VLF-LF PROPAGATON MEASUREMENTS DURNG THE 11 AUGUST 1999 SOLAR ECLPSE R. Fleury, P. Lassudrie-Duchesne Ecole Nationale Suptrieure des TClCcommunications de Bretagne, France ABSTRACT A survey of the VLF-LF spectrum evolutions has been conducted during the 11 August 1999 solar eclipse in the vicinity of the totality region. The measurements consisted of amplitude spectrum acquisitions in the frequency band 10-80 khz with a repetition period of 1 min. The field strength received from various transmitters could thus be monitored both during the eclipse and control days. The time signatures of the eclipse appear to be highly variable according to the frequency and radio path characteristics. The main features of the eclipse signatures are interpreted by using a wave-guide propagation model. NTRODUCTON Solar eclipses are known to produce disturbances in the ionosphere, which result in noticeable effects on HF, LF and VLF radio wave propagation. This paper is concemed with the effect of the 11 August 1999 total solar eclipse on the propagation VLF-LF radio waves. Such waves are propagated in the Earth-ionosphere wave-guide delimited by the ground and the ionospheric D region. Since direct measurements of the D region characteristics are sparse, this solar eclipse was a unique opportunity to analyse the effects of a localised D region disturbance on the propagation of VLF-LF waves. The 11 August 1999 solar eclipse started at 9:30 off the American Eastern coast. The zone of totality swept across the Atlantic Ocean and reached Comwall at 1O:lO, then moved on Eastwards over Central Europe. A VLF-LF spectrum monitoring system was operated continuously in Lannion, Northein Brittany, both during the eclipse and surrounding periods. This made it possible to survey a nuniber of radio paths from transmitters located in Western Europe and Eastem America. Fig. 1 shows the radio paths surveyed for this study as well as the path of totality of the eclipse at D region heights. EXPERMENTAL ARRANGEMENT Radio spectrum monitoring system The VLF-LF spectrum monitoring system located in Lannion (48"45'N, 3'27'W) is composed of an active whip antenna, an anti-aliasing low-pass analogue filter and a 16-bit A D converter with a sampling rate of 200 khz. The system is controlled by a PC with a DSP card for real-time data analysis. Acquisitions of 150 ms data sets are Fourier transformed to yield spectra in the frequency range 10-80 khz with a frequency step of 25 Hz. The repetition period was set to 1 spectrumlmin. Selected transmitters During the period of interest, 7 VLF-LF transmitters were identified and their signal levels continuously surveyed. The corresponding radio paths are listed in Table 1. EXPERMENTAL RESULTS Measured radio spectra An example of measured spectrum is given in Fig. 2. Peaks from various transmitters are clearly observed. The selected peaks are indicated by arrows. The field strengths of these transmitters are seen to be well above the background noise level (SNR > 20 LB). The peak values relative to each selected transmitter are first integrated over a 200 Hz bandwidth. Values obtained for successive spectra then yield field strength time series with a resolution of 1 min. Eclipse signatures Plots of the measured field strength vs. are given in Fig. 3 for each selected transmitter. The time when the eclipse totality reaches the receiver in Lannion is indicated on each graph together with the time of nearest approach of totality to the transmitter. Also shown in Fig. 3 are the measurements for the previous day, taken as control day. Variations in the field strength can be clearly identified on each graph around the time of the eclipse. Although unambiguous, the eclipse signature takes various shapes. t appears as one marked peak in Graphs (a), (b) and (e) but shows a drop in Graphs (c) and (d). n Graphs (f) and (g) a double peak structure is observed. The following is an attempt to interpret these signature shapes by modelling the propagation of the VLF-LF waves in the Earth-ionosphere wave-guide. For this purpose, we will select Graphs (a), (d) and (8) as HF Radio Systems and Techniques, Conference Publication No. 474 0 EE 2000 39 1
representative examples of the observed eclipse signatures. The corresponding radio paths are all crossing the path of totality of the eclipse. DATA ANALYSS Wave-guide propagation model The propagation model used for the data analysis is based on the mode theory [Morfitt and Shellman (l)]. n this theory, the waves are considered to propagate between the Earth and the ionosphere as normal modes. The influence of the Earth's magnetic field is taken into account in our calculation. Assumed electron density profiles One of the most important input parameters to the propagation model is the D region electron density profile. Following Wait and Spies (2), we assume an exponential profile with gradient parameter p and reference height 11'. Values for p and h' have been deduced from measurements by Morfitt (3) and Ferguson (4). For daytime in Summer, p and h' are found to take constant values p = 0.5 km-' and h' = 70 km. For Summer night-time, p and h' are found to vary in the range 0.38-0.91 km-l and 87km-88km respectively, according to the frequency. We assume that to the first order, the effect of the eclipse was to lift the reflecting height towards its nighttime value. For simplicity, the height disturbance was assumed to be uniforni along the whole radio path. The