Scientific objectives of ALIS

The list below tries to summarise the scientific objectives of ALIS, from the first proposal [Steen, 1989] until the present time.

Non-stable auroral forms:

Auroral events such as: substorm onsets, Westward Travelling Surges (WTS), surges, folds, spirals (or auroral vortices), rays and omega bands, and the not well-understood pulsating and flickering aurora belong to this category. These phenomena require observations with high temporal and spatial resolution. So far, ALIS has been used infrequently to make this type of observations, due to a combination of technical limitations (Section 3.3.4), as well as difficulties related to the scientific understanding of these phenomena. However, a theoretical paper on auroral vortices was published (Section 6.5.3).

Stable auroral forms:

The stable auroral arc has been the topic for many studies during the last five decades. Despite this, the ionospheric environment in and around it is not yet fully understood. Detailed field-aligned measurements of ionospheric parameters combined with tri-static plasma-drift measurements by EISCAT, produce profiles on electron density, electron and ion temperatures ranging from some 90  km to 1000 km, as well as, horizontal electric-field estimates in the ionospheric F-region. To relate such measurements to the optical data from ALIS, as well as to satellite and other ground-based instruments would increase the scientific yield of studies of stable auroral forms. Some results of such coordinated observations with ALIS, EISCAT and the FAST satellite are discussed in Section 6.5.2. Another stable auroral form is the diffuse aurora, (added to the scientific objectives in Steen et al. [1990]) where the equatorward edge of the diffuse aurora has been used to estimate the size of the auroral oval. The main ionospheric trough is often associated with the equatorward edge of the auroral oval. Examples of ``accidental'' measurements of the trough are discussed in Section 6.5.4. Other studies (however, not always of stable aurora) that could be included here are : enhanced aurora [Hallinan et al., 1985], black aurora, statistical studies of arc-thickness etc.

Characteristic energy of particles:

By the use of spectroscopic ratios, the characteristic energy of the precipitating particles can be obtained [Strickland et al., 1994; Rees and Luckey, 1974; Meier et al., 1989; Hecht et al., 1989]. To do this properly would require simultaneous measurements at two wavelengths at each station, thus doubling the data-flow and requiring at least two imagers at each station. Results obtained so far involve a single camera at each station and rapid filter changes (Section 6.5.1).

A 3-D image:

Since the fields-of-view of the ALIS stations overlap, it is possible to estimate the altitude distribution of auroral emissions by utilising triangulation and tomographic inversion techniques. Steen [1989] expected that ALIS would be able to produce 2-D maps of the altitude-distribution of the different auroral emissions. This would represent a pseudo 3-D image of the aurora. To develop techniques for visualising the variations in time and space of 3-D aurora was characterised as ``an interesting but non-trivial exercise^6.1''. While the latter still lies in the realm of the future, ALIS-images have frequently been used for obtaining heights and volume distributions through triangulation and tomography-like inversion techniques. The progress in this field is reported in Section 6.3 and references therein.

Relation between electron and proton aurora:

The main auroral particle species are electrons and protons. The intensity of the more diffuse proton aurora is much lower than the electron aurora. In order to study the relation between these two types of aurora, it was suggested to use the $N^+_{2}$ 1Neg. (4278 Å) and $H_\beta$ (4861 Å) emission lines [see for example Galand, 2001, and references therein]. Although ALIS would be well-suited for studies of proton aurora, no measurements have been carried out to date.

The relation between the neutral wind and the aurora:

Auroral intensifications appear to be related to rapid variations in the thermospheric neutral wind on a time scale which excludes contribution from the ion-drag force. This objective was added in Steen et al. [1990] and some initial studies were carried out (Section 6.5.6).

Non-auroral studies:

It was envisioned that ALIS would make mainly auroral observations during dark periods. The system would also be available for other types of measurements. One such type of measurement is to study the formation of Polar Stratospheric Clouds (PSC) which is important for the understanding of ozone depletion. The results of some of these studies are summarised in Section 6.6.1. It was later proposed [Steen et al., 1990] to study gravity wave modulation of airglow emissions. However, no such studies have been carried out to date. It was also speculated that ALIS could be used for other studies, for example high-altitude flashes, clouds and comets, etc. ALIS acquired some images of the Hale-Bopp and the Hyakutake comets; however these data have not yet been analysed. In summary, observations of HF pump-enhanced airglow quite unexpectedly ended up as the main topic for ALIS.

The majority of published ALIS results to date concerns studies of HF pump-enhanced airglow and therefore constitutes the main part of this chapter. When ALIS measurements stopped for the season each year due to the midnight sun period, one detector participated in a joint study to attempt daytime auroral imaging using an imaging spectrometer. This generated some interesting first results, as outlined in Section 6.5.5. Another sidetrack that might become productive in the future is the study of meteor trails. Some rather promising observations have already been carried out (Section 6.6.2).