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All-sky auroral images acquired at Ny-Alesund were used in conjunction with observations of a Polar overflight on November 30, 1997, to determine where prenoon, 0900-1000 magnetic local time (MLT) auroral structures map to the outer magnetosphere. Polar observations at midaltitudes are used to constrain the mapping between the aurora and the magnetosphere. The Tsyganenko 96 magnetic field model (T96), driven by interplanetary conditions and D ST , is used for that mapping. When the T96 model is driven by conditions on this day, the open/closed field line boundary maps 2-3 degrees lower in latitude than observed for this day. By making an ad hoc adjustment to match the location of the model open/closed field line boundary to observations, we find that the agreement with other aspects of the observations is also improved. These include (1) Polar observations of the dayside extension of the boundary plasma sheet (BPS) (structured low-energy electrons, detected in a region of sunward convection and region 1 field-aligned currents) mapped to the ionosphere in a region of discrete aurora, (2) Polar observations of the central plasma sheet (high-energy electron precipitation) maps to diffuse green auroral emissions equatorward of the discrete aurora, and (3) Polar electric field observations are consistent with convective motions of the auroral forms and changing interplanetary magnetic field (IMF) conditions. When the all-sky images are mapped into the magnetosphere, we find that the discrete aurora identified with the BPS mapped to the outer dawnside edge of the magnetosphere. This mapping indicates that earlier Geotail reports of a sunward flowing mixing region in the equatorial magnetosphere Fujimoto et al., 1998] is really within the dayside extension of the BPS, compatible with both the low-altitude BPS observations of Newell and Meng [1992] and type 4 dayside aurora of Sandholt et al. [1998]. Since the Polar observations place the sunward flowing mixing region as a source of region 1 currents for this IMF By positive case [Farrugia et al., this issue], these results extend the source of the region 1 currents from the antisunward flowing low-latitude boundary layer into the sunward flowing BPS, commensurate with Yamauchi et al. [1998] and Sonnerup [1980].

We present a technique which combines time series of line-of-sight (LOS) velocity and electron density measurements from the Sondrestrom incoherent scatter radar (74.5° invariant latitude) to reconstruct the large-scale horizontal structure of the F region ionosphere during polar cap patch events. This reconstruction technique provides a new density-based means of examining patch morphology. Its wide region of coverage also facilitates comparison of radar measurements with other observational data sets. For two periods when patches were present and convection conditions in the nightside polar cap could be adequately approximated by the simple velocity model used in this initial implementation of the technique, we compare reconstructed radar data montages with in situ ion density data from DMSP satellites and 630.0 nm all-sky images taken at the radar site. The satellite data agree well with the radar reconstructions near the observation site, and show general agreement well beyond the radar field of view (FOV). Data from satellite passes to the east and west of the radar coverage suggest high densities were present over many hours of magnetic local time in the nightside polar cap. Many of the patches observed in the radar data were elongated perpendicular to the convective flow and may have extended well beyond the radar FOV. The characteristics of density enhancements observed by the satellites in the nightside auroral zone were found to be consistent with the creation of auroral zone blobs by distortion of patches seen exiting the polar cap in the radar data. Radar data reconstructions also show reasonable qualitative agreement with instantaneous 630.0 nm all-sky images regarding the location, size, and shape of patch features. In addition to validating the streamline-mapping technique, this result also lends support to previous optical studies of patch occurrence and morphology.

This work describes an application of incoherent scatter (IS) radar and all-sky imaging techniques to the identification of signatures for the nightside auroral zone/polar cap boundary. This identification is made by estimating the characteristic energy of precipitating electrons across all latitudes measured by the IS radar. The primary method used to discern the boundary is based on an algorithm wherein the detailed shape of E region ionization profiles is mapped to the required number flux from a library of monoenergetic ionization profiles. This procedure results in a reconstructed energy distribution for the precipitating electrons from which a characteristic energy is derived. The latitudinal gradient of the characteristic energy is subsequently used to identify the boundary. Monochromatic all-sky imagers are used to establish the time history of the boundary and estimate characteristic energy (at magnetic zenith). Additional plasma signatures such as auroral ionospheric cavities and localized F region Te enhancements are shown to provide contextual clues for the location of the boundary. Three in-depth case study periods are used to qualify the various boundary identification methods. A detailed study of 30 IS radar scans in the geomagnetic meridian is used to conclude that a latitudinal gradient in characteristic energy of −5 to −11 keV per degree, when measured adjacent to an extended region of low-energy precipitation, can locate the boundary with a latitudinal precision of 0.2°.

