Post-sunrise Equatorial Spread-F & Radio Scintillations - PDF Document

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  1. Indian Journal of Radio & Space Physics Vol. 12, December 1983, pp. 184-189 Post-sunrise Equatorial Spread-F & Radio Scintillations R G RASTOGI Indian Institute of Geomagnetism, Colaba, Bombay 400005 12 April Received 1983 Experimental evidence isshown of the continuation and even intensification of the F-region irregularities after the sunrise 011 the basis of data from HF ionosonde, multifrequency radio scintillations and VHF backscatter radar at equatorial region. The data confirm that the F-region irregularities can coexist at the topside regions even during the presence of conducting E- region at low altitudes. These events seem to be associated with the early morning counter-electrojet events. scintillation scintillation paper phenomenon at Huancayo. records activity well after the sunrise. The present describes this post-sunrise in relation to the ionospheric revealed a few cases of 1 Introduction The scintillations satellite received at ground are known to be strong and very frequent during the nighttime ±25 dip latitude.' -] Comparing scintillation records ofATS-6 and 360 MHz with the Rastogi" has shown that it is the incidence of the range type of spread-F with multiple on the ionogram which are associated scintillations exceeding fluctuations scintillation index (SI). The frequency does not produce scintillations = 20~,o. Rastogi" has described scintillations of ATS-6 as received at Huancayo during 1974-75. The scintillations were maximum (median Sf = 60/.,) a couple before noon and at about 2200 hrs LT with median SI = 38~/;,. The 140 MHz radio lations of about 10% or (l dB) around about 351'0 or (4 dB) around scintillation of 360 MHz phenomenon with maximum 2300 hrs LT. In Fig. 1are shown the daily variations the mean SI as well as the percentage exceeded the levels of 1O~;" 20/., and 140 MHz beacons from ATS-6 received at Huancayo. As seen from the diagram the average had minima at about 1800hrs LT. At the morning exceeded 10~~ level on occurrence of SI >201'0 reduced to less than I% of time by 0800 hrs LT and SI > 30% reduced 11'0of time by 0700 hrs LT. Thus on the average could assume that scintillation around 2200-2300 hrs LT had after sunrise. This is also general acceptance that the spread-F disappears after sunrise. A closer examination of radio waves from an artificial scintillation conditions hours over a belt of the amplitude beacons on 41, 137, 140 ionograms at 2 Data and Results In Fig. 2 are shown the variations index of ATS-6 (137 and 360 M Hz) beacons vertical incidence records at Huancayo 7-8 Nov. 1974. The suddenly at 0200 hrs UT (2100 hrs LT) and reached the saturation level in less than half an hour. The activity Huancayo, of the scintillation and the layers of strong echoes with strong of 20 dB or IOO~~ type ofspread-F exceeding 2 dB or SI the variations on the night of activity scintillation started of ~ 5O'-'--'--'--'-'--'--r-'r-.--.--~ ~ 30 ~ 20 c (75'W) waves of hours of 41 MHz co o c u V\ waves suffered midday 2300 hrs scintil- and of LT. a nighttime The was basically median SI of 25% around of SI of time 30% for the the scintillation activity on .. c '" v•... '"a.. 0800 hrs LT and at minimum the scintillation about 7% of 4 time. The 2 1 00 19 to frequency of 06 01 12 07 18 13 UT LT one activity disappeared in accordance which peaked Fig. I-Daily percen! OCcurrenceof SI exceeding the levelsof 10"n' :!O"n and 30 "of the 140MHz radio beacon from satelliteATS- 6 received at Huancayo during the period Dee. 1974-Ma\ 1975 variations of the mean SI as well as the shortly with the phenomenon of the . 184

