200 Tuy§n tap cac cong trinh nghien cu-u Vat ly Dia c i u 2008
MOT SO DAC
T R I T N GCUA BIEN THIEN NGAY DEM TRlTOfNG ' DIA TLT TREN LANH THO VIET NAM
V6 Thanh Sun Vien Vdt ly Dia cdu
18 Hodng Qudc Viet, Cdu gidy, Hd Npi Mail: vtson@igp. vast. ac. vn
Tdm tdt: Trong khodng thai gian 15 ndm (1989-2003), vai nhicng ngdy truang tic yen tinh (Ap < ~) vd sd lieu ghi biin thiin tic tgi hai trgm dia tic Bgc Liiu (ndm trong viing xich dgo tie) vd trgm dia tie Phu Thuy (ndm ngodi viing xich dgo tic), chiing tdi dd xdc dinh duac tdng sd 422 CEJ (Counter Electrojet - Ddng dien ngicac xich dgo) vai hai kiiu xudt hien: CEJ xudt hien vdo budi sdng trong khoang thai gian 6' - 7^ (gia dia phuang) vd CEJ xudt hien vdo budi chiiu trong khodng thai gian 15^-17' (gia dia phuang).
Sic xudt hien ciia CEJ nhiiu nhdt vdo ndm hoat dpng mat trai yiu vd it nhdt vdo nhicng ndm mgt trdi hogt dpng mgnh. Hien tuang CEJ xudt hien vdo budi sdng Id nhiiu nhdt vdo nhicng ndm mat trai hogt dpng mgnh, trong khi dd hien tuang CEJ xudt hien vdo budi chiiu Id nhiiu nhdt trong nhicng ndm mat trdi hogt dpng yeu. Su phu thupc theo miia ciia CEJ Id: CEJ xudt hien vdo budi sdng Id nhiiu nhdt vdo miia ddng cdn CEJ xudt hien vdo budi chiiu Id nhieu nhdt vdo miia he. Chiing tdi dd nghiin cicu su xudt hien cua ddng diin nguac xich dgo igi hai vimg xich dgo tie Id Viet Nam vd An Dp trong ndm Mat Trdi hogt dpng mgnh (1990) vd trong ndm Mat trai hogt ddng yeu (1996). O cd Viet nam vd An Dp thi sicxudt hien ciia CEJ vdo ndm mat trdi hogt dpng yiu Id nhiiu han so vdi ndm mat trai hogt ddng mgnh;
CEJ xudt hien a viing An Dp Id nhiiu hon so vdi a viing Viet nam; su xudt hien ciia CEJ vdo budi sdng a Viet Nam Id it nhdt vdo miia he nhung a An Dp thi Igi it nhdt vdo thai ky phdn diem; ddi vdi CEJ xudt hien vdo budi chiiu thi a cd hai vimg Viet Nam vd An Dp nd xudt hien nhiiu nhdt vdo thai ky phdn diim. Chimg tdi dd nghien cicu dnh huang ciia ddng diin nguac xich dgo ddi vai biin thien tic SqZ ted trgm dia tie Dd Lat trong khodng thai gian 15 ndm (1989 - 2003).^ Anh huang ciia CEJ ddi vai biin thien tie ngdy dim yen tinh thdnh phdn ndm ngang H Id lan han so vdi dnh huang ciia nd ddi vdi biin thien ciia SqZ. Chiing tdi cimg da nghien cicu quy ludt biin ddi ciia biin thien tic ngdy dim yen tinh thdnh phdn thdng dieng Z (SqZ) trin ca sd sd lieu trung binh gid tgi bdn trgm dia tic trin lanh thd Viet Nam trong vdng 15 ndm (1989 - 2003). O Viet Nam, bien dp ciia SqZ tgi trgm dia tie Dd Lat (ddy Id trgm dia tic ndm a vimg ria cua xich dgo tic) Id lo'n nhdt so vai cdc trgm khdc. Biin thien cua SqZ trong thai ky mgt trdi hogt dpng mgnh la lan han so vdi thai ky mgt trdi hogt ddng yiu. Trong tieng ndm, biin dp cua SqZ Id lan nhdt vdo thai kj' phdn diem. Qua viec nghien cim nhiiu loan tic
Phan II: DjATU"- DIEN LY
hinh vinh tgi hai trgm dia tic .Sa Pa vd Phii Thuy trong khodng thai gian II nam (1987-1997) chiing tdi thdy rdng nhiiu logn tic hinh vinh ciig thdnh phdn thdng dicng Z liidn luon nguac pha vdi nhiiu logn tic hinh vinh ciig
thdnh phdn ndm ngang H.
Tir khoa: trucmg dia tie, ddng dien xich dgo, ddng diin nguac xich dgo, biin thien tic ngdy dimyin tinh, nhiiu logn tic hinh vinh.
