Science objectives of the Stare experiment

The Scandinavian Twin Auroral Radar Experiment (STARE) consists of two coherent radar stations located in Midtsandan (10.7oE, 63.4oN) in Norway, and in Hankasalmi (26.9oE, 62.30N) in Finland. The field of view of the radars is in the E-region ionosphere over Northern Scandinavia as shown in Figure 1. Each radar makes observations with good spatial resolution along eight antenna lobes. The resulting dense grid of observation 'points' from the two radars makes it possible to obtain electron velocity estimates with good spatial resolution (~20*20 km) over the large field of view (= a geographic area of 200000 km2).

The radars operate at 140 MHz and at 143.8 MHz, respectively, and are sensitive to magnetic field aligned electron density fluctuations in the ionospheric E-region of scale length of about one meter. Such density fluctuations (often refered to as 'irregularities') are caused by plasma instabilities, as the two- stream- and the gradient-drift instabilities. The phase velocity of the unstable plasma waves is related to the electron drift velocity, and with measurements from two directions (the two radar stations) in the same ionospheric region, an estimate of the electron velocity can be obtained. In this way spatial maps of ionospheric drifts are determined. Since the radar integration time is typically 20 seconds, this is the typical time resolution of the maps. The intensity of the radar backscatter is also related to geophysical parameters, namely the ionospheric electron density and the amplitude of the electron density fluctuations in the instable plasma waves.

The STARE system has been modernised, and started operations again in May 1998. The major advances over the 'old' STARE system are a associated with advances in hardware and software techniques. It is now quite easy to modify the observation program to accomodate special requirements arising for example in connection with joint measurements wih another experiment: near and far ranges, range resolution, pulse width, pulse sequences and power level, are variables. The gaun in each of the eight receiver channels are automatically controlled. The observations can be accessed remotely, also in real time.

[STARE Image]
The figure shows the STARE receiver array in Midtsandan. The grey box contains a Butler Matrix, which forms the 16 narrow antenna lobes that provides directional resolution in the measurements.

Observational parameters

Each radar measures three kinds of parameters: Intensity, Doppler Velocity, and Auto-Correlation Function of the backscattered signal as a function of range and direction, with a time resolution of typically 20 seconds. A unique capability of this experiment and an important point is that the spatial structures (or variations) of the three parameters can be determined at a given time, and that the development in time of these spatial structures are also measured.

In the field of view common to the two radars estimates of the ionospheric flow velocities can be obtained with a spatioal resolution of about 20x20 km These flow velocities have been used to study convection, substorms, geomagnetic pulsations, and many other aspects of ionospheric/magnetospheric physics. Ground based expriments are essential to study such phenomena, because they can yield informations about spatial scales and spatial (bulk) velocities, which satellite measurements can only with difficulty or not at all provide.

The three kinds of observable parameters are being used to study plasma instabilities exciting the one meter plasma waves in the E region, which gives rise to the backscattered signal.

Cooperation

The STARE facility is oprated jointly by Max-Planck-Institut für Sonnensystemforschung (MPS) in Germany ( Dr. E. Nielsen), and the ( Finnish Meteorological Institute (FMI) in Finland, in cooperation with ELAB (University of Trondheim) in Norway.

STARE measurements are used in cooperation with several other experiments, for example:

Rio-Imager
DASI
DE 1 and DE 2
VIKING
EISCAT
HEATING
INTERBALL
CLUSTER
GEOTAIL
POLAR
WIND