I M Wright, B J Fraser, F W Menk
Cooperative Research Centre for Satellite Systems
Department of Physics
University of Newcastle
NSW 2308
Australia

Abstract

Apparent drift motion of polar cap arcs may be indicative of plasma convection processes in the ionospheric plasma. The dynamics of a number of individual polar cap events observed by a dual wavelength allsky imager at Scott Base have been analysed in detail. The imager, which records optical emissions at 630 nm and 428.7 nm has been operated at Scott Base, (-79.96°, 327.61° magnetic), during the winters of 1996 to 1998. The events presented have been selected to show representative examples of arcs observed primarily in the dayside. Of interest is the speed and direction of the polar cap arc drift motion with respect to solar wind and interplanetary magnetic field conditions. Events were observed between 10:00 and 19:00 local time (mid-morning to dusk), where drift velocities varied between 60 m/s away from the invariant pole and 1000 m/s polewards. Another example shows a drift velocity of 300 m/s perpendicular to the pole direction. Each of the examples shows different velocities and drift direction, for differing solar wind conditions, and different times. The small number of images in the initial data set needs to be expanded into a full three year data set, and include data from other instruments, in order to fully understand the correlations of polar cap arc drifts with solar wind/interplanetary magnetic field (IMF) conditions.

1. Introduction

The observation of optical auroral forms that occur in the polar cap has led to the identification, and naming, of a wide range of emissions including: polar cap arcs, extremely high latitude auroras, sun aligned arcs, transpolar arcs, horse collar auroras, and theta aurora. It is difficult to interrelate these as they have been defined by various authors under a wide range of conditions and over a period of observations that began in 1907. In a recent review paper, Zhu, et al., [1997] use the term polar cap arc to differentiate aurora seen in the polar cap or poleward of the auroral oval from auroral oval emissions. Unlike auroral zone arcs these are seen more during northward IMF and quiet magnetic conditions . Early observations, made visually, and by film based allsky camera, identified properties of polar cap arcs including the Sun-Earth orientation, transverse motions, the occurrence only at high latitude, and the negative correlation with magnetic activity.

The launching of low Earth orbit satellites in the 1970's, for example the DMSP series, enabled particle measurements to be related to optical emissions [Hardy et al, 1982 ]. However, the orbits limited these studies to relatively local regions, and spin scanning resulted in coarse resolution in time and position. UV imaging and the high altitude elliptical orbit of DE 1 in the 1980's led to the first view of the theta aurora spanning the entire polar cap, a phenomenon that ISIS 2 and DMSP could not see. The DE-1 and 2 satellites found that polar cap arc precipitation had plasma sheet or plasma sheet boundary layer (PSBL) characteristics suggesting the arcs occur on closed field lines [Peterson and Shelley, 1984; Frank, 1988]. This was supported by the observed conjugacy of transpolar arcs [Craven, et al., 1991]. However, others, (e.g. Hardy et al ,1982; Gussenhoven and Mullen, 1989), have seen polar rain and relativistic electrons near polar arcs suggesting a source on open field lines. It is still not clear whether polar cap arcs occur on open or closed field lines, or whether the same phenomenon can arise under either of these conditions. (The review by Zhu, et al., [1997] contains comprehensive references)

Continued observations of high latitude optical emissions are needed to resolve questions about their relationship to open/closed field lines, and equally importantly, to the ionosphere convection that accompanies them. This study uses images of polar cap aurora acquired at Scott Base to investigate the scale, the motions, and dynamics of these phenomena to examine these questions. In particular the behaviour of small scale, relative to the polar cap, arcs of less than 1000 km length are noted.

2. Scott Base Imager project

2.1 Imager site

An Allsky digital imager has been operating at Scott Base, Antarctica since 1996 to record optical phenomena during the darkness of the Antarctic winter. The first season of operation produced some 28 days of data when aurora was visible for some time during the 24 UT hours, over the 1996 Austral winter. In 1997, the second season, 22 days of useful data were obtained. All the data from the first two seasons were recorded during quiet magnetic conditions, Kp ~ 2. The presence of cloud, or interference from sun or moon light, prevented collection of image from other times. For 1998, there were 36 days of good data recorded, some during more active conditions. Out of the data from the three operation periods, on some 38 occasions the drift motion of detected arcs have been selected for further study. The intensity and structures of arcs can vary during their lifetime. The average lifetime is some tens of minutes, but they occasionally exist for longer than 1 hour.