[. Mff dau
Viet Nam nam a vimg vT do thap va xich dao tir, do do viec nghien cuu quy luat bien doi ngay dem ciia truong dia tir bien thien; chang ban nhu nghien cim anh hu6ng ciia d6ng dien xich dao (equatorial electrojet - EEJ) va d6ng dien ngugc xich dao (counter electrojet - CEJ) doi voi bien thien ngay dem ciia truong dia tir, bien thien tir hinh vinh,...; Iu6n la van de dugc gioi khoa hoc Viet Nam va thi gioi dac biet quan tam.
Da CO mgt so nghien ciai ve anh hu6ng ciia d6ng dien xich dao doi voi truong dia tir bien thien ngay dem tren lanh tho Viet Nam [5, 6, 12, 13], tuy nhien van de anh hirong ciia d6ng dien ngugc xich dao d6i voi tru6ng dia tir bien thien ngay dem van c6n kha moi me [15-17]. Quy luat bien d6i ciia bien thien ngay dem yen tinh ciia thanh phan thang dimg Z cung chua dugc quan tam nghien ciru nhieu, gan day moi c6 mgt s6 c6ng trinh de cap den van de nay [5, 13]. tuy nhien so lieu diing de nghien cim c6n it. Mat khac nhirng bien thien tir c6 chu ky tir vai chuc phiit den vai ba gio, chang ban nhu nhieu loan tir hinh vinh, van cdn chua dugc quan tam nghien cim nhieu tren lanh tho Viet Nam, moi c6 mot cong bo ve nhieu loan tir hinh vinli tren lanh tho Viet nam [13], tuy nhien thoi gian va so lieu sir dung c6n It.
Trong khu6n kho bai bao nay, chimg t6i niuon trinh bay ket qua nghien cim ve hien tugng d6ng dien ngugc xich dao va anh huong ciia no doi vo'i bien thien tir ngay dem yen tinh ciia thanh phan nam ngang H va thanh phan thang dimg Z, mot so ket qua nghien ciru thing ke ve bien thien tir ngay dem yen tinh thanh phan thang dimg Z tren lanh thi Viet Nam trong v6ng 15 nam tir 1989 den 2003; va ket qua nghien cuu thong ke vl nhieu loan tir hinli vinh tai hai tram dia tir Sa Pa va Ha Ngi trong khoang thofi gian mot chu ky boat dong Mat Troi (11 nam, tir nam 1987 den nam 1997). D I SO sanh hien tugng CEJ quan sat dugc a nuoc ta voi hi^n tugng CEJ quan sat dugc o' vimg xich dao tit khac tren thi gioi, chiing toi cung trinh ba\- mot so ket qua nghien cim ve quy luat xuit hien ciia CEJ 6 Viet Nam va An Do trong nam Mat Troi boat dong manh (1990) va jrong nam Mat Tro'i boat dong yeu (1996).
2. Dong dien nguffc xich dao
Viino xich dao tir la viing c6 truong bien thien tir khac biet so voi viing vT do trung l^inh va vi do cao. Mpt trong nhirng khac biet do la sy xuat hien ciia ddng dien ngugc
*ich dao. Lan diu tien vao nam 1962, Gouin [3] da mo ta tru6ng hgp dao ngugc dinh
^ biln thien thanh phin nam ngang H vao thai diem gan bii6i trua theo gi6f dia Phuong tai dai dia tir Addis Ababa thuoc viing Trung Phi nam a khu vyc xich dao tir.
202 Tuy§n tap cac cong trinh nghien cu-u Vat ly Dja c l u 2008
Den nam 1967, Gouin va Mayaud [4] da mo ta hien tugng nay khi quan sat gian do tii tai Addis Ababa va ggi no la d6ng dien ngugc xich dao. Nhirng nghien cim sau do cua cac nha khoa hgc dia tir tren the gioi da khang dinh hien tugng dong dien ngugc xich dao chi t6n tai trong vimg xich dao tir va doi khi dugc Idioanh vimg trong mgt dai hep dgc theo kinh tuyen (nho hon 2 gio) [9, 10].
2. 1. Phuang phdp nghien ciru vd co' sd so lieu
Muon nghien ciai bien tugng dong dien ngugc xich dao, chiing ta can phai co so liei ghi bien thien tir ngay dem tai mot tram nam trong viing xich dao tir va mgt tram nam ngoai vimg xich dao tir tren ciing mgt kinh tuyen. Chimg toi da sir dung so lieu bien thien tir ngay dem tai hai tram dia tir Bac Lieu va Ha Ngi de phuc vu cho viec nghien ciru.
