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[1] The present lightning location system of the Finnish Meteorological Institute has been in operation since 1998. We present statistics from the 10-year period 1998-2007. The main emphasis of this study is to give a comprehensive analysis of the development of the location system. Our results complement previous studies by providing 10-year lightning statistics from high latitudes in Europe (60-70°N). The changes in the detection efficiency have also been discussed. Our results indicate that in Finland (1) the median (average) peak currents in 2007 were −11.3 (−16.0) kA and +13.3 (+20.7) kA for negative and positive flashes, (2) the percentage of positive flashes in 2007 was 17%, (3) the average multiplicity for negative (positive) flashes in 2007 was 2.1 (1.2), (4) the median semimajor axis of the 50% location error ellipse in 2007 was 600 m (for 92% of flashes it was below 5 km), and (5) the average number of sensors reporting a flash in 2007 was 6.3. The network has experienced several configuration changes, leading to a more stable and better performing system, but these technical changes have also caused significant changes in the statistics of the measured lightning data. Clearly, lightning statistics over a 10-year period cannot be handled without considering the impact of technical changes on the detection network.

The effective area of an antenna collecting the vertical air-Earth current is a coefficient of proportionality between the collected current and the air-Earth current density. The effective area can be correctly defined only if the model behind the proportionality relation is adequate. The current collected by a horizontal long-wire antenna is not exactly proportional to the vertical air-Earth Maxwell current density because of the different behavior of the displacement and conduction components of the current. Thus, two different effective areas are separately defined for the displacement and nondisplacement components. First, the dynamic effective area characterizes rapid variations of the displacement current. It is calculated assming that the air does not contain any space charges and that the electric current flow lines match the electric field lines. Second, the static effective area characterizes the nondisplacement current, and it is calculated by taking into account the facts that the conductivity close to the wire surface is unipolar, that the wind-determined horizontal ion trajectories do not match the electric field lines, and that there are space charges due to the electrode effect of the wire and of the ground. Traditionally, the atmospheric electric vertical current density measurements have been interpreted by using the dynamic effective area as a calibration coefficient. This turns out to be a satisfactory approximation in the case of strong turbulence when the nearground space charge layer is high and the static effective area approaches the dynamic effective area. In the limit of low turbulence the traditional interpretation results in errors of several tens percent. A reduced height of the antenna helps to keep the static effective area close to the dynamic effective area and to suppress the errors. 0148-0227/96/96JD-02131 $09.00 ground-based sensors are the Wilson plate [see I•ra•l, 1973], the horizontal long-wire antenna [Kasemir, 1955; Ruhnke, 1969], and the Burke-Few sensor [Burke and Few , 1978]. The Wilson plate is a well-insulated horizontal plate with an area of typically a few square meters. The plate is flush with the ground and often covered to simulate the surrounding ground surface. The horizontal long-wire antenna is a well-insulated metal wire of a diameter of typically 1-3 mm spanned over the ground at a height of typically 1-3 m. The length of the antenna is much greater than its height and exceeds 1 km in some measurement setups. The Wilson plate and the horizontal long-wire are connected to the ground through an electrometric operational amplifier, and their potential is mainrained near the ground potential. The ambient electric field is strongly disturbed in the vicinity of the wire antenna. The Burke-Few sensor is a metal sphere typically a few tens of centimeters in diameter, divided horizontally into two wellinsulated hemispheres and installed at a height of a few meters. An electrometric amplifier is connected between the two hemispheres, and the whole sensor is well in...

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