Abstract. The four Cluster s/c passed over Northern Scandinavia on 6 February 2001 from south-east to north-west at a radial distance of about 4.4 R E in the post-midnight sector. When mapped along geomagnetic field lines, the separation of the spacecraft in the ionosphere was confined to within 110 km in latitude and 50 km in longitude. This constellation allowed us to study the temporal evolution of plasma with a time scale of a few minutes. Ground-based instrumentation used involved two all-sky cameras, magnetometers and the EISCAT radar. The main findings were as follows.Two auroral arcs were located close to the equatorward and poleward edge of a large-scale density cavity, respectively. These arcs showed a different kind of a temporal evolution. (1) As a response to a pseudo-breakup onset, both the up-and downward field-aligned current (FAC) sheets associated with the equatorward arc widened and the total amount of FAC doubled in a time scale of 1-2 min. (2) In the poleward arc, a density cavity formed in the ionosphere in the return (downward) current region. As a result of ionospheric feedback, a strongly enhanced ionospheric southward electric field developed in the region of decreased Pedersen conductance. Furthermore, the acceleration potential of ionospheric electrons, carrying the return current, increased from 200 to 1000 eV in 70 s, and the return current region widened in order to supply a constant amount of return current to the arc current circuit.Evidence of local acceleration of the electron population by dispersive Alfvén waves was obtained in the upward FAC region of the poleward arc. However, the downward accelerated suprathermal electrons must be further energised below Cluster in order to be able to produce the observed visible aurora.Correspondence to: A. T. Aikio (anita.aikio@oulu.fi) Both of the auroral arcs were associated with broad-band ULF/ELF (BBELF) waves, but they were highly localised in space and time. The most intense BBELF waves were confined typically to the return current regions adjacent to the visual arc, but in one case also to a weak upward FAC region. BBELF waves could appear/disappear between s/c crossings of the same arc separated by about 1 min.

Enhanced ion acoustic fluctuations have been recently observed by incoherent scatter radars in the upper ionosphere. Here it is suggested that these fluctuations can be due to the Langmuir wave decay L→L’+S. In this process the “pump” Langmuir waves (L) are produced by a beam instability due to streaming soft electrons. Those waves then produce backscattered Langmuir waves (L’) and ion acoustic waves (S). Estimations of the required electron beam energy and flux, as well as the beam number density are given. It is found that satellite and rocket observations support this interpretation.

This paper presents a numerical study of the effect of Langmuir turbulence on incoherent scatter spectra. The Langmuir turbulence is driven by low energy beams of electrons in the Earth’s upper ionosphere above 300km. The nonlinear coupling between Langmuir waves and ion-acoustic waves is governed by the Zakharov system of equations. The model is enhanced with stochastic forcing in order to estimate by how much over the thermal level the spectrum seen by an incoherent scatter radar will be enhanced. This also allows us to directly compare the modeled spectra to the observed spectra collected by the incoherent scattering technique, as well as to statistically investigate the signature of the modeled spectra through an exploratory data analysis. Results for different beam energies are presented, covering the regimes of weak as well as strong turbulence. The incoherent scatter spectra signature is discussed in light of these regimes. It is shown that incoherent scatter radar observations of enhanced ion-acoustic and/or Langmuir waves compared to thermal level can provide good estimates of the beam parameters and of the type of turbulent regime. The cascade regime leads to strongly asymmetric spectra with enhancements over a limited range of wave numbers. The cavitation regime leads to marginally asymmetric spectra, with enhancement over a wide range of wave numbers, and features a central peak for a limited range of wave numbers. Finally, it is shown that the Langmuir turbulence should be preferentially observed for scattering wavelengths large compared to the Debye length.

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Observations by the ElSCAT and Millstone Hill radars of strongly enhanced, often asymetric, ion acoustic line spectra in the topside auroral ionosphere have been reported recently by a number of researchers. Such strongly enhanced ion acoustic line spectra are shown to arise naturally in a plasma unstable to the ion‐ion two‐stream instability. A linear theory of density fluctuations is used for the calculations, which should be applicable to weakly unstable non‐magnetized plasmas near the onset threshold of the instability.