D6ng dien ngugc xich dao huong ve phia Tay, vi vay no anh huong tryc tiep va ro net nhat den bien thien ciia thanh phan nam ngang H tai cac tram dia tir nam trong vimg xich dao tir. Doi voi moi mgt ngay, de xac dinh mgt each day dii bien thien ngay dem ciia thanh phan nam ngang H, chiing t6i da lay gia tri trung binli gio ciia Fl trir di mirc trung binh giti'a dem ciia ngay do. De tien cho viec sir dung sau nay, chiing toi ky hieu bien thien ngay dem ciia thanh phan H la AH, bien thien ngay dem ciia thanh phan H tai Bac Lieu la AHBI., bien thien ngay dem ciia thanli phan H tai Ha Ngi la AH[-[N. Voi muc dich xac diiili dugc nhung ngay xuat hien d6ng dien ngugc xich dao mgt each ro net va tranh anh hirong ciia nhieu nguon khac nliau. chimg toi da chgn nhung ngay truong tir yen tinh voi chi so boat dgng tir trung binh A,, < 7.
Nhung nghien ciai ciia cac nha khoa hgc tren the gioi [8, 9, 10] da khing dinh ring ngay xuat hien dong dien ngugc xich dao la ngay ma trong thoi gian ban ngay hieu so ciia AH tai mgt tram nam trong vimg xich dao tir trir di AH tai mgt tram khac nim ngoai vimg xich dao tir c6 gia tri be hon 0 (hai tram nay phai nam gan nhu tren ciing mgt kinh tuyen). Vi nhirng ly do tren, de xac dinh dugc nhung ngay xuit hien dong dien ngugc xich dao tai Viet Nam, chiing toi da chgn nhung ngay yen tinh co chi sl boat dgng tir trung binh Ap < 7 va vao thoi gian ban ngay hieu so ciia AH tai Bac Lieu trir di AH tai Ha Ngi co gia tri am, tire la (AHBL - AHHN ) < 0,
Chimg ta deu biet rang vao thai gian ban ngay, tai vimg xich dao tir luon ton tai mot d6ng dien hirong ve phia Dong ma nguoi ta goi la dona dien xich dao, don^ dien nay lam cho bien thien ciia thanh phan nam ngang H tai cac tram dia tir a viing xich dao tir CO bien do Ion hon han cac tram nam ngoai vimg anh huong ciia xich dao tir, dac biet la trong nhirng ngay truong tir yen tinh, Chinh vi vay, trong thoi gian ban ngay hieu img am ciia AH tai !hgt tram nam trong viing xich dao tir trir di AH tai mgt tram khac nim ngoai viing xich dao tir chi co the dugc giai thich bang sy tin tai mgt dong dien c6 hirong ngugc lai voi d6ng dien xich dao va co cuong do Ion hon dong dien xich dao.
Tram dia tir Bac Lieu chinh thirc boat dgng tir culi nam 1988, con tram dia tir Ha Ngi bat dau hoat dgng tir nam 1967, vi vay chiing toi da tiln hanh nghien cim dono dien ngugc xich dao tren lanh tho Viet Nam tir nam 1989 din nam 2003.
Tmoc tien chimg toi tien hanh tinh toan hieu sl (AHBL - AHHN ) tren co so gia tri trung:
binh gio da quy toim tai hai tram dia tir Bac Lieu va Ha Ngi. Sau do tir sl lieu boat dgng til
Phin II: DjA T i r - DIEN LY 1Q2
ciia Trung Tam so lieu Qu6c te c6ng bo, chimg t6i da chgn tat ca cac ngay biln thien hi ngay dem yen tinh co gia tri trung binh ciia chi sl boat dong hi Ap < 7. Tir cac ngay biln thien tix yen tinh, ket hgp vai_ gia tri hieu sl (AHBL - AHHN ) ciia cac ngay nay chimg toi xac dinh dugc nhung ngay xuat hien d6ng dien ngugc xich dao.
2. 2. Phdn tich ket qud
Trong v6ng 15 nam tir 1989 din 2003, chimg toi da xac dinh dugc ting sl 422 hien tugng d6ng di?n ngugc xich dao (CEJ) tren lanh thi Viet Nam trong do c6 196 hien tugng CEJ xuat hien vao bull sang va 226 hien tugng CEJ xuit hien vao bull chilu [15]. Hinh ye 1 bieu dien mgt so hien tugng CEJ xuat hien trong khoang thoi gian chung toi tien hanh nghien cim. Nhin chung hien tugng d6ng dien ngugc xich dao xuit hien chii yeu vao thai dilm 6h - 7h sang va 15h - 17h chilu (theo thai gian dia phuong).
