An Isis 2 pass is singled out for a detailed examination of the particle fluxes, optical emissions, and ionospheric parameters observed during a quiescent period (late recovery) between two substorms. This pass was chosen because it was part of a coordinated data acquisition period between the Air Force Geophysics Laboratory (AFGL) Airborne Ionospheric Observatory, Isis 2, and DMSP (Defense Meteorological Satellite Program). As a result, both long‐duration measurements (aircraft) and transient, snapshot (spacecraft) data are available. This allows, on a macroscopic level, the separation of space and time effects. On the basis of the joint data set it was established that the latitudinal morphology observed by the satellite is basically spatial in nature. It is concluded that the observed particle fluxes are most easily understood in terms of precipitation from the quiet time plasma sheet without intervening acceleration. The observed optical emissions and ionospheric parameters are found to be in good qualitative and quantitative agreement (within experimental error) with the electron fluxes, although establishment of this point has required careful determination of the viewing direction of the optical instruments, removal of scattered light (albedo) from underlying cloud and snow, and consideration of the effects of photon‐counting statistics on contour plots of the optical data.
This paper and two companion papers describe a case study of multiple auroral phenomena in the midnight sector. The duration of the study was 12 hours on December 9, 1971. For 9 of the 12 hours the Air Force Geophysics Laboratory airborne ionospheric observatory flew near local midnight across North America so as to intersect many successive passes of Defense Meteorological Satellite Program (DMSP) and Isis 2 satellites which were also in the midnight sector. The large‐scale DMSP auroral photographs have been combined with the temporally continuous aircraft‐ and ground‐based measurements to provide extensive observations of long duration compared to the lifetimes of the two substorms observed. All the types of observations (satellite pictures, aircraft ionospheric soundings, all‐sky camera photographs, photometer recordings, as well as particle and other satellite‐ and ground‐based measurements) have been combined to describe the temporal histories of a large number of phenomena and of their interrelations. The phenomena are the discrete auroras in the oval and polar cap, the continuous (E layer) aurora, auroral absorption (nearly absent on this day), and (in the companion papers) the F layer irregularity zone and 6300‐Å emission, particle precipitation, the electrojet, and interplanetary magnetic field and magnetospheric phenomena. DMSP photographs are found to detect both discrete and continuous auroras; uses and limitations of the data are described. In terms of the overall perspective of this study the phenomena are found to fall into six periods of 1–3 hours each. Two of these periods contain substorms of differing characteristics; the remaining four periods are relatively quiet but also differ from one another. Throughout each of the periods the polar cap, oval, and continuous auroras as a whole retain the same configuration as described, in terms of whether each of the auroras is present or absent and whether adjacent auroras are joined or separated. These configurations are described as phenomenological states of the aurora, and the periods are regarded as the lifetimes of these states. The large‐scale, long‐term features of the auroral states suggest that they reflect individual states of the magnetosphere.
Airborne observations of auroral activity at midnight were conducted for a period of 9 hours by employing an ionospheric sounder and all‐sky cameras. During the observation period, two substorms occurred. The first substorm was associated with a compression of the magnetosphere (as measured by Dst) and with oscillations of the earth's bow shock. At this time, auroral activity was characterized by a series of poleward and equatorward motions and by the absence of a breakup phase. Magnetic disturbances were confined to a small region in the midnight sector. The second auroral substorm displayed many of the features associated with a large‐scale polar magnetic substorm. For selected times the locations of the eastward and westward electrojets were inferred from a number of high‐latitude magnetic records. All‐sky photographs and ionosonde data indicate that the poleward edge of the westward electrojet was bounded by a westward moving auroral surge accompanied by a strong sporadic E ionization. The equatorward electrojet boundary was less well defined by visual auroral forms. Soft particle spectrometer measurements from the Isis 2 satellite were made in conjunction with the aircraft measurements and indicate that large fluxes of field‐aligned electrons gave rise to the bright auroral surge, more isotropic fluxes of less energetic electrons contributing to the auroral E layer.
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