Abstract. Geomagnetic and auroral activity vary seasonally with maxima at equinoxes, as has been known for more than a century. The cause remains under debate. The angle made by the Earth's dipole axis with the typical direction of the interplanetary magnetic field (IMF) can explain a portion (about 17%) of the effect. To explain the majority of the equinoctial effect, we suggest that geomagnetic activity peaks when the nightside auroral zones of both hemispheres are in darkness, as happens at equinox. Under such conditions, no conducting path exists in the ionosphere to complete the currents required by solar wind-magnetosphere-ionosphere coupling, and geomagnetic disturbances maximize. To test this theory, the Universal Time (UT) variation of geomagnetic activity was explored. As our model predicts, geomagnetic activity in December, measured by the Am index, evinces a deep minimum around 0300-0600 UT when the auroral oval of both hemispheres are in darkness and a maximum around 1500-1600 UT when the southern nightside oval is sunlit. In June, complementary effects are predicted and observed.Previous studies using the AE index have shown more ambiguous results. Here we show that if AE is resolved into the AU and AL components, the discrepancy disappears, with the AL component following the same pattern as does Am. We thus conclude that the intensity of global geomagnetic activity is well ordered by whether the nightside auroral oval is sunlit in one hemisphere or neither.

[1] Polar cap ionospheric measurements are important for the complete understanding of the various processes in the solar wind-magnetosphere-ionosphere system as well as for space weather applications. Currently, the polar cap region is lacking high temporal and spatial resolution ionospheric measurements because of the orbit limitations of space-based measurements and the sparse network providing ground-based measurements. Canada has a unique advantage in remedying this shortcoming because it has the most accessible landmass in the high Arctic regions, and the Canadian High Arctic Ionospheric Network (CHAIN) is designed to take advantage of Canadian geographic vantage points for a better understanding of the Sun-Earth system. CHAIN is a distributed array of ground-based radio instruments in the Canadian high Arctic. The instrument components of CHAIN are 10 high data rate Global Positioning System ionospheric scintillation and total electron content monitors and six Canadian Advanced Digital Ionosondes. Most of these instruments have been sited within the polar cap region except for two GPS reference stations at lower latitudes. This paper briefly overviews the scientific capabilities, instrument components, and deployment status of CHAIN. This paper also reports a GPS signal scintillation episode associated with a magnetospheric impulse event. More details of the CHAIN project and data can be found at http:// chain.physics.unb.ca/chain.

We have used a mode-coupling hypothesis to study the nonlinear evolution of E region irregu-!axities at high latitudes. Conservation of energy and the identification of two distinct time scales for the problem at hand has allowed us to obtain an expression in the fluid regime for the mean frequency and the spectral width of different types of echoes observed by coherent radars. In this particular paper we have applied our results to a few simple cases, naxnely, to situations that are free of large-scale gradients and for which aspect angles axe close to zero. Even though anomalous diffusion effects were also neglected, our theory nevertheless predicts that in the absence of density gradients, strongly driven Farley-Buneman waves should normally saturate at a mean speed between 70% and 100% of the ion-acoustic speed of the medium. The theory also predicts that zero-frequency type 2 waves in strongly turbulent situations should have a frequency width, which when translated to a Doppler width, should be approximately equal to the ion acoustic speed of the medium. Those results are a direct consequence of assuming that mode coupling is responsible for the saturation of all linearly unstable waves in the E-region plasma. A companion paper will consider the modifications introduced by large-scale density gradients. INTRODUG'IION Linear theories have been successful in determining the cause of most E region irregularities seen by coherent scatter radars at low and high latitudes. Theories that have survived the test of time include most prominently theFarley-Buneman mechanism [Farley, 1963; Buneman, 1963] and the gradient-drift instability [Rogister and D'Angelo, 1970]. At high latitudes, it has also been proposed that intense parallel currents carried by thermal electrons could lead to coherent echoes moving at the ion acoustic speed of the medium even though the local E x B drift could be markedly smaller than that same ion acoustic speed [Chaturvedi et al., 1987; Villain et al., 1987, 1990]. While linear theories have been successful at providing basic explanations for the existence of large-amplitude plasma waves (for example, see the review by Fejer and Kelley [1980]), they are unable to predict many other important properties of the observed waves. For example, it is well known that linear theory cannot be used to compute wave axnplitudes (it produces a positive growth rate for all times) or to compute spectral widths (the modes predicted by linear theory are made of separate and independent eigenfrequen-Paper number 92JA02836. 0148-0227/93/92JA-02836505.00 cies). In addition, linear theory cannot, in principle at least, even produce the right value for the mean frequency of the waves: once the waves have grown to large amplitudes and approach saturation, individual wave trains can no longer be simply described in terms of coherent fluctuations. Nonlinear theories must therefore be used in order to un-11.587 11,588 HAMZA AND ST-MAURICE: FULLY TURBULENT FARLEY-BUNEMAN WAVES f0 in the presence of electrostatic proce...