  2. & RADIO RASTOGI: POST-SUNRISE EQUATORIAL SPREAD-F SCINTILLATIONS contrary partially ionograms. electrojet other electric sunrise had not occurred at 0700 hrs LT a strong blanketing This suggests the presence of the counter- conditions during post-sunrise words, the normal reversal field towards the eastward on this day, till 0730 hrs LT. blanketing was recorded Es-layer on the started reached LT). These features spread-F The scintillations the lowest slowly after 0900 hrs UT (0400 hrs LT) and reached a level of 40° o shortly before and scintillations disappeared examination of the ionogram the presence of frequency spread with some indication oflayering within the spread region, giving rise to weak scintillations. At 0515 hrs LT, the formation ionization in the lower F-region group retardation at the lower frequencies. LT clear retardations on 0 and X components start of the F-trace were continued at higher frequencies. the Es-layer traces below the F-layer group retardation at frequencies critical frequencies were also evident and the spread-F echoes were still present above the new F-Iayer. spread continued at frequencies critical frequencies. It is interesting up to 0730 hrs no Es-q layer was indicated declining a minimum at 0400 hrsUT level by 0700 hrs UT (0200 hrs are the normal induced scintillations of 137M Hz beacon value of almost zero (2300 hrs LT) and the F-trace characteristics at post-evening after reaching started of hours. period or in of the equatorial direction increasing after 1200 hrs U'T (0700 hrs LT) by 0800 hrs LT. The at 0400 hrs LT indicated :: 10 0 ,..-.,.......--...,....--~-------------, ; ~ 10 .... c .2 137MHz of new by the ~;: c 'u 11'1 was indicated At 0530 hrs at the 00 03 06 09 01 04 12 I~ 07 10 13 18 21nrs UT 16 nrs LT seen, but the spread-F At 0545 hrs LT even were recorded, close to F-region The close to the F-region to note that even and on the 100~--~r7--------------------------~ AT5-6 beacons ••80 .60 • .,s c .2 ~ C ;:; II> 40 ..J!'': ' . - 20 ~"~ ~~Sl --- - '_. -_._+4-+-1-+-if+t - -+r-"R'+-f--+-:-++ 21 hrs UT 16hrs LT 15 10 18 13 12 07 00 03 06 01 09 04 - , - 0545 I;E~ QQQQ.·+---t-I--t-++++t-H 0600 . - *~citw onOl T . E~i?l!-~- . ; -- 6 7 8104Hz 2 '5 11 iii I f<- 'B3ffj I 0700 0700 ' km ~----~-+--·~~·~tTTH 400~----~--4--t~~~,i~~H1 I-------t-----~ 20C __ - " -l::::~ =r=: ,.- ! ! ~~ ) 00 _'-'-:1= 200 ---+-- ~ .. '00 - ---t-----! a t 2 .o5}O.LT .'-~' , ." .,.'--f-J II I -1--- O~----~2--~~4~~6"'~M~Hz ,. '-.- )' ~+ ~ IL 5 ; . , I ) 6 7 B lC Fig. 3-Daily ATS-6 ionograms variation and Huancayo 23 Nov. of scintillation 360 MHz) on 1974 index of and of (137 beacons the the 22- Fig. 2- 360 MHz beacons ionograms at Huancayo Variation of received the scintillation at Huancayo on the night of 7-1l Nov. index and the vertical of ATS-6 137 and incidence 1974 at night 185

  3. INulAN J RADIO & SPACE PHYS. VOL. 12,DECEMBER 1983 In Fig. 3 are presented another example of post- sunrise scintillations, showing the temporal variation of the scintillation index of 137and 360 MHz beacons from ATS-6 at Huancayo , and the ionograms Huancayo corresponding 23 Nov. 1974. After the usual evening activity of scintillations. and later disappearance of scintillations by 0400 hrs LT. there was a resurgence of scintillations continued up to 0800 hrs LT. The ionograms indicated strong spread-F at 0445 hrs LT. Further, the evidence of fresh ionization at lower frequencies and the spread- F at frequencies beyond the critical frequencies were seen at 0515 hrs LT. At 0530 hrs LT even E21ayer was clearly seen suggesting ionization below the F-Iayer having been generated following sunrise. The spread-F was clearly seen above the F-region up to 0600 hrs LT. The continuation of scintillations suggest that the spread-F did exist on the topside of the F-region but was hidden to ground station due to the uniform bottoms ide F-region ionizations. It is also to be noted that even in this case clear E-Iayer