Radio Science, Volume 34, Number 4, Pages , July-August 1999

Transcription

1 Radio Science Volume 34 Number 4 Pages July-August 1999 Sunrise effects on VLF signals propagating over a long north-south path Mark A. Clilverd Neil R. Thomson and Craig J. Rodger British Antarctic Survey Cambridge England Abstract. We present a detailed study of the times of amplitude minima observed on the 12-Mm path from NAA (24 khz 1 MW Cutler Maine) to Faraday Antarctica during the period (NAA is a naval transmitter call sign.) This study represents the first account of the effect of the sunrise terminator when it is parallel to a propagation path at some times of the year. Since the NAA-Faraday path is within 3 ø of the northsouth meridian parallel orientation happens close to the equinoxes while the maximum angle of incidence occurs during the solstices. During the solstices the terminator takes a significant length of time to cross the entire propagation path so modal conversion effects are observed over a range of hours. During the equinoxes however the leading edge of the night-day transition region crosses the whole propagation path within 20 min. The interpretation of the timing of minima is consistent with modal conversion taking place as the sunrise terminator crosses the NAA-Faraday transmission path at specific consistent locations. The timing of minima is remarkably consistent from year to year. Long wave propagation modeling is used to show that the location of nightside minima at an altitude of km in the subionospheric waveguide represents the location of the sunrise terminator on the great circle path when dayside minima occur. 1. Introduction Diurnal variations of VLF signals propagated over long distances (>5 Mm) were first observed by Yokoyama and Tanimura [1933] with diurnal phase variations first reported by Pierce [1955] and Crombie et al. [1958]. The experimental results showed that the phase advanced during sunrise with pronounced steps coincident with amplitude minima. The fieldstrength variations during sunrise the sunrise effect could not be explained by simple single-mode propagation theory but Budden [1961] and Wait [1962] pointed out that multiple modes are needed to explain VLF propagation over long paths. A model using two waveguide modes was proposed by Crombie [1964] from a study of the transmissions from NPM Hawaii to Boulder Colorado. (NPM NBA NLK and NAA are call signs for naval transmitters.) Crombie modeled the sunrise effect by assuming a nonnegligible second mode in the nighttime portion of the Paper number 1999RS /99/1999 RS points on the dayside of the dawn discontinuity experienced a signal minimum simultaneously using a study of transmissions from NBA Canal Zone to Nairobi and also a mobile receiver in the Atlantic. Ries [1967] used VLF transmitter signals at khz to show that the converted second mode be- comes stronger with increasing frequency. A model of lowest-order sky wave propagation failed to describe the observed phase and amplitude behavior during the sunrise effect although previously the coincidence of phase advances with the time of sunrise at the ionospheric reflection points of a very simple low-order sky wave had been noted [Rieker 1963]. Regular annual variations in the times of amplitude minima caused by changing illumination conditions on a west-east propagation path were first noted by Israel and ROssler [1966] and Ries [1967]. Thomson et al. [1963] and then Lynn [1967] discussed observations from a more north-south orien- the day-night boundary. Walker [1965] verified that all I ' L1... tateta propagation [/iltll....] They u cu uata obtained from the path from NLK (18.6 khz Seattle Wash- 1 Also at Department of Physics University of Otago Dunedin ington) to Australia where the angle of the sunrise New Zealand. terminator to the propagation path attained a minimum value of about 20 ø in the middle of the path in Copyright 1999 by the American Geophysical Union. December. Using 1 year of observations Lynn found 939 that sunrise fading depended on the angle made by the propagation path with the terminator and that

