Drift & Anisotropy Parameters in Sporadic-E & Normal-E Layers"

Transcription

1 ndian Journal of Radio Space Physics Vol. 5, December 1976, pp Drift & Anisotropy Parameters in Sporadic-E & Normal-E Layers" S P MANOHAR RAO & B RAMACHANDRA RAO Department of Physics, An dhra University, Waltair Received 5 April 1975; revised received 9 September 1975 Measurements of drift motions in the ionosphere have been made at Waltarr at the E-region level using the spaced receiver technique for the sporadic-e and norrnal-e layers. The records from this experiment ere analyzed by the correlation method. The drift directions and speeds ere obtained as ell as the characteristic velocities, the orientations of the correlation ellipses, their ellipticities and sizes. The results indicate the drift to be toards north-east for sporadic-e and to the south-est for norrnal-e during daytime. At night, the drifts for both the layers have the same trend of variation. A comparative study of the drift parameters for these to layers revealed them to be greater for sporadic-e. An attempt has been made to explain these results. 1. ntroduction A familiar technique for determining motions in the ionosphere uses fading records of pulsed vertically reflected radio signals received on three spaced receivers. Fading records of the signals reflected from the Es layer ere taken at Waltair during the summer months of 1964 and 1965, a period of minimum solar activity. Observations ere confined to the summer months only because E, is most active during these months at this latitude, hile during other seasons its occurrence is infrequent. Records ere taken on a frequency of 5'6 MHz for E, and on 2'0 MHz for normal-e layers, respectively. That the observations refer to E, layers is identified by manually scanning the frequency and observing the absence of group retardation. All observations above the maximum value of /0 E naturally refer to E s- Belo fo E the distinction beteen the to layers could be easily made by observing manually p'f records, and by the type of fading. The E, layer at Waltair is characterized by fast fading during daytime. A comparison is made beteen the irregularities of these to layers. 2. Observation and Analysis Records ere taken for short time intervals of 3-4 min and analyzed by taking amplitude values at intervals of e 3 8 sec. Amplitude sampling at intervals as short as O' 318 see has been accomplished by dividing the length of the amplitude record by the total number of ordinates read (260 in this case). Scaling the amplitudes at closer intervals did not enhance the accuracy of the estimate of the correla- Paper presented at the Symposium on Earth's Near Space Environment, February 1975, held at National Physical Laboratory, Ne Delhi tion coefficient, hereas scaling at ider intervals increased the variance of the distribution. No appreciable differences ere observed hen the scaling intervals ere altered. The 260 observations of amplitudes thus obtained are fed to a CDC 3600 Fortran computer to obtain the auto and cross-correlation coefficients. The drift and anisotropy parameters have been determined by the correlation method of Phillips and Spencer! together ith the modifications of Sales" and Fooks." Tn presenting the results the data for the to years, viz and 1965, are taken together to yield combined results for the entire period, as no significant difference as noticed in the variation of these parameters from 1964 to Results 3.1 Velocities The diurnal variations of the apparent drift velocity Va, the true drift velocity V, the critical velocity Vc, and the ratio (Ve/V), for E. and normal-e are shon in Fig. 1. All the four parameters present higher magnitudes, in general, for Es at all times of the day. Both the layers present three maxima for the complete day, the noon maximum being the most prominent. There is not much variation in the random motions in normal-e as can be seen from the curves of Fig. 1 (C) and CD), hile on the other hand the E. reflections sho marked variations in both these figures, hich in turn mean that the drift in E, has relatively large random motions than the usual steady drift, as in the case of normal-e. The high magnitude of Vel V is itself a piece of evidence for the randomness of the drift motions that take place in Es.

