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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.
One of the traditional emission lines of choice for monochromatic
auroral imaging is the 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 state is quenched at lower altitudes, the ratios of the
column emission rates for the 6300 Å and 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 6300 Å emission line makes comparisons with the rapid 1Neg.
4278 Å and 5577 Å emission-lines difficult, particularly for
active auroral events.
An estimate of the auroral electron spectra
In 1996 the first ALIS station was equipped with filters for the
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
An eastward travelling fold similar to that described by
Steen et al.  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 6300 Å, 5577 Å, 1Neg. 4278 Å, and
8446 Å. The observed fold appeared almost identical in all
emission-lines, except, as expected in the slow emission.
This initial study concluded that the 8446 Å
auroral morphology appears similar to that of 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
the other method is based on an inversion of the 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).
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.
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
. [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 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.
An estimation of the energy flux by inversion of 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]
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.
Coordinated observations with satellite and radar
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 .
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.
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
s. The orientation of
the Kiruna imager is
. Nikkaluokta had
no CPS at this time (hence the rotation) and was imaging zenith.
The remaining stations are in the magnetic zenith position
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]
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,
, is the measure of rotation in a fluid,
is the fluid velocity). Pudovkin et al.  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.
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. ; Hedin et al. . 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
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''.
Studies of the ionospheric trough
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 ; Cocks et al. ; Barmore ; Conde et al. ; Conde and Jacka ; Bens et al. .
Daytime auroral imaging
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
[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
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
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.
Daytime auroral image in 6300 Å acquired on April 28, 2000 at
19:08:34 UTC. The solar elevation angle was
features of the 6300 Å emission-line are seen in north-east
as well as in south-west. (Courtesy of David Rees)
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 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.
The relation between the thermospheric neutral wind and auroral events
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
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 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.
Next: Other studies
Up: Scientific results from ALIS
Previous: HF pump-enhanced airglow
copyright Urban Brändström