2 940 CLILVERD ET AL.: SUNRISE EFFECTS ON VLF SIGNALS fade spacing anomalies could be interpreted as arising from anomalously high mode conversion along the path in the vicinity of the geomagnetic equator. Pappert and Snyder [1972] used a mode-conversion signals or "round-the-world-the-other-way" signals. Lynn [1967] found this to be especially true during the sunset conditions on his east-west path although this would depend on individual transmitter-receiver geometry. One solution to this problem would be to undertake a study where the geometry of the direct path results in high attenuation of the indirect path. This is the case for direct paths that are orientated north-south because the indirect path would have to pass over the Antarctic ice mass to arrive at the receiver and hence would suffer very high attenuation. This paper describe such a study. In this paper we report a detailed study of the times of amplitude minima observed on the 12-Mm northsouth path from NAA (44.6øN 67.3øW 24 khz 1 MW Cutler Maine) to Faraday Antarctica (65øS 64øW) during the period We use 1-min average amplitude data to investigate the propagation conditions throughout the annual cycle of changing solar illumination. We present the first study of a propagation path that lies parallel to the sunrise terminator for some of the year. We show that the majority of the features observed are consistent with mode conversion at sunrise at ionospheric altitudes (---75 km) at locations where modal minimum conditions are found. 2. Observational Results Subionospheric signals from the VLF transmitter NAA were recorded on an omega and minimum shift keying (OMSK) receiver [Dowden et al. 1994] at Faraday Antarctica from 1990 to The locations of the transmitter receiver and transmission path are shown in Figure 1. The phase and amplitude of the signals were logged with a time resolution of 1.25 s. In this paper we present 1-min averaged amplitude values in decibels referenced to an arbitrary but constant receiver voltage level where 15 db is equiv- alento about 50/zV m -1 at the vertical loop aerials. This work deals only with amplitude although stepprogram to model the occurrence of simultaneous wise phase advances are also expected to coincide minima during sunrise with allowance for four modes. with the amplitude minima [Rieker 1963]. Diurnal Good agreement was found when modeling the ex- variations of the signal amplitude from NAA are perimental configuration described by Walker [1965] shown in Figure 2. Twenty days of data from January with a simple model of terminator transition thickness 1994 are overplotted to indicate the reproducibility of of 1000 km and linear changes in the ionospheric the daytime values from 1200 to 2000 UT when few parameters of/3 ( km -1) and h' ( modes are present to interfere with one another. The km) from night to day conditions. variability in the nighttime values occurs at least One difficulty in interpreting the sunrise interfer- partly because of the increase in the number of ence effect was the potential influence of indirect significant modes present (five or more) and the consequent interference between them. Around the time of sunrise along the transmission path ( UT) a series of deep minima are observed. These features are associated with modal conversion effects as the sunrise terminator crosses the transmitter- receiver path. In order to study the variation in the time of occurrence of these minima we undertook an analysis of ---5 years of data including solar maximum and minimum conditions. A simple five-point comparison algorithm was written to detect amplitude minima and to log the time at which they occurred. Significant efforts were made to exclude any artificial minima caused by changes in transmitted power and also when the transmitter was off air. The results during sunrise conditions ( UT) for the whole of 1994 are shown in Figure 3a. In this plot each symbol represents a detected minimum. During nighttime propagation conditions there are high levels of variability and the influence of this can be seen in the rather uniform spread of points between 0700 and 0900 UT. During the daytime there is relatively little variability compared with night and the influence of this can be seen in the relative scarcity of points between 1200 and 1300 UT. During the sunrise transition period ( UT depending on time of year) several discrete features can be discerned some very well defined and some less so. In December-January (days ) discrete features are apparent from about 0700 to 1100 UT and in June (days ) similar structure is discernible from 0800 to 1200 UT. However in March-April (particularly days 75-90) and also in September-October (particularly days ) most of the discrete features are confined to the period from about 0930 to 1030 UT. Overall the structure could be described as being like a "sweet wrapper" or "candy wrapper."

3 CLILVERD ET AL.' SUNRISE EFFECTS ON VLF SIGNALS ' Figure 1. The positions of NAA and Faraday and the great circle VLF transmission path between them. (NAA is a naval transmitter call sign.) The generation mechanism for sunrise modal min- terminator and at a fixed location a receiver would ima is generally accepted to involve significant mode observe a series of modal features as they passed conversion where the sunrise terminator crosses the overhead. propagation path (modeled by Crombie [1964] and Both Walker [1965] and Ries [1967] showed that the validated by Walker [1965]). For the case where the seasonal variations in the occurrence of minima could transmitter is in darkness and the receiver is in be reasonably described by considering the seasonal daylight the magnitude of the sum of the interfering change in sunrise times at lower ionospheric altitudes modes at all altitudes at the location of the terminator (60-90 km) for fixed locations. In Figure 3b we have determines whether receivers on the same great plotted curves which represent the times of sunrise as circle path anywhere on the dayside of the termina- seen at 75 km altitude as a function of day of the year tor and more than 1000 km from it are simulta- parametric in distance from the NAA transmitter. neously experiencing maxima or minima. If the trans- The curves for the particular distances in Figure 3b mitter is in daylight while the receiver is in darkness are those that most closely match the observations in then typically the dominant day mode at the termi- Figure 3a. The distances from the transmitter that the nator will be converted into a range of nighttime discrete features in Figure 3a can be associated with modes. These will interfere with each other thus are and producing a succession of maxima and minima at km. The curves include the sunrise variation for NAA receivers located on the nightside of the path. The and Faraday as there is evidence that this is associsuccession of maxima and minima move with the ated with the times of modal minima observed at the