Thuong thi hien tugng CEJ xuat hien vao buoi chieu keo dai hon so voi bu6i sang va anh huong ciia no doi voi bien thien ngay dem thanh phan H ciing Ion hon (thong qua hieu so (AHBL - AHH^' )). Hau bet sy xuat hien ciia hien tugng CEJ la vao buoi sang hoac vao buoi chieu ciia timg ngay rieng re, tuy nhien ciing co mot so ngay hien tugng CEJ xuat hien dong thoi ca vao bu6i sang va buoi chieu (o day chiing toi da thong ke dugc 28 ngay xuat hien dong thai CEJ vao ca buoi sang va buoi chieu trong khoang thai gian tir 1989 den 2003). Nhirng ngay xuat hien CEJ vao ca buoi sang va buoi chieu duong nhu khong tuan theo mgt quy luat nao ca.
Mgt dac diem de dang nhan thay la vao nhiing nam boat dgng mat tr6i yeu, sy xuat hien cua dong dien ngugc xich dao nhieu hon (hinh ve 2). Dieu nay ciing phii hgp voi nhiing nghien cim ciia cac nha khoa hgc khac tren the gioi [10]. Mat khac chimg toi thay rang vao nhirng nam mat Uoi boat dong yeu thi sy xuat hien ciia hien tugng CEJ vao buoi sang la it hon so voi nhung nam boat dong mat tr6i manh. 0 nhiing nam boat dong mat Uoi ylu thi hien lugng CEJ xuat hien vao bu6i chieu la nhieu hon.
Phin Ion anh hu6ng ciia hien tugng CEJ doi voi bien thien ngay dem thanh phan H tai Viet Nam chii yeu la duoi 10 nT, doi khi anh hu6ng ciia no ciing co the dat toi mgt vai chuc nT. Trong th6'i gian xuat hien hien tugng CEJ vao buoi sang, gia tri thap nhat ciia biln thien ngay dem thanh phan H so voi mirc giira dem la am; trong khi d6 doi voi hien tugng CEJ xuat hien vao buoi chieu thi gia tri thap nhat ciia bien thien ngay dem thanh phin H so voi mire giira dem nhieu khi la duong, tham chi co the dat toi mot vai chuc nT. Dilu nay da dugc Rastogi [4, 6] giai thich rat ro rang boi sy bien mat ciia lop Es-q tren ting dien ly tren co so s6 lieu dien ly ghi dugc tai tram dien ly Thumba (tram nay nim trong viing xich dao tir) va tai tram dien ly Kodaikanal (tram nam ngoai vimg Xich dao tir) tren lanh tho An Do.
Kit qua thins ke cung cho thay doi voi hien tugng CEJ xuat hien vao bu6i sang thi Vao miia he (cac thang 5, 6, 7 va 8) n6 xuat hien it nhat va vao miia d6ng (cac thang I, 2, 11 va 12) no xuit hien nhieu nhat, Doi voi hien tugng CEJ xuat hien vao buoi chieu fti vao miia he no lai xuit hien nhieu nhat va dac biet ro net vao nhiing nam boat dgng pat troi yeu.
204 Tuyln tap cac cong trinh nghien CLFU Vat ly Dja clu 2008
1989
I I I I I I I I I I I I I I I I I M I I I I I I I I I I I I I
0 2 4 6 8 10 12 14 16 18 20 22 24 L T 0 2 4 6 8 10 12 14 16 18 20 22 24 L T 0 2 4 6 8 10 12 14 16 18 20 22 24 L T
1995
I M I I I II M II
0 2 4 6 8 10 12 14 16 18 20 22 24 L T 0 2 4 6 8 10 12 14 16 18 20 22 24 L T 0 2 4 6 8 10 12 14 16 18 20 22 24 L T
nT
200 — 160 — 120 —
80 — 40 — 0 —
H
1 1 1
1998
1 i 1 1 1 1 1 1
0 2 4 6 a 10 12 14 16 13 20 22 24 L T 0 2 4
I I I I
10 12 14 16 18 20 22 24 L T 0 2 4 6 8 10 12 14 16 18 20 22 24 L T
nT
200 — 160 — 120 — 80 — 40 — 0 —
H
1 1 1 2001
1 1 i 1 i 1 1 1 1 I 1 I I i M 1 I I I I I I II II I I I I I
0 2 4 6 8 10 12 14 15 18 20 22 24 L T 0 2 4 6 8 10 12 14 16 13 20 22 24 L T 0 2 4 6 8 10 12 14 16 18 20 22 24 L T
Hinh 1. Vi du y§ s u x u i t hien ciia dong dien ngug-c xich dao theo s6 lieu cua thanh phB n i m ngang H tai tram dia tii' Bac Lieu tii- nam 1989 d i n nam 2003.
