Ake Steen

First tomographic estimate of volume distribution of HF‐pump enhanced airglow emission

This report presents the first estimates of the three-dimensional volume emission rate of enhanced O(1D) 6300 A airglow caused by HF radio wave pumping in the ionosphere. Images of the excitation show how the initially speckled spatial structure of excitation changes to a simpler shape with a smaller region that contains most of the excitation. A region of enhanced airglow was imaged by three stations in the Auroral Large Imaging System (ALIS) in northern Scandinavia. These images allowed for a tomography-like inversion of the volume emission of the airglow. The altitude of maximum emission was found to be around 235 ± 5 km with typical horizontal and vertical scale sizes of 20 km. The shape of the O(1D) excitation rate varied from flatish to elongated along the magnetic field. The altitude of maximum emission is found to be approximately 10 km below the altitude of the enhanced ion line and 15 km above the altitude of maximum electron temperature. Comparisons of the measured altitude and temporal variations of the 6300 A emission with modelled emission caused by O(1D) excitation from the high energy tail of a Maxwellian electron distribution show significant deviations. The 6300 A emission from excitation of the high energy tail is about a factor of 4 too large compared with what is observed. This shows that the source of O(1D) excitation is electrons from a “sub-thermal” distribution function, i.e. the electron distribution is Maxwellian at low energies and at energies above 1.96 eV there is a depletion.

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.

Vortex structures in the ionosphere and the magnetosphere of the Earth

Abstract Model nonlinear equations, which have stationary solutions in the form of monopole and dipole vortices and vortex chains, are presented. We show that there is a qualitative agreement with satellite observations in the magnetosphere, as well as with the all-sky camera ionospheric data.

Geomagnetic substorms as perturbed self-organized critical dynamics of the magnetosphere

The e0ect of self-organized criticality (SOC), known from the theory of complex nonlinear systems, is considered as an internal mechanism of geomagnetic 4uctuations accompanying the development of magnetospheric substorms. It is suggested that spatially localized current sheet instabilities, followed by magnetic reconnection in the magnetotail, can be considered as SOC avalanches, the superposition of which leads naturally to the 1=fpower spectra (f — frequency, � — numerical parameter) of geomagnetic activity. A running 2D avalanche model with controlled dissipation rate is proposed for numerical investigation of the multi-scale plasma sheet behavior in stationary and nonstationary states of the magnetosphere. Two basic types of perturbations have been studied, the 9rst induced by an increase in the solar wind energy input rate and the second induced by a decrease in critical current density in the magnetotail. The intensity of large-scale perturbations in the model depends on accumulated energy level and internal dissipation in a manner similar to the dependence characteristic of real magnetospheric substorms. A spectral structure of model dynamics exposed to variations of solar wind parameters reveals distinctive features similar to natural geomagnetic 4uctuations, including a spectral break at 5h separating frequency bands with di0erent spectral slopes. c � 2001 Elsevier Science Ltd. All rights reserved.

Seasonal and solar cycle variations in high- latitude thermospheric winds

Thermospheric wind measurements have been collected systematically every winter for over nine years from a high-latitude site at Kiruna, Sweden (67.8{degree}N, 20.4{degree}E). The database contains 1,242 nights of data collected with a Fabry-Perot Interferometer (FPI), perhaps the largest single-site database of thermospheric winds. This analysis shows a marked seasonal and solar cycle variation. Particularly at high solar activity, sunward winds of the evening period (16-20 UT) are more than 50% stronger at Spring than at Autumn equinox. This large asymmetry in the behavior of high-latitude thermospheric winds at spring and autumn equinox has not yet been predicted by model simulations.

Optical observations of water in Leonid meteor trails

[1] Two simultaneous filtered images (589 and 423 nm) of a meteor trail were recorded during the 2002 Leonid storm. The first image shows Na atoms and the second Ca and Fe atoms and signals at altitudes much higher than can give rise to ablation of metals, in agreement with other observations of high altitude visible trails [Spurný et al., 2000a; Spurný et al., 2000b]. Ablation models [McNeil et al., 1998] and analysis of the history of the 2002 Leonid meteoroids [McNaught and Asher, 1999] support the conclusion that the high altitude emissions are due to H 2 O + and H α,β,γ formed through the decomposition in the hyperthermal collision between H 2 O from meteoroid ice [Kresak, 1973] and atmospheric N 2 [Dressier et al., 1992].