A unique pass of the DMSP F11 satellite, longitudinally cutting through the cusp and mantle, combined with simultaneous optical measurements of the dayside cusp from Svalbard has been used to determine the width in local time of the cusp. We have shown from this event study that the cusp was at least 3.7 hours wide in magnetic local time. These measurements provide a lower limit for the cusp width. The observed cusp optical emissions are relatively constant, considering the processes which lead to the 630.0 nm emissions, and require precipitating electron flux to be added each minute during the DMSP pass throughout the local time extent observed by the imaging photometer and probably over the whole extent of the cusp defined by DMSP data. We conclude that the electron fluxes which produce the cusp aurora are from a process which must have been operable sometime during each minute but could have had both temporal and spatial variations. The measured width along with models of cusp precipitation provide the rationale to conclude that the region of flux tube opening in the dayside merging process involves the whole frontside magnetopause and can extend beyond the dawn-dusk terminator. The merging process for this event was found to be continuous, although spatially and temporally variable.

Coordinated radio and optical measurements of the structure and dynamics of the postsunset equatorial ionosphere were conducted on October 1, 1994, from Agua Verde, Chile (11.3°S magnetic latitude (MLat)). The measurements clearly show a north-south aligned undulation or ripple on the bottomside of the F layer at 2000 LT, appearing as an eastward propagating decrease in the 630.0-nm airglow, resembling a traveling ionospheric disturbance in the digital portable ionosonde measurements and causing a total electron content decrease in the Global Positioning System (GPS) satellite measurements. The initial development of this feature, toward the east and away from the magnetic equator, took place in an otherwise smooth, unstructured ionosphere. Spread F began to develop in the ionograms at 2020 LT, and, at this same time, local onset of satellite signal scintillation was detected using the multiple ray paths throughout the sky available from the GPS satellite constellation transmitting at L band frequencies. UHF scintillation measurements from Ancon, Peru, along the same magnetic field line, show that intense scintillation and ionospheric irregularities had developed over the magnetic equator almost 60 min prior to their development at 11°S MLat. The observations suggest that the east-west electric field expected to be present within the earlier developed depletion and scintillation region at the magnetic equator mapped along magnetic field lines to lower altitudes and higher latitudes, resulting in an undulation or dome-shaped structure, before evolving into a fully developed depletion (with associated ionospheric irregularities) all along the magnetic flux tube.

Combined optical and radio sensors provide a unique characterization of the structure of equatorial emission depletion regions connected to rising bubbles over the magnetic equator. In Chile, as part of the MISETA campaign in fall 1994, a CCD-enhanced all-sky imaging photometer provided optical images of the postsunset appearance and motions of the depletion bands at a magnetic dip latitude of 11°S. Concurrently, a Digisonde collocated with the photometer monitored the appearance of spread F. In between the ionograms, the sounder operated as a Doppler interferometer identifying the locations of F layer irregularities associated with the spread F. They were found to lie inside the emission depletion regions. The HF sounder, requiring orthogonality with the field-aligned F layer irregularities to generate the spread structure, tracked these irregularities inside the emission depletion bands as they drifted eastward. Ray tracing simulations show that the radio waves become trapped within the depletion regions when the depletions are within 300 km of the sounder site. Model calculations indicate that the sounder rays encounter orthogonality with the Earth's magnetic field within the depletion bubble southward from the site, consistent with the local dip angle. The combination of optical images with HF radio sounding demonstrated that radio imaging in the equatorial ionosphere can be done with a digital ionosonde that operates as a Doppler interferometer. The Digisonde measurements and ray tracing show for the first time that the spread F signatures on ionograms are the result of coherent scatter from irregularities primarily within the walls of the depletion.

