Auroral studies

Although ALIS was built for auroral observations, and consequently most of its observing time was spent on auroral observations, the amount of published scientific results in this field have been sparse. One explanation is that once the HF pump-enhanced airglow experiments proved successful, it was deemed far more productive for a small group to spend time publishing these results. Other contributing factors are the technical problems affecting the maximum framerate (Section 3.3.4). Furthermore, as ALIS is a new type of instrument, much remains to be learned about how to exploit it in the optimal way. The amount of data is also somewhat limited as auroral studies often are dependent on a variety of coordinated observations with other instruments.

When planning coordinated studies with satellites as well as ground-based radars and optical instruments, many criteria need to be fulfilled; near new-moon, clear skies, the satellite magnetic footprint needs to be reasonably close to the ground-based instruments, radar observing-time needs to be applied for and scheduled [Hedin et al., 1999]. Furthermore, the probability of observing the desired auroral phenomena must be maximised for the ground-based sites taking part in the study. For campaigns involving satellite passes, slow magnetic midnight passes are optimal. Yet, fulfilling all of these criteria for a successful observation might take a very long time. Therefore patience, long-term stability and flexibility in changing the scientific objectives are key-issues. An auroral event cannot be made to occur at the desired time or place to fit a particular measurement. However, well-planned observations will increase the probability of making useful observations. In this section some of the results related to auroral studies are summarised.

An estimate of the auroral electron spectra

One of the traditional emission lines of choice for monochromatic auroral imaging is the $O(^1D)$ 6300 Å emission line [Meier et al., 1989; Solomon et al., 1988]. However, the excitation mechanism of that line is still under some debate [Meier et al., 1989]. As the excited $O(^1D)$ state is quenched at lower altitudes, the ratios of the column emission rates for the $O(^1D)$ 6300 Å and $N^+_{2}$ 1Neg. 4278 Å emission-lines have been taken as an indicator of the characteristic energies of the precipitating electrons. The long radiative lifetime of the $O(^1D)$ 6300 Å emission line makes comparisons with the rapid $N^+_{2}$ 1Neg. 4278 Å and $O(^1S)$ 5577 Å emission-lines difficult, particularly for active auroral events.

In 1996 the first ALIS station was equipped with filters for the $O(3p^3P)$ 8446 Å emission line. Initial observations confirmed a sufficient signal strength for the ALIS-Imagers, and subsequently all stations were equipped with filters for this emission-line.

A preliminary study [Steen et al., 1999a] presents data from 25 March 1998. Two ALIS stations (Kiruna and Merasjärvi) provided data from an auroral event occurring at the end of a geomagnetically disturbed period. An eastward travelling fold similar to that described by Steen et al. [1988] first appeared in the west and traversed the Kiruna zenith at about 1000 m/s as estimated from all-sky images. After that, the arc broke up and was replaced by a diffuse auroral band. The fold was observed in $O(^1D)$ 6300 Å, $O(^1S)$ 5577 Å, $N^+_{2}$ 1Neg. 4278 Å, and $O(3p^3P)$ 8446 Å. The observed fold appeared almost identical in all emission-lines, except, as expected in the slow $O(^1D)$ emission. This initial study concluded that the $O(3p^3P)$ 8446 Å auroral morphology appears similar to that of $N^+_{2}$ 1Neg. 4278 Å.

In a more extensive study [Gustavsson et al., 2001b], two methods for estimating characteristics of primary electron spectra are compared and used to describe the auroral event of 25 March 1998. One method uses the spectral information in the images (Figure 6.12), while

Figure 6.12: The right column displays meridional cuts through images from the Kiruna ALIS station. These ``keograms'' are projected to an altitude of 120 km and the axes are in km relative to Kiruna. As can be seen the arcs drift slowly northward while brightening. After $900 \pm 10$ s a fold sweeps eastward and the arc rapidly moves south and fades. The left columns shows characteristic energy (top) and oxygen scaling factor (bottom) as calculated from the spectroscopic ratios of $I_{8446}/I_{4278}$ and $I_{6300}/I_{4278}$. [After Figure 6 in Gustavsson et al., 2001b]

the other method is based on an inversion of the $N^+_{2}$ 1Neg. 4278 Å altitude distribution. Using the second method, ALIS can currently give estimates of the primary electron distribution with a time resolution of about 10 s (Figure 6.13).