New Zealand's Scott Base Station is at Pram Point on Ross Island in the Ross Sea, and the US base, Mc Murdo, is a few kilometres away at Discovery Point. Both New Zealand and the USA each have a permanent observatory/laboratory at Arrival Heights which is about 60 metres above sea level, and four kilometres inland from the coast. Arrival Heights is located at 77 ° 49' 46" S, 166°39' 40" E geographically and is -79.96°, 327.61° CGM magnetic. This geographic location experiences total darkness at local noon for several weeks as the Sun is below the horizon during winter. These conditions allow the study of dayside emissions that normally are invisible.

Figure 1: Imager view looking upwards.

Figure 1, which superimposes the quiet time (Kp ~ 2) auroral oval of Feldstein and Starkov [1967] for each 6 hour interval over the map of Antarctica, shows Scott Base, (SB), is completely within the polar cap, except for the hours around 1800 UT, when the quiet time auroral oval may also enter the field of view. The circle, which is centred on Scott Base, represents a 1000 km diameter field of view (see section 2.3), and the auroral oval is shown rotating around the fixed imager location. The direction to the sun is indicated by the arrows. This Figure shows the upwards looking imager view, and the superimposed map of Antarctica is reversed left to right. In the images, the direction to the geomagnetic pole (MP) is to the upper left at 45°. The geographic pole is labelled "SP". At Scott Base, local solar noon is at 0056 UT.

2.2 Dual Wavelength Imager

Observations are recorded as two side by side circular horizon to horizon images of the sky. The left image contains 630 nm wavelength information and the right hand image is of 427.8 nm. There are examples presented in the later Figures. The Allsky system is housed in Antarctica New Zealand's Arrival Heights laboratory using a light trap below a glass dome in the roof. Room temperature air from the laboratory is circulated through the light trap and dome to minimise icing inside the dome. Figure 2 shows the schematic arrangement of the imager. It was manufactured by Keo Consultants of Boston and is similar to units used by the US at South Pole station and the US PENGUIn research project. Keo Consultants also supplied a different version of the instrument to the Australian Antarctic Division. A Canon 16 mm focal length "fish eye" lens creates a horizon to horizon image of the sky. This image is collimated so essentially parallel rays are passed through a dual pass band interference filter that only passes 630 nm and 428.7 nm wavelengths to a beam splitting prism. The prism produces the two side by side images of the whole sky, one image has the 427.8 nm wavelength removed by another interference filter, and the other has the 630 nm information removed.

Figure 2: Allsky imager schematic diagram.

These side by side images, are focussed onto an image intensifier. Keo Consultants documentation states the system detects minimum emissions of 50 Rayleigh. An intensifier is needed to detect optical emissions of this level. This intensifier was developed for night time military uses and has ramifications that will be discussed later. The output intensifier image is scanned by a CCD television camera. The camera includes a power supply for the intensifier, the TV camera, a rough calibration light, and a mechanical shutter below the objective lens that is interlocked with a light detector. The shutter and light dependant resistor protect the image intensifier from the damage that would result if an image of the sun was allowed to fall on the input.

Under normal operation the Allsky camera will take a pair of exposures every thirty seconds. One exposure of 0.9 seconds duration, and the other 0.3 s are achieved by allowing the CCD to integrate for these periods by suspending the normal television scanning at 25 frames per second. These automatically gain controlled images are 8-bit digitised, compressed, and then written to magneto-optical disc. In this paper we concentrate on the 0.9 s exposures which provide more detailed images than the 0.3 s exposures. The operation of the system is software controlled from a dedicated PC, with a video monitor for image display.