An equatorial campaign was conducted during September 25 to October 7, 1994, to investigate the neutral and plasma dynamics in the equatorial ionosphere after sunset in relation to the day-to-day variability of the occurrence of equatorial spread F (ESF). The campaign was organized under the auspices of National Science Foundation's Multi-Instrumented Studies of the Equatorial Thermosphere Aeronomy program (MISETA), which included the Jicamarca radar, spaced-antenna satellite scintillation, digisonde, all-sky imager, and Fabry-Perot interferometer (FPI) measurements near the magnetic equator in Peru. During a part of the period September 27 to October 3, the Geophysics Directorate of Phillips Laboratory performed measurements away from the magnetic equator at Aguaverde, Chile (magnetic latitude: 11°S) located 800 km to the east of the Jicamarca meridian using geostationary and GPS satellite scintillation, digisonde and all-sky imager systems. The incoherent scatter radar results indicate that the postsunset enhancement of upward plasma drift, even though of the order of only 20 m s−1 during the solar minimum period, is a necessary condition for the generation of ESF. In view of the extreme difficulty of determining the neutral wind speed during the early evening hours by the FPI due to low airglow intensity, it was not possible to unequivocally associate the observed postsunset enhancements with strong eastward neutral winds. However, considering a few observations contiguous to the campaign period, it appears that such a causal relationship may exist. The scintillation drift measurements in Peru and Chile indicated that the zonal irregularity drift was smaller away from the magnetic equator, implying a variation of neutral wind with latitude. This is reproduced in the altitude variation of zonal drift observed by the Jicamarca radar. During a magnetic storm, scintillation measurements indicated that eastward drifts near the magnetic equator are accompanied by westward drifts near the anomaly peak, which is consistent with the effects of a disturbance dynamo. The campaign results indicate that in order to resolve the variability of ESF, a careful probing of neutral dynamics as a function of latitude needs to be undertaken during the postsunset period.

The extended flight of the Airborne Ionospheric Observatory during the Geospace Environment Modeling (GEM) Pilot program on January 16, 1990, allowed continuous all-sky monitoring of the two-dimensional ionospheric footprint of the northward interplanetary magnetic field (IMF) cusp in several wavelengths. Especially important in determining the locus of magnetosheath electron precipitation was the 630.0-nm red line emission. The most striking morphological change in the images was the transient appearance of zonally elongated regions of enhanced 630.0-nm emission which resembled “rays” emanating from the centroid of the precipitation. The appearance of these rays was strongly correlated with the Y component of the IMF: when the magnitude of By was large compared to Bz, the rays appeared; otherwise, the distribution was relatively unstructured. Late in the flight the field of view of the imager included the field of view of flow measurements from the European incoherent scatter radar (EISCAT). The rays visible in 630.0-nm emission exactly aligned with the position of strong flow jets observed by EISCAT. We attribute this correspondence to the requirement of quasi-neutrality; namely, the soft electrons have their largest precipitating fluxes where the bulk of the ions precipitate. The ions, in regions of strong convective flow, are spread out farther along the flow path than in regions of weaker flow. The occurrence and direction of these flow bursts are controlled by the IMF in a manner consistent with newly opened flux tubes; i.e., when |By| > |Bz|, tension in the reconnected field lines produce east-west flow regions downstream of the ionospheric projection of the x line. We interpret the optical rays (flow bursts), which typically last between 5 and 15 min, as evidence of periods of enhanced dayside (or lobe) reconnection when |By| > |Bz|. The length of the reconnection pulse is difficult to determine, however, since strong zonal flows would be expected to persist until the tension force in the field line has decayed, even if the duration of the enhanced reconnection was relatively short.