Figure 6.13: An estimation of the energy flux by inversion of $N^+_{2}$ 1Neg. 4278 Å altitude distribution. About 400 s after 23:20:00 UTC there appears to be a slight increase in both total flux and typical energy. [After Figure 6 in Gustavsson et al., 2001b]

The paper concludes that only two dimensional measurements are able to follow the dynamics of this type of event, as single-point measurements cannot distinguish between temporal and spatial variations. Estimating the characteristic energy of precipitating electrons by combining measurements in 4278 Å, 6300 Å and 8446 Å provides useful results for temporal variations slower than 100 s. The primary electron spectra retrieved from the altitude distribution of the $N^+_{2}$ 1Neg. 4278 Å emission can give estimates of the electron characteristics with 10 s time resolution, but only for discrete auroral structures. To fully exploit the information obtainable from these kind of observations requires a combination of spectral and spatial analysis and to compare the results with two-dimensional electron transport and ionospheric models.

Coordinated observations with satellite and radar

During 7, 8 and 16 February 1997, ALIS, a five-channel, meridian scanning photometer [Kaila, 2003c], and EISCAT performed simultaneous observations during several auroral events when the FAST satellite [Carlson, 1992] passed in orbits with close magnetic projections to the ground-based instruments.

A preliminary collection of data from these observations was presented by Brändström et al. [1997a]. Later these events were also discussed by Andersson [2000, Chapter 4] A recent, more comprehensive study is in the preparation-phase, as briefly outlined below from an abstract by Sergienko [2003].

On 16 February 1997 the FAST satellite passed through the auroral oval over Northern Scandinavia one hour before a local substorm onset. The magnetic projection of the satellite orbit was within the field-of-view of ALIS. Given the high temporal resolution of FAST particle detectors combined with the sensitivity and spatial resolution of ALIS, studies of fine auroral structures became an interesting possibility. The FAST electron spectrogram displays three well-defined regions with clear borders between them. The most polarward region is a multiple inverted-V structure with an electron characteristic energy of 3-5 keV, while the middle region is characterised by homogeneous Maxwellian electron spectra with a characteristic energy of about 3.5 keV. Chaotic, weak fluctuations in the precipitating electron energy flux are also observed. The third, most equatorward region, involves two different populations of electrons. The first of them is an extension of the particles from the second region while the other population consists of energetic electrons (about 10 keV) with a perceptible periodic structure in the precipitating part of the particle flux.

The auroral data from ALIS correlate nicely with the electron spectrum peculiarities, as observed by FAST. The most poleward region of the electron spectra corresponds to the bright auroral arcs moving towards the equator, while the second part of the electron spectra conjugates with a region where the short-lived auroral rays and small patches appear chaotic against the weak diffuse background. In the equatorward parts of the auroral images, a very regular spatial luminosity structure, consisting of thin and weak auroral stripes is seen. A sample image of the auroral situation during the FAST pass at 20:10:00 UTC appears in Figure 6.14.

Figure 6.14: Example auroral images from 16 February 1997 during the FAST pass at 20:10:00 UTC; Abisko (top left), Nikkaluokta (middle-left), Kiruna (middle), Silkkimuotka (middle-right and Tjautjas (bottom). All images are in 5577 Å and with $t_{\mathit{int}}=2$ s. The orientation of the Kiruna imager is $a_{\phi}=0^{\circ}$ and $z_{\theta}=40^{\circ}$. Nikkaluokta had no CPS at this time (hence the rotation) and was imaging zenith. The remaining stations are in the magnetic zenith position ( $a_{\phi}=180^{\circ}$ $z_{\theta}=12^{\circ}$).