"phin II: DIA Tir - DIEN LY
4 0
1988
20
205
So' lan xuat hien CEJ theo n a m
1992 1996 2000 Chi sd A p trung binh n a m
2004
10
1988 1992 1996 2000
Sd vet den mat trdi trung binh nam
2004
100
1988 1992 1996 2000 2004
Hinh 2. Su xuit hien hien tugng CEJ, chi so hoat dong tie Ap trung binh nam va s l vlt den mat trcyi theo nam.
3. So sanh hien tuang dong diin nguac xich dao a VietNam vd An Dp
Bk so sanh bien tugng CEJ quan sat dugc a nuoc ta voi hien tugng CEJ quan sat roc o vimc xich dao tir khac tren the gioi, chiing toi trinh bay mgt s6 ket qua nghien fu vl quy luat xuit hien ciia CEJ o Viet Nam va An Do trong nam Mat Troi boat dgng anh (1990) va trong nam Mat Troi hoat dgng yeu (1996). Tai Viet Nam, chung toi da Uuncr s l lieu biln thien tir ngay dem tai hai tram dia tir Bac Lieu (nam trong viing ch dao tir) va Phii Thu\' (nim ngoai vimg anh huong ciia xich dao tir); a An Do chiing ida sir dung sl lieu biln thien tir ngay dem tai hai tram dia tir Trivandrum (nam trong
•ng xich dao tir) va Alibag (nam ngoai viing anh hirong ciia xich dao tir) de phuc vu pyiec nghien ciru.
206 Tuyin tap cac cong trinh nghien cu'u Vat ly Dja c l u 2(M
2. 3. I. Cdc dgc diim gidng nhau vi sicxudt hien CEJ tgi Viet Nam vd An Dp
Nhin chunR. sy xuit hien ciia d6ng dien ngugc xich dao chii yeu vao buoi sai^
(khoang 6^ - 1^) va buli chilu (khoang 15'' - H*") theo gio dia phuong o ca VigJ Nam va An Do. Vao nam Mat Troi boat dgng manh (1990) sy xuat hien cua CEJlj it ban bin so vo'i nam Mat Troi boat dgng yeu (1996). Cu the la o Viet Nam nam
1990 quan sat dugc 19 lin xuit hien CEJ trong khi do nam 1996 quan sat dugc 37 lin; c6n tai An Do nam 1990 quan sat dugc 59 Ian xuat hien CEJ trong khi do nam 1996 quan sat dugc 101 lin (bang 1). Dilu nay cung phii hgp voi nhtrng nghien ciru vl hien tugng CEJ tren thi gioi [10]. Nhiing ngay c6 sy xuat hien ciia CEJ vao ca buli sang va buli chilu trong nam Mat Troi boat dgng yeu (1996) la nhieu hon so voi nam Mat Troi boat dong manh (1990). Ket qua thong ke ciing cho thay hien tugng CEJ 6 ca Viet Nam va An Do xuat hien it hon vao thai ky phan diem. Trong nam Mat Trai boat dgng ylu (1996) thi sy xuat hien ciia CEJ vao buoi sang a thai ky miia dong lai it hon so voi nam Mat Troi boat dgng manh (1990). Trong khi do hien tugng CEJ xuat hien vao buoi chieu lai nhieu nhat vao thoi ky miia he. 0 zi hai viing Viet Nam va An Do, doi voi hien tugng CEJ xuat hien vao buoi chieu thi chiing xuat hien nhieu ban trong nam Mat Troi hoat dgng yeu (1996) 6 ca ba miia trong nam (miia dong, miia he va phan diem). Cac ket qua nay cQng phii hgp voi quy luat xuat hien theo miia ciia CEJ da dugc nghien tren the gioi [9]. Nhirng ngaj quan sat dugc hien tugng xuat hien CEJ 6f Viet Nam thi hau nhu cung deu quan sal dugc o' An Do (bang 1), dieu nay da cho phep khang dinh sy ton tai ciia d6ng dien ngugc xich dao (d6ng dien huong Tay) 6f vimg xich dao tir
Bang 1. K i t qua t h i n g ke hien tugng CEJ a Viet Nam va An Do theo cac mua trong namj
Qu6c Gia
Viet Nam An Dp
•
Nam
1990 1996 1990 1996
Tong s6 Ngay
19 34 52 71
T6ng s6 CEJ
19 37 59 101
Dong
6 3 23 21
Buoi He
0 1 11 23
s^ng Phan diem 2 1 10 10
Tong s6
8 5 44 54
Dong
1 11 1 11
Bu6i He
7 14 14 27
chi4u Phan diem 3 7 0 9
Tong s6 11 32 15 47
Ca sang
chilu 0 3 7 30 ,
\
Anh huong ciia dong dien ngugc xich doi voi truong bien thien tir cua thanh pha nim ngang H 6 Viet Nam va An Do (thong qua viec so sanh hieu s l cua (AHBL - AHp va (AHTRI - AHALI) phan Ion la be hon 10 nT; chi c6 mgt sl ngay anh huong nay co' dat toi vai chuc nT.
