Abstract. The shape of the electron energy distribution has long been a central question in the field of highfrequency radio-induced optical emission experiments. This report presents estimates of the electron energy distribution function, f e (E), from 0 to 60 eV, based on optical multiwavelength (6300, 5577, 8446, 4278Å) data and 930-MHz incoherent scatter radar measurements of ion temperature, electron temperature and electron concentration. According to our estimate, the electron energy distribution has a depression at around 2 eV, probably caused by electron excitation of vibrational states in N 2 , and a high energy tail that is clearly supra-thermal. The temporal evolution of the emissions indicates that the electron temperature still plays an important role in providing electrons with energies close to 2 eV. At the higher energies the electron energy distribution has a nonthermal tail.

Abstract. This paper presents the first direct empirical evidence that mesoscale variations in ion velocities must be taken into consideration when calculating Joule heating and relating it to changes in ion temperatures and momentum transfer to the neutral gas. The data come from the first tristatic Fabry-Perot Interferometer (FPI) measurements of the neutral atmosphere co-located with tristatic measurements of the ionosphere made by the European Incoherent Scatter (EISCAT) radar which were carried out during the nights of 27-28 February 2003 and 28 February until 1 March 2003. Tristatic measurements mean that there are no assumptions of uniform wind fields and ion drifts, nor zero vertical winds. The independent, tristatic, thermospheric measurements presented here should provide unambiguous vector wind information, and hence reduce the need to supplement observations with information obtained from models of the neutral atmosphere, or with estimates of neutral parameters derived from ionospheric measurements. These new data can also test the assumptions used in models and in ion-neutral interactions. The FPIs are located close to the 3 radars of the EISCAT configuration in northern Scandinavia, which is a region well covered by a network of complementary instruments. These provide a larger scale context within which to interpret our observations of mesoscale variations on the scales of tens of kilometres spatially and minutes temporally. Initial studies indicate that the thermosphere is more dynamic and responsive to ionospheric forcing than expected. Calculations using the tristatic volume measurements show that the magnitude of the neutral wind dynamoCorrespondence to: A. L. Aruliah (a.aruliah@ucl.ac.uk) contribution was on average 29% of Joule heating during the first night of observation. At times it either enhanced or reduced the effective electric field by up to several tens of percent. The tristatic experiment also presents the first validation of absolute temperature measurements from a common volume observed by independently calibrated FPIs. Comparison of EISCAT ion temperatures at an altitude of 240 km with FPI neutral temperatures show that T i was around 200 K below T n for nearly 3 h on the first night during a period of strong geomagnetic activity. This is inconsistent with energy transfer. Comparison with FPI temperatures from surrounding regions indicate that it could not be accounted for by height variations. Indeed, these first results seem to indicate that the 630-nm emission did not stray too far from 240 km. There were also apparent drops in T e at the same time as the anomalous T i values which are energetically implausible. Incorrect assumptions of composition or nonMaxwellian spectra are likely to be the problem.

[1] The physics of the interaction of bursty bulk flows (BBFs) with the near-Earth plasma is not yet known in detail. We address this issue with comprehensive observations made with the MIRACLE network during a well-documented streamer event. In this case an equatorward intruding auroral streamer inclined from northwest (NW) to southeast (SE) reached the region of the proton oval of 486 nm (H b ) luminosity. This H b band is assumed to map poleward of the !30 keV proton isotropic boundary, which is at geocentric distances $7-8 R E according to the appropriate magnetic field models. A significant enhancement (30%) in the H b luminosity was recorded just at the time and in the location where the equatorward end of the streamer reached the H b band. This enhancement implies a corresponding pressure enhancement in the equatorial magnetosphere. The simultaneous poleward shift of the H b emission suggests that also a magnetic field dipolarization took place in the same region. An NW-aligned electrojet (300-400 km in width) accompanied the equatorward development of the streamer (with streamer locating at the dusk flank of this electrojet), and it produced a sharp negative magnetic bay ($300 nT) resembling a substorm onset. Both auroral and equivalent current observations suggest that the plasma jet was not completely dissolved or stopped during this interaction, but rather diverted in the azimuthal direction. Equivalent currents reconstructed from ground magnetic field observations indicate that a R1-type field-aligned current system (upward current $0.5 MA) accompanied the streamer development. However, our observations do not show any signatures of a R2-type current system expected to build up during the flow braking and diversion according to previously published simulation studies.

Abstract. Global ultraviolet auroral images from the IM-AGE satellite were used to investigate the dynamics of the dayside auroral oval responding to a sudden impulse (SI) in the solar wind pressure. At the same time, the TV all-sky camera and the EISCAT radar on Svalbard (in the pre-noon sector) allowed for detailed investigation of the auroral forms and the ionospheric plasma flow. After the SI, new discrete auroral forms appeared in the poleward part of the auroral oval so that the middle of the dayside oval moved poleward from about 70 • to about 73 • of the AACGM latitude. This poleward shift first occurred in the 15 MLT sector, then similar shifts were observed in the MLT sectors located more westerly, and eventually the shift was seen in the 6 MLT sector. Thus, the auroral disturbance "propagated" westward (from 15 MLT to 6 MLT) at an apparent speed of the order of 7 km/s. This motion of the middle of the auroral oval was caused by the redistribution of the luminosity within the oval and was not associated with the corresponding motion of the poleward boundary of the oval. The SI was followed by an increase in the northward plasma convection velocity. Individual auroral forms showed poleward progressions with velocities close to the velocity of the northward plasma convection. The observations indicate firstly a pressure disturbance propagation through the magnetosphere at a velocity of the order of 200 km/s which is essentially slower than the velocity of the fast Alfvén (magnetosonic) wave, and secondly a potential (curl-free) electric field generation behind the front of the propagating disturbance, causing the motion of the auroras. We suggest a physical explanation for the slow propagation of the disturbance through the magnetosphere and a model for the electric field generation. Predictions of the model are supported by the global convection maps produced by the SuperDARN HF radars. Finally, the interchange instability and the eigenmode toroidal Alfvén oscillations areCorrespondence to: A. Kozlovsky (alexander.kozlovsky@oulu.fi) discussed as possible generation mechanisms for the dayside auroral forms launched by the SI.

Abstract.On 7 December 2000, TV ASC camera in Barentsburg (Svalbard) observed pre-noon (at 09:00-10:00 MLT) rayed auroral arcs, which occurred at the poleward edge of the auroral oval after an IMF transition from B y -dominated (B y = +8.8, B z = +4.3) to strongly northward dominated (B y = +2.7, B z = +8.6). The arcs appeared from the area of enhanced luminosity seen in the western (nightside) horizon, and developed to the east, progressing at a velocity of about 1.5 km/s. Simultaneously, the arcs were drifting poleward at a velocity of 300-500 m/s, whose value was equal to the F-region ionospheric plasma drift velocity observed by the Incoherent Scatter Radar (ESR). The arc appearance and motion corresponded well to the poleward expansion of the auroral oval following the IMF shift, which was observed by the UVI on board the Polar satellite. The observed auroras were associated with closed LLBL indicated by the particle precipitation data from DMSP satellites showing also several-keV electrons of PS origin. The observations allow us to suggest that the arcs arise due to the interchange instability that starts to develop at the boundary between the magnetospheric plasma and the magnetosheath flux tubes entering the closed magnetosphere due to the reconnection beyond the cusp after the IMF changes. The interchange instability can be suggested as a possible mechanism for the formation of the LLBL.