L. L. Cogger

Far Ultraviolet Imaging from the Image Spacecraft: 1. System Design

Direct imaging of the magnetosphere by the IMAGE spacecraft will be supplemented by observation of the global aurora, the footprint of magnetospheric regions. To assure the simultaneity of these observations and the measurement of the magnetospheric background neutral gas density, the IMAGE satellite instrument complement includes three Far Ultraviolet (FUV) instruments. In the wavelength region 120-190 nm, a downward-viewing auroral imager is only minimally contaminated by sunlight, scattered from clouds and ground, and radiance of the aurora observed in a nadir viewing geometry can be observed in the presence of the high-latitude dayglow. The Wideband Imaging Camera (WIC) will provide broad band ultraviolet images of the aurora for maximum spatial and temporal resolution by imaging the LBH N2 bands of the aurora. The Spectrographic Imager (SI), a monochromatic imager, will image different types of aurora, filtered by wavelength. By measuring the Doppler-shifted Ly-α, the proton-induced component of the aurora will be imaged separately. Finally, the GEO instrument will observe the distribution of the geocoronal emission, which is a measure of the neutral background density source for charge exchange in the magnetosphere. The FUV instrument complement looks radially outward from the rotating IMAGE satellite and, therefore, it spends only a short time observing the aurora and the Earth during each spin. Detailed descriptions of the WIC, SI, GEO, and their individual performance validations are discussed in companion papers. This paper summarizes the system requirements and system design approach taken to satisfy the science requirements. One primary requirement is to maximize photon collection efficiency and use efficiently the short time available for exposures. The FUV auroral imagers WIC and SI both have wide fields of view and take data continuously as the auroral region proceeds through the field of view. To minimize data volume, multiple images are taken and electronically co-added by suitably shifting each image to compensate for the spacecraft rotation. In order to minimize resolution loss, the images have to be distortion-corrected in real time for both WIC and SI prior to co-adding. The distortion correction is accomplished using high speed look up tables that are pre-generated by least square fitting to polynomial functions by the on-orbit processor. The instruments were calibrated individually while on stationery platforms, mostly in vacuum chambers as described in the companion papers. Extensive ground-based testing was performed with visible and near UV simulators mounted on a rotating platform to estimate their on-orbit performance. The predicted instrument system performance is summarized and some of the preliminary data formats are shown.

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

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.

Mapping using the Tsyganenko long magnetospheric model and its relationship to Viking auroral images

The Tsyganenko long magnetospheric model (1987) has been used in conjunction with ultra-violet images taken by the Viking spacecraft to investigate the relationship of the auroral distribution to different magnetospheric regions. The model describes the large-scale structure of the magnetosphere reasonably well for dipole tilt angles near zero, but it appears to break down at higher tilt angles. Even so, a wide variety of auroral configurations can be accurately described by the model. It appears that the open-closed field line boundary is a poor indicator of auroral arc systems with the possible exception of high-latitude polar arcs. The auroral distribution typically called the oval maps to a region in the equatorial plane quite close to the Earth and can be approximately located by mapping the model current density maximum from the equatorial plane into the ionosphere. Although the model may break down along the flanks of the magnetotail, the large-scale auroral distribution generally reflects variations in the near-Earth region and can be modeled quite effectively.

Observations in the vicinity of substorm onset: Implications for the substorm process

Multi-instrument data sets from the ground and satellites at both low and high altitude have provided new results concerning substorm onset and its source region in the magnetosphere. Twenty-six out of 37 substorm onset events showed evidence of azimuthally spaced auroral forms (AAFs) prior to the explosive poleward motion associated with optical substorm onset. The azimuthal wavelengths associated with these onsets were found to range between 132 and 583 km with a mean value of 307±115 km. The occurrence rate increased with decreasing wavelength down to a cutoff wavelength near 130 km. AAFs can span 8 hours of local time prior to onset and generally propagate eastward in the morning sector. Onset itself is, however, more localized spanning only about 1 hour local time. The average location of the peak intensity for 80 onsets was 65.9±3.5 CGMlat, 22.9±1.2 Mlt, whereas the average location of the AAF onsets was at 63.8±3.3 CGMlat, 22.9±1.1 Mlt. AAF onsets occur during time periods when the solar wind pressure is relatively high. These low-latitude wavelike onsets appear as precursors in the form of long-period magnetic pulsations (Pc 5 band) and frequently occur on the equatorward portion of the double oval distribution. AAFs brighten in conjunction with substorm onset leading to the conclusion that they are a growth phase activity causally related to substorm onset. Precursor activity associated with these AAFs is also seen near geosynchronous orbit altitude and examples show the relationship between the various instrumental definitions of substorm onset. The implied mode number (30 to 135) derived from this work is inconsistent with cavity mode resonances but is consistent with a modified flute/ballooning instability which requires azimuthal pressure gradients. It is suggested that this instability exists in growth phase but that an additional factor exists in the premidnight sector which results in an explosive onset. The extended source region and the distance to the open-closed field line region constrain reconnection theory and local mechanisms for substorm onset. It is demonstrated that multiple onset substorms can exist for which localized dipolarizations and the Pi 2 occur simultaneously with tail stretching existing elsewhere. Further, the tail can be less stretched at geosynchronous orbit during the optical auroral onset than during the precursor pseudobreakups. These pseudobreakups can be initiated by auroral streamers which originate at the most poleward set of arc systems and drift to the more equatorward main UV oval. Observations are presented of these AAFs in conjunction with low- and high-altitude particle and magnetic field data. These place the activations at the interface between dipolar and taillike field lines probably near the peak in the cross-tail current. These onsets are put in the context of a new scenario for substorm morphology which employs individual modules which operate independently or couple together. This allows particular substorm events to be more accurately described and investigated.

