Characteristics of the Equatorial VLF Emissions

Transcription

1 Indian Journal of Radio & Space Physics Vol. 10, April 1981, pp Propagation Characteristics of the Equatorial VLF Emissions RAM PRAKASH, S K JAIN & BIRBAL SINGH Department of Physics, R B S College, Bichpuri. Agra Received 10June 1980; accepted 5 January 1981 Propagation characteristics of the day and nighttime equatorial VLF emissions observed in the satellites as well as on the ground are determined with the help of ray-tracing computations by employing ionospheric models which yield electron density distributions similar to those found in the equatorial anomaly. The results show that these emissions are propagated under the influence of the negative horizontal density gradients of the anomaly. Propagation characteristics of the daytime VLE emissions are almost similar during both quiet and disturbed periods. However, the propagation characteristics of the nighttime VLF emissions are influenced by the anomaly during disturbed periods only, and during quiet periods, the observation of the emissions on the ground depends upon the wavenormal angle at which they are generated. I. Introduction Recently, Bullough et af.1 have reported, from their Ariel 3 and 4 satellite experiments, the observation of VLF emissions at frequencies 3.2, 9.6 and 16 khz at high, middle and low latitudes, respectively, in the ionosphere. As the occurrence region of low latitude emissions was found to lie in the latitude range below ± 30, they named these emissions as equatorial emissions. Further, since the time of occurrence of these emissions coincided with the time of fully developed equatorial anomaly [l5()()"1800 hrs ML T (Ref. 2 and 3)], they suggested that these emissions are ducted through the field lines along which the anomaly peaked. Hayakawa and Tanaka4 and Hayakawa and Iwais have also assumed a similar type of propagation for low latitude ground observed VLF emissions. On the basis of intensity calculations Ram Prakash et ai.6 have suggested that the low latitude ground observed emissions could be the same as the equatorial emissions observed on the Ariel 3 and 4 satellites. Although the trapping and ducted propagation of the equatorial VLF emissions in the equatorial anomaly has not been studied so far, there are certain questions which rule out this type of phenomenon to occur. Firstly, the electron density variation across the Fieldline at all the altitudes inside the anomaly does not represent exactly the Gaussian type of distributions "7 as found in actual ducts'. Hence tbe ray-patbs of the emissions cannot remain confined to the same field tine along the whole propagation path. During the magnetic disturbances. there are reports from bottomside ionograms to indicate that the equatorial anomaly is either weakened considerably or not developed at an', while it is not clearly established in the topside ionosphere, as the anomaly can be either developed wen duri~g certain storms or not developed at all during other stormslo However, for the present study of whistler propagation we take the case of the_ complete absence of the anomaly to represent the stormtime.conditions. Secondly, the equatorial emissions are also observed in the nighttime and early morning hours during which the anomaly is confined to a very narrow latitude range (± 5 ) with a single crest of enhanced ionization at the equatorll 12. During these periods, the question of the ducted propagation of VLF emissions does not arise. In the present paper, we calculate the propagation paths of day and nighttime equatorial emissions observed during quiet and disturbed periods with the help of ray-tracing computations and then discuss their ground observations on the basis of final wavenormal orientation at the base of the F-region ionosphere. For this purpose we assume that these emissions are propagated under the influence of negative horizontal density gr,adients of the equatorial anomaly. The effect of horizontal gradients on VLF propagation was, in fact, suggested by Aubrey13 who found from the FR-l satellite experiment that the wavenormals of the downcoming VLF waves at 750 km were oriented towards the equator. Later on, Fijalkow et al.14 also studied the effect of horizontal gradients on VLF.propagation. However, the results of these workers correspond to the VLF propagation in the middle latitude ionosphere above 40, whereas in this paper we study the propagation characteristics of the VLF waves which are observed below 30 where the horizonal density gradients are provided by the equatorial anomaly. 2. ~ MotIeIs am *c..p.tll' Pw r. 2.1 ~ ~ Dmsily M..w For the computation of ray-paths of tbeequatorial emissions., we employ a background electron density

