31{4)[C£)], 368-373 Tap chi CAC KHOA HOC VE TRAI D A T 12-2009
MOT SO DAC DIEM CUA SPREAD F XICH DAO QUAN TRAC TAI VIET NAM
HOANG THAI LAN, NGUYEN THU TRANG
I. GlOl THIEU
Trong tu trudng Trai Ddt, ehuyen ddng cua cac hat mang dien Id dj hudng. Nd dupe xac dinh bdi su phan phdi ede hat mang dien va cac hat trung hda. Nhu vay, lire diiu khien cac ion va cac electron ehuyen ddng vdi nhung tde dp khae nhau san sinh ra ddng phan ky. Nhung nhung trudng dien phan cue dupe sinh ra lam cho ddng phan ky tdng cdng tu do khdp noi. Sau dd, cac trudng dien biln ddi lai phan bd lai su ion hda va thay ddi nhung ehuyen ddng dj hudng dd.
Hien tupng Spread F xich dao (ESF) la hien tupng bieu hien cae bat dn djnh eua plasma dien ly, xay ra dudi nhung diiu kien dien ddng luc di thudng cda dien ly vao thdi gian sau hoang hdn.
Nhung khao sat ddu tien ve spread F bdng phuong phap tham dd vd tuyen dupe H. G. Booker va H. W. Wells (1938) [2] phat hien tren dien ly do thu nhan dupe tai Huancayo. Dien ly dd ed vet phan xa ldp F bj "khuylch tan" vd "trai rdng"
thanh vet day thay vi la vlt manh thdng thudng.
/ li 1 1 / M ,
Thuat ngu "spread F" (Fs) dupe sd dung d l gpi ten vet. theo dd da cd bdt thudng vl mat dp plasma.
Vao nhung nam 1970 [3], bieu dd radar (bilu do dp cao - thdi gian - mat dp) sd dung thang mau xam thjnh hanh nhu mdt phuong thuc hieu qua dl theo doi vi tri va cudng dp cua bat thudng nay (hinh 1).
Su xudt hien cua nhung cau true cd hinh dang
"ldng vu" tren bieu dd giup cho Woodman va La Hoz (1976) [11] dua ra gia thie't ve eP c h l sinh bat dn djnh d phdn dinh ldp F; theo dd, mdt nhilu loan ed quy md Idn trong phdn day khdng dn dinh cda ldp F cd the phat trien mdt each phi tuyln tinh thanh "bpt bong bdng" theo each thde gan gidng nhu bat dn dinh Rayleigh - Taylor trong ddng chat long trung hda, khi mdt chdt long nang hon tua tren mdt chat long nhe hon nd. Bpt bong bdng sau dd se dang len do anh hudng cua hieu ung ndi va tie'p tiic dien tie'n nhu vay, tham chi ngay ca trong cac khu vuc cd gradient dn dinh vl hap ddn. Y tudng nay ldn ddu tien dupe Woodman trinh bay tai hdi nghj vl vat ly plasma khdng gian nam 1975.
<— Hinh 1. Dien ly dd ddu tien ghi nhan sir xudt hien eua Fs (a) va edu true ldng vu tren bieu dd dp cao - thdi gian - mat dp (b)
Ldp F xich dao sau hoang hdn nang len eao do su tang cua dien trudng hudng ddng, dupe bie't din mdt each rdng rai gpi Id su tang ddng dien ngupc xich dao ma su tang dd la do vai trd uu the cua dynamo Idp F. Viing gradient ndng dp dien tu duong tinh cua day ldp F tang nhanh ehdng lam cho vung dien ly trd nen bat dn djnh va hdn loan ddn tdi su tang trudng cua bpt plasma khdng theo quy luat bdi cO ehe bdt dn djnh ndi tieng Rayleigh- Taylor {hinh 2).
Cac bpt plasma phat trien trong khi dng ddng theo phuong thdng ddng tang su triet tieu plasma d pham vi ldn vdi ede bat quy luat thu cdp phat trien bdi nhung qua trinh ndi tdng khi eae gradient mat dp bpt khi trd nen dde. Bdng cac ky thuat khae nhau ngudi ta quan trde thdy pham vi bat dn cd the td hang chuc xentimet tdi hang tram kildmet [5, 7].
