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Propagation characteristics of low latitude whistler duct characteristics have been investigated based on day-time measurements at Jammu. The morphogical characteristics of low latitude whistlers are discussed and compared with characteristics of middle and high latitude whistlers. The Max. electron density (N_{m}) at the height of the ionosphere obtained from whistler dispersion comes out to be higher than that of the background which is in accordance with the characteristics of whistler duct. The equivalent width is found to be close to the satellite observations and the characteristics of whistler duct in low latitude ionosphere are similar to those in middle and high latitude ionosphere. The width of ducts estimated from the diffuseness of the whistler track observed during magnetic storm is found to lie in the range of 50 - 200 Km.

Ground based whistlers are known to generally attribute to propagate trapped in field aligned whistler ducts at high and middle latitudes [1,2] and also lower latitudes [3,4]. Whistlers which are VLF electromagnetic signals, travel through the ionosphere-magnetosphere coupled system along the geomagnetic field lines to the magnetically conjugate points in the opposite hemisphere have become a very important tool for probing the plasma sphere and beyond. During their propagation through the magnetosphere these whistler waves acquire dispersion characteristic typical of the electron density inhomogenieties present along the whistler path. The physical features and propagation characteristics of low latitude whistlers are the important and interesting subjects in the work of atmospheric research [

Ray tracing and intensity studies by Tanaka and Hayakawa [

The first direct evidence for the existence of whistler ducts comes from in situ observations on board OGOI [

The recording of whistlers at our low latitude ground station Jammu was started in the month of December, 1996 on routine basis. At low latitude, the whistler occurrence rate is low and sporadic. But once it occurs, its occurrence rate becomes comparable to that of mid latitudes [

For spectral analysis the normal procedures of analysis by means of sonographic equipment was used. Detailed spectrum analysis has been done for the day time whistlers on February 14, 1998 in order to study the propagation characteristics of these whistlers. From the detailed analysis, it is found that the measured values of recorded whistlers on this day have dispersion of order of 38 sec^{1/2}. The recorded whistlers observed on February 14, 1998 is shown in

The tweak trace explains the position of whistler source from the observatory is given by the equation.

where ∆t is the time delay of frequency f = 1.16f_{c} from the front of the tweak here f_{c} = 1.70K Hz is the lower cut-off frequency of the tweak and c is the velocity of light in vacuum. Substituting the values of ∆t in Equation (1) we obtain d = 4650 Km. The distance between the stations and their magnetic conjugate points is about 4745 Km for Jammu. The closeness of the two values of the distance can be taken as an indirect support of the ducted propagation of day-time observed whistlers.

The Max. density of electrons in F-region is calculated from the whistler data and ionospheric sounding. For the quasi longitudinal propagation of VLF electromagnetic waves, the dispersion is related to the parameter of the medium [

Here f_{p} f_{H} are the plasma frequency and electron gyro frequency respectively.

We consider that the configuration of the earth’s magnetic field is similar to that of magnetic dip-dipole, for simplicity. Using the equation of magnetic line, we obtain that the Max. height h_{T} of magnetic field line passing through Jammu, is 1158 Km. An ionospheric model with parabolic distribution of electron density is adopted i.e.

where

Here h_{m} is the height of Max. electron density of ionosphere, N_{m} the Max. electron density at h_{m}, Y_{m}, the half thickness of the parabolic ionospheric model and h_{o} the height of the lower boundary of the ionosphere.

From Equations (2) and (3), we obtain

where B = geomagnetic induction

where R_{E} = 6370 Km, the average radius of earth = 24.33˚N, the^{ }magnetic dip latitude of station. By inserting D = 38 sec^{l/2}, h_{m} = 283.8 Km, h_{m} = 150 Km, Y_{m} = 133.9 Km and B = 0.38 Guass. We get N_{m} = 14O × IO^{5} cm^{−3} for Jammu. In comparison with N_{m} = 0.99 × 10 s cm^{−3} obtained directly from data of ionospheric sounding. The value obtained of N_{m} from whistler observations to the result is larger by a factor of about one. Thus the electron density along the propagational path of whistler is higher in case of Jammu, which is the characteristic of the whistler duct observed at day-time.

From the width of the trace, the equivalent width of the whistler duct at Max. height of its path can be calculated. By using the definition of dispersion and the Equations (2) and (3) we have

where ∆S_{O} is the difference of the equal vent path length and ∆b_{m} is the trace width of the whistler. From the width of the trace at frequency f obtained from the sonograms. ∆S_{O} is calculated, Knowing the latitude of whistler propagation, it is then possible to calculate the effective width of the ducts by using the calculated value of ∆b_{m} from the sonogram. It is found that equal vent width of whistler duct at Max. height of its propagational path is approximately 30 Km estimated from satellite observations (Park, 1980). In some case the width of duct turns out to be approximately 200 Km. This result is in reasonable agreement with that reported by Somayajulu and Tantry (1968) and Okuzawa et al. (1971).

The estimation of duct width by using the low latitude whistlers and Satellite observations has been discussed by many workers. In some cases the value is of the order of 80 Km from Satellite observation [21,22]. In some cases its thickness turns out to be approximately 200 Km for stormy days. On quite days the whistler traces are so sharp that the determination of duct width is likely to be in error, because of incoherent limitations in time resolution. Here we have determined the duct width for normal days of day-time duct and it comes out to be 10 - 25 Km which is in good agreement with the results of Somayajulu and Tantry and Okuzawa [6,17].

Hence it has been concluded that the morphological characteristics of low latitude whistlers are same as middle and high latitude whistler. The position of whistler source can be derived from tweak received from Jammu. The Max. electron density of F2-region can be determined from whistler data and ionospheric sounding. The N_{m} value is obtained higher from the whistler dispersion than the background which is the characteristic of the whistler. The equivalent width of whistler duct at the Max. height of its path is found to be very close to the value obtained from satellite observations. It may be shown indirectly that the characteristics of whistler duct in low latitude ionosphere. Duct width estimated from the diffuseness of the whistler traces observed during magnetic storm is found to be in the range of 50 - 200 Km.

The author is grateful to Director, N I T, Srinagar-190006. Kashmir for his encouragement and keen interest in publishing this research work