[en] This paper reviews some recent observations made with all-sky cameras, photometers and electronic imaging devices, both from spacecraft and from the ground. When the diffuse aurora is included, the instantaneous distribution of auroras differs considerably from the classical Feldstein auroral oval. Contrary to currently held views, the diffuse aurora appears to be the same as the mantle aurora (Sandford, 1964, 1968). New ground-based observations are presented which show that the dynamics of the instantaneous auroral distribution is also very different from the classical substorm picture when the dynamic behavior of the diffuse aurora is included. The Feldstein oval should therefore not be used as a reference frame for anything except averaged data. The instantaneous equator-ward boundary of the auroral oval as defined by the diffuse aurora is perhaps a more suitable coordinate for ordering single sets of observations. (Auth.)

[en] A clearer picture of the dynamical nature of the post-noon aurora has been obtained with instrumental studies during rocket flights over the Northwest Territories of Canada. A sequence of meridian scans showing the measured intensities of the emissions along the meridian from south to north is presented. An all-sky picture of an auroral event is presented which shows a very narrow arc running through the zenith in an east-west direction, typical of many occurring during the period 1-3 hrs after local magnetic noon. A contemporaneous meridian scan taken over the same location shows a narrow spike in the zenith. The frequent occurrence of these narrow, transient auroral arcs with lifetimes of 1-2 minutes, resulting from inverted V events, is discussed

[en] Detailed plasma and field measurements acquired from a recent sounding rocket within regions of transverse ion acceleration near 500 km altitude during a large auroral substorm at Churchill (Canada) are described. Data from simultaneous ground-based observations of the optical auroral morphology are presented as well. Ionospheric ions were energized to about 300 eV, perpendicular to the local geomagnetic field to within about 1-2 deg. The results are suggestive of parametric decay of large-amplitude lower hybrid waves into H(+) Bernstein waves, resulting in the transverse ion acceleration. 21 references

[en] A sounding rocket carrying 100 kg of high explosives and plasma diagnostic instrumentation was launched from Churchill Research Ranch on 6 April 1980 over a premidnight auroral arc. The object of the experiment was to produce an ionospheric hole or plasma density depletion near 300 km altitude on field lines connected to an auroral arc. The plasma depletion is produced when the explosive by-products (mostly water) charge-exchange with the ambient O⁺ ions and then rapidly recombine. It was speculated that the presence of the 'hole' would interfere with the field-aligned current systems associated with the arc and would in turn perturb the auroral source mechanism. The release occurred about 10 km poleward of the auroral arc field lines. As expected, a large ionospheric hole was detected by the rocket-borne plasma sensors. Within a few seconds following the release, (a) the energetic electron precipitation observed in the hole dropped to background levels, (b) the luminosity of the auroral arc observed by a ground-based auroral scanning photometer decreased by a factor of two, and (c) the ionospheric E region density below the hole decayed at a rate consistent with a sudden reduction in particle precipitation. The simultaneous onset of these gross changes in electron precipitation coincident with the release strongly suggests a cause and effect relationship and demonstrates the intimate relationship that exists between the state of the ionospheric plasma and the auroral acceleration mechanism

[en] We present ionospheric ion convection measurements in a series of four rocket payloads in and near dayside and nightside auroral arcs: one at Cape Parry (75.4⁰N invariant latitude) near 1300 MLT and three at Churchill (70.0⁰N invariant latitude) between 1900 and 2200 MLT. Direct measurements were made of the ionospheric ion velocity distribution function, and the observed ion convection velocities and equivalent convective electric fields were correlated with the energetic particle precipitation, the optical morphology of the aurora, and the topology of the geomagnetic field. Both in the postnoon and premidnight sectors it was observed that (1) equatorward of the region(s) of precipitation the ion flow was predominantly westward, with velocity of about 1 km/s; (2) poleward of the region(s) the flow was predominantly westward, with velocity of about 1 km/s; (2) poleward of the region(s) the flow was predominantly eastward: (3) the change in the flow direction, where observed, occurred near though not exactly at the edges of the precipitation region; (4) the flow inside the precipitation region was lower; (5) the reversal of the ion flow, where observed, occurred on closed magnetic field lines; and (6) the convective electric field typically dropped from 40 to 80 mV/m outside the precipitation region to 10 to 30 mV/m within. In the dayside Cape Perry flight, where quantitative photometric measurements were available, detailed anticorrelation between the ion convection speed and the green line emission intensity was also observed