traces were seen (0700 hrs LT) without any indication suggesting absence of eastward electric field in the ionosphere. In Fig. 4 are reproduced some of the ionograms at Huancayo on II Dec. 1974showing the continuation of spread-F, even after sunrise, which had produced fresh ionization in the F-Iayer. It is to be noted that at 0630 hrs LT there were clear reflections with no Es-q layer. In Fig. 5are reproduced some sample recordings of the amplitude of 137MHz radio wave from ATS-6 to Huancayo as well as the variation of the Sls of 137 and 360 MHz beacons on II Dec. 1974. It is seen that the SIs at both 137 and at on to the same period E and E2-layer 0515LTI "1 t 0530- ~ , I 0545 I "I'" ,l,cL ,Li.J . .~ of Es-Iayer ~~T TTTTTT ~- = ~ 0615 IAI. "I' , 1\ 1'1.1" 0630 1\ I'" 0645 '"' , .r 0530 .. ~ 0700 0715 . o t 0745 dB 0600 8 -1' minl_ 100.-~------------------------, . :: 80 I " I .2 40 .. I .. c 20 II i 'u en lL_._i1. I 00 03 06 01 ATS-6 beacons -' 1 I )( ~ c- c 137 MHz 60 I km 400 0630 j JI ." 'II~ ~!: ~~ ,~ ~ ':: ". I - 0 ~ ----~ 360MHz 200 I ~. " ''''•...•. 09 04 - _1.._ •.. .J_~_ .•_1_ .••__ .J 15 18 10 13 ' ' I I o I 21hrsUl 16hrsLT 12 07 2 3456789 y (MHz) 1 Fr.qu.nc Fig. 5 ATS-6 to Huancayo and some sample recordings of the amplitude of 137 MHz radio wave on II Dec. 1974 Variation of SIs of 137 and 360 MHz radio waves from Fig.4-lonograms continuation ofspread-F even after the sunrise effect in the F-region at Huancayo on 11 Dec. 1974 showing the 186

  4. RASTOGI: POST-SUb/RISE EQUATORIAL SPREAD-F & RADIO SCINTILLATIONS LT, F-Iayer higher frequencies layer reflections frequencies The spread-F 0730 hrs LT. It is to be noted that no reflections was very clear only. At 0545 hrs LT even the E2- were recorded with close to critical frequencies can be seen even on the ionogram trace with spread-F at 360 MHz had decreased night to a very low value by 0500 hrs LT and there was a spurt of activity reaching a peak around The amplitude records of 137 M Hz beacon very little fluctuations at 0515 hrs LT which later grew stronger and at 0615 hrs LT the fluctuations strong as 6-8 dB together imposed on it. This indicates continuation of the nighttime that was there, but irregularities the sunrise on some special condition associated with the absence irregularities in the lower E-region We now describe the event on the night of 5-6 Nov. 1974. In Fig. 6 is shown the f-plot at Huancayo night of 5-6 Nov. 1974. It is seen that the range type of spread-F had started at transformed into frequency 0300 hrs LT the range regenerated and spreadiness frequencies was present even up to 0800 hrs LT. It is also to be noted that the q-type of Es had appeared only after 0800 hrs LT. In Fig. 7 are reproduced Huancayo as well as the variations and 360MHz radio waves from ATS-6 to Huancayo 5-6 Nov. 1974. It is seen that the ionograms 0300 and 0400 hrs LT show spread-F: the group retardation clearly indicated and the scattered virtual heights greater than ionogram at 0430 hrs LT showed some layers of strong scatter suggesting the generation irregularities in the F-region. sunrise effect was first seen on the F-Iayer. At 0530 hrs from their high values in the 0630 hrs LT. showed spread-F of the F-Iayer. at at were as from with that it is not only the F-region irregularities were intensified which could be of normal at the same time. rapid fluctuations 100 ,--------------------, ATS-6 be-aeons •• . •. " c ''2 " c " '" II I~" I I ~, I ~ \ :' , I I I , , 60 ,,, after .=. 137MHz ' 60 I I n I 'I " I, I, /1 / "II'l I Es-q 40 , 360MHz ~: ',I \ \ 20 for the \ , , ·1\LI_"'-1~ 12 07 I 1900 type by midnight type spread-F near hrs LT which but at again critical 21hrs UT is hrs LT 00 03 09 04 15 10 18 13 ~~~20;0;l~T;;~:t=t:t=!i mil was the some ionograms of the Sis of 137 at ~~ ~~~iRB ~ on for 0200, type of the frequency of the F-trace signals that of was ..','~~ were at h~linF. The of range At 0515 hrs spread-F LT the ,. _;:.