Calculations of the emission intensities with an auroral 5577 Å emission model using the measured electron spectra have given a good quantitative agreement between the structured high energy electrons and the auroral stripes. The stripes stretch along the geomagnetic latitudes, have a width less than 2 km, and are separated by the background luminosity by approximately the same distance. The intensities of the stripes vary in longitude and with time, but their positions do not change in latitude, at least not during the 15 minute observation by ALIS. Possible formation mechanisms for the regular striped auroral structures will be discussed in another publication in preparation [Sergienko, 2003]

Auroral vorticity

The east-west elongated auroral arc is frequently destroyed by an instability process involving the development of a series of vortex structures of various scales. Vorticity, $\mathbf{\zeta}$, is the measure of rotation in a fluid, $\mathbf{\zeta}=\mathbf{\nabla} \times \mathbf{u}$ (where $\mathbf{u}$ is the fluid velocity). Pudovkin et al. [1997] published a theoretical and experimental review of data connected with vorticity in the magnetospheric plasma and on its signatures in the auroral dynamics. Early ALIS images were used in this paper to exemplify what can be learned from studying vortex structures in the aurora. Such studies can yield an abundant information on the physical state of the magnetospheric plasma as well as the processes developing in it. For example, by observing auroral forms with high temporal and spatial resolution, it is possible to determine the distribution of the electric field and field-aligned currents in the magnetosphere. The shape of auroral vortices, carry information on the nature of the Magnetohydrodynamical (MHD) instabilities responsible for the excitation of the observed turbulence. Subsequent experimental work by for example Trondsen [1998, and references therein] is an excellent example of research in this field. Due to technical problems affecting the maximum achievable temporal resolution (Section 3.3.4), ALIS has not been perfectly suited to continue studies in this field.

Studies of the ionospheric trough

An example showing that ALIS sometimes can provide usable supporting measurements, even when not operated in a scientific mode is presented by Hedin et al. [2000]; Hedin et al. [1999]. In this study of the main ionospheric trough, all the EISCAT radars were for the first time operated in a four-beam configuration close to the meridian plane in order to obtain a wide area of observation without the loss of temporal resolution [Hedin et al., 2000]. This combined ``meta-radar'' has a huge fan-like observation area, of about $70^{\circ}$N- $80^{\circ}$N in geographic latitude [Hedin et al., 2000]. Supporting measurements were provided by the FAST satellite and ALIS. During the night of 14 March 1997, ALIS was only operated occasionally since there were no significant aurora, and the conditions at some stations were partly cloudy. The images were obtained randomly in order to check cloud conditions etc. However, some images happened to be acquired at relevant times for the trough study, displaying faint, diffuse auroral structures around the time when the trough poleward boundary passed over the field-of-view of ALIS. The position of the diffuse aurora obtained from ALIS was consistent with extrapolated data from the more direct measurements by EISCAT [Hedin et al., 2000]. This observation of the trough was found to be typical as it had all the common features as compared to earlier studies. Calculations of the apparent southward motion of the trough were consistent with it having an oval shape. The trough was seen to be wider towards magnetic midnight. The paper concludes by stating that ``the earlier proposed linear equations for trough motions are shown not to be valid over the latitude range in question''.

Daytime auroral imaging

The problem of isolating atmospheric airglow and auroral emissions from scattered sunlight has been solved in a number of ground-based experiments, such as: Noxon and Goody [1962]; Cocks et al. [1980]; Barmore [1977]; Conde et al. [1992]; Conde and Jacka [1989]; Bens et al. [1965].

Normally measurements with ALIS stop for the season around 15 April, and operations resume in late August or early September. This is because the bright summer nights at arctic latitudes prevent ALIS from performing low-light measurements. However, some experiments have been made to try to image the aurora from ground even during this off-season. In this case the normal ALIS imagers (Chapter 3) are not usable, instead the camera head was dismantled and used together with a prototype imaging spectrometer for daylight auroral imaging. The main part of this new imaging instrument was an imaging optical spectrometer based on two 50 mm diameter capacitance-stabilised Fabry-Perot etalons [Rees et al., 1981; McWhirter, 1993; Rees et al., 1996; Rees et al., 1999] placed directly in series with each other and with a narrow-band (2 Å) interference filter. In front of this a wide-angle (35 mm) lens with telecentric optics was mounted. The output image consisting of interference fringes was imaged by a 300 mm lens onto a CCD camera-head from an ALIS-Imager (Chapter 3) operated with $2 \times 2$ on-chip binning. The spectrometer had a wavelength bandwidth of 3.7 pm [Rees et al., 2000]. Since two capacitance-stabilised etalons were used, it was possible to tune the spectrometer by varying the optical path-differences of both etalons, placing the narrow annulus corresponding to 6300 Å emission line at any radius within the image. By scanning the position of the 6300 Å annulus and taking ten successive images at each scan position, it was possible to construct a two-dimensional sky-image at 6300 Å (Figure 6.15). This image was the main

