The Kilpisjärvi Atmospheric Imaging Receiver Array (KAIRA) is a dual array of omnidirectional VHF radio antennas located near Kilpisjärvi, Finland. It is operated by the Sodankylä Geophysical Observatory. It makes extensive use of the proven LOFAR antenna and digital signal-processing hardware, and can act as a stand-alone passive receiver, as a receiver for the European Incoherent Scatter (EISCAT) very high frequency (VHF) incoherent scatter radar in Tromsø, or for use in conjunction with other Fenno-Scandinavian VHF experiments. In addition to being a powerful observing instrument in its own right, KAIRA will act as a pathfinder for technologies to be used in the planned EISCAT_3-D phased-array incoherent scatter radar system and participate in very long baseline interferometry experiments. This paper gives an overview of KAIRA, its principal hardware and software components, and its main science objectives. We demonstrate the applicability of the radio astronomy technology to our geoscience application. Furthermore, we present a selec-Manuscript

Abstract. We present here the results of a statistical study of the ionospheric trough observed in 2003 by means of satellite tomography. We focus on the seasonal morphology of the trough occurrence and investigate the trough latitude, width and the horizontal gradients at the edges, at different magnetic local times, as well as their relations to geomagnetic activity and the interplanetary magnetic field. A seasonal effect is noticed in the diurnal variation of the trough latitude, indicating that summer clearly differs from the other seasons. In winter the troughs seem to follow the solar terminator. The width of the trough has a diurnal variation and it depends on the season, as well. The broadest troughs are observed in winter and the narrowest ones in summer. A discontinuity in the diurnal variation of the trough latitude is observed before noon. It is suggested that this is an indication of a difference between the generation mechanisms of morningside and eveningside troughs. The density gradients at the edges have a complex dependence on the latitude of the trough and on geomagnetic activity. The photoionization and the auroral precipitation are competing in the formation of the trough walls at different magnetic local times. An important finding is that the interplanetary magnetic field plays a role in the occurrence of the trough at different levels of geomagnetic activity. This is probably associated with the topology of the polar cap convection pattern, which depends on the directions of the IMF components B y and B z .

Abstract. The present standard EISCAT incoherent scatter experiments are based on alternating codes that are decoded in power domain by simple summation and subtraction operations. The signal is first digitised and then different lagged products are calculated and decoded in real time. Only the decoded lagged products are saved for further analysis so that both the original data samples and the undecoded lagged products are lost. A fit of plasma parameters can be later performed using the recorded lagged products. In this paper we describe a different analysis method, which makes use of statistical inversion in removing range ambiguities from the lag profiles. An analysis program carrying out both the lag profile inversion and the fit of the plasma parameters has been constructed. Because recording the received signal itself instead of the lagged products allows very flexible data analysis, the program is constructed to use raw data, i.e. IQsampled signal recorded from an IF stage of the radar. The program is now capable of analysing standard alternatingcoded EISCAT experiments as well as experiments with any other kind of radar modulation if raw data is available. The program calculates the ambiguous lag profiles and is capable of inverting them as such but, for analysis in real time, time integration is needed before inversion. We demonstrate the method using alternating code experiments in the EISCAT UHF radar and specific hardware connected to the second IF stage of the receiver. This method produces a data stream of complex samples, which are stored for later processing. The raw data is analysed with lag profile inversion and the results are compared to those given by the standard method.

Differential energy flux of electrons precipitating into the high‐latitude ionosphere can be estimated from incoherent scatter radar observations of the ionospheric electron density profile. We present a method called ELSPEC for electron spectrum estimation from incoherent scatter radar measurements, which is based on integration of the electron continuity equation and spectrum model selection by means of the Akaike information criterion. This approach allows us to use data with almost arbitrary time resolutions, enables spectrum estimation with dense energy grids, avoids noise amplifications in numerical derivatives, and yields statistical error estimates for all the output parameters, including the number and energy fluxes and upward field‐aligned currents carried by the precipitating electrons. The technique is targeted for auroral energies, 1–100 keV, which ionize the atmosphere mainly between 80 and 150 km altitudes. We validate the technique by means of a simulation study, which shows that Maxwellian, kappa, and mono‐energetic spectra, as well as combinations of those, can be reproduced. Comparison study for two conjugate satellite measurements to the EISCAT UHF radar are shown, for Reimei and Swarm, showing an agreement with the results. Finally, an example of a 2‐hr measurement by the EISCAT radar is shown, during which we observe a variety of precipitation characteristics, from soft background precipitation to mono‐energetic spectra with peak energies up to 60 keV. The upward field‐aligned current varies from 0 to 10 μAm−2 and the total energy flux from 0 to 250 mWm−2.

The high‐latitude ionosphere‐thermosphere system is strongly affected by the magnetospheric energy input during magnetospheric substorms. In this study, we investigate the response of the upper thermospheric winds to four substorm events by using the Fabry‐Perot interferometer at Tromsø, Norway, the International Monitor for Auroral Geomagnetic Effects magnetometers, the EISCAT radar, and an all‐sky camera. The upper thermospheric winds had distinct responses to substorm phases. During the growth phase, westward acceleration of the wind was observed in the premidnight sector within the eastward electrojet region. We suggest that the westward acceleration of the neutral wind is caused by the ion drag force associated with the large‐scale westward plasma convection within the eastward electrojet. During the expansion phase, the zonal wind had a prompt response to the intensification of the westward electrojet (WEJ) overhead Tromsø. The zonal wind was accelerated eastward, which is likely to be associated with the eastward plasma convection within the substorm current wedge. During the expansion and recovery phases, the meridional wind was frequently accelerated to the southward direction, when the majority of the substorm WEJ current was located on the poleward side of Tromsø. We suggest that this meridional wind acceleration is related to a pressure gradient produced by Joule heating within the substorm WEJ region. In addition, strong atmospheric gravity waves during the expansion and the recovery phases were observed.