What is a global auroral substorm

The departure of the aurora from quiet levels in a dynamic manner constitutes some type of auroral “breakup” event. Research into the auroral breakup predates the International Geophysical Year (1957/1958). This feature of the aurora, and the later, more global concept of the auroral substorm, has become a focus for much of the auroral research that occurs today. New instrumentation and global collaborations continue to refine our knowledge of the substorm process and how it proceeds in the ionosphere. In particular, global auroral imaging has advanced our understanding of the dynamics of the process and has given us the ability to put localized observations into a global perspective. Fundamentally new cycles of auroral activity are now understood to exist, and this has provided a means by which auroral activity can answer questions about magnetospheric substorm dynamics. Along with this wealth of observations has come a wide range of theories purporting to explain the mechanism of the onset of this phenomenon. There is, however, no single theory which stands out as clearly explaining the wide range of active auroral phenomena. A synthesis which combines these theories and allows them to each explain individual aspects of the problem appears to be required. This has led to a new way of understanding the active aurora as a set of processes or modules which occur either coupled together or independent of one another to form a particular event. This view represents a fundamental departure from the view of the substorm as a single unchanging entity. Auroral activity can rather be thought of as the earthward end of a diverse set of ionospheric and magnetospheric processes which couple together to form different cyclical patterns. A symbolic representation of this modularization is presented to simplify future schematics of large-scale auroral dynamics.

The double oval UV auroral distribution: 1. Implications for the mapping of auroral arcs

During the later stages of the auroral substorm the luminosity distribution frequently resembles a double oval, one oval lying poleward of the normal or main UV auroral oval. We interpret the double oval morphology as being due to the plasma sheet boundary layer becoming active in the later stages of the substorm process. If the disturbance engulfs the nightside low-latitude boundary layers, then the double oval configuration extends into the dayside ionospheric region. The main UV oval is associated with the inner portion of the central plasma sheet and can rapidly change its auroral character from being diffuse to discrete. This transition is associated with the substorm process and is fundamental to understanding the near-Earth character of substorm onset. On the other hand, the poleward arc system in the nightside ionosphere occurs adjacent to or near the open-closed field line boundary. This system activates at the end of the optical expansion phase and is a part of the recovery phase configuration in substorms where it occurs. These two source regions for nightside discrete auroral arcs are important in resolving the controversy concerning the mapping of arcs to the magnetosphere. The dayside extension of this double oval configuration is also investigated and shows particle signatures which differ considerably from those on the nightside giving clues to the magnetospheric source regions of the aurora in the two local time sectors. Near-Earth substorm onsets are shown to be coupled to processes occurring much further tailward and indicate the importance of understanding the temporal development of features within the double oval. Using “variance images,” a new technique for the investigation of these dynamics is outlined.

Simultaneous optical observations of transpolar arcs in the two polar caps

Optical observations with DE 1 and Viking demonstrate conclusively that transpolar arcs can be present simultaneously at polar latitudes in both hemispheres. The arcs observed here are aligned parallel to one another on opposite sides of the magnetic pole, and relative to the noon-midnight meridian are rotated slightly in the counterclockwise direction when viewed from above the North Pole.

Width and structure of mesoscale optical auroral arcs

Arc widths were calculated for 3126 stable auroral arcs observed by an all-sky camera located in Gillam, Manitoba. The camera is filtered to accept 5577-A emissions and has a single-pixel spatial resolution of 1.7 km at zenith. The measured mean width of stable mesoscale arcs located within ±5° of magnetic zenith is 18 km with a standard deviation of 9 km. The width distribution exhibits a steep cutoff below 8 km; when combined with studies of small-scale auroral structure this cutoff suggests a gap in the occurrence of arcs with widths of order 1 km. This feature of the arc width spectrum argues against the notion of a turbulent cascade of energy from larger to small scales. Residuals from the Gaussian fits are only about 3% of the arc amplitude on average, indicating little sub-structure within arcs at scales down to the measurement resolution.

A survey of small‐scale spatially periodic distortions of auroral forms

High-resolution television observations by the University of Calgary portable auroral imager of the small-scale spatially periodic distortions of auroral forms known as “curls” are presented. During winter 1995 and 1997 field trips to Rabbit Lake, Saskatchewan, Canada, a large amount of video data containing auroral curls was acquired. Analysis of this data set has led to much improved insight into the properties of these features. Observed spatial and temporal characteristics are presented, with results compared to those of earlier surveys wherever applicable. Striking parallels between the characteristics of these ∼1 km scale size curls and the characteristics of the ∼ 100 km scale size auroral “spirals” observed by the Freja UV imager are pointed out and discussed in terms of shear-driven instabilities. In addition, observations of the interesting new phenomena of “auroral kinks” and “spinning auroral patches” are briefly described.

High-resolution television observations of black aurora

In view of a recent revival of interest in the black aurora and related phenomena, and a striking lack of information on the phenomenon in the literature, we present high spatial and temporal resolution optical observations of the black aurora made by the University of Calgary portable auroral imager. A variety of black auroral phenomena, such as black vortices, arcs, and eastward drifting black auroral patches and arc segments, were observed in the evening and midnight sector diffuse auroral oval during a field trip to Rabbit Lake, Saskatchewan, from February 25 to March 7, 1995. Observed spatial and temporal characteristics are reviewed, with statistical data presented in the form of histograms.