Abstract. In the study of E region irregularities the standard procedure is to Fourier analyze the irregularities in both time and space, that is, to describe them as a superposition of plane waves. This introduces di•culties when the amplitude of the plane waves becomes large, thereby adding nonlinear terms to the original equations and forcing all the plane waves to become coupled to one another. In the present work we stay away from Fourier analysis and use the standard fluid description of the instabilities in the limit of perturbed electric fields that are strictly perpendicular to the geomagnetic field. We obtain a nonlinear generalization of the standard results whereby the diffusion-like operator found in linear theory is now a function of the density itself. Therefore, as the structures grow, the net electric field seen by the ambient plasma inside the structures changes in time: it rotates and its amplitude decreases. Consequently, one possible saturation mechanism for the instabilities is a. reduction in the net electric field inside the structures, which brings them to threshold velocity conditions. This being stated, other nonlinear saturation mechanisms remain possible if they require smaller saturation amplitudes than the present work. Either way, our work is consistent with intermittency and implies that the largest amplitude structures in the medium should be rotated away from zero flow angle conditions by a measurable amount. Finally, we show that when compared to an irregularity-free situation, there should be a measurable reduction in the average Hall current carried by the plasma, while the average Pedersen current should not be affected.

Many types of coherent radar spectra have a width in Doppler velocity units that is less than the ion acoustic speed of the medium. In spectra labeled as type 1 the mean Doppler shift of these narrow spectra matches the ion acoustic speed of the medium. There also exist narrow high-latitude spectra for which the mean Doppler shift is either markedly less or markedly more than the ion acoustic speed. We propose that electron density gradients with scale lengths as small as 100 m are at the origin of a large fraction of these narrow spectra near 50 MHz. The sharp density gradients in that case are created in regions of discrete auroral precipitation associated either with multiple narrow arcs or with sharp edges of broader features. Using the same principle at radar frequencies in the 10-to 20-MHz range, we find that gradient scales from 20 to 30 km in size create a combination of fast and slow phase velocities closely resembling the spectral characteristics expected from NO + ion cyclotron waves. However, gradients are not always responsible for slow narrow spectra; a detailed analysis of available observations has led us to conclude that the high-latitude E region cannot always be considered as fully turbulent even when appreciable coherent echo returns are registered by the radars. In particular, slow narrow spectra at 50 MHz are at times produced under gradient-free weakly turbulent conditions. In addition, at lower radar frequencies (10 to 20 MHz) the narrow spectral width of slowly moving waves and the morphology of these waves both suggest that the irregularities are generated indirectly via mode coupling of linearly unstable modes and that these "secondary" waves are themselves not coupling eflqciently. This implies that processes other than mode coupling are contributing to the overall wave energy budget. In that case we suggest that the convective properties of the slowly growing modes are an important factor in removing wave energy• even for waves as small as a few meters in wavelength. We also propose that there may be two distinct generation mechanisms for secondary waves at 10 MHz, each with its own mean Doppler shift behavior. doupis et al. [1991] were not convinced by that explana-6447 6448 ST.-MAUR/CE ET AL.: NARROW E REGION SPECTRA weakly or strongly turbulent [Hamza and St.-Maurice, 1993a] (hereinafter referred to as paper 1). For nonion-acoustic Doppler shifts, on the other hand, it may not be so obvious that narrow wave spectra are made of primaries particularly if their mean Doppler shift is ST.-MAURICE ET AL.: NARROW E REGION SPECTRA 6449smaller than the ion acoustic speed. As we will show, a more precise probing of the waves properties or of the circumstances that lead to their occurence can help sorting out whether the observed waves are primaries or secondaries.Weakly turbulent ideas for the generation of secondary waves are rather well known, and we only raise a few points about the role of anomalous diffusion in complicating the interpretation of the mean Doppler shifts somewhat for thes...