2 INDIAN 1 RADIO & SPACE PHYS, VOL 10, APRIL 1981 model similar to that employed earlier by Singh 15, Singh et al. 16 and Singh and Singh 17. This is a diffusive equilibrium model18 which is represented at a reference level of 400 km by electron density of 1.5 x 105 el. cm -3, 0 + of 95%, He + of 4.75%, H + of 0.25 % and by a temperature of 1000 K. Since the computations are done down to 120 km (base of the F region ionosphere), the ionosphere-exosphere model given by Kimura 19 is also employed. 2.2 Equatorial Anomaly Model Topside sounder data have shown that the electron density distribution around the equator is highly variable This has been named as equatorial anomaly. During daytime the anomaly indicates two peaks, one on each side of the equator and confined to a particular field line22. The location of the peaks shifts to other field lines as the day progresses. During nighttime the two peaks coalesce into one that lies on the equator and the anomaly is confined to a limited latitude rangell 12 (± 50). In order to simulate such a model of the equatorial electron density distribution in the present calculations, the following expression for the horizontal electron density variation given by Singhl5 is introduced.... (1) Here Neo is the reference level electron density, CI. an arbitrary constant whose value depends upon the 0 '" z0...j 0:: f-uuj 'E u M '" ~~ 0:: ~ zujuz0uz 2 1 3'7 /-.....,, / / \ I ~ \ magnitude of the gradient to be applied, () the colatitude and {3a constant whose value is given by the cosine of the latitude at 120 km where the horizontal gradient becomes zero. Initially the value of C( is taken to be 13 in the latitude range 22 to 11a and then - 2 between 11 and the equator such that a profile similar to that shown by curve A in Fig. 1 is obtained. This curve represents equatorial anomaly in the latitudinal distribution of the electron density around the equator at 400 km in which the electron density peaks at 11 latitude on both sides of the equator at the geomagnetic field line corresponding to 18 geomagnetic latitude. A geocentric dipole field model is assumed for magnetic lines of force. Profile B is obtained by taking the value of CI. to be 11 in the latitude range 22 to 50 and then zero between 50 and the equator. This profile corresponds to the one observed during the period of magnetic disturbances when the trough of the anomaly is eliminated and a flat distribution is obtained between ± 50 around the equator. Incidentally, these two profiles fit well with the integrated electron content profiles between 420 km and the height of the Alouette-l satellite on quiet and disturbed days, respectively, as reported by King et al.20 and Matuura Computer Programme The differential equations used for the ray-tracing computation are similar to those given by Haselgrove23 and later on as used by Yabroff24, Kimural9, Shawhan25 and Taylor and Shawhan26. The computer programme for two-dimensional ray tracing is similar to that employed by Taylor and Shawhan 26. The programme makes use of Runge-Kutta integration technique for the calculation of first four points along the path and then the Adam Bashforth predictor corrector method is used for calculating the remaining points. The programme was run on the IBM 360/44 tomputer installed at the Delhi University, Delhi. 3. Results and Discussion GEOMAGNETIC LATITUDE, deg Fig. I~Plots showing electron density distribution of the equatorial anomaly during quiet (curve A) and magnetically distrubed (curve B) periods simulated in the computation of raypaths (The dotted curve represents the field line along which the quiet time anomaly exists.) 3.1 Propagation Characteristics of Daytime Emissions Assuming that the equatorial emissions are generated in the equatorial plane at L = 1.2 (maximum altitude at the equator being 1274 km), we computed initially, in the equatorial anomaly model A, the raypaths of these emissions for the frequencies 3.2, 5 and 9.6 khz from the generation region to the base of the F region ionosphere (:::,:120 km). It is worthwhile to mention here that Ariel 3 and 4 satellites had different frequency channels between 0.75 and 17.8 khz to observe both the transmitted signals from the ground (at 16.0 and 17.8 khz) as well as natural VLF and ELF emissions which ha ve their origins in the ionosphere. In 50

