Before data from incoherent scatter radars became available to the space physics community, the identification of substorms relied heavily on those from ground-based magnetometers. The construction of radars and their subsequent upgradings in operational techniques during the last two decades have made it possible to unveil the different roles that electric fields and conductivities play separately within the auroral electrojets, which gave rise to ground magnetic perturbations during substorms. By briefly reviewing progress in substorm research, this concise review attempts to emphasize the importance of having data from incoherent scatter radars for deducing (1) “true, ” not “equivalent, ” ionospheric currents at normal latitudes; (2) substorm changes in the global as well as local patterns of the electrostatic potential; and (3) the spatial/temporal relationship between “directly-driven” and “unloading” processes in substorms. Radar data will offer a unique opportunity to understanding quantitatively the convection/ substorm relationship if examined properly as well as effectively along with other ground-based and satellite measurements of ionospheric/magnetospheric processes. Several near-future topics which can be pursued using data from new modes of radar operation are also listed.

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The recently developed imaging riometer technique has proven a valuable tool for investigations of the complex dynamics of auroral and polar radiowave absorption events. These events mostly relate to the precipitation of energetic particles into the upper atmosphere to create enhanced ionization in the D-region at low altitudes. A further mechanism which may cause enhanced ionospheric radiowave absorption is electron collision frequency enhancements due to processes like E-region heating by large electric fields. The imaging riometer observations are particularly well suited to separate spatial and temporal variations in the ionospheric phenomena causing radiowave absorption, and thus to define their extent and motions. This work will discuss the poleward expansion of substorm absorption events, and the variable daytime absorption events related to the modulated precipitation of substorm electrons. The high-latitude nightside observations support the model of substorms being a series of enhancements distributed in latitude and longitude rather than single onset, monotonously expanding events. The individual enhancements could be explained by an augmented current wedge mechanism. The dayside observations indicate that disturbances associated with a strong convection shear may cause enhanced and strongly variable precipitation of the eastward drifting cloud of high-energy electrons generated during substorms. Basis for the analysis are events observed, among other, by imaging riometers at the cusp-latitude observatories in Sdr. Strømfjord (inv. lat.= 73.7°) and Danmarkshavn (inv. lat. = 77.3°) in Greenland, and in Ny-Ålesund (inv. lat. = 76.1°) at Svalbard, and at the auroral zone observatory in Tjørnes (inv. lat. = 66.9).

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The understanding of the magnetosphere-ionosphere couplings requires the concomitant use of experimental facilities and numerical tools. The European Incoherent Scatter radar (EISCAT), able to provide instantaneous vertical profiles of the ionosphere, makes possible the development of data processing methods to get boundary parameters, in particular the topside downward electron heat flow: this boundary parameter is crucial for the purpose of quantitative modelling of the ionosphere. It characterises at auroral latitudes the energy transfer from the magnetosphere to the ionosphere and is determinant for the development of fluid numerical models of the ionosphere. This paper presents how with such a boundary condition obtained from the data, the steady-state and the dynamic behaviour of the ionosphere-from 100 to 3000 km-can be quantitatively reproduced with a fluid description of the plasma. Coupling between atmosphere and ionosphere is included through chemistry and collisions while the particle and energy transfers from the magnetosphere to the ionosphere are included through ionisation and energy deposition resulting from electron precipitation.

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In the presence of high electric fields in the auroral ionosphere, the ion distribution functions depart from their usual Maxwellian shape. The incoherent scatter radars have observed the signature of this phenomenon in the form of distorted spectra which have lost their double-humped characteristics. In consequence, the classical procedure of spectrum analysis, which assumes Maxwellian distributions to fit the plasma parameters, is no longer valid and has to be modified to account for a non-Maxwellian plasma. The observations of the European incoherent scatter radar EISCAT were at the origin of substantial developments, which are still in progress. We briefly review the main steps in the experimental and theoretical approaches of the non-Maxwellian behaviour of the ionospheric plasma, and we discuss particularly the most recently published results.

