27(3), 274-281 Tap chf CAC KHOA HOC VE TRAI DAT 9-2005

? / ? A' A , /

PHAN VNG CUATANG DIEN LYXICH DAO QUA.N sAT TAI Tp HO cHf MINH ·

DOI v6I BAo TU 17-19/8/2003

r. Md nA.u Cac nghien c(ru ve moi ttrong quan giiia bao ttr

va nhieu lo~n di¢n ly c6 y nghia quan tr9ng trong trng dung thuc tien . Nhieu l o~n di¢n ly thuang anh huang Mn nhiing hoat di)ng hang ngay cua chung ta, dac bi¢t Ia doi v6[ h¢ thong thong tin giiia cac tram mat da't Slr dung dai song YO tuye'n HF truyen trong tang di¢n ly vii h¢ truyen thong v¢ tinh v6i cac dai tan so cao han xuyen qua tang di¢n ly. Trong thai gian xay ra bao ttr, di) dan di¢n, di) cao vii do day cua cac 16p di¢n ly thay d6i. Vi vay, de dam bao thong tin lien tuc, che' di) truyen song cung can phai thay d6i tuong ung v6i tr~ng thai cua di¢n ly. Nhfrng bien d6i m~nh trong h¢ thong nhi¢t quyen - di¢n ly vung vy di) thap va xich dao lien quan Mn di¢n d()ng 111c chuyen ti e'p tlr vy di) cao de'n vy do thap va nhiing thay d6i th iinh phan h6a hQC cua khi quyen tang cao trong thai gian Xfly ra bao tlr rat quan trQng trong vi¢c nghien ClrU tha i ti e't khong gian hi¢n nay.

Su xuat hien cua "bao ttr c6 li en quan chat che de'n hoat di)ng cua M::tt T ra i, nhat Ia v6i cac vu bung n6 trong sac d u cua M::tt Trai. Bao ttr bat dau tlr tlr li en quan de'n su xuat hi¢n cua cac ve't den tren M~t Tra i. Bao tu bat dau bat nga lien quan de'n d tc vu bung n6 trong sac d u cua khi quyen M~t Tra i. Bao ttr xay ra hau nhu cling m6t luc tren Trai Da't, nhung 6 cac vi tri khac nhau, hinh thai cua xung quan sat dugc khong giong nhau . Do d6, phan ung cua tang di¢n ly cung se khac nhau a nhiing vj tri khac nhau. Cac ke't qua nghien cuu cho thay hi¢u trng cua bao tlr d6i v6i di¢n ly YO

cling phtrc t~p va cac nha khoa hQC tren toiin the gi6i cung dang n6 1!1C hgp tac M tim hieu moi' " quan h¢ phfrc tap nay. Bii i bao gioi thi¢u ke't qua phan tich bien d6i dien ly tren CO' sa so li¢u quan trac t<;t i dii i H6c Mon (Long. I 06°34 E, Lat.l0°5l N,

274

HOANG THAI LAN

dip Lat. 2°9 N) trong tran bao tv xay ra viio thang 8/2003 voi ml;l c dich g6p phan b6 sung ke't qua viio b(rc tranh di¢n ly toiin d u cung nhu dan dan lam sang to CO' che' anh huang dja phuong a khu VUC phia nam.

II . PHVONG PHAP PHAN TICH VA SO LIEU

Kh i c6 cac bung n6 xay ra tren Mat Trai, cac hat •tich di¢n chuyen d()ng ve phia Trai Da't voi van toe rat 16n. Cac Iuong hat ph6ng ra ttr M::tt Trai nen cac duang SlrC tlr truong Trai Dfrt t~ i ranh gioi xay ra tuong tac. Du6i tac dt,mg cua su tuong tac d6, tt'r truang cua Trai Dat bien d6i va t<:to ra cac nhieu loan dia ttr. Cac phan tich toan di¢n ve hinh thai bao tt'r da dugc tie'n hanh ttr cuoi the' ky 19 II , 5, 7, 8, III vii da cho thay rang a xich d~o vii vy d('> trung binh, Sli suy giam cua thanh phan H trong tha i gian c6 bao tt'r c6 the bieu dien giong nhu tu truang song song v6i true cua Iuong Cl1C dia tlr c6 hu6ng ve phia nam. Di) 16n cua truang nhi~u loan doi xung guanh true nay bie'n thien theo thai gian bao tlr vii dugc xac djnh theo tha i gian do dugc tinh tli kh i bat dau tran biio . Su bat dau cua m¢t tran bao tt'r dugc d::tc trung bai su tang di)t ngot thanh ph an H tren toan cau (SC). Tie'p theo SC Ia pha ddu cua bao ttr. Trong ph a nay, thanh ph an nam ngang H c6 the tang vii i chuc gamma vii keo dai trong viii gia. Sau d6 Ia pha chinh v6i dac trung Ia Sli giam manh cuong di) th iinh phan nfun ngang H. Du6i tac dung cua tt'r truang Trai Da't, cac h~t tich di¢n t~o ra m6t dong di¢n chay theo hu6ng tay trong m~t ph~ng xich dao Trai Dat va lam suy giam cuang di) tu truang. Sau khi dat Mn gia tr! cue tieu, cuang di) tu truang tu tu tra Iai gia tr! binh thuong, giai doan nay goi Ia pha h6i phuc .

