Bjorn Gustavsson

Unambiguous evidence of HF pump‐enhanced airglow at auroral latitudes

Simultaneous observations by up to three low-light imaging stations belonging to the Auroral Large Imaging System (ALIS) have provided the first strong evidence of high-frequency (HF) pump-enhanced airglow at auroral latitudes. The airglow was enhanced by an ordinary mode 4.04 MHz electromagnetic wave with an effective radiated power (ERP) of about 210 MW that was transmitted from the EISCAT-Heating facility near Tromso, Norway. While often observed at low or mid-latitudes, and despite numerous earlier experiments, no unambiguous observations of pump-enhanced airglow have been reported at auroral latitudes. On February 16, 1999, the first successful results were obtained, and this paper concentrates on discussing optical data from this event. Triangulated estimations of the altitude and position of the enhanced airglow are also presented. Auroral-latitude observations of HF pump-enhanced airglow are important in order to better understand the underlying excitation mechanisms.

First results of auroral tomography from ALIS-Japan multi-station observations in March, 1995

Auroral tomography observations have been carried out in March, 1995, as a joint international campaign between Sweden and Japan. Three unmanned Swedish ALIS stations (Kiruna, Merasjärvi, Tjautjas) and two Japanese JICCD sites (Abisko, Nikkaluokta), geographically separated by about 50 km at higher latitudes, were operated to capture multi-station monochromatic tomography images at 557.7 nm wavelength using CCD cameras. All cameras were pointing to one of the predetermined directions to secure a common field of view. Several images of auroral arcs, mostly for the core region right above Kiruna, have synchronously been taken by the multi-station imaging system. Tomographic inversion analysis for four-point images was carried out using the algebraic reconstruction technique. Reconstructions of a curved arc and of a double arc system suggest promising application of this technique to the retrieval of three-dimensional auroral luminosity.

Estimate of auroral electron spectra, the power of ground-based multi-station optical measurements

Abstract Ground-based multi-station optical imaging has a great inherent scientific potential for investigations of the magnetosphere-ionosphere-thermosphere interactions by using inversions of the ionospheric optical signal, the aurora. Two methods for estimating characteristics of primary auroral electron spectra are compared and used to describe an auroral event. One method uses the spectral information in the images and the other method is based on the inversion of the N 2 + 1 NG 4287 A altitude distribution. With the second method ALIS can currently give estimates of the primary electron distribution with medium time resolution (10 s). The auroral event, a passage of an eastward moving fold in a pre-existing auroral arc, is analysed and characteristics of the precipitating electrons show regions with different fluxes, e.g. a soft region that has previously been found inside the fold appears to belong to a wider region of soft precipitation that emerges as the arc activates.

Alis, a state-of-the-art optical observation network for the exploration of polar atmospheric processes

Abstract An optical group at the Swedish Institute of Space Physics in Kiruna, Sweden has been developing the ALIS (Auroral Large Imaging System) multi-station optical observing network which makes it possible to obtain composite monochromatic 2-D images over a fairly wide field-of-view (FOV), and more interestingly, a CT (Computed Tomography) image set for the retrieval of 3-D structure of aurora by adjusting vergence angles of cameras to a common volume. National Institute of Polar Research, Japan is collaborating in observation and analysis. At the moment, the network has 6 stations separated from each other by about 50 km. Each station houses a monochromatic CCD (Charge Coupled Device) imaging system mounted on the steerable azimuth/elevation drive along with a house keeping unit and supervising computer linked to the control center via a telephone line. Altitude profiles of luminosity for stable arc and aurora vortex at 557.7nm and recently at 427.8nm are analysed by the algebraic reconstruction technique and compared with sophisticated numerical modelling of auroral emission rate. Conjunctions with satellites and radars are now intensively explored towards comprehensive understanding of the formation and electrodynamics of aurora. Imaging of polar stratospheric clouds is also attempted in relation to arctic environmental studies.

Investigations of the polar atmosphere with use of dynamical characteristics of the processes and self -organizing neural networks on ground-based multi-position optical observations

Abstract This article discusses questions connected with investigations of the polar atmosphere via ground-based multi-position optical observations. For this investigation a specially developed method is applied. It use the calculated dynamical parameters of the processes and self-organizing artificial neural networks. With self-organizing neural networks 5–7 classes have been extracted which can be associated with the regions having different physical properties. In particular, have been extracted regions intuitively coincident with polar stratospheric clouds and aurora.

Optical studies of polar stratospheric clouds

Polar Stratospheric Clouds (PSC) appear in the polar zones of the Earth in the winter. These clouds are known to cause enhanced chemical ozone destruction. Methods for optical remote-sensing of PSC in use or under development at the Swedish Institute of Space Physics are discussed with respect to their advantages and limitations. Especially multistatic imaging may become a valuable additional tool for PSC studies.

Classifying auroras using artificial neural networks

In Auroral Large Imaging System (ALIS) there is need of stable methods for analysis and classification of auroral images and images with for example mother of pearl clouds. This part of ALIS is called Selective Imaging Techniques (SIT) and is intended to sort out images of scientific interest. Its also used to find out what and where in the images there is for example different auroral phenomenas. We will discuss some about the SIT units main functionality but this work is mainly concentrated on how to find auroral arcs and how they are placed in images. Special case have been taken to make the algorithm robust since its going to be implemented in a SIT unit which will work automatic and often unsupervised and some extends control the data taking of ALIS. The method for finding auroral arcs is based on a local operator that detects intensity differens. This gives arc orientation values as a preprocessing which is fed to a neural network classifier. We will show some preliminary results and possibilities to use and improve this algorithm for use in the future SIT unit.

ALIS: a multistation imaging facility with possibilities for future VI applications

ALIS (Auroral Large Imaging System) is an imaging facility in Northern Sweden. The system consists of six unmanned, remote controlled stations. Each station has a high performance CCD imager, and some stations also have other scientific instrumentation (e.g. pulsation magnetometers). ALIS is capable of producing large amounts of data in a short time. For that reason, novel (AI/VI) techniques for data analysis, are of high priority in order to be able to handle the large data sets. In this paper we will try to describe the current implementation and address the questions of how to interface AI/VI applications to an existing multi station research facility, in terms of real- time experiment control, selective imaging, real-time data analysis, etc.

Automatic recognition of auroral forms

A method for recognition of geometrical shapes in auroral forms is presented. The method is based on the analysis of isolines of auroral luminosity shapes. The basic variables used are the angle, (phi) (s), between the tangent of the contour and the x-axis of an arbitrary coordinate system, and the differential, d(phi) (s), as a function of the distance, s, along the contour. The analysis also includes Fourier transformation of the experimental function d(phi) (s) obtained for the observed auroral forms, and the comparison of the power spectrum, F(k), with those for a series of model contours. Some dynamical characteristics of the aurora are also discussed.