3 PRAKASH et at.: PROPAGATION CHARACTERISTICS OF EQUATORIAL VLF EMISSIONS 1500 ~ (11 t.o = _50 t.f = _173 (21t.o =_70 t.f =+ 179 (3) t.o =_90 t., =+177 (4\t.o=-1100 o t.,=+174 \ 5) tj.o =-130 tj., =+170 o " MAGNETIC LATITUDE, deg Fig. 2-Plots showing the ray-paths of the daytime equatorial emissions computed at frequency S khz from 1274km altitude in the equatorial plane in the quiet time equatorial anomaly model [Parameters Ao and AI represent the initial and final wave normal angjes; AI is taken at a height 120km. The arrows indicate the wavenormal directions of the ray-paths.] this paper we are concerned. with natural VLF emissions.observed by Ariel 3 and 4 satelijt,es in the frequency range khz only. m initial wavenormal angles Ao were chosen to'be in the range of - 50 to (Ao is the angle between the vertical and the wavenormal which is measured from the vertical, positive in clockwise direction and negative in anticlockwise direction). The computed ray-paths' for the frequency of 5 khz are shown in Fig. 2. Since the ray-paths corresponding to other frequencies indicate similar results as that of 5 khz, they are not shown in the Fig. 2. The wave transmission through the lower ionosphere and its observation on the ground requires a fmal wavenortnal at the base of the F-region ionosphere almost vertically downward (A, ~ 180 ) and lie in a transmission cone We find that the final wave normal angles of ray-paths 2 and 3 are 179 and 177, respectively, which are very close to Since the transmission cones at low latitudes are 5-6 wide30, the fmal wavenormalsof the waves corresponding to these ray-paths would lie in the transmission cones and would be observed on the ground. The waves corresponding to the ray-paths 1,4 and 5 cannot be observed on the ground because the final wavenormals are much away from the downward vertical direction and hence such waves would be accessible to the satellites only. Further, since the arrival latitudes of the majority of the ray-paths lie well within 30, the confinement of the equatorial emissions below this latitude is verified. Now _we calculate the ray-paths in the equatorial anomaly model B which corresponds to the period of magnetic disturbances. The wave frequency and the initial wavenormal angles are chosen to be the same ai above. The results show that the final wavenormal angles of ray-paths 2 and 3of Fig. 2 are now and + 178, respectively. These angles are also very nearly ±180 and hence the corresponding waves would be accessible to the ground. The final wavenormal angles of other ray-paths are slightly different from the above but they also cannot penetrate the lower ionosphere because of their large deviation from the downward vertical direction. Thus the propagation characteris tics of daytime equatorial emissions are almost similar during both the quiet and magnetically disturbed periods. 3.1 Propapti08 CIIaracterilIti of N'apttime Ii' _ During nighttime the two peaks of the equatorial anomaly coalesce into one that lies on the equator and the anomaly is confmed to a very limited range of latitude11 12 (± S ).Since the occurrence region of the equatorial emissions is much away from this anomaly region, the equatorial anomaly during nighttime may not affect the propagation characteristics of nighttime emissions. Thus we calculated the ray-paths of the nighttime emissiens in the background electron density model (Sec. 21~ The ray-paths are presented in Fig. 3.The fmal wavenormals ofthe ray-paths 1to 4 are far away from the downward vertical direction and hence the corresponding waves would not be observed on the ground. The fmal wavenormalangle ofray-path 5 (computed with 40= -130 ) is+178 and is very close to the downward vertical direction. The wavenormal corresponding to this ray-path would1ie in the transmission cone and the corresponding waves w o ~l-sc 15001"tll 60 _-so: 12IAo ( ('IAo ( ẹ. 500 ~ ~ ~ ~ t MAGNETIC LATITUDE. H. Fig. 3-PIots showing the ray-paths of the VLF emissions of frequency S khz computed from 1274 km altitude in the background density model SI

4 would be observed on the ground. Thus the nighttime equatorial emissions generated at large wavenormal angles may be observed on the ground without any influence of density gradients of the anomaly. Singh et al.!6 have also arrived at similar conclusion while discussing the propagation characteristics of quiet time low latitude VLF emissions. During disturbed periods the nighttime anomaly extends to a wide latitude range around the equator (although the peak remains intact on the equator). This produces negative horizontal density gradients in the ionization at low latitudes. Experimental evidences of such type of electron density distribution have been reported by Singh 15 and Singh et a1 16. Singh et al. 16 have computed the ray-paths of low latitude nighttime emissions employing an ionospheric model similar to that described in Sec. 2.1, which consisted of horizontal density gradients representing disturbed conditions. They have shown that the final wavenormals of the nighttime VLF emissions are tilted almost along the downward vertical direction as a result of such horizontal density gradients. On the basis of these