Chi so Dst [9] dugc dung de bieu dien su bie'n d6i cua th iinh phan ttr truong H 6 vung vy di) thap

va xich d::to do tac d()ng cua v6ng dong xich d::to . Chi s6 Dst cung dp th6ng tin djnh lm;mg ve nhieu lo::tn ttr c6 lien quan din cac th6ng s6 d!a v:lt ly va M~t Troi . Vi d6i rat kh6 xac djnh S1.! bat dau cua pha hoi phuc nen ngvoi ta thvong xem xet dong thai dtc hi~u frng trong pha chinh va phan dau cua ph a hoi phuc (hlnh 1 ).

::r :or:r: f·

;o..

;'r:u [

1C{'

i ,, L

sc

? ha h6i phuc

., u.l ' Ll.' ' -.L..i.:...C'c:J' '--'-' -'-' Ll .LI !..l,l i ~· ! lllJ.!! J..L! : ..J_;! !...JI.w.· ' lJ.I I''-'-' - 'L.:'-'.' _ · ....11...!.'"-' ' ....:·.u· I '-'-' ' -'-"" "

0 60 '1 2

Hinh 1. Cac pha dac tnmg cua m¢t tr(tn bao tU:

Ngay 31/8/2003 Ia ngay yen tinh dvQ'c ch9n M so sanh trong bao cfto nay.

III. KET QuA PHAN TfCH so ut;:u Hinh 2 bieu dien trvong dia ttr bie'n thien quan

S6 li~u di~n ly dvQ'c phan tich trong bai bao la cac thong s6 cua lap F:

• foF2 - tan so toi h<?n cua lop F • h'F- d<? cao b i~u kien cua lop F • hpF - d<? cao x&y ra ct,Jc d<?i nang d<? di$n tlJ'

cua lop F

Cac di~n ly do dvQ'c ghi tai dai H6c M6n 15 phut/lan va dVQ'c xi1ly theo cac th6ng s6 tren .

Cac gia tr! cua thanh phan ttr trvong H ghi t::ti dai H6c M6n dvQ'c phan tich song song v6i cac gia tr! Dst (lay tl.r website : http:/Jswdcdb.kugi.kyoto­u.ac . jp/wdc ).

Cac thong s6 di~n ly trong dieu ki~n yen tinh cling dVQ'c siX dung M so sanh. Thai gian c6 dieu ki~n yen tinh dVQ'C xac djnh d!Ja tren chi' s6 Kp (chi' s6 hanh tinh, bieu dien m6i lien quan giiia gi6 M~t Trai va muc nhieu lo<:tn cua trvong d!a ttr) .

Chi' s6 Kp [4] dVQ'c Jay ttr website: http://www. gpz-postdam.de/kp.html

trac dVQ'C tai dai H6c Man trong cac ngay 16-17-18-19/8/2003. S6 li~u quan trac cho tha'y cac bie'n d6i bat thvong the hi~n trong ngay 17 va 18/8. De xem xet phan ung cua tang di~n ly trong dtc pha cua tr:ln bao ttr nay, s6 li~u ttr va s6 li~u di~n ly dvQ'c phan tich Ia ba ngay 17-18-19/8.

