Trond S. Trondsen

Far ultraviolet imaging from the IMAGE spacecraft. 2. Wideband FUV imaging

The Far Ultraviolet Wideband Imaging Camera (WIC) complements the magnetospheric images taken by the IMAGE satellite instruments with simultaneous global maps of the terrestrial aurora. Thus, a primary requirement of WIC is to image the total intensity of the aurora in wavelength regions most representative of the auroral source and least contaminated by dayglow, have sufficient field of view to cover the entire polar region from spacecraft apogee and have resolution that is sufficient to resolve auroras on a scale of 1 to 2 latitude degrees. The instrument is sensitive in the spectral region from 140–190 nm. The WIC is mounted on the rotating IMAGE spacecraft viewing radially outward and has a field of view of 17° in the direction parallel to the spacecraft spin axis. Its field of view is 30° in the direction perpendicular to the spin axis, although only a 17°×17° image of the Earth is recorded. The optics was an all-reflective, inverted Cassegrain Burch camera using concentric optics with a small convex primary and a large concave secondary mirror. The mirrors were coated by a special multi-layer coating, which has low reflectivity in the visible and near UV region. The detector consists of a MCP-intensified CCD. The MCP is curved to accommodate the focal surface of the concentric optics. The phosphor of the image intensifier is deposited on a concave fiberoptic window, which is then coupled to the CCD with a fiberoptic taper. The camera head operates in a fast frame transfer mode with the CCD being read approximately 30 full frames (512×256 pixel) per second with an exposure time of 0.033 s. The image motion due to the satellite spin is minimal during such a short exposure. Each image is electronically distortion corrected using the look up table scheme. An offset is added to each memory address that is proportional to the image shift due to satellite rotation, and the charge signal is digitally summed in memory. On orbit, approximately 300 frames will be added to produce one WIC image in memory. The advantage of the electronic motion compensation and distortion correction is that it is extremely flexible, permitting several kinds of corrections including motions parallel and perpendicular to the predicted axis of rotation. The instrument was calibrated by applying ultraviolet light through a vacuum monochromator and measuring the absolute responsivity of the instrument. To obtain the data for the distortion look up table, the camera was turned through various angles and the input angles corresponding to a pixel matrix were recorded. It was found that the spectral response peaked at 150 nm and fell off in either direction. The equivalent aperture of the camera, including mirror reflectivities and effective photocathode quantum efficiency, is about 0.04 cm2. Thus, a 100 Rayleigh aurora is expected to produce 23 equivalent counts per pixel per 10 s exposure at the peak of instrument response.

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.

Asymmetric multiple auroral arcs and inertial Alfvén waves

High-resolution optical observations by the University of Calgary Portable Auroral Imager show a frequent occurrence of asymmetric multiple small-scale auroral arc structures during auroral substorms. Whereas the classical multiple arc array tends to exhibit a fairly symmetrical configuration, with parallel motions within individual discrete arcs being opposite in direction across the center of the arc array, the multiple arcs to be discussed herein are distinguished by the presence of discrete arcs strictly equatorward of the two bright counter-streaming arcs that would ordinarily define the center of the arc array. The intensity of these parallel equatorward-lying arcs were in most cases found to decrease rapidly in the equatorward direction. By considering the topology of the structures and the spacing between arcs, observations are found to be consistent with recent theories suggesting inertial Alfven waves as a possible cause of fine-scale auroral arcs.

Interplanetary magnetic field By control of dayside auroras

Abstract The effect of the interplanetary magnetic field (IMF) By component on the dayside auroral oval from Viking UV measurements for March–November 1986 is studied. Observations of dayside auroras from Viking UV images for large positive (15 cases) and negative (22 cases) IMF By (∣By∣>4 nT), suggest that: (1) the intensity of dayside auroras tends to increase for negative IMF By and to decrease for positive By, so that negative IMF By conditions seem preferable for observations of dayside auroras; (2) for negative IMF By, the auroral oval tends to be narrow and continuous throughout the noon meridian without any noon gap or any strong undulation in the auroral distribution. For positive IMF By, a sharp decrease and spreading of auroral activity is frequently observed in the post-noon sector, a strong undulation in the poleward boundary of the auroral oval around noon, and the formation of auroral forms poleward of the oval; and (3) the observed features of dayside auroras are in reasonable agreement with the expected distribution of upward field-aligned currents associated with the IMF By in the noon sector.