2 RAO & RAO : Es & NORMAL-E DRFf & ANSOTROPY PARAMETERS u 280f {'OOf >0120[ 40L ~ ~-L 200~ ~ < E ~»: 'Of L- -L2- ~ -J J 0 -Es X-.-x E 360 ix 300 '. Z '0 200 Ol Ql "0 -s- ~ (8) -- f) u 160 G OL -L ~ ~ ~ ~ ~ 80 z '0 Ol Ql "0 -e TME,hrs LT Fig. 2-Diurnal variations of (A) ~J (apparent drift tion) and (B) ; (true drift direction) (A) direc- Fig. -Diurnal variations of Va, V, Ve, and VelV 3.2 Drift Direction. The diurnal variations of the apparent drift direction q,a and the true drift direction q, for the sporadic-e and norrnal-e layers are shon in Fig. 2. The normal-e layer presents peak values of both the apparent and true drift directions at 1000 and 1700 hrs ith a minimum in the early afternoon hours (around 1400 hrs). At these very same hours, the true drift directions for E, present minima. A common feature observed in the diurnal variations of the apparent and true drift directions for E, is the afternoon minimum u~ ~ -100 S E ;- 100 t: uọ... UJ 60 > 20 i... "'- x"-.ị Ị ~TME)hrs 3.3 Harmonic Analysis of the True Drift Speeds The diurnal variations of the NS and EW components of the true drift velocity for Es and normal-e layers are presented in Fig. 3. The NS component has the same general trend throughout the day for both layers ith the exception of a time lag in the direction of reversals. This velocity component tends to the north for a longer period during the day for Fig. 3-Diurnal variation of the NS component (A) and EW component (8) of the true drift velocity 273

3 NDAN J. RADO SPACE PHYS., VOL. 5, DECEMBER 1976 Ea. At night, the variation is essentially the same in both cases. On the other hand, the EW component for E, is predominantly to the east during hrs, hile it is to the est for normal-e during the same period. The variation follos the same pattern during the night for both layers until they split out in opposite directions around sunrise. The post midnight maxima occur at the same time (0200 hrs) for both layers hile the pre-midnight maxima have a small time lag. The harmonic analysis of the NS and EW components of the true drift speed for both layers is presented in Table. A study of Table reveals that the NS component of the true drift speeds, in both cases, has the semidiurnal component dominating over the diurnal Table 1- Results of Harmonic Analysis of the NS and EW Components Layer Direc- Steady Diurnal Semi-diurnal tion compo- component component nent E. NS sin (1+267 ) sin (2t+ 76 ) EW sin (,+255 ) sin (2/+337 ) E NS sin (t+291 ) sin (2t+225 ) EW sin (/+255 ) sin (2t+25) component. The steady NS component is smaller than the amplitudes of the diurnal and semi-diurnal components for E s, hereas the amplitude of the NS diurnal component is smaller than the amplitude of the semi-diurnal component and the steady NS component, for normal-e. The steady EW components for both layers are directed toards the east. There is a major difference observed hen the EW components of the drift speeds are compared for the to layers. For E" the semi-diurnal component predominates over the diurnal component hile the diurnal component dominates over the semi-diurnal component, in the case of normal-e. The amplitude of the semi-diurnal component is by far greater than the diurnal and the steady EW components for Es, hile the steady EW component for normal-e is greater than the amplitude of both the diurnal and semi-diurnal components. For E s, the phase difference beteen the NS and EW diurnal components is very small (about 12 deg) indicating the diurnal vector to be in a clockise sense. This can be clearly noticed in the polar diagram shon in Fig. 4. The high degree of ellipticity of the polar plot of the diurnal drift vector for Es is due to this small difference in the phase angle beteen the NS and EW diurnal components. The corresponding phase difference for normal-e is quite large, but the diurnal drift vector also rotates in a elockise sense. -ES ----E N N s STEADV COMPONENT m/sec o L- '...1 s DURNAL COMPONENT SEM-DURNAL C.OMPONENT 274 Fig. 4-Polar plots of harmonic components of drifts