I . HI

Hinh 2. Bie'n thien cua th~mh ph an H trong cac ngay 16-17-18-19/8/2003, quan trac t::ti dai H6c Mon

275

Hinh 3 bieu dien chi s6 Dst va bie'n thien cua thanh phan H quan tr;ic t<;1i dai H6c M6n trong cac ngay 17-18-19/8/2003. Gia trj Dst cho thay d¢ suy giam ClfC d<;ii cua tlr tnrang dat 168 nT vao 16 UT ngay 18/8 , L: Kp =52+, Ap =108 . Hinh thai bie'n d6i cua thanh phan H quan triic tai dai H6c Mon cho thay SC xay ra vao 14.36 UT ngay 17/8. D¢ Sli y giam CU'C dai cua thanh ph an H d~t gia trj 217 nT vao Iuc 13:42 UT(20:42 LT) ngay 18/8.

Nhieu lo<;1n di~n ly xuat hi~h vao thai diem SC, foF2 tang d<!t ngiil t voi gia trj II ,6 MHz so voi giii tri trung binh 8,5 MHz (hinh 4) . Tr~n bao tv nay c6 pha dau va pha chinh xay ra vao d. tha i gian ban ngay va ban dem. Vao thai gian ban ngay, trong cii pha dau va pha chinh , nang d.¢ ion deu cao han rat nhi€u - tan s6 chenh l ~ch eve d<;ii len toi 6 MHz. Yao giai do~n ttr 20 LT de'n 03 LT ngay 18/8 quari trac thay gia tri foF2 suy giarh so' voi gia tri foF2 6 dieu ki ~n yen tinh. Thai gian nay ttrang frng voi giai doan suy giam cu:e dai cua Dst va phan dau cua pha hoi phuc.

Hi~u frng true ti e'p len su d!ch chuye'n d i~n Jy c6 the' de dang quan trac thong qua sv bie'n d6i d¢ cao cua lop F. Hinh 5 bieu dien cac bie'n thien cua h'F trong thai gian xay ra bao ttr va so sanh voi bie'n thien ngay dem yen tinh . Trong pha dau, cac

nT 17KlS/2003

100

0 .00

·1 00

-200

-500

-40 0

·500

100

0 .00

· 100

-200

-:.oo

·400

-500

0 8 12 16 20 0 8

gia trj h'F cao han binh thtrang vao tha i gian ban dem. Thai gian ban ngay khong the hi~n ro net d~c trtrng anh htr6ng tren h'F 6 cii pha dau va pha chinh. D<;> cao h'F tang d<;>t ng<)t vao bu6i toi (20 LT) cao han binh thtrang khoang 70 km va giam dot ngot vao thai diem Dst dat d¢ suy giam CU'c dai. Ke't qua cho tha'y lop F2 sau d6 l<;1i djch chuyen len cao vao sau nua dem (00 - 03 LT) ngay 18/8 voi d¢ cao chenh l ~ch khoang 80 km. Day Ia giai do<;in dau cua pha hoi ph[lC. Hi~n ttr<;mg tang d¢ cao h'F luon di kern voi Sl! giam tan so foF2.

Hinh 6 bie'u dien Slf bie'n thien ngay dem cua cac th6ng s6 foF2 (a), h'F (b) trong tha i gian bao tlr, dong thai so sfmh voi cac gia tri trung binh cua tharig. Ta tha'y , n6i chung vao thai gian ban ngay 6 cii pha dau va pha chinh, cac thong s6 foF2 va h'F deu cao han rat nhieu so v6'i gia trj trung binh thang. Chenh l ~ch ctrc dai ve Uin so toi h<;1n foF2 khoang 5 MHz, ve d<;> cao h'F- khoang 70 - 80 km. Vao thai gian hoang h<Jn , trong dieu ki~n binh thtrang, d<;> cao cua lop F tang. Nhtrng trong thai gian xfly ra bao tlr, do cao h'F quan trac dUQ'C lai c6 xu huang giam Yao bu6i toi , 6 pha chinh , foF2 giam va h'F tang so voi gia trj trung binh. NgUQ'C l ~ i , vao trtr6'c nua dem, foF2 tang va di kern la hi~n tuang h'F giam.