[en] In a neutral sheet with a positive B_z, X lines and O lines occur in pairs. If it is assumed that discrete auroral arcs are the mapping along field lines of X line-O line pairs, then much of the complicated structure and behavior of discrete arcs can be readily explained. The authors apply this model to representative events in nightside data from a meridian-scanning photometer chain showing the growth, expansive and recovery phases of two substorms, all-sky imager data, and dayside data including Viking UV images and also to a Defense Meteorological Satellite Program photograph of auroral activity. Features explained include the appearance, brightening, fading, disappearance, dividing of one arc into two, the uniting of two arcs into one, branching of arcs, two classes of short-lived rapidly propagating arcs, a newly identified basic substorm intensification structure, and the reaction of two arcs to a westward traveling surge. All these features can be explained in terms of forward or reverse merging at one or more X lines. Forward merging is defined as the usual flow direction for nightside merging, and reverse merging is with the flow through the X line reversed. The motion of an arc is the resulting of a poleward velocity due to flux transfer through the X line and arc and the large-scale equatorward convection (E x B/B² drift). Magnetic islands which become entrapped in dipolar fields dissipate by fast merging with rapid motion of the island and associated arc. On the nightside these islands are a likely explanation of substorm injection events

[en] Data obtained during the IMS with a ground-based chain of meridian photometers which measured the H/sub β/ and OI 5577-A emissions have been analyzed quantitatively to yield the mean distributions of proton and electron aurora in geomagnetic latitude and time as functions of magnetic activity. The total energy inputs from electron and proton precipitation around the nightside auroral oval are estimated as functions of AE from the 5577-A and H/sub β/ intensity data. Average latitude profiles for the two emissions are given as a function of AE for the four night time sectors between 1800 and 0600 local magnetic time. In agreement with previous studies it is found that the average flux due to protons is an order of magnitude less than that due to electrons under disturbed conditions although under very quiet conditions the two are comparable. Finally, the H/sub β/ observations have been fitted to a quantitative empirical model giving H/sub β/ intensity distributions as a function of AE. The peak values of H/sub β/ intensity increase linearly with AE for all magnetic time sectors. copyright American Geophysical Union 1988

[en] A series of perturbation experiments (Waterhole I, II, and III), in which ''holes'' were created in the F region ionosphere by explosive releases of large quantities of water vapor, has been conducted to test theories of the electrodynamic structure of auroral arcs. The water vapor releases created large (roughly-equal50-km diameter) holes in the ionosphere in and near structured premidnight auroral arcs. It was anticipated that these holes would interrupt or perturb the ionospheric current systems associated with the arcs and that this perturbation would in turn affect the acceleration mechanism responsible for the aurora. Results from the two successful rocket flights (Waterhole I and III) are presented, and it is shown that significant modifications of the energetic electron precipitation patterns were induced by both releases. The first release was made 10 km poleward of a discrete early evening auroral arc, and the perturbation caused a cessation of electron precipitation through the hole and a significant modification of the arc. The second release, which occurred on magnetic field lines connected to the center of a series of arcs, produced a much smaller hole mainly due to the preexisting low electron density and also induced a less striking energetic electron response. In this case the flux was enhanced. Various models and theories of the perturbation mechanism are discussed, and it is shown that both responses are consistent with current theories of the electrodynamic structure of auroral arcs

[en] 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 59.5⁰ to 73.3⁰N (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_βlambda4861 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

[en] Observations of pulsating auroras in the magnetic zenith and proton and electron auroras along the magnetic meridian were conducted from three stations covering from 58⁰ to 71⁰ invariant during the pulsating aurora campaign in Saskatchewan in January and February of 1980. Pulsations were observed only south of or on the southern edge of the proton precipitation region. This is in general agreement with other uncoordinated studies of proton and pulsating auroras in the late morning hours, but it also appears to be true for isolated events at other times as well. (auth)