High time-resolution imaging of auroral arc deformation at substorm onset

Abstract Observations of auroral disturbances such as westward-travelling surges, omega bands and auroral folds, are by necessity usually made when the luminous features have acquired their characteristic geometry, i.e. after the actual formation process. We demonstrate an event in which the initial brightening of an arc developed into a series of auroral spirals and finally into a westward-travelling surge. Two intervals of spiral formation were observed, but only the second of these generated a surge-like feature. Since the surge formation occurred on a time-scale of a few seconds, this type of observation can only be made with high time-resolution imaging devices. The auroral developments took place in the zenith above Kiruna, where they were recorded by ground-based monochromatic imagers. The subsequent poleward expansion of the activity was observed by the EISCAT incoherent scatter radar 200 km to the North of the initial activation. These observations are discussed in the light of existing substorm theories, and it is proposed that the instability responsible for the final break-up of the arc is self-destructive.

Observations of the variations of thermospheric winds in Northern Scandinavia between 1980 and 1986: A study of geomagnetic activity effects during the last solar cycle

Abstract Since late 1980, a ground-based Fabry-Perot interferometer has been in operation near Kiruna, Sweden (20.4° E, 67.8° N). This instrument has recorded upper thermospheric neutral winds under a wide range of geophysical conditions which have occurred during six observing winters. These data cover the period from the time of maximum geomagnetic activity during the last solar cycle, 1980/82, to the 1985/86 period of generally quiet geomagnetic activity near sunspot minimum. A statistical analysis of the data from about 400 nights of observation provides a graphic description of the generally rapid time-dependence of upper thermospheric winds in the vicinity of the mean auroral oval to individual geomagnetic disturbances. The data also provide an excellent data base describing the auroral oval. The data describe ion convection patterns as they respond to variable geomagnetic activity, and also the mean distribution of OI 630 nm emission as a function of local time, latitude and geomagnetic activity. These results can be used to examine the geomagnetic input parameters to a global thermospheric model (for example the semi-empirical global models of magnetospheric convection) as required to bring the simulations of thermospheric circulation into overall improved agreement with the observations.

The first daytime ground‐based optical image of the aurora

Aurorae, spectacular phenomena in the polar night sky, also provide a convenient projection of effects of complex and energetic plasma processes of the outer magnetosphere. Much has been learned about the ionosphere and magnetosphere from night-time auroral images. However, similar imaging is extraordinarily difficult by day, due to the overwhelming background of atmospherically-scattered sunlight. This is unfortunate, since many auroral plasma processes may be unique to the sunlit ionosphere. A visible-light image of the aurora at λ630-nm wavelength was obtained from Kiruna, Sweden, at sunset on May 2, 1999, by an imaging spectrometer featuring excellent spectral resolution and out-of-band rejection. We believe this to be the first such image obtained from the ground under near-daytime conditions. These observations were obtained as a test of principle during the development of a prototype instrument. We believe this technique holds great promise for future ground-based studies of the daylit ionosphere and magnetosphere.

On the development of folds in auroral arcs

Abstract The development of an auroral arc in the midnight sector, from diffuse to discrete with subsequent large scale folding, is studied with the aid of several ground-based observations, including incoherent scatter radar, and data from a HILAT satellite pass. Ion drift velocities in the F -region, as measured by EISCAT, were consistently eastward throughout and after the whole period of development, whilst the ion temperature showed two large enhancements just prior to the appearance of the main auroral fold. The fold moved eastwards and crossed the EISCAT antenna beam, appearing as a short-lived spike in electron density at altitudes between about 100 km and 400 km. The spike in electron density came progressively later at higher altitudes. The observations are interpreted as the result of enhanced convection in the ionosphere and in the magnetosphere. The auroral arc folding is suggested to be caused by the Kelvin-Helmholtz instability in a velocity shear zone in the magnetosphere.