4 RAO & RAO : s, & NORMAL-E DRFT & ANSOTROPY PARAMETERS The phase difference beteen the NS components of the to layers is quite large. This led to the interesting result, viz. the NS semi-diurnal vector for normal-e rotates in an anticlockise sense hile the corresponding E, drift vector rotates in a clockise sense. The same result can also be obtained from the diurnal variation curves hich sho that the overall drift speeds for E, are directed north-east hile they are to the south-est for normal-e. The phase difference in the EW semi-diurnal components is very large, and as a result the semi-diurnal drift vectors of the to layers rotate in opposite directions, ith that of normal-e in an anticlockise sense. This result is further supported by the diurnal curves hich revealed the drift in the to layers to be complimentary, especially during daytime. 3.4 Parameters of the Characteristic Ellipse The diurnal variation of the length a of the semimajor axis of the characteristic ellipse (taken as a measure of the size of the ground diffraction pattern), the axial ratio r and the orientation ~ of the characteristic ellipse are shon in Fig. 5. The diurnal variation of a reveals greater magnitudes for sporadic-e from a little after 0800 hrs till sunset. An interesting feature seen in Fig. 5 is the greater magnitude of normal-e irregularities during the early morning hours ( hrs). The diurnal variation of r shos high values for Es during us hile those of normal-e are fairly constant during this period. No significant variation is seen in this parameter for both layers at night. The diurnal variation of t presents lo values for E, during hrs, hile high magnitudes are possessed by normal-e during the same period. At night, the magnitudes of'~ are greater for Es. 4. Discussion Whitehead" proposed a theory for the formation of sporadic-e from vertical gradients in horizontal inds. This as considered in terms of gravity aves by Hines." The neutral ind motion directed toards the east at a particular level ill set the positive ions into motion toards the east by collisional interaction. But because of the earth's magnetic field the positive ions tend to move upards. An overlying oppositely directed ind results in an opposite motion of the positive ions, and thus ionization is accumulated in beteen these to levels. The resultant electric field tends to get the electrons follo the positive ions. n the ground-based experiment, the bottom part of the Es layer reflects the signal, and hence it can be taken that the drift velocity and direction obtained correspond to the bottom of the Es layer. The eastard sense of the EW component 300 E ~ 200 c 200 (il).... x 0,x l( " 'x-x Z, 'x X /, ". Cl 100 'x < i. u. 'S oj( ' o~ ~ ~ ~ ~ ~ ~ OB TME, hrslt Fig. 5-Diurnal variations of a, y and r of E, drifts observed in this investigation, therefore, agrees ith the theoretical agreement for the formation of sporadic-e. t may be noted that the essential process considered in the above ind shear theory for the accumulation of E, ionization is a vertical transport of ionization hich is caused by the horizontal movement of ionization along the earth's magnetic fielcl. 5. Conclusions (i) Drifts in E, are mainly directed to the northeast during the day hile they are to the south-est for normal-e. (ii) The fading observed in E, is inferred to be mainly due to random changes of the patterns. (iii) The behaviour exhibited by the parameters of the characteristic ellipse proves that the sporadic- E behaviour is different from that of normal-e. (iv) Results of harmonic analysis indicate the steady NS component to be oppositely directed for the to layers (south for Es and north for normal-e). The steady EW component is directed to the east for both layers. The NS semi-diurnal component dominates over the NS diurnal component for 275

5 NDAN J. RADO SPACE PHYS., VOL. 5, DECEMBER 1976 both layers, but the EW semi-diurnal component of E' dominates over its diurnal component, hile the EW diurnal component of the normal-e layer dominates over its semi-diurnal component. (v) A comparison of all the drift parameters of the to layers revealed them to be greater for sporadic-e. Acknoledgement One of the authors (S P M R) is thankful to the Council of Scientific and ndustrial Research, Ne Delhi, for financial assistance. References 1. Phillips G J & Spencer M, Proc, phys, Sac., 68 (1955), Sales G S, Penn. State. Univ. Rep., No. 131, Fooks G F, J. atmos. terr.phys.; 27 (1965), Whitehead J D, J. atmos. terr. Phys., 20 (1961), 49; 24 (1962), Hines C 0, J. atmos. terr. Phys., 2S (1963),