Dense Plasma
3. Tinh ddn Pedersen tieh hop cda dng ddng bdt dn, bj kilm soat bdi thermospheric meridional/transequatorial winds.
Less Dense Plasma Hinh 2. Ca e h i Rayleigh-Taylor
Dua tren cac sd lieu quan trde, ede nghien cdu ly thuyet va cac md hinh md phdng [1, 3, 4, 8, 10, 13], mdt so dd dan gian ve eae qua trinh cd lien quan dupe trinh bay tren hinh 3.
Cac nhan td phu thudc Idn nhau cda he thdng Ionosphere - Thermosphere kilm soat su phat trien cua Spread F g d m :
1. Chieu cao cua ldp F vao budi tdi va sir trdi dat plasma thang ddng do sir tang cda dien trudng ngupc (PRE), bj kiem soat bdi Thermospheric zonal wind (hudng ve phia ddng vao budi tdi) vd gradient kinh tuyen (gradient thdi gian dia phuong) trong Idp E ddn dien luc hoang hdn (SS.AE).
2. Su phat trien cua mdt gradient mat dp dde d day Idp F, noi ma sir bat dn djnh dupe bat ddu bdi eae mam mdng hdn loan (dupe cho Id td sdng trpng trudng).
Hinh 3. Cac qua trinh lien quan din su xudt hien ESF
II. KET QUA 1. Sd lieu
Bao cao sd diing chudi dien ly dd phut thu thap dupe nhd may tham dd dien ly thing dung (CADI) tai Dai quan trdc khi quyln Hoc Mdn, giai doan 2002 - 2006. Spread F dupe phan loai thanh cac dang chinh nhu sau {hinh 4):
a) Dang F (trai rdng ve tdn sd) b) Dang Q (trai rdng vl dp cao)
c) Dang L (trai rdng ca ve tdn sd va dp cao) d) Dang P (vd djnh hinh)
Tuy nhien, Fs xudt hien tren dien ly dd thuc te thu nhan tai Dai Hoe Mdn khdng gidng het nhu hinh ve ma phdn ldn ed dang nhu sau {hinh 5).
2. Ket quii phan tich a. Cac dang Fs {hinh 6)
Dd thi d hinh 6 tdng hpp tat ca cac trudng hpp Fs xudt hien. Dang P xudt hien thudng xuyen hon ca, cac vlt cd su trai rdng ca vl dp cao va tdn sd nhung khdng cd hinh dang rd ret nhu da md ta.
Cac ke't qua quan trde trude day d nhung vung vT dp khae eho thdy dang Fs trai rdng vl tdn sd chie'm uu the d viing vi dp cao (dang F).
Trong khi dang Fs trai rdng vl dp eao (dang Q) chie'm uu the d vung xich dao. Tuy nhien, d Viet Nam, dang F xudt hien nhilu hon dang Q la mot dae diem dang ehu y.
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Hinh 5. Cae dang Fs xudt hien tren dien ly dd quan trac tai Dai Hoc Mdn
Hinh 6. Su xudt hien cua eae dang Fs
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b. Bie'n thien ngay dem {hinh 7)
Spread F tai Viet Nam xudt hien nhilu vao thdi gian sau nda dem. Cue dai trong khoang td 01 LT de'n 02:15 LT. Cue tieu vao thdi diem ngay sau khi Mat Trdi Ian va trude khi Mat Trdi mpe, cung trong khoang thdi gian nay, sd ldn Fs xudt hien biln ddi dang ke, trong khi lai rdt dn djnh vao khoang td 22 LT den 02:30 LT. Trong thdi gian khao sat ghi nhan dupe 46 trudng hpp trong dd Fs vdn tdn tai sau khi Mat Trdi mpc; Fs chi xudt hien vao khoang hon 1 gid sau khi Mat Trdi lan vdi tan sudt tang lien tuc vd rd ret de'n sau nda dem.
