Auroral imaging

Following the invention of photography the first successful picture of the aurora was taken by Brendel in 1892 [Baschin, 1900]. This paved the way for multi-station auroral imaging. One of the first large-scale auroral observation campaigns involving several stations was carried out by Birkeland [1908]; Birkeland [1913].

The most well-known imaging instrument is maybe the All-Sky Camera (ASC), which consists of a camera together with an optical arrangement of one or several mirrors providing near $180^{\circ}$ field-of-view. This instrument exists in a variety of designs [for example Elvey and Stoffregen, 1957; Stoffregen, 1955; Hyppönen et al., 1974; Stoffregen, 1956, and others] and was pioneered by Gartlein [1947]. During the International Geophysical Year (IGY) of 1957-1958 a ground-based network of all-sky cameras was operating at 114 stations around the polar regions [Stoffregen, 1962]. Since then, improved versions of the all-sky camera have been the main observatory instruments for ground-based imaging of the aurora. Examples of present state-of-the-art digital all-sky cameras are the all-sky optical imager (ASI) in use at the Amudsen-Scott South Pole station [Ejiri et al., 1998; Ejiri et al., 1999] as well as the cameras used in the Finnish MIRACLE network^1.3[Syrjäsuo, 1996; Syrjäsuo, 2001; Syrjäsuo, 1997]. These all-sky cameras represent a considerable improvement over earlier instruments. Since a filter-wheel is present, spectroscopic measurements are possible. A somewhat different, and less advanced approach, is presented in Section C.1, where a commercial digital colour camera is used.

The intense development of television cameras starting in the late 1940's led to the emergence of better low-light imaging detectors based on television image-tubes, for example image orthicons and intensified vidicons. This enabled direct electronic recording of auroral image data. Absolute measurements with this class of detectors is very difficult, mainly due to calibration difficulties related to their non-linear response. Therefore these detectors have mainly been used for white-light imaging.

According to Jones [1974], the first use of image orthicon television camera systems for auroral observations were by Davis and Hicks [1964]. Image-intensified vidicon tubes were introduced by Scourfield and Parsons [1969]. Since then, technology has improved considerably and there exists a plethora of auroral imagers based on television type cameras, often in a combination with an image intensifier. An example of a modern television-type imager is the excellent Portable Auroral Imager (PAI) intended for high-resolution auroral imaging [Trondsen, 1998]. A few more examples of television-type imagers are mentioned in Section 3.2.1.

Space-borne optical imagers simplified the monitoring of large-scale auroral features. The Viking imager [Anger et al., 1987] may serve as an excellent example of this [see Pellinen and Kaila, 1991, for a more complete listing of space-borne imagers]. Polar/VIS^1.4 is a more recent example of the versatile capabilities of space-borne auroral imaging techniques. For global auroral imaging, the capability to use UV-emissions to measure sunlit day-side aurora is a great advantage [Steen, 1989]. However, auroral imaging from space does not make ground-based and rocket-borne studies obsolete, they are both powerful and complementary methods that should not be underestimated. For example, small- and medium-scale phenomena are difficult to study from space due to the spatial smearing caused by the orbital motion, as well as imprecisely known value of the effective albedo [Steen, 1989]. Furthermore the orbital motion prohibits continuous studies in a certain local time sector, as well as along a certain magnetic field-line. The best results tend to emerge when different observing methods are combined.

Auroral height estimations

The number of reliable height estimations of the aurora before those obtained from photographic methods are very few [Størmer, 1955]. The first measurement of the height of an aurora was made between 1726 and 1730 by de Mairan [1733] resulting in an estimated height of about 400-1300 km. Further reading on early height determinations is found in the works of Størmer [1955]; Wilcke [1778], and references therein.

By obtaining auroral photographs simultaneously from two or more locations, it is possible to employ triangulation techniques to estimate the height of the aurora. The first results from this method were obtained by Størmer [1911]. Later on the methods were improved and simplified [for example Vegard and Krogness, 1920], as described in the cornerstone work by Størmer [1955].

For examples of more recent height-determinations of the aurora see Aso et al. [1990]; Frey et al. [1996]; Aso et al. [1993]; Brandy and Hill [1964]; Aso et al. [1994]; Stenbaek-Nielsen and Hallinan [1979]; Romick and Belon [1967]; Steen [1988a]; Kaila [1987]; Jones et al. [1991]; Brown et al. [1976]; Steen [1988b], and references therein. However, embarking onto a detailed discussion of the many recent measurements extend far beyond the scope of this introduction.

Spectroscopic techniques

The auroral signal contains a considerable amount of spectral information. The first measurements of the auroral spectra were carried out by Ångström [1869]; Ångström [1868]. He also named the convenient unit Ångström (1 Å = 0.1 nm). Another important contributor to auroral spectroscopy was Vegard [1913]. Further information on auroral spectra, as well as more references are provided by Jones [1974] and Chamberlain [1995].

Sadly, spectrographs and spectrometers are rare instruments in present day auroral studies. As much more sensitive detectors exist today, a re-examination of the spectral features of the aurora might prove rewarding.

At the present time, the dominating instrument for spectroscopic studies of aurora is the interference filter photometer. This instrument is used either for fixed single-point measurements, or in a scanning or imaging configuration. Examples of contemporary instruments are found in Kaila [2003a].