A specialized incoherent scatter radar scanning mode has been developed for use in conjunction with simultaneous real-time all-sky images. These complementary diagnostics are used to examine the aeronomy and electrodynamics of stable auroral arcs that delineate the boundary between the polar cap and the auroral oval. The first arc discussed, observed at 2000 MLT, represents the boundary between antisunward plasma flow in the polar cap and sunward return flow equatorward of the arc. The arc defined an equipotential in the high-latitude convection pattern in that no plasma flowed across the arc. The radar line-of-sight velocity measurements also indicate that this arc is consistent with a convergent electric field and an associated weak upward field-aligned current. The second arc was observed at 2330 MLT and was associated with a nightside gap or magnetic reconnection region. Strong antisunward flow was observed directly across the arc, although a velocity shear was superposed on this steady flow along the poleward edge of the arc. Detailed plasma density, temperature, and line-of-sight velocity measurements from the radar are presented for both arcs to define the electric field, horizontal and field-aligned currents, and thermal plasma parameters associated with these arcs.

Radio wave and optical experiments were conducted onboard a U.S. Air Force research aircraft in March 1977 and March 1978 at low magnetic latitudes to investigate the effects of F region electron density irregularities on transionospheric communications links. Imaging photometer, ionosonde, and satellite amplitude scintillation measurements were used to monitor the development and motion of F region 6300-A O I airglow depletions, spread F, and scintillation producing irregularities that are all associated with low-density bubbles in the postsunset equatorial ionosphere. The 6300-A airglow depletions are the bottomside signature of low plasma density within the bubbles. Examples of multiple airglow depletions and their relation to variations in the F layer virtual height (h′F) and to the occurrence of amplitude scintillations on 250-MHz satellite signals are described. Estimates of the average bottomside electron density, from simultaneous ionosonde measurements and 6300-A airglow intensities, show electron density decreases of ∼66% within the bubbles. These decreases are approximately the same for bubbles observed at the magnetic equator and near Ascension Island (18°S magnetic latitude). The measurements at Ascension Island show that airglow depletions extend away from the magnetic equator into the southern 6300-A intertropical arc. Variations in the maximum poleward extent of airglow depletions and of associated ionospheric irregularities that give rise to amplitude scintillations were observed. These latitudinal variations are interpreted, using field line mapping considerations, as variations in the maximum altitude of plasma bubbles over the magnetic equator. A north-south flight confirms that the overall pattern of airglow depletions and associated ionospheric irregularities extends continuously across the magnetic equator to ±15° magnetic latitude.

Results are presented from the Polar Ionospheric Irregularities Experiment (PIIE), conducted from Sondrestrom, Greenland, on March 15, 1985, designed for an investigation of processes which lead to the generation of small-scale (less than 1 km) ionospheric irregularities within polar-cap F-layer auroras. An instrumented rocket was launched into a polar cap F layer aurora to measure energetic electron flux, plasma, and electric circuit parameters of a sun-aligned arc, coordinated with simultaneous measurements from the Sondrestrom incoherent scatter radar and the AFGL Airborne Ionospheric Observatory. Results indicated the existence of two different generation mechanisms on the dawnside and duskside of the arc. On the duskside, parameters are suggestive of an interchange process, while on the dawnside, fluctuation parameters are consistent with a velocity shear instability.

Auroral photometric measurements were carried out at South Pole, Antarctica (74.5degree invariant latitude), throughout the austral winter of 1972. Observed auroral emissions included 4278 A N/sub 2/ /sup +/, 4861 A H/sub ..beta../ 5577 A I, and 6300 A I. A comprehensive study of auroral emissions and riometer absorption associated with the solar flare activity of August 1972 has been carried out. Energetic solar protons (>1 MeV) measured by satellite detectors over the polar caps were found to be reasonable for the measured 30-MHz absorption in both the northern and the southern hemispheres, except for two substorm-related events. These protons account for only a fraction of the measured 4278 A N/sub 2/ /sup +/ with the remainder of the emissions excited by protons and electrons of lower energies. Simultaneous photometric measurements and riometer observations allow separation of precipitating particle species and characteristic energy, which in turn allows separation of substorm effects from energetic solar proton effects. (AIP)

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.