Ph^n II: DjA TU" - DIEN LY
207
150 -^
100 -^
50 —
H / -
1 1 1 1 1 1 1 1 1 1 1
\ CEJ 1 ' 1 ' 1 ' •
Bac Li£u 1 - 7 . 1990
' 1 ' 1 ' 1
Hinh 3. Vi du v l s u x u i t hien cua dong dien nguo'c xich dao tai Bac Lieu v^ Trivandrum cac ngay 1 - 7 -1990 va 29 -12 -1996.
Z 3. 2. Cdc dgc diim khdc nhau ve suxudt hien CEJ tgi VietNam vd An Dp
Su khac nhau ro ret nhat ciia hien tugng CEJ a Viet Nam va An Do la su xuat hien ciia CEJ tai An Do la nhilu hon hin so vod a Viet Nam (trong hai nam 1990 va 1996, a Viet Nam thing ke dugc 56 Ian xuat hien ciia CEJ; trong khi do 6 An Do sy xuat hien cua CEJ la 160 lin). Dilu nay c6 the dugc gia dinh la do d6ng dien xich dao (equatorial electrojet - EEJ) a vimg Viet Nam la manh hon so vod vimg An Do; day la dong dien co huong Dong a
<Jo cao khoang 107km co huong ngugc vod huong cua CEJ 6 do cao khoang 100km [10, 11], vi vay no lam cho anh huong ciia CEJ doi vod truong bien thien hi cua thanh phan nam ngang H bi giam di. Mat khac, gia tri ciia thanh phan nam ngang H tai Trivandrum ciia An
£)o (Idioang 39900 nT) ciing be hon so vod tai Bac Lieu cua Viet Nam (khoang 41000 nT) Va no ciing co thi gop phan lam cho sy xuat hien ciia CEJ a An Do la nhieu ban. Tuy nhien
•iay chi la nhimg gia dinh ciia chimg toi, de hieu tuong tan ban ve van de nay co le chimg ta cin phai nghien cim sl lieu bien thien tir va s6 lieu dien ly co dong th6d tai nhieu vimg xich
^0 hi khac nhau tren the giod.
Tai Viet Nam. nhung ngay co CEJ xuat hien ca vao buoi sang va buoi chieu la it ban nhilu so vdi o An Do (6 Viet Nam thing ke dugc 3 ngay vao nam 1996, Uong khi do a An
208 Tuyln tap cac cong trinh nghien CLPU Vat ly Dia c l u 2008
Do thong ke dugc 37 ngay va rieng nam 1996 co toi 30 ngay). Mat khac co the thay rang a Viet Nam, hien tugng CEJ xuat hien vao buoi chieu la nhieu hon so vod buoi sang trong ca hai nam 1990 va 1996; trong khi do o An Do, hien tugng CEJ xuat hien vao buoi chieu lai it hon so vod buoi sang vao nam Mat Trdi boat dgng manh (1990) va dilu nay phii hgp vdi kit luan ciia Rastogi [9].
Quy luat xuat hien theo miia ciia hien tugng CEJ tai Viet Nam va An Do con co khac biet niia la: doi vdi hien tugng CEJ xuat hien vao buoi sang thi d Viet Nam vao miia he la it nhat, trong khi do d An Do thi no xuat hien it nhat vao thai ky phan dilm; dii vdi hien tugng CEJ xuat hien vao buoi chieu thi d An Do xuat hien it nhit vao thdi ky phan dilm, Uong khi do d Viet Nam quy luat nay kh6ng dugc the hien ro ret (vao nam Mat Trdi boat dgng yeu thi xuat hien it nhat vao thdi ky phan diem, nhung vao nam Mat Trdi boat dgng manh thi no lai xuat hien it nhat vao miia dong). Co le nhung sy khac nhau nay mang tinh dia phuong va no ciing phii hgp vdi ket qua nghien cuu ve sy xuit hien theo miia cua CEJ d cac khu vyc khac nhau tren the gidi (vi du nhu vimg Nam My, vimg Tnmg Phi hay vimg An Do) [9].