4 942 CLILVERD ET AL.' SUNRISE EFFECTS ON VLF SIGNALS 2O l0 oo Figure 2. Amplitude data from NAA-Faraday for January Data values of -15 to -20 db at UT represent the noise levels when the transmitter was off air. receiver [Ries 1967]. On day 1 the line representing predicted terminator effects throughout the year are sunrise at 75 km at 1125 UT is calculated for the NAA closely matched to the data plot. location the line at 1055 UT representsunrise at The consistency of the timing of minima from year 2300 km from NAA and so on. The earliest line to year is indicated in Figure 4. All of the minima (visible on the plot only from day 50) represents detected from 0700 to 1300 UT during are sunrise at 75 km above Faraday. Figure 3b shows a plotted. If significant changes in the timing of minima very similar variation to the data shown in Figure 3a were occurring from year to year we would expect and it is clear that whole can be well modeled by that the well-defined features in Figure 3a would be associating the discrete curves in Figure 3a with smeared out in Figure 4 which is not the case. Given sunrise at 75 km altitude for a specific latitude and the intrinsic variability of nighttime amplitudes the therefore location. In particular the UT ranges of extremes of influence of geomagnetic activity whis-

5 CLILVERD ET AL.: SUNRISE EFFECTS ON VLF SIGNALS 943 (a) lo o loo 200 3oo (b) day (1994) NAA o loo 200 3oo day Figure 3. (a) Times of detected minima during UT in (b) Sunrise times at 75 km altitudes for locations along the NAA-Faraday great circle path where distances are given from NAA. tier-induced electron precipitation and solar radiation from solar maximum in 1990 to solar minimum in 1995 this consistency remarkable. The plot also allows subtle details to be discerned including some not so clearly observable in Figure 3a. Featuresuch as the three semicircular inflection-point patterns occurring at UT in March (days 80-90) are not modeled in Figure 3b. This is probably caused by the terminator never being fully parallel to the prop- agation path at an altitude of 75 km although its precise explanation is unclear. 3. Comparison With Modeling The study presented in this paper represents the first account of the effect of the sunrise terminator when it is parallel to a propagation path at some time of the year. Since the NAA-Faraday path is within 3 ø

6 944 CLILVERD ET AL.: SUNRISE EFFECTS ON VLF SIGNALS o loo 200 5oo day Figure Times of detected minima during UT for -- 5 years of data collecteduring of the north-south meridian parallel orientation hap- time over which modal features are seen is only 0.4 pens close to the equinoxes while the maximum angle hours. The transition is identified through a point of of incidence occurs during the solstices. During the inflection in the timing of minima at about 1000 UT. solstices the terminator takes a significant length of A characteristic of the equinoctial feature is that the time to cross the entire propagation path so modal earliest September one is at 0936 UT while the effects are observed over a range of hours. However equivalent March feature is at 0948 UT. This is during the equinoxes the leading edge of the night- reproduced in Figure 3b and can be understood by day transition region crosses the whole propagation consideration of the relationship between siderial path almost instantly. This can be seen in Figure 4 time and UT (the"equation of time" [Clay 1731]. near days and where the range of Previous modeling of this phenomenon has been