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Measurements of the European incoherent scatter facility (EISCAT) occasionally indicate the presence for brief periods of ionospheric small-scale structures and turbulence at scales smaller than the typical space and time resolution of the radar. Such events violate the assumption of space and time uniformity used in the analysis of the received signal. This results in an erroneous estimate of the derived ionospheric parameters, or even in the failure of the analysis when the measured spectrum departs too much from its usual shape. We review recently published observations from EISCAT data of two types of such phenomena, namely the small-scale auroral structures of intense flow bursts, and the ion-acoustic turbulence. The large differences in their spectral signature and origin illustrate both the variety of the small-scale phenomena occuring in the ionosphere, and the capability of the incoherent scattering technique to detect them and to infer original informations on the mechanisms responsible for them.

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Ion composition in the lower-F region for the daytime period of summer season has been obtained by correcting the EISCAT composition model with the ratio between the measured and modeled ion temperatures. The model profile of ion temperature was estimated with a fitting procedure of tan^-1 function on the EISCAT measurements under an assumption that the temperature is almost free from any additional heating. In order to warrant this assumption, the data periods during which the effect of frictional heating was negligibly small have been selected under the criterion that the F-region d.c. electric field remained below 4 mV/m in magnitude. The result reveals that the transition altitude between molecular and oxygen ions has a clear diurnal variation with a minimum altitude of about 187 km around 13 LT.

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In this work we have made a comparison between quiet and disturbed time values of the auroral zone E-field ionospheric conductances and E-region neutral winds as derived from EISCAT-UHF measurements. The average diurnal mean values of the E-field are rather unchanged between quiet and disturbed days while the amplitude of the 24-hour diurnal oscillation is enhanced by a factor of 3 from averaged quiet to disturbed days. The E-region neutral wind has an eastward component between 90 and 110 km which is seasonal dependent, showing highest values in summer and lowest in winter with spring and fall values in between. The most outstanding difference between quiet and disturbed time E-region neutral winds is an increase in the eastward diurnal fide of a factor of 3-4 above 100 km. The mean wind as well as the higher order tidal modes (12-, 8- and 6-hour modes) display rather minor differences.

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This paper focuses on the meridional wind in the F region deduced from EISCAT experiments. Comparisons of this radar deduced neutral wind with optical measurements (interferometer MICADO) and analytical models (HWM and FTH) show a very good agreement both during active and quiet ionospheric conditions and over several consecutive days of experiment.

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The Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) was inaugurated in June 1994 and is located on the island Andoya (69°30'N, 16 00' E) in Northern Norway. ALOMAR is an international centre for studies of the arctic middle atmosphere (the height range 10-100 km). Through the use of three ground-based lidar instruments (with five lasers), one VHF and one MF radar system it will provide information about the dynamical and photochemical processes in the middle atmosphere, including the state and development of the ozone layer. The lidars (light detection and ranging) use laser light scattered from atmospheric molecules, atoms and aerosols to measure density, temperature, winds, aerosol content and selected trace gases. Ad hoc and routine measurements will be efficiently carried out at ALOMAR, using both the basic instrumentation mentioned above and other supporting experiments. The facility provides the international science community with the opportunity for year-round, in-depth studies of the arctic middle atmosphere. The paper will briefly describe the new facility and its scientific aims.

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This paper describes scientific subjects important in the middle atmosphere in the polar region, considering the effects of phenomena unique to the high latitudes as well as the coupling processes between the neutral and ionized atmosphere. We are particularly pointing out the relevant application of an MST (mesosphere-stratosphere-troposphere) radar for observations of middle and lower atmospheric structure and dynamics. We briefly summarize fundamental techniques of the MST radar. Then, we survey phenomena that are exclusively detected in the polar atmosphere, such as precipitation of high energy particles, aurora) electrojet, stratospheric sudden warming, the polar vortex or a polar night-jet, PSC (polar stratospheric cloud) associated with ozone depletion and PMSE (polar summer mesospheric echoes), related to NLC (noctilucent clouds). Among these phenomena there are certain topics that need to be studied with an MST radar in addition to the multitude of other complementary experiments which are already applied for these studies in the Arctic and Antarctic. Finally, we discuss a realization of an MST radar operated on 50 MHz, which should be established in conjunction with the Svalbard incoherent scatter radar operated on 500 MHz.

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