In general the solutions to the azimuthal equations of motion of the solar wind possess a singularity at the radial Alfvenic critical point if the fluid is non-viscous and has an infinite electrical conductivity. This singularity can only be removed by a proper choice of boundary conditions. If a plasma with a large but finite conductivity is considered, the singularity is removed in the mathematical sense. However, it is shown that for this latter case the actual solution does not differ significantly from the former model and that the boundary condition is still determined from the behavior of the solution at or near the Alfvenic critical point.

The steady-state motion of a quasi-neutraln-ion plasma is investigated using a fluiddynamical model. The main results obtained are that there are only two distinct ways in which such a plasma can make a transition from a subsonic state to a supersonic one. There is one unique possibility for which there exists one critical point where all the ion gases have their Mach numbers exactly to unity and where the individual ion forcing functios (the inhomogeneous terms) are non-zero but linearly related to each other. The other possibility which we find is that at a critical point all inhomogeneous terms are identically equal to zero and there exist as many critical points as there exist simultianeous zeros of the ion forcing functions. These results are necessary and sufficient forn≥3. For one- and two-ion plasmas special results can be obtained, some of which fit into the above two classes and some of which do not.

A model of the solar atmosphere is presented in which we discuss the conservation of angular momentum for the two basic states in which the solar gas can be: namely, either confined by closed field lines or outflowing along open magnetic field lines. It can be shown that the boundary conditions are in general different for these two cases. From this we obtain the results that in the closed configuration the gas can corotate at the solar surface with the magnetic field lines and its angular velocity will then increase with height, whereas for a gas flowing along an open field line the angular velocity will decrease. An exception to the latter case can be found where the open magnetic field lines are strongly nonradial and where the density is a slowly varying function of radius. In such regions the angular velocity may initially increase with height, reach a maximum and then decrease.

Steady state symmetrical model for solar wind near equatorial plane, investigating effects of magnetic forces, viscosity and anisotropic pressure on azimuthal motion

The singularities and critical points of the polar-wind equations in the hydrodynamic model of Banks and Holzer are discussed. They are shown to be functions of the relative ion composition and the ratios of the electron to ion temperatures, and can be described in terms of a ‘reduced’ Mach number. As a result of these singularities, polar-wind flows are restricted to O+ velocities on the order of 50 m sec−1 near the critical point.

In a previous paper (Cosmic Electrodyn.3, 116) we had suggested the use of the radialr −2/5 dependence rather than the often usedr −2/7 one in approximations of the electronic temperature profile in some two-fluid models. This note is intended to clarify and restate some points on this subject.

The solutions of a radially symmetrical solar wind model with thermal conductivity are discussed in the neighborhood of the loci of the critical points, called the critical line, and at infinity. It is shown that for constant mass and total energy flux the conditions at infinity are related uniquely to the conditions at the critical line. Furthermore the requirement that the solution is continuous and has the proper behavior at infinity will determine uniquely one specific point on the critical line through which the solution has to pass.

The first sounding rocket flight into the dayside cusp with dark ground and northward interplanetary magnetic field (IMF) conditions was launched from the new SvalRak range at Ny-Alesund in the Svalbard archipelago in early December 1997. Extensive ground observations of auroral emissions and radar backscatter provided contexts for in situ rocket measurements. Real-time interplanetary measurements from the Wind satellite aided launch selection with foreknowledge of impending conditions. NASA rocket flight 36.153 was launched near local magnetic noon while the IMF was dominated by positive B X and had lesser northward B Z and negative B Y components. The rocket's westward trajectory carried it toward auroral forms associated with morningside boundary layers. The rich set of vector dc electric and magnetic fields, energetic particles, thermal plasma, plasma waves, and optical emissions gathered by the rocket reveal a complex electrodynamic picture of the cusp/boundary-layer region. Four factors were important in separating temporal and spatial effects: (1) Near the winter solstice the Earth's north magnetic pole tilts away from the Sun, (2) at the UT of the flight the dipole axis was rotated toward dawn, (3) the variability of solar wind driving was low, and (4) B X was the dominant IMF component. We conclude that no signatures of dayside merging in the Northern Hemisphere were detected in either the rocket or ground sensors. Electric field variations in the interplanetary medium directly correlate with those observed by the sounding rocket, with significantly shorter lag times than estimated for simple propagation between Wind and the ionosphere. The correlation requires that the observed Northern Hemisphere convection structures were stirred in part by merging of the IMF with closed field lines in the Southern Hemisphere, thereby adding open flux to the northern polar cap. Subsequent motions of adiaroic polar cap boundaries were detected in the rocket electric field measurements. The observations indicate that IMF B X significantly affected the location and timing of merging interactions.