C6 nhung ngay mac dii deu quan sat dugc sy xuat hien ciia CEJ nhung d Viet Nam no xuat hien vao buoi chieu, trong khi do tai An Do lai quan sat dugc vao buli sang va ngugc lai CO ngay d Viet Nam hien tugng CEJ xuat hien vao buoi sang thi d An Do lai thiy no xuat hien vao buoi chieu. Trong hai nam 1990 va 1996 co ting sl 14 ngay xay ra hien tugng nay va chi co 1 ngay (ngay 28-4-1996) la d Viet Nam hien tugng CEJ xuit hien vao buoi sang c6n d An Do lai thay no xuat hien vao buoi chieu. Trong nam Mat Trdi boat dgng manh (1990) thi hien higng CEJ xuat hien vao cac thdi diem khac nhau (buoi sang va buli chieu) Uong cimg mgt ngay d Viet Nam va An Do la It hon bin (4 ngay trong ting sl 14
ngay thong ke dugc) so vdi nam Mat Trdi boat dgng yeu (1996). ' Co mgt sl ngay anh hudng ciia CEJ doi vdi trudng biln thien tix cua thanh phin nim
ngang H d An Do la Idn ban so vdi d Viet Nam th6ng qua viec so sanh hieu sl ciia (AHBL -
AHPT) va (AHTRI - AHALI); tuy nhien cung c6 mgt so ngay anh hudng ciia CEJ d Viet Nam lai Idn hon.
Theo quan diem cua chung toi nhung dac diem khac nhau vl sy xuit hien cua hien tugng d6ng dien ngugc xich dao cung nhu anh hudng cua no doi vdi biln thien hi ciia thanh phin nim ngang H d Viet Nam va An Do la nhung dac trung mang tinh dia phuong ciia moi vimg.
2. 4. Anh hudng cua CEJ doi v&i bien thiin tu- ngdy dim yin tinh cua thdnh phdn thdng dung Z
Chiing ta diu biet rang d6ng dien ngugc xich dao tin tai vao thdi gian ban ngay va CO hudng Tay [3], vi vay n6 lam cho trudng biln thien tir ciia thanh phin n°im ngang H d viing xich dao tir bi giam di, ding thdi no lai lam cho trudng biln thien tir ciia thanh phan thang dirng Z d vimg ria xich dao tir tang len. Vi Iy do°nay va tranh anh hudng ciia nhirng trudng ben ngoai co nguln glc khac nhau chiina 'tdi da sir dung so lieu SqZ tai tram Dia tir Da Lat dk nghien ciru. 0 phin tren chiina toi da trinh bay ket qua nghien ciru anh hudng ciia dong dien nguoc xich dao dii vdi
Phin II: DjA TLT - DIEN LY
209
trudng bien thien tir ciia thanh phin nim ngang H trong khoang thdi gian tu nam 1989 den nam 2003 [15]; vi vay tren co sd nhirng ngay xuat hien CEJ chiing toi da danh gia anh hudng ciia no dii vdi trudng biln thien tir ciia thanh phan thing dung Z. M^t khac do s6 lieu ciia tram Dia tir Da Lat vao nhirng gid buoi sang sdm thuong bi mat (nguyen nhan mat c6 thi la do chit lugng cua giay anh khong tot hoac ciing CO the la do thay giay anh vao thdi dilm xung quanh 7h sang gid dia phuang), vi vay de danh gia mgt each khach quan anh hudng ciia CEJ doi vdi SqZ, chiing toi chi nghien ciru anh hudng ciia CEJ xuit hien vao buoi chieu doi vdi bien thien tir cua thanh phan thang dung Z trong nhirng ngay yen tinh. Chung ta deu biet rang bien thien tir ciia thanh phan thang dirng Z trong nhtrng ngay yen tmh bao gid ciing giam dan khi mat trdi mgc va giam den cue tieu vao buoi trua, sau do no bat dau tang len va dat mirc khong vao khoang thdi gian giua dem [10]. Nhu vay vao thdi gian ban ngay khi xuat hien d6ng dien ngugc xich dao thi d thdi diem d6 gia trj ciia bien thien tir thanh phan thang dirng Z se khong tuan theo quy luat chung nira ma no se tang manh (no khong giam manh nhu dii vdi bien thien tir ciia thanh phan nam ngang H) so vdi nhiing gia tri xung quanh thdi diem xuat hien CEJ.
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4 I I 10 11 14 I I 18 10 22 24 LT 0 2 4 « I 10 11 14 H 18 10 11 14 LT 0 1 4 8 8 10 11 14 18 IS 10 11 14 LT
Hinh 4. Anh huo-ng cua CEJ d i i vo'i biln thien tif SqZ tai tram Da Lat.