7 CLILVERD ET AL.: SUNRISE EFFECTS ON VLF SIGNALS 945 restricted to considering a few modes only. Both Crombie [1964] and Muraoka [1982] used two modes while Pappert and Snyder [1972] allowed four. Here we used the National Ocean Systems Center (NOSC) long wave propagation capability code (LWPC) [Ferguson and Snyder 1990]. The code computes modal conversion along the propagation path on a continual basis for typically 10 or more modes and is widely regarded as the most realistic model for VLF propagation problems. We use parameters for nighttime ionospheric conditions of/3 = 0.5 km - and h' = 84 km similar to the values NOSC prescribe in the LWPC code for the conditions studied here. In this paper we use LWPC to model the nighttime propagation conditions along the path (described immediately below) and also the amplitude of signals received at Faraday during sunrise (in the next section). The variation in NAA 24.0 khz nighttime amplitude with distance along the great circle path toward Faraday is shown in Figures 5a and 5b. In Figure 5a we show the amplitude variation in the upper half of the waveguide although it should be noted that LWPC is an approximate model expected to be reasonably accurate near the ground and not necessarily at higher altitudes. There is a gradual decrease in amplitude with distance but there are also regions where the amplitude is significantly reduced due to interference between propagation modes. The distances from the transmitter to the sunrise terminator when modal minima were observed at Faraday are indicated as vertical dotted lines. These distances are in close agreement with the locations of modal minima features predicted by LWPC. In Figure 5b we show the amplitude variation in the lower half of the Waveguide. The pattern is similar to Figure 5a but the vertical dotted lines tend to coincide with modal maxima not minima. The waveforms depicted in Figures 5a and 5b can be thought of as a standing wave pattern as long as the nighttime ionospheric conditions remain unchanged. During the austral summer months (i.e. October-February) sunrise occurs at the receiver end of the great circle path first. The night-day terminator moves toward the transmitter encountering successive nighttime modal minima each in turn generating an amplitude minimum along most of the length of the dayside portion of the path. As the locations of the nighttime standing wave minima are essentially unchanging the time at which the nightday terminator reaches them will vary with time of year and thus the times of the occurrence of dayside amplitude minima will similarly vary. The yearly sunrise times at 75 km altitude for locations along the NAA-Faraday great circle path are plotted in Figure 3b. Each location can be closely associated with modal minima at km deter- mined by LWPC as in Figure 5a. However it is not clear that the occurrence of modal minimum features just beneath the lower ionospheric boundary is the critical factor in determining whether simultaneous daytime minima are generated. The electric field produced by the sum of the modes present at each altitude on the nightside of the dawn discontinuity must be conserved across the boundary. Hence the association of the location of nightside minima at km altitude with the location of the sunrise terminator on the great circle path when dayside minima occur is suggestive but the mechanism is not fully understood. The apparent association is worthy of further study. 4. Discussion and Summary We have shown that it is possible to model the gross behavior of the discrete minima features shown in Figure 4. The interpretation of the timing of minima is consistent with modal conversion taking place as the sunrise terminator crosses the NAA- Faraday transmission path. The apparent association of the predicted location of nightside minima at km altitude with the location of the terminator on the great circle path when dayside minima occur is worthy of further study. About 5 years' worth of detected minima from 1990 to 1995 were overplotted and shown in Figure 4. The consistency of the timing of minima is remarkable given the extreme geomagnetic and solar influences that occurred from solar maximum in 1990 to mini- mum in A detailed study of the timing of the particularly well-defined minimum at about 1030 UT seen in Figures 2 and 4 during December-January indicates that there is a shift of only 2 min from solar maximum to minimum with solar maximum conditions producing earlier times. This result has been confirmed using the 1.25-s resolution data. This observation is consistent with changes in the location of mode conversion as a result of changing ionospheric conditions with solar cycle. Any change in location would need to be of the order of 100 km toward the transmitter in order to explain the timing change. The movement of mode conversion sites is

8 946 CLILVERD ET AL.' SUNRISE EFFECTS ON VLF SIGNALS (a) loo 75 km 8o 6o 60 km - - ; ' \. - - i it 45krn -- I ¾1'11 ' z 4o (b) I I ' I I ' I distance from NAA (kin) I- --- Okm km - I i '-" i O 0 I d[stonce from NAA (km) Figure 5. (a) Long wave propagation capability (LWPC) output for receivers at altitudes of and 45 km with/3-0.5 km - and h' = 84.0 km on the NAA-Faraday great circle path with a 50-km resolution in distance. (b) A similar LWPC output for receivers at altitudes of 0 15 and 30 km. The locations of the terminator when modal minima were observed at Faraday are indicated by vertical dotted lines in both panels. inconsistent with increasing nighttime D region alti- Ultimately the goal of studies such as ours is to tudes from solar maximum to minimum a way enhance our ability to understand and model VLF similar to that observed during daytime by Mechtley et propagation over long distances. The current status of al. [1972] as this would tend to increase the distance modeling the complex propagation conditions on of modal conversion from the transmitter opposite paths like NAA-Faraday during sunrise can be seen that inferred from the observations. The cause of the in Figure 6. The predicted amplitude of NAA as a solar cycle shift in timing is worthy of future attention. function of time calculated using LWPC is shown for

9 o o o CLILVERD ET AL.: SUNRISE EFFECTS ON VLF SIGNALS 947 2O 15 - \ data 10 model "' Time (UT) Figure 6. The calculated amplitude as a function of time near sunrise for the NAA-Faraday path on January 10 using LWPC compared with the average of the observed amplitude data from January 1 to shown in Figure 2. January 10 and is compared with the average of the amplitude data from January 1 to shown in Figure 2. The LWPC values have been offset by -25 db to compensate for different reference levels compared with the OMSK and to allow more easy comparison. LWPC predicts a deep well-defined minimum occurring just after 1100 UT about 6 db down on daytime levels. The timing is in good agreement with the observed data. A small minimum about 1.5 db deep can be seen slightly before 1030 UT in the model output. The observed data indicate that in reality the corresponding minimum is slightly later (a few minutes) and deeper (about 5 db). The observed data also exhibit a minimum at 0930 UT; this is possibly present in the LWPC result but lacks definition and any significant depth. The accuracy of the timing of minima in the LWPC results is encouraging suggesting that in many ways the gross features of the nighttime propagation conditions are being well modeled. The disparity between the depths of the ob-