-.' -- '. .<. "" Ir 070o--'-I---'--:'-['TTl - i I - i - ---=-'i u, Ii-;- Q3L __ : __ :: '=._' _~_==r:::=:_ = - .~- l~ :-----~T+-~ i ~---r--- -"-, .'. .~-.+-- irtr "',"~ :' t T .ill ! j-:-:J i- t:-- ~ - . - +---- -_ .• --, 1 TrTJ km Q~IL:Lll 400-~-~ T i .-- 200-- ----- 10 12 6 6 10MHz Frequ~ncy 2 3 4 6 8 Fig. 7-Variation 6 to Huancayo on the night of 5-6 Nov. 1974 and some of the corresponding ionograms at Huancayo of the SIs of 137and 360MHz beacons from ATS- 0500hrs 0530 0600 0630 0700 0730 0800 l krn 600= -""'liI-.- .•..._---- •....•... r- 400~_.!-- 200- - 0"' •••• _ Amplitude- Fig. 8-- Modified range time intensity profiles of the 50 MHz beackscatter echoes of Jicamarca Radar on the morning of 6 Nov. 1974 187

  5. J RADIO VOL. 12,DECEMBER 1983 & SPACE INDIAN PHYS. obtain backscatter echoes at intervals of each 30 see. In the diagram only selected profiles at every 30 min are reproduced from 0500 to 0800 hrs LT. At 0500 hrs LT there were backscatter echoes between 280and 400 km altitude; the height of the base ofF-layer was about 280 the altitude profile of the intensity of 'the q-type of Es layer were recorded on this day. The scintillation indices were almost zero around 0200- 0300 hrs LT and had increased to high values after 0400 hrs LT. The scintillations had continued up to about 0800 LT. In Fig. 8 are reproduced some modified range time intensity (MRTI) records echoes by Jicamarca radar on the morning of 6 Nov. 1942. By attenuating the intensity of the backscatter echoes linearly in dB from 0 to 70 dB in 30 sec one can of 50 MHz backscatter ~100 0 x CIJ '0 .£ c 0 +- IV.- +-c2.0 \oJ (/) 80 60 , I . I I , I II . I I I I I' • -o--r--r- 0315 40 '-'" frt-itt=ntr r iI I I I.TIT °00 0315:_ ::_, -; ~ ~--Lr---'-t-' •..'" I~ I , 0400 I I ,... , Il1o I I +I I I .. 0500 _. I,,", I 0800 1\ 1/1' , ~ v ..•~ ...' , 'IV '1/ r I I I I J + " \081S·aJI I I 0530 " I I I I ~f.- 0830 '"' ~,,-- 1084S=ooEi 0945 LT , 0630 I - 1 i n - - I , I 0700 w. "" , -- 1\0 11'1 •......' - - I, 'J. , I , I I I +-+- km 300 20 10 ::>--'i "1+'-' --1 \ I d!3 o 4 0730LT '1. R' 0 V1'\o h- :~ """11~t - I I 7 ~ H-r I I o1 I ........-4 lmin I ! 8 ! I I I I I 2 3 4 5 9 MHz Fig. IO-Variations beacon at Huancayo on II Mar. 1'J15 and some of the ionograms at Huancayo corresponding to the morning scintillation event of the scintillation index of ATS-t> (137 MHz) Fig. 9-Some beacon from ATS-6 received at Huancayo on 6 Nov. 1974 sample records of the amplitude of 137 MHz radio 188

  6. & RADIO RASTOGI: POST-SUNRISE EQUATORIAL SPREAD-F SCINTILLATIONS the conclusion region of the ionosphere short circuiting developed after the sunrise. The present that F-region irregularities sunrise when both the FI, F2 and E layers are fully developed. Indications are also there that these events of the occurrence of post-sunrise associated with the early morning events when the horizontal region is westward during regions. It is recommended of the topside irregularities equatorial ionosphere during the post-sunrise It is already known that period of strong scintillation the range type of spread-F. period, viz. 0300-0600 hrs again may be strong on some occasions. data show still another period shortly after the sunrise, i.e. after the F-layer has generated ionizations, when intensified. The irregularities altitude of the F-layer peak topside of the F-Iayer. These irregularities associated with the early morning suggesting that the presence field at the equator after sunrise helps the continuation of topside F-region irregularities. that any irregularities are smoothened by the present in the F- out due to the conductivities paper shows after the krn and the Huancayo indicate thin layers at strong region. At 0530 hrs LT strong backscatter