2.3 Data Characteristics

Auroral properties that can be observed from the ground include; apparent extent, apparent velocity, and apparent intensity. From a single observation point, assumptions need to be made to estimate actual values for these parameters. These assumptions depend on the performance of the of the observing device, and experience based on observations and results from previous work.

The electron - oxygen reaction that produces the red 630 nm radiation occurs at altitudes from 200 km to as high as 450 km, depending on atmospheric conditions such as the concentration of oxygen. [Carlson and Egeland, 1995]. The nitrogen - electron reaction responsible for the blue 427.8 nm emissions only occurs lower in the ionosphere, at about 120 km altitude. Estimates of size and movement of auroral events need to take the altitude into consideration.

An arbitrary assumption we make is that the red image occurs at an altitude of 250 ± 50 km, a Figure used by other workers e.g. Sandholt, et al., 1998, which is twice the assumed altitude of the blue image. A 1000 km diameter field of view used in this study corresponds to the 120 km altitude maximum field of view (FOV) of the lens. The geometry of the fish-eye lens compresses the image to the edges of the field so that the central 250 km diameter of sky occupies half the image. The resolution of the 256 by 256 pixel system is very coarse at the edge of the FOV and only very large scale features can be discerned away from the centre of the images. Small scale arcs, relative to the scale of the polar cap, can be well resolved in the centre 250 km area, and it is the behaviour of narrow arcs of less than 1000 km in length that is studied here.

The recorded image is dependent on the instantaneous response of a photon cascade device (the image intensifier) and consequently there is an inherent uncertainty in the level of the recorded signal. The effect of the image intensifier is shown in Figure 3. This shows the response of the imager system during tests made to back calibrate the first season's data on the laboratory bench. A low light intensity test target using light emitting diodes (LED's) was constructed. This consisted of a grey square mounted in a matt black light proof box. The camera could be operated seeing only this square, filling the central third of the field of view, with all extraneous light excluded. The voltage fed LED light source could be varied to produce images with the same intensities as natural signals found in observed arcs. The images recorded from this test box consist of a square in the centre of each of the red and blue side by side images. Allowing for the uneven illumination in the test box, a horizontal scan of intensities across the centre line of this test image should show an increase and then a decrease at the target square boundaries in each of the red and blue images, with a reasonable plateau between.

Figure 3: Imager system responses to test target. (a) response for one frame and (b) stacked responses from six frames (c) upper and lower limits from (b).

The plot in Figure 3(a) shows the response across line 128 (out of 256 lines) in one 0.9 second image recorded from the test target. The y-axis values are arbitrary pixel intensities which vary between 0 and 255, as determined by the intensifier gain and the compression software. In this instance the maximum is about 65, which is comparable with intensities from many auroral events. The x-axis is the pixel location number across the image. The edges of the square in the red image are seen at pixel 48 and at pixel 90, and in the blue image at about 153 and about 200 in Figure 3(a). Ideally, all test images recorded under identical conditions should be the same. In Figure 3(b), the response for another five images (0.9 s) recorded successively every 2 seconds with the same target are superimposed on the original plot of Figure 3(a). In Figure 3(b), the single trace response is not replicated. A broadly similar response to Figure 3(a) is seen, but due to the variability of the intensifier, the only boundary edge that remains well defined is at pixel 90.

In Figure 3(c) the range of the six traces from Figure 3(b) is shown, with arrows marking points of interest. The edges remain reasonably well defined in the red and blue images, provided that the response is above the noise level. This is indicated by the vertical arrows. The approximate background noise (intensity level 22 ±3 in the field data, and in the tests) is indicated by the horizontal dashed line. The oblique arrow shows where the response in the blue image is sufficiently low to be lost in system noise even when a blue target is present.

It is seen that successive images of an unchanging target are not identical pixel by pixel, but can vary by about 25%. Averaged over time and area the variable response of the intensifier should, for a constant signal, provide a meaningful average response. However, in this application of an image intensifier a significant error must result when capturing single images of aurora that vary in both time and space. The recorded intensity could be anywhere within the 25% band. The recorded position of edges of arcs in the images has an error range of about 5% however. By estimating the centre of an arc in the red images, and allowing for altitude, reliable velocity estimates can be made.

