[1] A new method utilizing stochastic inversion in determining the electric field and neutral wind from monostatic beam swing incoherent scatter measurements is described. The method consists of two stages. In the first stage, beam-aligned ion velocities from a chosen F region height interval and a set of subsequent beam directions are taken as measurements. The unknowns are the two electric field components and the field-aligned ion velocity profile. The solution gives the most probable values of the unknowns with error estimates. In the second stage, the measurements consist of beam-aligned ion velocities from the E region, and the electric fields given by the first inversion problem are also used as measurements. The number of applied beam directions may be greater than in the first inversion problem. This is a feasible approach since the neutral wind usually changes more slowly than the electric field. The solution of the second inversion problem gives the most probable values of the three neutral wind components. Results of the method are shown for 11 September 2005, when the European Incoherent Scatter (EISCAT) UHF radar was running in the CP2 experiment mode, which is a four-position 6 min monostatic cycle. In addition, from each beam direction a tristatic measurement at one F region range gate was made using two additional receivers. That allowed comparison between the monostatic and tristatic electric field results, which were in excellent agreement. The calculated neutral wind components were in good accordance with previous measurements during disturbed conditions from the same site.Citation: Nygrén, T., A. T. Aikio, R. Kuula, and M. Voiculescu (2011), Electric fields and neutral winds from monostatic incoherent scatter measurements by means of stochastic inversion,

We describe the electrodynamics of a postmidnight, high‐latitude ionospheric trough, observed with the European Incoherent Scatter radar in northern Scandinavia on 24–25 June 2003 around 22:00–02:30 UT during quiet conditions. The UHF radar made meridian scans with a 30 min cadence resulting in nine cross sections of ionospheric parameters. The F region electric field was also determined with the tristatic system. Ionospheric equivalent currents, calculated from ground magnetometer data, mostly show an electrojet‐like current that is reasonably uniform in the longitudinal direction. Combined analysis of the conductances and equivalent current with a local Kamide‐Richmond‐Matsushita (KRM) method yields the ionospheric electric field and field‐aligned current (FAC) in a 2‐D (latitude‐longitude) area around the radar. We conclude that the most likely scenario is one where the trough is initially created poleward of the auroral oval by downward FAC that evacuates the F region, but as the trough moves to lower latitudes during the early morning hours, it becomes colocated with the westward electrojet. There the electron density further decreases due to increased recombination caused by enhanced ion temperature, which in turn is brought about by a larger convection speed. Later in the morning the convection speed decreases and the trough is filled by increasing photoionization.

[1] Incoherent scatter measurements were carried out on 9 November 1987, showing the presence of an ionospheric trough in the F region. The experiment was made using the EISCAT UHF radar, and it consisted of an azimuthal scan with constant beam elevation and a meridional scan. Since the radar rotates with the Earth, beams with different directions from subsequent scans meet in the same MLT-CGMLat pixel in nonrotating frame. If the ionosphere is not too variable, these can be combined to give an average value of electron density and ion/electron temperature in each pixel. Furthermore, since different beams passing through the same pixel give different ion velocity components, it is also possible to obtain the velocity vector. The geomagnetic conditions during the observations were quiet enough for assuming a quasi-stationary ionosphere. It was found that both ion and electron temperatures have minima within the trough region and increase at the poleward wall. Ion velocity measurements, together with a convection model, suggest that the density depletion within the trough is due to recombination of F region plasma convecting for a long time in the dusk convection cell beyond the terminator. The northern edge of the trough is associated with soft particle precipitation. The southern edge is steeper than the northern edge, and is built by sunlit plasma brought to the trough region by corotation. The trough is thus a result of a combination of transport and precipitation processes rather than stagnation.Citation: Voiculescu, M., T. Nygrén, A. Aikio, and R. Kuula (2010), An olden but golden EISCAT observation of a quiet-time ionospheric trough,

[1] When ionospheric electric field and neutral wind are determined from ion velocities given by monostatic beam swing incoherent scatter measurements, a stationary and horizontally homogeneous ionosphere is assumed. These assumptions are not necessarily valid during a single beam cycle or within the whole measurement region. Disturbances in the receiver may also cause errors. Thus the results may contain errors which are not of statistical nature. If most of the data come from regions where temporal and spatial variations are small, more reliable results are expected, if observations which violate the basic assumptions are rejected. This paper shows a means of finding such measurements. Electric fields and neutral winds with their standard deviations are first determined from measured ion velocities, and these results are used backwards to calculate reconstructed ion velocities and their standard deviations. Then the differences between the measured velocities and the reconstructed ones as well as the standard deviations of these differences are computed. If a given difference lies beyond a selected statistical limit, the corresponding measurement is regarded as unreliable and is rejected. New results are obtained by repeating the analysis with the cleaned data set. The process is repeated until either all remaining measurements are statistically reliable or the determination of the parameters fails. In the latter case, there will be a gap in the results. The paper presents the underlying statistical theory, the statistical evaluation of the results, and demonstrates the use of the method with incoherent scatter data.Citation: Nygrén, T., A. T. Aikio, M. Voiculescu, and R. Kuula (2012), Statistical evaluation of electric field and neutral wind results from beam-swing incoherent scatter measurements,

Abstract. During the late evening and night of 14 September 2004, the nightside auroral oval shows a distinct double oval configuration for several hours after a substorm onset at ∼18:45 UT. This structure is observed both by the IM-AGE satellite optical instruments focusing on the Southern Hemisphere, and by the MIRACLE ground-based instrument network in Scandinavia. At ∼21:17 UT during the recovery phase of the substorm, an auroral streamer is detected by these instruments and the EISCAT radar, while simultaneously the Cluster satellites observe a bursty bulk flow in the conjugate portion of the plasma sheet in the magnetotail. Our combined data analysis reveals significant differences between the ionospheric equivalent current signature of this streamer within a double oval configuration, as compared to previously studied streamer events without such a configuration. We attribute these differences to the presence of an additional poleward polarization electric field between the poleward and the equatorward portions of the double oval, and show with a simple model that such an assumption can conceptually explain the observations. Further, we estimate the total current transferred in meridional direction by this recovery phase streamer to ∼80 kA, significantly less than for previously analysed expansion phase streamer events. Both results indicate that the development of auroral streamers is dependent on the ambient background conditions in the magnetosphere-ionosphere system. The auroral streamer event studied was simultaneously observed in the conjugate Northern and Southern Hemisphere ionosphere.