optical tomographic techniques, using data from a photometer on a horizontal, spinning rocket and a line of three meridian scanning photometers. The ground-based scanner data at 4709, 5577, 8446 and 6300 Å were successfully inverted to provide a set of volume emission rate distributions in the plane of the rocket trajectory, with a basic time resolution of 24 s. Volume emission rate profiles, derived from these distributions peaked at about 150 km for 5577 and 4709 Å, while the 8446 Å emission peaked at about 170 km with a more extended height distribution. The rocket photometer gave comparable volume emission rate distributions for the 3914 Å emission as reported in a separate paper by McDade et al. (1991, Planet. Space Sci. 39, 895). Instruments on the rocket measured the primary electron flux during the flight and, in particular, the flux precipitating into the auroral arc overflown at apogee (McEwen et al., 1991; in preparation). The local electron density and temperature were measured by probes on the rocket (Margot and McNamara (1991; Can. J. Phys. 69, 950). The electron density measurements on the downleg were modelled using ion production rate data derived from the optical results. Model calculations of the emission height profile based on the measured electron flux agree with the observed profiles. The height distribution of the N₂⁺ emission in the equatorward band, through which the rocket passed during the descent, was measured by both the rocket and the ground-based tomographic techniques and the results are in good agreement. Comparison of these profiles with model profiles indicates that the exciting primary spectrum may be represented by an accelerated Maxwellian or a Gaussian distribution centered at about 3 keV. This distribution is close to what would be obtained if the electron flux exciting the poleward form were accelerated by a 1–2 kV upward potential drop. The relative height profiles for the volume emission rate of the 5577 Å OI emission and the 4709 Å N₂⁺ emission were almost indistinguishable from each other for both the forms measured, with ratios in the range 38–50; this is equivalent to I(5577)/I(4278) ratios of 8–10. The auroral intensities and intensity ratios measured in the magnetic zenith from the ground during the period before and during the rocket flight are consistent with the primary electron fluxes and height distributions measured from the rocket. Values of I(5577)/I(4278) in the range 8–10 were also measured directly by the zenith ground photometers over which the arc system passed. These values are slightly higher than those reported by Gattinger and Vallance-Jones (1972) and this may possibly indicate an enhancement of the atomic oxygen concentration at the time of the flight. Such an enhancement would be consistent with our result, that the observed values of I(5577) and I(8446) are also significantly higher than those modelled on the basis of the electron flux spectrum measured at apogee.     

Eclipse observations in the rocket ultraviolet

An Aerobee 150 was launched into the umbra from Wallops Island at 18:36:15 UT 7 March, 1970, carrying two intensity calibrated EUV Wadsworth grating spectrographs. The experiment was a collaborative venture between groups from A.R.U. Culham, H.C.O., Imperial College and York University, Toronto. This was the first attempt to record stigmatic spectra of the chromosphere and corona in the EUV, analogous to the visible eclipse Flash Spectrum.A wealth of new observational data has demonstrated the importance of a new instrumental and experimental technique for recording the ultraviolet flash spectrum. Lyman- limb spectroheliograms reveal complex activity extending high into the corona in contrast to the modest level of activity observed in H. Preliminary intensity reduction shows that Lyman- dominates as the principle radiation loss for these features.     

OGO-4 observations of the ultraviolet auroral spectrum

Auroral ultraviolet spectra in the range 1200–3200 A have been obtained by the spectrometer onboard the OGO-4 satellite. Emissions of N₂, H, O and N are readily identified. Atomic and molecular intensities are deduced from the comparison with a synthetic spectrum and compare reasonably well with some previous measurements and calculations. A feature at 2150 A is assigned to the (1-0) NO γ band. Taking into consideration the various excitation mechanisms of NO(A²σ) we propose that the energy transfer from N₂ metastable molecules to oxygen accounts for the excitation of the NO γ bands. In particular, we suggest that the resonant reaction between O₂ and highly metastable N₂(W³Δ) molecules may be a major source of NO(A²σ).