There are many examples in the field data of blue arcs apparently switching on or off in successive exposures which is explained by the system's blue response shown in Figure 3(c). As the result of the testing, it is apparent that absolute calibration of the system is not possible, and intensity ratios of instantaneous red/blue images are unreliable. Intensity ratios have not been used in this study.

3. Observations

The Allsky data recorded over the Antarctic winters has provided many images where the aurora has been well enough defined, and of sufficient intensity, to make reasonable deductions of spatial and temporal characteristics using the altitude assumptions mentioned above. The motion of polar cap arcs may display an aspect of overall ionosphere convection , or they may be quite localised. Examples of polar cap auroras discussed here have been selected for events displaying a definite drift motion. The prevailing IMF conditions (projected from upstream), which indirectly drive ionospheric convection are also presented for these events. These examples show polar arc drift motion can be quite varied.

3.1 1996 June 20

Figure 4: 1996 June 20. (a) raw data image; (b) arc positions; (c) WIND MFI data.

On this day between 23:23:00 and 23:31:30 UT, as shown in Figure 4, a very bright, broad, roughly sun aligned arc formed from the SSE, where it was attached to the auroral oval, extending to the NNW. It moved poleward, (to the west) as a loop, and then collapsed to the south within 6 minutes. At the south east, where this arc was connected to the auroral oval, it did not show much movement. The auroral oval, which should be seen in the lower left of the images is partly obscured by icing inside the dome. The thumbnail, Figure 4(a), shows the raw image of 23:27:00 UT, and the composite negative image, (b), shows the positions of the arc every 30 seconds, moving from east to west for the period in the red image. The blue image (right side) shows positions of the arc between 23:25:00 and 23:28:30 UT. Here is an example of the blue image apparently switching on and off, as mentioned above. Assuming an altitude of 250 km for the red emissions, the westward velocity was 500 ±80 m/s over the centre of the field of view. At this time the direction to the sun is just east of north, and is indicated by the arrowhead. Figure 4(c) shows the solar wind conditions from the WIND spacecraft Magnetic Field Instrument (MFI) data. The vertical line labelled 23:24 eq relates to the conditions expected at the Earth at the time of the event, taking into consideration the prevailing solar wind velocity and travel time to the magnetopause. Kp was 1-, Bz was weakly positive, having been so mostly for the previous hours, By was also weakly positive, Bx at 23:24 eq was negative. The total B field was about 5 nT. There were two other similar arc events, with similar short lives, though not as intense, during the period 22:00 June 20 to 00:35 June 21, around mid day local time, with drift motion towards the invariant pole.

3.2 1996 July 17

Figure 5: 1996 July 17. (a) raw data image; (b) arc positions; (c) WIND MFI data.

The thumbnail, Figure 5(a), shows the raw image seen at 21:52 UT. This event also occurred just before local noon. A sun aligned arc is visible in both red and blue images above dawn and cloud interference. The arc drift motion is eastwards, away from the magnetic pole. The composite image, Figure 5(b) shows positions of this arc, which had a slower drift motion, every 2 minutes between 21:37 and 21:59 UT. At the end of this period the arc collapses lengthwise to a vestige patch in the south, after experiencing eastward drift velocities of between 60 ±8 m/s and 200 ±25 m/s. The higher velocity was seen in the south. Kp= 2 at 21:00 UT. As seen in Figure 5(c), where the vertical line indicates the conditions projected to a time within this event, Bz = 0, and had been for the preceding hour. By, Bx_ -2 nT with Btot = 5 nT. For the drift direction of this arc By was negative while for the 1996 June 20 event described above By was positive.

3.3 1997 July 22

Figure 6: 1997 July 22. (a) raw data image; (b) arc positions; (c) WIND MFI data.