Coincident DE 2 satellite plasma-drift and particle measurements and all-sky imaging photometer data were used to construct two-dimensional convection patterns in a study of polar ionospheric convection in the presence of subvisual-intensity soft-particle excited 6300-A sun-aligned polar cap arcs. Results show a highly anisotropic temporally stable convection with greater order than has previously been suspected.

The use of SF6 releases to excite the airglow from atomic oxygen in the ionosphere is demonstrated by experiment and by theory. Enhanced 777.4-nm emissions from O(5P) states were measured during the AFGL-sponsored Ionospheric Modification Studies campaign at Wallops Island, Virginia. For this rocket campaign, 8×1025 molecules of SF6 were released at 350-km altitude into the midnight ionosphere on November 9, 1983. The 777.4-nm airglow increased by over 150 R. The temporal evolution of the airglow is simulated using an ion chemistry model involving six positive and negative ion species and 16 reactions. The model indicates that substantial enhancements in 844.6 nm and 135.6 nm will be triggered by SF6 injections into the ionosphere. The model also demonstrates that the intensity of the 777.4-nm line is strongly dependent on the temperature of the neutral atmosphere.

A new instrument for all-sky spectrophotometric imaging of aurora and airglow has been installed in the Air Force Geophysics Laboratory's Airborne ionospheric Observatory. Initial observations of equatorial and near-equatorial 6300-A O I airglow show the existence of north-south aligned regions of airglow depletion. These dark bands often extend more than 1200 km in the north-south direction and 50–200 km in the east-west direction. They are observed to drift toward the east during the evening-midnight hours, with one observation of westward drift after local midnight. Airglow fine structure associated with the boundaries of the dark bands has been observed down to the 2.5-km resolution limit of the instrument. Simultaneous airborne ionospheric soundings indicate that these regions of airglow depletion are characterized by an increase in the virtual height of the F layer. A simple model of field-aligned electron density depletion in the bottomside of the F layer explains both the airglow observations and the ionospheric soundings.

Photometric keograms from South Pole Station are used to investigate dayside auroral precipitation. Identification of cusp auroral signatures allows cusp movements to be monitored and correlated with substorm activity and interplanetary magnetic field direction. We conclude that any erosion of dayside field is not a direct result of southward turning Bz. On the other hand, cusp position is intimately related to substorm timing and intensity. We suggest the associated dayside field reconfiguration results from diversion of near-tail magnetopause currents through the nightside electrojet because of enhanced auroral ionosphere conductivity. This interpretation implies that substorms result from increased efficiency of the extraction of energy from the solar wind rather than from a release of internally stored magnetic energy in the tail.

Le laboratoire ionospherique de l'Air Force a effectue des mesures radio, optiques et de scintillation selon un trajet N-S le long du meridien de Chatanika. Les mesures obtenues ont ete comparees aux donnees du radar incoherent (densite electronique, temperature, derive des ions). L'intensification de la region F observee ne correspond pas a la precipitation d'electrons mais plutot a des mecanismes de convection