Figure 6.15: Daytime auroral image in 6300 Å acquired on April 28, 2000 at 19:08:34 UTC. The solar elevation angle was $0.6^{\circ}$. Bright features of the $O(^1D)$ 6300 Å emission-line are seen in north-east as well as in south-west. (Courtesy of David Rees)

accomplishment over earlier studies. In this initial work a suitably-normalised previously published [Delbouille et al., 1973] solar spectrum was subtracted since no suitable arrangement for measuring the solar spectrum across the entire image was present for the prototype instrument. Due to this, the intensities of the 6300 Å emission line were only roughly estimated, so neither useful Doppler-shifts (corresponding to wind) nor line-widths (corresponding to temperature) could be extracted.

With modest modifications, this instrument could be used to make optical auroral observations under all day-time solar illumination conditions, albeit with low temporal resolution.

The relation between the thermospheric neutral wind and auroral events

The thermosphere and ionosphere are coupled to the magnetosphere via electric fields and field-aligned currents [Aruliah et al., 1996]. Intensification of the aurora is found to be related to rapid variations in the thermospheric neutral wind on a time scale that excludes contribution from the ion-drag force [Steen and Collis, 1988]. Instead the neutral wind variations must be understood in the context of large-scale coupling between the ionosphere and magnetosphere. ALIS can provide high resolution auroral images, that combined with neutral wind vector fields obtained by Fabry-Perot interferometer measurements can yield a better understanding of the relation between the thermospheric neutral-wind and the aurora.

Measurements of the F-region neutral wind using Fabry-Perot interferometers have been carried out in Kiruna since the early 1980's in collaboration with groups in the U.K. [Aruliah et al., 1996]. In 1997, two scanning mirror Fabry-Perot interferometers were operated in collaboration with the Atmospheric Physics Laboratory at University College London [Aruliah et al., 1996; McWhirter, 1993], as well as one Doppler Imaging System (DIS) [Rees et al., 1997] in collaboration with Utah State University and Hovemere Ltd. In a preliminary study [Brändström et al., 1997b] data from the latter instrument was compared to auroral images obtained from ALIS. The Doppler Imaging System (DIS) used in Kiruna consists of an all-sky lens, an interference filter for the 6300Å airglow/auroral emission and a Fabry-Perot etalon. The resulting interference fringes are imaged by an intensified Peltier-cooled CCD camera. In the subsequent analysis, each of the six interference fringes are divided into 24 sectors for which the Doppler shift of the input signal is calculated. Each sector thus corresponds to the Doppler-shift in a certain region of the sky, making it possible to `image' the neutral wind velocity.

During the winter of 1996/1997 events with conjugate all-sky, ALIS and DIS observations were collected. Events when there was clear sky, auroral activity and preferably discrete auroral forms with a strong signal in the $O(^1D)$ 6300 Å emission line were selected. A collection of data from 10 January 1997 was described by Brändström et al. [1997b]. (Other promising events also exist for example 7-9 February 1997). When a stable arc was seen, the neutral wind was mainly northward. Preliminary analysis of the dataset suggests that the wind is directed perpendicular to the arc, with very low wind speeds parallel to the arc. This observation is supported by observations from 10 January. Another interesting feature in the data is the apparent westward drift of diffuse auroral structures at the same time as the wind turns eastward around 19:30 UTC. However these results are to be considered only as a demonstration of measurement possibilities. These studies have not been followed up so far.