!9/08/2003

12 16 8 12 16

Hinh 3. Dst va bie'n thien cua thanh ph an H quan trac tai dai H6c Mon ngay 17-18-19/8/2003 (ph an b6ng rna Ia thai gian ban dem : 18 - 06 L T)

276

0 4

Km

0 4

17/0812003 18/0812003 19/0&/2003

8

8

12 16 20 0 4 8 12 16 20 0 4 3 12

Hinh 4. Tan so toi h~n foF2 quan tr:ic tai dai H6c Mon ngay 17-18-19/2003 Duong dam bieu dien bien thien ngay dem trong dieu ki ~n yen finh

12 16 20 0 4 8 12 Hi 20 0 " 4 8 12

Hinh 5. Bien thien cua d¢ cao h'F trong thoi gian bao tLr Duong d~m bieu dien bien thien ngay dem trong diet;~ ki~n yen tinh

16 20

16 20

Hinh 7 bieu dien ket qua phan tich di) cao xay ra CIJC d~i nong d¢ di~n tii hpF trong thoi gian bao tLr va so sanh voi cac gia tri hpF trong dieu ki~n yen

tin h. Ket qua cho thay , vao thoi gian ban dem, cac gia tri cua hpF2 cao han so voi cac gia tri hpF2 6 dieu ki~n tinh ca 6 pha dau va pha chlnh . Vao

277

Mllz

18

s

6

4

2

Km

800

700

600

100

0 3 4 5 6 7 8 9 10 11 12 lJ 14 15 16 17 1& 19 20 21 22 23 24 UT

Hihh 6a . So sanh bie'n thien ngay dem cua th6ng so fo F2 trong tha i gian bao tu va gia tr! trung binh cua thang 8/2003

Ngay

····i.

' ' '

o 2 3 4 6 7 s 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 UT

Hz'nh 6b. So sanh bie'n thien ngay dem cua th6ng so h'F trong tha i gian bao tlt' va gia tr! trung binh cua thang 8/2003

thai g ian ban ngay , d¢ cao hpF giam nhe so voi a dieu ki¢n tinh .

Hinh 8 bieu d ien slf khac bi¢t gifra bie'n thien cua d¢ cao h 'F vil. d¢ cao cue d~ i hpF trong tha i

278

gian xiy ra bao tlt' . Ta c6 the tha'y rang cac hi¢u Ll'ng bao tu len d6 cao bieu kie'n va d6 cao cue d~i giong nhau vao tha i gian tlt' bu6i toi cho de'n ni1a dem a pha dau. Tuy nhien, trong pha

Km

Km soo

700

600

500

400

100

0

17/0S/2003 lll/0&12003 19.10812003

3 12 16 20 0 4 8 12 16 20 0 4 8 12 16 20

Hlnh 7. Bie'n thien hpF2 trong thai gian bao tli' (duang manh) so sanh v6i hpF2 trong dieu ki~n yen tlnh (duang d~m)

17/0Sl2003 1810&12003 19/0&12003

o 4 12 16 20 4 12 16 20 4 12 16 20 24 UT

Hinh 8. So sanh gifra d¢ cao hi~u d1,mg h'F(duang dam) va d¢ cao cue d~i hpF2 trong thai gian bao tlr (duang manh)

chinh , dieu khac bi~t ve hi ~u frng bao tlr len hai thong SO tren the' hi~n ro net 0 SU' giam chenh l¢ch gifra do cao hi ¢u dung va do cao cue d\li.

D¢ chenh lech CU'c d'ai cua d¢ cao hpF gil1a dieu ki~n bi~h th~o~g va khi c6 bao tlr d\lt t6i 150km.

279

M(>T s6 NH~N XET

Ket qua phitn tich s6 Ii¢u quan tnic bao tt! 17-19/8/2003 cho thay nong dQ di~n ly tang cao vao thai gian ban ngay. Tang di¢n ly chuyen djch len cao mQt each dang ke vao ban dem, d~c bi~t Ia khoang thai gian tn.r&c va sau nira dem. Vao thai diem xily ra suy giam ttr truang cgc dl,ii, d<;> cao giam d¢t ng(lt va nong d<;> di~n ly cilng suy giam.

Trang nam 2003 da c6 3 bao ttr m<;1nh xay ra vao thang 8, thang 10 vil thang 11. Ph an frng cua tang di~n ly quan td.c tai Tp. Ho Ch\ Minh trong tran bao cue ml,J.nh vao thang 10 da duqc bao do trong [ 17]. V&i eac ket qua pha.n tich qua eac tr~n bao tt1 nay bu&c dau c6 the ru.t ra nhfrng phan frng d~C trtrng CUa di~n Jy khu Vl,I'C phfa nam nhu sau :

1. Trong thai gian xay ra bao ttr, gia trj tan so toi han foF2 vao ban ngily phan Ion deu tang so voi binh thuong.

2. Vao thai gian sau nua dem, nhfrng Sl1 bie'n deli cua d(> cao bi€u kie'n (h'F) the hi~n m(>t su dich chuyen len nhfrng d¢ cao Jon va d~c bi~t nhanh chong vao thai gian suy giam cgc di;ii cua thanh phan H. S\1 djch chuyen len cao Iuon di kern voi Sl1 giam cua foF2.