results, they have explained the large VLF activity at low latitude ground stations during the periods of magnetic disturbances. In the light of this it may be inferred that during. the periods of magnetic disturbances the nighttime equatorial emissions are propagated under the influence of negative horizontal gradients of the equatorial anomaly and that their raypaths are similar to that of Singh et a[16. INDIAN J RADIO & SPACE PHYS, VOL 10, APRIL Intensity of Equatorial Emissions Bullough et al.: reported the intensity of the equatorial VLF emissions at 3.2 khz to be "" W m -2Hz -I at the Ariel 3 sateliite height. Earlier, Iwai et al. 31 and Ondoh and Isozaki.. reported the intensities of the ground observed equatorial VLF emissions at 5 khz to be 5.5 x to- 18 W m -2Hz- I (at Moshiri, geomag. lat., 34.3 N) and 4.4 x 1O- 19 Wm-2Hz- I (at Hiraiso, geomag. lat., 26.2 N) respectively. In order to determine the generation mechanism of these emissions, Ram Prakash et al. 6 calculated- their intensities initialiy in the equatorial plane where they are generated, and found to be 5 x W m -2Hz -I per electron (maximum intensity for too ev electrons). Then, by carrying out ray-tracing computations which' accounted for both the collisional and spreading losses along the propagation path, they determined the intensities of these emissions at the satellite height (500 km) which were found to be in good agreement with the observed intensities. The calculated intensities were also found to be in good agreement with the ground observed intensities when the appropriate losses in the lower ionosphere were included. Of course, at wave 52 frequencies much lower than the electron gyrofrequency, Landau damping varies directly with the wavenormal angle along the propagation path 33. However, in the present ray-tracing calculations, although the ray-paths are nonducted, the wavenormal angles along the propagation paths are small and losses due to Landau damping are not significant. Acknowledgement The authors are thankful to the Director, Computer Centre, Delhi University, Delhi, for giving permission to use their IBM 360/44 computer. Two of the authors (RP and SKJ) are grateful to the University Grants Commission, New Delhi, for providing financial support in the form of junior and senior research fellowships, respectively. The authors also wish to thank Shri D P Singh and Shri Had Singh for the valuable discussions they had with them during the course of this study. References I Bullough K, Hughes A R W & Kaiser T R, Magnetospheric Physics, edited by B M Mc Cormac (0 Reidel Publishing Co. Ltd, Oordrecht, Holland), 1974, Bullough K, Hughes A R W & Kaiser T R, Proc R Soc London Ser A (GB), 311 (1969) Eccles 0 & King J W, Proc IEEE (GB), 57 (1969) Hayakawa M & Tanaka Y, Replonos&Space ResJpnl/apan),27 (1973) Hayakawa M & Iwai A, J Atmos & Terr Phys (GB), 37 (1975) Ram Prakash, Singh 0 P & Singh B, Planet & Space Sci(GB), 27 (1979) Smith R L, Helliwell R A & YabrofTI, J Geophys Res (USA), 65 (1960) Angerami J J, J Geophys Res (USA), 75 (1970) Matuura N, Space sn Rev (Netherlands), 13 (1972) Raghavarao R & Sivaraman M R,J Atmos & Terr Phys (GB), 35 (1973) II Appleton E V, J Atmos & Terr Phys (GB), S (1954) Rastogi R G, J Geophys Res (USA), 64 (1959) Aubrey M P, J Atmos & Terr Phys (GB), 30 (1968) Fijalkow E, Altman C & Cory H,J Atmos &. Terr Phys (GB), 3S (1973) Singh B, J Geophys Res (USA), 81 (1976) Singh 0 P, Jain S K & Singh B, Ann Geophys (France), 34 (1978) Singh 0 P & Singh B, Ann Geophys (France), 34 (197S) 113. IS Angerami J J & Thomas J 0, Tech rep no , Stanford University, Stanford, California, Kimura I, Radio Sci (USA), 1 (1966) Kingh J W, Reed K C, Olatunji EO & Legg A J,J Atmos & Terr Phys (GB), 29 (1967) Eccles 0 & King J W, Proc R Soc London Ser A (GB), 281 (1964) Lockwood G E K & Nelms G L,J Atmos & Terr Phys (GB), 26 (1964) Heselgrove J, Reports of conference on the r'lysics of the ionosphere, Physical Society, London, 1955, YabrofTI, J Res Natl Bur Stand (USA), 65D (1961) Shawhan S 0, Res rep No , Department of Physics and Astronomy, University of Iowa, Iowa, 1966.

5 PRAKASH et al.: PROPAGATION CHARACTERISTICS OF EQUATORIAL VLF EMISSIONS 26 Taylor W W L & Shawhan S D,J Geophys Res (USA), 79 (1974) Maeda K & Oya H, J Geomagn & Geoelectr Vapan), 14 (1963) IS!. 28 Helliwell R A, Whistlers and related ionospheric phenomena (Stanford University Press, Stanford), Scarabucci R R, Tech rep no (Stanford Electronic Laboratories, Stanford, California), Jain S K, Singh D P & Singh 8, Agra Univ J Res (Science) (India), 27 (1978) r. 31 Iwai A, Ohtsu J & Tanaka Y, Proc Res lnst Atmos, Nagoya Univ Vapan), 11 (1964) Ondoh T & Isozaki S, Radio Res Lab Uapan), 15 (1968) Kennel C F & Thorne R M,J Geophys Res (USA), 72 (1%7)