height increase Ah' will be determined by data fit. The only apriori information we use on Ah' is that it should not exceed the night-time limit of 88 km. Since the influence of p is less than that of t', we assume for convenience a constant value of p = 0.5 m-] during the eclipse. We further need to define some dependence law for Ah' with time during the eclipse. At this stage, we made the crude assumption that dh' was proportional to the eclipse obscuration value with a pre-eclipse value of 70 km. We then assumed that the maximum value for Ah' took place at mid-eclipse and was independent of the frequency. Simulation of the received field strength The propagation software was run using a set of h' values ranging from 70 to 88 kin by steps of 1 km. An example of results is shown in Fig. 4. As expected, multimode propagation produces interference that results in deep signal fades at particular locations. The calculated signal level was then analysed as a function of 11' for the distances corresponding to the selected transmitters. t was found that the maximum value for Ah' that best fits the data lies around Ah'z 5 km. This value seems to be in accordance with the previous results of Meisel et al. (5). Fig. 5 shows that the simulation results are in good qualitative agreement with the data. t is noted, however, that the agreement for the peak-to-peak amplitudes of the eclipse signatures deteriorates as the frequency increases. n addition, the simulated signatures appear systematically more spread in time than the measured ones. We interpret these discrepancies as the consequence of the over simplified model we assumed for the D region disturbance. CONCLUSON Field strength VLF-LF measurements conducted during the 11 August 1999 total solar eclipse in Europe have been analysed using the guided wave propagation model. t is found that the data are consistent with a reflection height increase of about 5 h at mid-eclipse. Discrepancies in the amplitude of the eclipse signatures suggest that more realistic variations of Ah' with time and position along the radio paths during the eclipse need to be incorporated in the ionospheric disturbance model. ACKNOWLEDGEMENTS The development of the spectrum monitoring system used in Lannion was supported by the DClCgation GCnCrale a 1'Armement. REFERENCES 1. Morfitt D.G. and Shellman C.H.,1976, MODESRCH, an improved computer program for obtaining ELFNLF/LF mode constants. Naval Electronics Laboratory Centre nterim Rep. 77 1, NTS Accession No. ADA032573. NTS Springfield, Va. 22 16 1, USA 2. Wait J.R. and Spies K.P., 1965, nfluence of finite ground conductivity on the propagation of VLF radio waves, Radio Sc., 3, 787-791 3. Morfitt, D.G., 1977, Effective electron density distributions which describe VLF/LF propagation data. Naval Ocean Systems Centre Tech. Rep. TR141, NTS Accession No. ADA047508. NTS Springfield, Va. 22 16 1, USA 4. Ferguson J.A.,1980, onospheric profiles for predicting nighttime VLF/LF propagation. NOSC TR 530, NTS Accession No. ADA 085399. NTS Springfield, Va. 22 16 1, USA 5. Meisel D.M., Duke B., Aguglia R.C. and Goldblatt N.R., 1976, Solar eclipse effects on HF and VLF propagation, JATP, 38,495-502 392
60: so: 40: 0' Fig. 1. Simplified map showing the path of totality at D region height for the eclipse of 11 August 1999 (hatched area), in relation to the surveyed VLF-LF radio paths. 060' 03b' 0 19 0000 0 is. Transmitter R~igby Rhatiderfehn Cutler, ME Rugby France-Sud Geneve-Prangins Mainflingen Latitude Longitude Country Frequency Path length (kh4 52'22" 01'11'W UK 16.0 (km). 433 53'01'N 07'36'E D 23.4 911 44'38" 67'17'W USA 24.0 4 740 52'22" 01'11'W UK 60.0 433 43'28" 02'00'E F 62.6 722 46'24" 06'15'E CH 75.0 772 50'01'N 09'00'E D 77.5 911 Table 1. Transmitter characteristics. Radio path lengths correspond to a receiver in Lannion (48'45'N,3'27W). 10 20 30 40 50 60 70 80 Frequency (khz) Fig. 2. Example of measured spectrum. The spectral lines corresponding to the transmitters surveyed during the eclipse are indicated by arrows. 393
-22 c7- -.,. -_-) -22 8 9 10 11 _----_ - - 7--- 12 13 U -26-28 -30-32 n -16 [--, (e) France Sid FTp---- -18.i 62.6 khz -20 c 7-20 6-22 U -24-26 -28 Y 7 -r-r-7-46 1 W -28 - -7T7-( 1- - 7 7 7 - ~~ -26 -- -28 {(g) Mainflingen (D) 77.5 khz 1 :,,:,.( 4-38,,,,,,,,,,,,,,,,,,. Fig. 3. Time evolution of the field strength measured in Lannion during the eclipse (continuous line) far 7 transmitters. The time when the eclipse totality reaches the receiver and the time of nearest approach of totality to the transmitter are indicated by circles and triangles respectively. Dotted lines refer to the previous day, which served as control day. 394
Relative level (db) Fig. 4. Example of propagation model output at 16 khz. The field strength level variations are shown against distance from the transmitter. The reference height varies from h' = 70 km (assumed daytime value) to 11' = 88 kin (corresponding to typical night-time value in Summer). v 0-5 -1 0-15 s -20 E. - -25 a -30 - -35-40 -45 940km o o o - 910 km -50 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 A Fig. 5. Comparison of simulated (dots) vs. measured data (lines) for 3 typical eclipse signatures. The reflection height was assumed to increase during the eclipse in proportion to the obscuration value and in a uniform inaimer over the radio path. The simulated data were adjusted to the measurements at first contact with an assumed reference height of 70 km. 395