3. Nhfrng Sl1 bien d6i trong chieu cao cua dlnh m~t dQ di~n tu (hpF2) khi c6 bao ttr khOng n6i b~t

. nhu a h'F, nhung chung mota d~c diem nhfrng dao d(>ng quan tr9ng.

Da c6 nhieu bao cao ve su bie'n deli cua mat d6 di¢n tlr 0 dinh Ibp F2, foF2, h'F vil hpF/hmF (2 , 3~· 6, I 0, 12-20]. Nhfrng S\1 bien d6i nay di kem·VOi Sl1 tang ho~c giam trong cac pha cua bao. De c6 the xac djnh toan b(> CC! che Yilt Jy xay ra trong tang di~n Iy khi c6 bao tlr can phai nghieri ClrU, khao sat hi~u frng di~n ly m¢t each chi tie't 6 nhieu dia diem va doi v&i ttrng tran bao ttr. Nhieu chuang trinh hQp tii.c nghien c(ru va so sanh ke't qua cua eac tr<;lm quan trac nflm trong vung xfch di;!O dang dl1QC Xil.y dung vil thuc hi~n. So sanh phiin frng dac trung cua di¢n ly khu vue phia nam voi cac d~c trung cua m¢t so vj tri xich d<;1o vil vy d¢ thap khac da buoc dau dUQC tie'n hanh [ 17 J va se Ia muc tieu cua nhfrng nghien c(ru sap t&i .

Bai bao dl1QC hoan thanh v&i Sl1 tili trQ kinh phi cua Chuang trinh Nghien cfru Co ban trong llnh vue Khoa hc,K Tu nhien, de tai rna s6 731504

280

"Nghien cfru moi quan h~ gifra tang di~n ly va tn.rong dja ttr bien thien t<;1i khu vgc phia nam".

Tac gia chan thanh cam on GsTsKh I....e Minh Triet vil Gs TsKh Nguyen Thj Kim Thoa ve nhfrng g6p y trong chuyen mon dja ttr M hoiln thi~n bili bao.

TAl ueu THAM KHA.O

[1] W.G. ADAM, 1892 : Comparision of simultaneous magnetic disturbance at several observatories, Phil. Trans. London, 183,131.

12] J.O. ADENIYI, 1986 : Magnetic storm effects on the morphology of the 'equatorial F2-layer •. Journal of Atmospheric and Terrestrial Physics 48 , 8, 695-702.

[3] S. BASU, KM. GROVES, H.C.YEH, S. SU, F.

1. RICH, P.J. SULTAN, and M.J. KESKINEN, 2001 : Response of the equatorial ionosphere in the South Atlantic region to the great magnetic storm of July 15 , 2000, Geophysical Research Letters 28, 3577-3580.

0

[4] F. BOBERG, P. WlNTOFT, H. LUNDSTEDT, 1999 : Real time Kp predictions from solar wind data using neutral networks , J. Physics and Chemistry of the Earth, 17.

[5] J.A. BROUN, 1861 :On the horizontal force of the Earth's magnetism, Proc. Roy. Soc. Edinburgh, 22, 511 .

[6] A.G. BUST, T.L. GUASSIRAN, and D.S. COCO, 1997 : Ionospheric observations of the November 1993 storm, Journal of Geophysical Research 102, 14293 -14304.

[7] S. CHAPMAN, 1935 : The electric current­systems of magnetic storms, Terr. Mag. Atmos. Phys., 40, 349.

[8] S. CHAPMAN, 1952 : The morphology of magnetic storms: an extension of the analysis of Dst, the disturbance local-time inequality, Amali di Geofisica, 5, 481 .

[9] J.K. HARGREAVES, 1992 : The solar­terrestrial environment, 279, Cambridge University Press.

[10] C. HO, M. MANNUCCI, U.J. LINDQWISTER, B.T. TSURUTANI, L. SPARKS, B.A. IIJIMA, B.D. WILSON, I. HARRIS, and M.J. REYES, 1998: Global ionospheric TEC variations during January I 0,

1997 storm, Geophysical Research Letters 25, 2589- 2592.