Sir xuat hien cua Fs giai doan 2002 - 2006
15 16 17 18 19 20 2122 23 0 1 2 3 .1 5 6 Gid dia phuang
Hinh 7. Su xudt hien cua Fs vao ban dem
S y xuat h i | n cua Fs giai doan 2002 - 2006
1200 1000 800 e - 600
400 200
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nil
3 4 5 6 7 T h i n g
10 11 12
Hinh 8. Biln thien mua cua su xuat hien Fs
c. Bien thien miia: Hinh 8 eho thay Fs xuat hien nhilu nhat vao eae thang 3, 4; cue dai thd cap dat dupe vao thdng 7 hoac thang 9, 10; cue tieu vao thang 11, 12. Xu the bien dieu nay the hien d hdu hit cac nam.
d. Do trai rdng ciia vet phan xa: quy djnh cho
su trai rdng cua vet phan xa la > 0.3 MHz va
> 30 km dp dung eho trudng hpp cac dien ly dd c6 Fs xudt hien tai TP. Hd Chi Minh thu dupe nhung kit qua rat thu vj, dd la nhilu khi dp trai rdng chi vda xdp xi gia trj quy ude nhung nhung vlt Fs rat rd rang, manh va tdn tai hang vai gid; cung ed khi vlt day hang tram km nhung hdu nhu chi tdn tai khoang 1 gid {hinh 9).
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1 0 I IHinh 9. Su trai rdng ve tdn sd va dp cao cua cac vet phan xa
Cde gid trj cua dp trai rdng the hien tren hinh 9 eho tha'y tan sd bi khuyech tan manh nhdt vao thdi gian khoang 23 LT - 01 LT, trong khi vao 05 LT - 06 LT dp khuyech tan khdng nhieu (< 1 MHz); dp cao cua vet bj khuyech tan nhilu vao trude nda dem (20 LT - 21 LT), giam ddn vao nda dem vd lai bj trai rdng nhilu vao 03 LT - 04 LT, ca biet vdo 05 LT (nam 2002), day cung la thdi diem thudng xuyen xudt hien vet dang Q day dae.
Mdi quan he gida su xudt hien Fs va dp boat ddng Mat Trdi (so sanh vdi so vet den vd thdng lupng bdc xa 10.7 em Mat Trdi) khdng the hien quy luat rd rang.
III. BAN LUAN KET QUA
Kit qua phan tich cac sd lieu quan trac dupe tai Dai quan trac Hdc Mdn cho thdy Spread F xich dao tai Viet Nam cd nhung dae trung sau:
1. Su khuyech tdn vl tdn sd xay ra nhilu hon sir khuyech tan ve dp cao.
2. Spread F bat ddu xudt hien vao 18:30 de'n 19:00 gid dja phuong va dat dinh xudt hien vao trude nda dem trong hdu hit tdt ca cae thang.
3. Dp trai rdng vl tdn sd va vl dp cao eua vet Fs khdng ldn nhung vlt tdn tai sudt nhilu gid, vet dang Q manh thudng vdo khoang 21 LT - 22 LT vd vlt dang F vao khoang 23 LT - 01 LT vdi dp trai rdng hang tram km va vai MHz tuong dng.
4. Spread F xudt hien nhieu nhdt vao thdng 7 (cue dai luc 22 gid LT), so vdi thang 8, thang 9 vd thang 4, thang 5. Khuynh hudng mua eua spread F ndi chung the hien xu hudng dae trung la cue tilu vdo thdng 12 va cue dai vao thang 7.
5. Vdo cac thang phan dilm, Spread F xay ra nhieu hon vdo thdi gian sau nua dem, xudt hien dn dinh td gdn niia dem d i n gdn sang.