Observations of electron and proton precipitation fluxes and Birkeland currents from Defence Meteorological Satellite Program spacecraft F7 and simultaneous ground-based optical measurements from Svalbard (∼75° geomagnetic latitude) of dayside aurorae (∼0900 MLT) are presented. In these cases, the auroral emissions within the field of view can be separated into different latitudinal zones with corresponding structures in particle precipitation and Birkeland current. The temporal evolution of these structures is monitored by the ground-based instruments, indicating the presence of both stationary cleft emissions produced by soft electron fluxes (energy of a few hundred eV) and multiple, discrete arcs and arc fragments of transient or more persistent nature, corresponding to the precipitation of keV electrons. The latter are associated with narrow sheets of upward flowing Birkeland currents and reduced proton precipitation, suggesting the presence of field-aligned potential drops above the spacecraft. Assuming that these low-altitude structures map to the low-latitude boundary layer of the magnetosphere, they correspond to a strong filamentation of the boundary layer, in the prenoon sector. Field-aligned current systems separated in latitude and associated with different particle populations, i.e., plasma-sheet-like and low-latitude-boundary-layer-like particles, respectively, are observed in one case, after a sharp interplanetary magnetic field Bz polarity change.

Ground-based optical and digital ionosonde measurements were conducted at Thule, Greenland to measure ionospheric structure and dynamics in the nighttime polar cap F layer. These observations showed the existence of large-scale (800-1000 km) plasma patches drifting in the antisunward direction during a moderately disturbed (Kp greater than or equal to 4) period. Simultaneous Dynamics Explorer (DE-B) low-altitude plasma instrument (LAPI) measurements show that these patches with peak densities of about 10 to the 6th el per cu cm are not locally produced by structured particle precipitation. The LAPI measurements show a uniform precipitation of polar rain electrons over the polar cap. The combined measurements provide a comprehensive description of patch structure and dynamics. They are produced near or equatorward of the dayside auroral zone and convect across the polar cap in the antisunward direction. Gradients within the large scale, drifting patches are subject to structuring by convective instabilities. UHF scintillation and spaced receiver measurements are used to map the resulting irregularity distribution within the patches.

A series of experiments conducted in December 1979 to investigate the structure of plasma depletions in the low latitude, nighttime ionosphere is discussed. Density biteouts of about one order of magnitude in the dominant ion, O(+), are mapped to lower altitudes along magnetic field lines for comparison with 6300-A and 7774-A O I airglow depletions. Owing to the different airglow production mechanisms (dissociative recombination of O2(+) for 6300 A and radiative recombination of O(+) for 7774 A), the 6300-A depletions reflect plasma depletions near the bottomside of the F layer, while those at 7774 A are located near the peak of the layer. The O(+) biteouts map directly into the 7774-A airglow depletions in the same hemisphere and also when traced into the opposite hemisphere, which suggests magnetic flux tube alignment over north-south distances of approximately 2220 km. The 6300-A (bottomside) depletions are found to be wider in longitude than the 7774-A (F-peak) depeletions near the equatorward edge of the Appleton anomaly.

Coordinated measurements of F region plasma patches were conducted on February 3/4, 1984, from Thule and Sondrestrom, Greenland. Optical, ionosonde, amplitude scintillation, total electron content (TEC), and incoherent scatter radar measurements were combined to reveal several new aspects of the structure and transport of these localized regions of enhanced F region ionization. For the first time these patches were directly tracked flowing in the antisunward direction over distances of 3000 km from the center of the polar cap to the poleward edge of the auroral oval. Quantitative measurements of TEC show increases of 10--15 TEC units within the patches, above a background polar cap value of 5 TEC units. Amplitude scintillation measurements show the presence of ionospheric irregularities through the entire patch, with a weak indication of stronger scintillation on the trailing (or E x B unstable) edge.

An Isis 2 pass studied in related experiments was 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. Since both long-duration measurements (aircraft) and transient snapshot (spacecraft) data are available, space and time effects can, on a macroscopic level, be separated. The latitudinal morphology observed by the satellite is found to be basically spatial in nature. It is suggested that the observed particle fluxes can be explained in terms of precipitation from the quiet time plasma sheet without intervening acceleration. The agreement of the observed optical emissions and ionospheric parameters with the electron fluxes is discussed.