Hemispheric asymmetry of the structure of dayside auroral oval

A comprehensive analysis of long-term and multispectral auroral observations made in the Arctic and Antarctica demonstrates that the dayside auroral ovals in two hemispheres are both presented in a two-peak structure, namely, the prenoon 09:00 magnetic local time (MLT) and postnoon 15:00 MLT peaks. The two-peak structures of dayside ovals, however, are asymmetric in the two hemispheres; i.e., the postnoon average auroral intensity is more than the prenoon one in the Northern Hemisphere but less in the Southern Hemisphere. The hemispheric asymmetry cannot be accounted for by the effect of the interplanetary magnetic field By component and the seasonal difference of ionospheric conductivities in the two hemispheres, which were used to interpret satellite-observed real-time auroral intensity asymmetries in the two hemispheres in previous studies. We suggest that the hemispheric asymmetry is the combined effect of the prenoon-postnoon variations of the magnetosheath density and local ionospheric conductivity.

Fine-scale optical observations of Aurora

Abstract High spatial and temporal resolution observations of auroral forms made by the University of Calgary Portable Auroral Imager are reviewed. Observations include auroral forms with apparent widths of on the order of tens to hundreds of meters and lifetimes of less than a second to several tens of seconds. Included here is a class of multiple auroral arc systems exhibiting a strikingly asymmetric internal topology. It is shown that such systems of multiple linear arcs may be explained by mode conversion of field-line resonance shear Alfven waves to inertial Alfven waves. A survey of small-scale, short-lived auroral vortices (curls) occurring on thin auroral forms within the active aurora has also been performed, with some results being reviewed here. The Kelvin-Helmholtz instability as a responsible mechanism is discussed in light of the observationally acquired statistical data. Observations of small-scale “black” auroral arcs and vortices are briefly reviewed as well.

Hyperspectral all-sky imaging of auroras

A prototype auroral hyperspectral all-sky camera has been constructed and tested. It uses electro-optical tunable filters to image the night sky as a function of wavelength throughout the visible spectrum with no moving mechanical parts. The core optical system includes a new high power all-sky lens with F-number equal to f/1.1. The camera has been tested at the Kjell Henriksen Observatory (KHO) during the auroral season of 2011/2012. It detects all sub classes of aurora above ~½ of the sub visual 1kR green intensity threshold at an exposure time of only one second. Supervised classification of the hyperspectral data shows promise as a new method to process and identify auroral forms.

Simulating the aurora

We present the first computer graphics algorithm designed to simulate the aurora, a natural phenomenon of great visual beauty and considerable scientific interest. The algorithm is based on the current understanding of the physical origin of this natural display. The aurorae are mainly caused by high-energy electrons originating in the sun and entering the earth’s atmosphere in narrow regions centered on the magnetic poles. These electrons collide with atmospheric atoms, which are excited to higher energy levels. The excited atoms emit rapidly varying visible light in a curtain-like volume as they return to lower energy levels, thereby creating the aurora. By simulating these light emissions along with the spatial and temporal distribution of the entering electrons, we are able to render the major visual aspects of auroral displays. The applicability of this auroral model for rendering and scientific purposes is illustrated through comparisons of synthetic images with photographs of real auroral displays. Copyright # 2003 John Wiley & Sons, Ltd.

Low-cost multi-band ground-based imaging of the aurora

Modern auroral research uses a variety of optical instruments ranging from photometers to spectral imagers. We report our results in developing an inexpensive auroral imager, which captures true-colour images using four wide-band channels. While not replacing dedicated highly sensitive cameras with filter wheels and narrow bandpass filters, the advantages of capturing the colour should not be underestimated. The colour not only provides more information about the physical processes in the ionosphere but also enhances both manual and automated image processing due to the discriminating power of colour information. We have operated our auroral imager RAINBOW in Athabasca, Alberta, Canada for over a year. RAINBOW can acquire images every ten seconds and operate even in moonlit conditions. A clever design using inexpensive optical components provides a field-of-view of approximately 150 degrees, and an external shutter provides protection from direct sunlight. We discuss the issues related to imager hardware and colour calibration. Future applications are also highlighted.