Da 70 nam ke td khi phat hien hien tupng bat quy luat Spread F, ngudi ta da nghien cdu hieu rd siJ hinh thdnh nhung bat dn djnh d ldp day, nhung nhung bie'n thien hang ngdy chua dupe giai thich thdu dao va nhilu van d l ly thuyet lien quan chua dupe giai quyet [6, 9, 12]. Nhung nghien cdu dinh lupng dua trin nen tang sd lieu quan trac do dae bdng nhieu phuang phap khae nhau vl trang thai ciia dien ly va quyen trung hda la vd eung can thie't, se giup lam rd vai trd cua nhiing ylu td vat ly
tae ddng de'n su hinh thanh va duy tri hien tupng nay. Chi khi cd nhdng ly giai vat ly hoan chinh thi mdi cd the tien tdi muc tieu du bdo xem khi nao thi Spread F xay ra, hoac it nhdt la xay dUng md hinh md phdng ve Spread F vdi nhung dae tinh eu the eua nd.
TAI LIEU DAN
[I] M. A. ABDU, R. T. MEDEIROS, J. A.
BITTENCOURT, J. A. BATISTA, 1983 : Vertical ionozation drift velocities and range spread F in the evening equatorial ionosphere, J. Geophys. Res.
88, 399 - 402.
[2] H. G. BOOKER and H. W. WELLS, 1938 : Scattering of radio waves by the F region of the ionosphere, / . Geophys. Res., 43, 249-256.
[3] D. T. FARLEY, B. B. BALSLEY, R. P WOODMAN, 1970 : Equatorial spread F - implications of VHP radar observations, J.
Geophys. Res. 75 (34), 7799.
[4] B. G. FE:rER, L. SCHERLIESS, E. R. DE PAULA, 1999 : Effects of the vertical plasma drift velocity on the generation and evolution of equatorial spread F, J. Geophys. Res. 104.
19859-19870.
[5] J. K. HARGREAVES, 1992 : The solar- terrestrial environment, Cambridge University Press.
[6] HOANG THAI LAN, M. A. ABDU, J.
MACDOUGALL, I. S. BATISTA, 2009 : Longitudinal differences in the Equatorial Spread F characteristics between Vietnam and Brazil, Advances in Space Research, ASR-D-09-00038R2, Elsevier Press.
[7] M. C KELLEY, 1989 : The Earth's Ionosphere: Plasma Physics and Electrodynamic, Academic Press, Inc.
[8] M. J. KESKINEN, S. L. OSSAKOW, B. G. FEJER, 2003 : Three-dimensional nonlinear evolution of equatorial ionospheric soread F bubbles. Geophys. Res. LeU. 30(16), 1855.
doi:10.1029/2003GL017418.
[9] A. J. LYON, N. J. SKINNER, R. W.
WRIGHT, 1961 : Equatorial spread-F at Ibadan - Nigeria, Journal of Atmospheric and Terrestrial Physics, 21, 100 -119, Pergamon Press Ltd.
[10] P. J. SULTAN, 1996 : Linear theory and modeling of the Rayleigh-Taylor instability
leading to the occurrence of equatorial spread F, J.
Geophys. Res. 101, 26875 - 26891.
[11] R. F. WOODMAN and C. LA HOZ, 1976 : Radar observations of F region equatorial irregularities, J. Geophys. Res. 81, 5447.
[12] R. F. WOODMAN, 2009 : Spread F - An old Equatorial Aeronomy Problem finally resolved? Annales Geophysicae, 27, 7975 -1934.
[13] S. T. ZALESAK, S. L. OSSAKOW, P. K.
CHATURVEDI, 1982 : Nonlinear equatorial spread F : the effect of neutral winds and background Pedersen conductivity, J. Geophys. Res. 87, 757.
SUMMARY
Some characteristics of the Equatorial Spread F observed over Vietnam
The occurrence of equatorial plasma instabilities is a manifestation of the plasma bubble
irregularities that occurs under the unique electrodynamical conditions of the post-sunset ionosphere. This phenomenon commonly is referred to as Equatorial Spread F (ESF), can cause radio signals propagating through the disturbed region to scintillate. This results a fade in signal power is received, even to a loss of the signal. Understanding of the ESF will open up possibilities for predicting on communication systems.
This paper presents statistical results about the occurrence of equatorial spread F (ESF) in South Vietnam based on analyzing the data observed by vertical ionospheric sounding at Hocmon Atmospheric Observatory during the period 2002 to 2006.
Ngdy nhdn bdi : 05-11-2009 Vien Vat ly Tp. Ho Chi Minh