Ill J Y. KAMIDE, 1997 : Magnetic storm, Geophysical Monograph 98 , 1-17, The American Geophysical Union.

[12] D.R. LAKSHMI, C.N. RAO, A.R JAIN., M.K. GOEL and B.M. REDDY, 1991 : Response of equatorial and low-latitude F-region to the great magnetic storm of 13 March 1989, Annales Geophysicae 9, 286- 290.

1131 D. R. LAKSHMI, B.VEENADHARl, R.S. DABAS, B.M. REDDY, 1997: Sudden post-midnight decrease in equatorial F-region electron densities associated with severe magnetic storms, Annales Geophysicae 15, 306 - 313.

[ 14] 1.1. LEE, K.W. MIN, Y.P. KIM, Y. HEGAI, K.l. OYAMA, F.J . RICH, and J. KIM, 2002 : Large density depletions in the nighttime upper ionosphere during the magnetic storm of July 15 , 2000, Geophysical Research Letters 29, I 0.1029/200 1GLOI3991.

115] LE HUY MINH, TRAN THI LAN, PHAM THJ THU HONG, 2004 : Bao tu va bao di~n ly ngay 6-7/2000 va ngay 31 /3/2001 quan sat dtr<;J'C tai Phu Thuy , Tap chi cac Khoa hoc ve Trai Da't, 26, 2, 122-135.

116] Y. SAHAI, P.R . FAGUNDES, J.A. BITTENCOURT, and M.A. ABDU, 1998 : Occurrence of large scale equatorial F-region plasma depletions during geo-magnetic storm, Journal of Atmospheric and Solar-Terrestrial Phys ics 60, 1593 - 1604.

[171 Y. SAHAI , P.R. FAGUNDES, HOANG THAI LAN et a! , 2005 : Effects of the major geomagnetic storms in October 2003 on the equatorial and low latitude F-region in two longitudinal sectors, Journal of Geophysical Research - Space Physics, 2004JA010999.

f 18] J.H. SASTRl, K. NIRANJAN and K.S. SUBBARAO, 2002 : Response of the equatorial ionosphere in the Indian (midnight) sector to the severe magnetic storm of July 15, 2000, Geophysical Research Letters 29, I 0.1029/2002 GL015 133.

119] G.O, WALKER and Y.W. WONG, 1993 : Ionospheric effects observed throughout East Asia of the large magnetic storm of 13 -15 March 1989,

Jour~af of Atmospheric and Terrestrial Physics 55 , 7, 985 -1008.

(20] YINN-NIEN HUANG and KANG CHENG, 1993: Ionospheric disturbances around East Asia region during the 20 October 1989 magnetic storm, Journal of Atmospheric and Terrestrial Physics 55, 7, 1009- 1020.

SUMMARY

Equatorial ionospheric response to the during 17-19 of August 2003 geomagnetic storm observed in

Hochiminh city

The effects of magnetic storm on the ionosphere are complex and large deviations from apparent average behaviors. These effects are common in any particular storm. The interest in understanding the response of the upper atmosphere to geomagnetic storms has stemmed from the need to predict the ionospheric response. This need arises for practical reasons: communication systems require signals to pass from ground - to - ground using HF radio via the ionosphere, and from ground - to satellite through the ionosphere at higher frequencies. Ionospheric disturbances may cause the influence on everyday human activity. In the low latitude regions, where many people live, the investigations related to these disturbances are of great importance.

Ionospheric sounding measurements from Ho Chi Minh City (Long.106.3°E; Lat.10SON , dip lat. 2.9°N), Vietnam, are presented for the period August 17 to 19, 2003. The observatory equipped with a Canadian Advanced Digital lonosonde (CADI) and a Japanese Fluxgate Magnetometer. The period selected for the present study related to storm effects on equatorial F-region parameters and irregularities includes the intense geomagnetic storm on August 18. A comparative study of the F­region ionospheric data from the Hoc Mon observatory for both the quiet and disturbed conditions are presented and discussed in this communication . It is pointed out that on the magnetic disturbed period the foF2 values are increased mostly during daytime. The h'F variations show a rapid and large uplifting during nighttime. Simultaneous with the maximum depression of magnetic field, the falls in h'F and foF2 were observed . The variations in peak heights hpF2 during magnetic storm period are not as striking as those in h'F , but are often characterized by significant fluctuations.

Ngay nh(m bai: 11-9-2004 Phdn vi¢n Vat ly tai Tp. H6 Chi Minh

281