The raw image, Figure 6(a), is shown for 22:10:00 UT, one of the images used to make the composite image in Figure 6(b). This shows the locations of this arc every 30 seconds, in the red image, between 22:08:00 UT, the initial position, and 22:14:00 UT when it faded from view. Again the blue image is visible for less time than the red one. There is interference from the sun at the top left of the images, and the moon at the top right, and again there is some ice inside the dome on the lower left side of the images. This arc, another example from before local noon, apparently rotated about its northern end. The velocity overhead was about 500 ±65 m/s while at the south end it could have exceeded 1000 ±125 m/s. Again in Figure 6(c) the MFI data with the prevailing conditions indicated shows the total B tot field strength was of the order of 5 nT and had been stable for the previous hour or two, By = +2 nT, and Bx_-2 nT this time. B z was positive, about +2 nT, before the event, and although fluctuating, it remained mostly positive throughout the event. While the north end of this arc remained fixed, the rotation of the southern end was towards the pole.

3.4 1998 July 7

Figure 7: 1998 July 7. (a) raw data image; (b) arc positions; (c) WIND MFI data.

This arc, shown in Figure 7 moved north at about 300 ±40 m/s from 06:50 UT to arrive overhead at about 07:02 where it remained until fading into cloud/interference. This corresponds to 19:00 local time, around dusk. The thumbnail Figure 7(a) shows the raw data at 07:05 UT. There is no information in the blue image due to the presence of the moon. (A small spot visible 180° from the moon in the red image is a reflection of the moon from the inside of the spherical dome). In the composite image, Figure 7(b), the positions at 2 minute intervals from 06:50 until 07:16 UT are shown. This arc is again essentially aligned with the sunwards direction, the sun being west-south-west at the time. The solar wind data are shown in Figure 7(c) with the projected time of the arc onset marked. The B tot field is greater than the previous examples at around 10 nT and is more variable. Bx was between 8 nT and 5 nT for the preceding period, and remained positive. By was strongly negative at -10 nT, and had been negative for some time. Bz shows a sudden increase from around 1 nT to 6 nT at the start of this event. However as the viewing conditions were not perfect it is not possible to know if we have detected the actual birth of this event, or if it became visible after it was established.

4. Discussion

The drift velocities of the first three arcs presented here were, 500 m/s poleward, 200 m/s anti-poleward, and a "rotating" arc with up to 1000 m/s poleward motion, all recorded around local noon. The relation of the By direction to the poleward/anti poleward drift motion in these specific examples is in agreement with the findings reported by Valledares et al., [1994], that the east-west component of the IMF can control drift motion. The fourth event, from 0700 UT (local dusk), shows a drift of 300 m/s northwards which is unrelated to the poleward direction. This arc is aligned with the sun direction. As stated before, all the polar cap arc examples presented here, and observed so far in the study are associated with northwards IMF. This is expected from reports in all the previous literature. The rotation of polar cap arcs is not well documented.

The four examples here were selected to show that there are several differing drift motion regimes. Detailed examination of the other identified events in our data is needed to establish trends. The images selected here represent some aspects of polar cap arc phenomena. Our data does contain aurora from the oval, and arcs that can be sufficiently long in extent to span the entire field of view. The straight or nearly straight localised arcs, aligned sunwards, or to the cusp (Valledares, et al., 1994) direction described here are unique to polar cap conditions. Specific examples that occur when reliable satellite data on IMF and solar wind are also available, are therefore potentially useful to examine in detail.

It is not clear whether these arcs occur on magnetic field lines that are "open" to the interplanetary magnetic field, or field lines that map back into the magnetotail. Where the field lines map depends on the model of convection that applies, and more evidence is needed to justify either or both propositions. This paper presents preliminary observation results and the relation of these events with IMF By, Bx, and solar wind conditions will be expanded. Specific events will be related to data from magnetometers, riometers, and other photometers from the polar cap PENGUIn and AGO program and ionospheric convection patterns.

Acknowledgments

The assistance of Antarctica New Zealand personnel at both Scott Base and Christchurch, New Zealand is gratefully acknowledged. This research was supported by a grant from the Australia Research Council, the University of Newcastle, and the Cooperative Research Centre for Satellite Systems with financial support from the Commonwealth of Australia through the CRC program.

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