recorded at altitudes and between 500 and 600 km. These definitely confirm the regeneration of the irregularities at heights 0630 hrs LT echoes were stronger echoes at 350-450 km altitudes. LT echo altitude decreased LT further to 250 km altitude. This further suggests the downward movement of the irregularities, due to the westward electric field as supported complete absence of the q-type of Es irregularities. In Fig. 9 are reproduced recordings of ATS-6 (137 MHz) Huancayo on 6 Nov. 1974. At 0315 hrs LT the signals were fairly steady, and fluctuations of the amplitude LT the signal fluctua tions were deeper but still slow. At 0400 hrs LT the signal fluctuations some fast fadings were also present. 0600 hrs LT, showing fluctuations, was very similar scintillation records due to range These correspond to the period when Jicamarca records showed strong scattering but no range spread was seen in the bottomside ionosphere sounded by periods the fluctuations decreased became normal by 0730 hrs LT. In Fig. 10 are shown scintillation index of ATS-6 (137 M Hz) beacon Huancayo on II Mar. corresponding ionograms the usual nighttime scintillation to a very low level (20'%,)by 0600 hrs LT after which it again shot up to saturation 0800 hrs LT and remains above the normal up to midday. The ionogram strong frequency spread with some indication layers of strong scatter. There had not been ionograms available for the period between LT due to equipment failure. The ionograms hrs LT and later up to 0945 hrs LT did record clearly the spread-F near the peak of the F-layer. This is the example of longest lasting spread-F detected during the examination Huancayo during the ATS-6 period. ionogram at 0500 hrs LT did scattering within the F- E-region echoes were at 450 km 275-400 km; another can exist hours strong above recorded plasma the at 275 km gradient F-region. At and spread-F are At 0700 hrs counter-electrojet fields in the E- daylight equatorial to find whether are present to 320 km and at 0730 hrs electric the probably by the evidences in periods. the some of the amplitude beacon besides activity, there LT, the post-evening associated is a pre-sunrise when scintillations The present received at with only were present. At 0330 hrs slow and small been affected scintillations seem to be either near the ionization by freshly can were very deep and The record rapid and to the post-evening type of spread-F. be at very deep or on the appear to be counter-electrojet of a westward MRTI electrojet layers up to 600 km, of the later and HF ionosonde. At in intensity Acknowledgement Grateful scientists excellent data collected at their observatories. are also due to US National the award of Senior Force Geophysics Laboratory, for the facilities provided The ionospheric data supplied by World Data (Colo), USA, under Arthur US Academy of Sciences. thanks at the Instituto are due to the authorities del Peru for the and the variations of the at the Geofisico Thanks 1975 of Huancayo. activity had decreased and some It is seen that of Academy Associate Bedford (Mass). USA, to the author used in the Center A for STP Boulder Day Grant of Sciences for and to Air Research for the study. analysis level between 0700 and level even were at 0315 hrs LT showed No. 30 of the of the 0330 and 0745 hrs References at 0800 1 Sinclair J & Kelleher 201. R F, J Atmos & Terr Phys (GBl. 31 (1969) after the sunrise of ionograms H & Rastogi 2 Chandra R G. Curr Sci (India). 43 (1974) 567. of & 3 Chatterjee S K. Bandyopadhyay P. Ann Geophys A K. Guhathakurta Bandyopadhyay (France). 30 (1974) 329. 3 Discussion The general belief that the spread-F scintillations occur only during the nighttime R G. Indian J Radio & Space Pliys, II (1982) 1. 4 Rastogi as well as radio has led to R G. Indian J Radio & Space 5 Rastogi 11 (1982) 159. Phys. 189