ON THE IDENTIFICATION OF NEW PHENOMENA OBSERVED ON SAFIR SYSTEM MEASUREMENTS

Transcription

1 ON THE IDENTIFICATION OF NEW PHENOMENA OBSERVED ON SAFIR SYSTEM MEASUREMENTS Hamid Nebdi, Jean-Claude Jodogne, Michel Crabbé and Henri Malcorps Royal Meteorological Institute of Belgium 3, avenue Circulaire 1180 Brussels. Belgium Abstract A lot of particular phenomena were observed on the SAFIR (Système d'alerte Foudre par Interférométrie Radioélectrique) system measurements in different locations in Belgium and in other stations of some European countries (France and Netherlands). In this paper, the typical morphologies of these phenomena are presented and the identification of their origin is given. Key words: SAFIR, LF, VHF, solar flare, x-ray Introduction SAFIR is the most accurate system used up to day in order to forecast thunderstorms and to alert for ground flash danger. Since August 1992, the Royal Meteorological Institute of Belgium acquired this system for a real time lighting location [1]. It has been constructed by the France firm "Dimensions" under the license of the ONERA (Office National d'etudes et de Recherches Aérospatiales). The above system locates by triangulation; the direction of the reception of the lightning radiation by at least three antennas is determined by interferometry in the VHF band (between 108 and 118 MHz). In the opposite of the all other systems using real time lightning localisation, the system locates clouds as well as ground flashes, but originally didn't make a distinction between both. As RMI expressed the need to measure the electrical parameters of the ground flashes also, the antennas were complimented with capacitive electrical antennas. The signal of these antennas is also used to discriminate the ground flashes from the cloud flashes. The discrimination is done on the basis of the waveform of the signal detected by the capacitive antenna. The waveform is approximated by a triangle and this triangle has to satisfy given conditions to be identified with a groundstroke [2]. Recently some data collected from these antennas and others in some European countries present particular phenomena, which lead us to search for an identification of their origin and to attempt to know how it was occurred. In the present paper, we describe the observed phenomena in the different Belgian stations in the first section. In the second one, we give the same observed phenomena in the following European countries: France and Netherlands. Some discussions and the identification will be given in the third section.

2 1. The observed phenomena in Belgium The RMI has installed four antennas that covered the complete Belgian and Luxembourgian territories as well as a zone about 100 kilometres along the France and the Nederlands borders (see figure 1). These antennas are installed at the Geophysics Centre of RMI at Dourbes, at the production centre of "Antwerpse Waterwerken" (Antwerp Hydraulic Constructions) at Oelegem, at the dam of La Gileppe, that belongs to the "Ministère Wallon de l'equipement et des Transports" and the last installed one is at Mourcourt. These four antennas are connected by phone to the data processing system situated at the RMI. Figure 1: Localisation of the four SAFIR's antennas in Belgium and the area covered by this system Recently, we have detected some phenomena, which appeared sometimes on the Belgian data collected from SAFIR system. These phenomena have different morphologies and durations. In the following figures, we present the variation of the amplitude and the azimuth on the received signals at four Belgian localisations: Localisation Latitude Longitude Dourbes 50 N E Oelegem 51 N E La Gileppe 50 N E Mourcourt 50 N E 25 18

3 The same phenomena are present on different European countries at the same time of detection. In the second section, we present these phenomena, which are observed in tree localisations in France and Netherlands. In figure 2, we can easily observe the particular phenomena detected on data collected in 21 August These phenomena have an M form on the orange curve, which describes the amplitude variations of the electric field, and a blue segment near 80 on the y-axis, which describes the azimuth. The phenomena were observed about 1 minute between 16:33 and 16:34 UTC (the x-axis). Figure 2: The phenomena observed in four Belgian stations in 21 August The same phenomena were observed in 9 September 1999 (see figure 3), but there duration is more important then the first ones; it s about 7 minutes. These phenomena have a bell form on the orange curve, near 90 on the y-axis and situated between 17:04 and 17:11 UTC on the x-axis. Another cases were detected in 9 April 2002 (see figure 4). There duration is less then the two other cases given before; it s about few seconds. They appear like a dashed rectangular flash for the amplitude variations of the electric field (orange curve), and a blue segment for the azimuth variations, which is near the 120 value on the y-axis. In figure 5, we present the phenomena observed in 25 April They are the short ones, only for a signal detected at the Oelgem station, which have a bell form for the amplitude variations in some few seconds. The blue curve, which situated about 90 on the y-axis, is the same for the following tree stations: Dourbes, Oelgem and La Gileppe; but for Mourcourt station is about 320. Another observation can be easily done for the tree first cases, we remark that the amplitude variations of the electric field are similar for data collected in Dourbes and Mourcourt stations, and approximately for La Gileppe station.

4 Figure 3: The phenomena observed in four Belgian stations in 9 September Figure 4: The phenomena observed in four Belgian stations in 9 April 2002.

5 Figure 5: The phenomena observed in four Belgian stations in 25 April The observed phenomena in some European countries The same phenomena are observed in different European countries. In this paper, we present some cases detected in France and Netherlands. Figures 6 and 7 present the similar phenomena detected respectively in 21 August 1999 and 9 September These phenomena have been observed in tree different stations in France: Localisation Latitude Longitude Arbois 43 N E Pic de Bure 44 N E Saint-Chaptes 43 N E In figure 6, the same phenomena were observed in Belgian collected data at the same day and instant (see figure 3). We can easily remark that the orange curves have the same morphology then the first ones observed in Belgian data, and the blue segment corresponding to the azimuth variation has the same value, which is near 90. For figure 7, the Pic de Bure s station represents the same behaviour then the Belgian ones for the same duration (+7 ), but in the Arbois and St-Chaptes stations the phenomena have a short duration. The blue curves have the same value, but the orange ones their amplitudes change significantly.