10 948 CLILVERD ET AL.: SUNRISE EFFECTS ON VLF SIGNALS served and predicted minima clearly suggests that more accurate paramaterization of dawn-dusk propagation conditions is required as well as consideration of the underlying assumptions involved in the linear ramping model incorporated in LWPC. Owing to the small range of incidence angles between terminator and propagation path (<27ø) we would anticipate that two classes of minima would occur in the data set. We have discussed modal minima in detail but also note that there should be an attenuation minimum when near-parallel propagation is occurring. The attenuation minimum would tend to have a longer lifetime than the modal minima as the passage of the night-day transition region across the path takes about 1-2 hours. In Figure 4 the broad band of minima events occurring at about 1200 UT on January 1 (day 1) and changing slowly to 1130 UT by March (days 60-90) is the signature of the anticipated attenuation minima. This will be the focus of a future study. (Version 1.0: Full FORTRAN code user's guide 1 April 1990) Tech. Doc Natl. Ocean Syst. Cent. Alexandria Va Israel H. and J. R6ssler Zum sonnenaufgangseffekt im lfingstwellengebiet Z. Geophy Lynn K. J. W. Anomalous sunrise effects observed on a long transequatorial VLF propagation path Radio Sci Mechtley E. A. S. A. Bowhill and L. G. Smith Changes of lower ionosphere electron concentrations with solar activity J. Atmos. Terr. Phys Muraoka Y. A new approach to mode conversion effects observed in a mid-latitude VLF transmission J. Atmos. Terr. Phys Pappert R. A. and F. P. Snyder Some results of a mode-conversion program for VLF Radio Sci Pierce J. A. The diurnal carrier-phase variation of a 16-kilocycle transatlantic signal Proc. IRE Rieker J. Sunset and sunrise in the ionosphere: Effects on the propagation of longwaves J. Res. Natl. Bur. Stand. Sect. D Acknowledgments. The authors thank J. A. Ferguson for his permission to use LWPC. One of the authors (C.J.R.) was supported by the New Zealand Fellowship Ries G. Results concerning the sunrise effect of VLF signals propagated over long paths Radio Sci contract BAS 701. Thomson A. H. R. W. Archer and I. K. Harvey Some observations on VLF standard frequency transmissions as References received at Sydney N.S.W. Proc. IEEE Budden K. G. The Waveguide Mode Theory of Wave Propagation Prentice Hall Englewood Cliffs N.J Clay F. An explanation of the nature of equation of time and use of the equation table for adjusting watches and clocks to the motion of the sun: Also the description of a time-keeper for astronomical and other uses F. Clay London Whipple Sci. Mus. Cambridge Univ. Cam- Wait J. R. Electromagnetic Waves in Stratified Media Pergamon Tarrytown N.Y Walker D. Phase steps and amplitude fading of VLF signals at dawn and dusk J. Res. Natl. Bur. Stand. Sect. D Yokoyama E. and I. Tanimura Some long-distance transmission phenomena of low frequency waves Proc. IRE bridge England Crombie D. D. Periodic fading of VLF signals received over long paths during sunrise and sunset J. Res. Natl. Bur. Stand. Sect. D Crombie D. D. A. H. Allan and M. Newman Phase variations of 16 kc/s transmissions from Rugby as received in New Zealand Proc. IEE Dowden R. L. C. D. D. Adams J. Brundell and P. E. M. A. Clilverd and C. J. Rodger British Antarctic Survey Madingley Road Cambridge CB3 0ET England U. K. ( m.clilverd@bas.ac.uk; cjro@mail.nerc-bas.ac.uk) N. R. Thomson Department of Physics University of Otago Dunedin New Zealand. ( thomson@physics. otago.ac.nz) Dowden Rapid onset rapid decay (RORD) phase and amplitude preturbations of VLF subionospheric transmissions J. Atmos. Terr. Phys Ferguson J. A. and F. P. Snyder Computer programs for (Received February ; revised April ; assessment of long wavelength radio communications accepted April )