6 Figure 6: The phenomena observed in tree French stations in 21 August Figure 7: The phenomena observed in tree French stations in 9 September 1999.

7 In Netherlands, the same phenomena have been detected in data collected from tree stations of observation: Localisation Latitude Longitude Valkenburg 52 N E De Kooy 52 N E Deelen 52 N E In figure 8, we can see easily the same phenomena observed in figure 3 and 6 representing respectively the data collected in Belgium and France. The blue curves are also at the same value of 90 for the azimuth variation, and the orange ones, which correspond to the amplitude of the detected electric field, represent the same morphology in the form of letter M. The duration of these phenomena is about few seconds as observing before, and the moment of the production is the same for the ten European stations (4 in Belgium, 3 in France and 3 in Netherlands). Figure 8: The phenomena observed in tree stations in Netherlands in 21 August 1999.

8 3. Discussion In the above figures, we have seen some phenomena detected in data collected from ten European stations distributed in tree countries. In order to decipher their origin, we are based on our previous work [3] in which we have studied the effect of the atmospheric and ionospheric phenomena on the propagation of some signals at very low (VLF) and low frequencies (LF). Among these phenomena, we found the very outstanding signature of the solar flares on these signals. So as the SAFIR system operates with the low frequencies (LF) and the very high frequencies (VHF), we have suspected that the effect of these eruptions is also felt by the electromagnetic waves associated to these frequencies. Indeed, the solar flares represent the sudden energy release in the solar atmosphere from which electromagnetic radiation and, sometimes, energetic particles (mostly protons and electrons) and bulk plasma are emitted. So a sudden increase of x-ray emissions resulting from a flare provokes a great increase in ionisation in the lower regions of the ionosphere on the sunlit side of our planet. This sudden ionospheric disturbance can affect the propagation of radio signals. Sometimes at VHF, the effect may appear as a short-wave fade, as seen in above figures. This disturbance may last from minutes to hours, depending upon the magnitude and duration of the flare. In other hand, solar flares also create a wide spectrum of radio noise; at VHF this noise may interfere directly with a wanted signal [4]. To confirm these interpretations, we have used the collected data from the GEOS satellites (Geosynchronous Operational Environmental Satellites) [5] concerning the detected solar flares. The data corresponding to the different dates given above are presented in figures 9, 10, 11 and 12, respectively for 21 Augustus 1999, 9 September 1999, 9 April 2002 and 25 April These data confirm exactly our predictions for the all phenomena observed on SAFIR system measurements. In the figures bellow, we indicate approximately the instant of the observed phenomena with a green arrow, which correspond at some given type of the solar x-ray produced at the same instant. The following GOES x-ray flux plots contains 5 minutes averages (you can visualise plots for 1 minute averages in the following internet address: ) of solar x-ray output in the 1-8 Angstrom ( nm) and Angstrom ( nm) passbands. Data from both operational GOES satellites are included. The y-axis of these plots corresponds to the x-ray flux in Watt/m², and the x-axis corresponds to the universal time. The capital letters A, B, C, M and X correspond to different kinds of x-rays which indicate their magnitudes as given in the following table: Kind of solar x-rays X-ray flux (W/m²) A 10-8 B 10-7 C 10-6 M 10-5 X 10-4 So if we have for example a kind of solar x-rays of M2, which is produced in 21 August 1999 and provoked the phenomena observed on SAFIR data, this correspond to x-ray flux of value 2x10-5 W/m². This value is the highest observed one for the four chosen dates, which explain the difference on the morphologies for the first case and the others seen in the previous figures. The highest kinds of the other dates are C3, C2 and C2 respectively for 9 September 1999, 9 April 2002 and the last one 25 April 2002.

9 Figure 9: The green arrow indicates the time and the magnitude of the solar x-ray produced in 21/8/1999. Figure 10: The green arrow indicates the time and the magnitude of the solar x-ray produced in 9/9/1999.

10 Figure 11: The green arrow indicates the time and the magnitude of the solar x-ray produced in 9/4/2002. Figure 12: The green arrow indicates the time and the magnitude of the solar x-ray produced in 25/4/2002.

11 Conclusions The origin of the particular phenomena observed on some data collected from the SAFIR system measurements, in different locations in Belgium, France and Netherlands, is identified to the effect of the solar flares on the very high frequencies (VHF). The typical morphologies of these phenomena and their durations are related to the magnitude and duration of the solar flare. Acknowledgement This work was supported by the Federal Office for Scientific, Technical and Cultural Affairs, and the Royal Meteorological Institute of Belgium. Thanks to all Institutions and people who offered us their data. References [1] H. Malcorps and M.Crabbé, 1995: Lightning localization in Belgium, Nouvelles de la Science et des Technologies, vol. 13, numéro 2/3/4, pp Bruxelles [2] H. Malcorps and M. Crabbé, 2002: Thunderstorm Warning by SAFIR, 2001 SAFIR Workshop. Bruxelles. [3] H. Nebdi, J.-C. Jodogne and J.-J. Delcourt, 2002 : Qualitative study of the seasonal behaviour of the ionosphere s D region from 3 years of VLF-LF signals monitoring. XXVIIth General Assembly of the International Union of Radio Science. 17 aug-24 aug, Maastricht. [4] N. Cohen and K. Davies, 1994: Radio wave propagation. Space Environment Laboratory. National Oceanic and Atmospheric Administration. Boulder. [5]