2 1 0 ' Tuy§n tap cac cong trinh nghi§n cu'u vat ly Dja c l u 2008
Tren co sd kit qua nghien cim sy xuit hien ciia CEJ trong khoang thdi gian tix nam 1989 din nam 2003 ma chimg toi da tiln hanh [11], chimg toi da danh gia anh hudng cua CEJ doi vdi bien thien tir ngay dem yen tinh thanh phan diang dung Z tai tram Dia tir Da Lat trong nhirng ngay CEJ xuit hien vao buli chilu. Hinh ve 4 la mgt so vi du bieu dien nhiing ngay xuit hien CEJ anh hudng den biln thien hir SqZ tai tram Dia tir Da Lat trong khoang thdi gian chiing toi tien hanh nghien cim. Nhin chung, anh hudng cua CEJ doi voi bien thien tir SqZ la nho hon 10 nT, tuy nhien co mgt so ngay anh hudng cua nd co the han 10 nT tham chi dat tdi 20 nT. Mgt kho khan cho chimg toi la so lieu bien thien tir tai tram Da Lat khong co trong mgt so ngay xuat hien CEJ, vi vay chimg toi khong danh gia anh hudng ciia no theo nam hoac theo miia. Neu so sanh vdi anh hudng cua CEJ doi vdi bien thien tir ngay dem yen tinh thanh phan nam ngang H thi c6 the thay rang anh hudng cua no ddi vdi bien thien tir SqZ la nho hon. Co le day la dieu hoan toan hgp ly vi co nhieu ngay xuat hien CEJ ma n6 the hien ro d bien thien tir SqH nhung lai khong thay bieu hien d bien thien tir SqZ. Neu nhu sy xuat hien ciia CEJ vao buoi chieu trong khoang thdi gian tir nam
1989 den nam 2003 la 226 Ian doi vdi bien thien tir SqH thi ciing trong khoang thdi gian nay con so thong ke sy xuat hien CEJ doi vdi bien thien tir SqZ chi la 127 lan (cd 48 ngay
mat so lieu). i
3. Bien thien ngay dem yen tinh cua thanh phan thang dung Z (SqZ) tren lanh thi Viet Nam
Tren lanh tho Viet nam co bon tram dia tir: Sa Pa, Ha Ngi, Da Lat va Bac Lieu; frong d6 tram Dia tir Bac Lieu nam d vimg xich dao hi, tram Dia tir Da Lat nim d ria xich dao tix, c6n hai tram Dia hi Sa Pa va Ha Ngi nam d vimg vi do thap va khong chiu anh hudng cua dong dien xich dao ciing nhu d6ng dien ngugc xich dao. Dl tim hilu nhimg quy luat chung nhit cua bien thien tix ngay dem thanh phan thang dimg Z va tranh anh hudng cua nhilu trudng ben ngoai co nguon goc khac nhau, chimg toi da lya chgn biln thien ngay dem thanh phin thing dung Z trong nhimg ngay yen tinh (SqZ) dl phuc vu cho viec nghien cuu cua minh. Cu the trong nghien cuu nay, chimg toi da sir dung sl lieu biln thien tix SqZ tmng bmh thang cua hing nam (hi nam 1989 den nam 2003). Chiing tdi da nghien cuu quy luat biln doi theo hing nam, ciing nhu hing miia trong nam, cu thi la trong moi nam chiing toi diu tinh gia hd tnmg binh nam ciia SqZ, gia tri trung binh ciia SqZ d cac thdi dilm miia he, miia dong va phan diem. Mgt luu y la cac gia tri tnmg binh gid ciia biln thien tir SqZ diu dugc trir di mirc trung binh giiira dem (cac gid: 16, 17 va 18 UT).
Hmh ve 5 bieu dien gia Ui bien thien ngay dem yen tinh SqZ trung bmh nam tai cac tram dia hi Sa Pa, Ha Ngi, Da Lat ya Bac Lieu trong khoang thdi gian hi nam 1989 den nam 2003. Mgt dac diem de nhan thay nhat la biln thien ngay dem yen tinh SqZ tai tram Da Lat la Idn nhat so vdi cac ti-am khac. Quy luat nay la khac so vdi biln thien ngay dem yen tmh ciia thanh phan nam ngang H (bien thien ngay dem yen tinh SqH Idn nhit tai tram xich dao hi: Bac Lieu). Dieu nay c6 the dugc iy giai la do anh hudng ciia dong dien xich dao:
dong dien xich di^o co hudng Dong vi vay n6 lam cho biln thien cua thanh phin nim ngang H d Bac Lieu (nam d viing xich dao hi) tang manh va lam cho biln thien ciia thanh phin thang dimg Z d Da Lat (nam d vimg ria xich dao tix) ciing tang manh.
Ph^n II: DjA TU" - DIEN LY
211
^ Nhimg nam Mat Trdi hoat dgng manh biln thien SqZ tai cac tram Sa Pa, Ha Ngi, c6 the dat tdi hon -20 nT; d Bac Lieu dat tdi ban -30 nT; d Da Lat dat tdi -50 nT. Vao nhimg nam Mat Trdi boat dgng ylu, biln thien SqZ tai cac Uam Sa Pa, Ha Noi chi khoang gin -20 nT; d Bac Lieu chi khoang -20 nT; con d Da Lat khoang -30 nT.
Viec nghien cim quy luat biln dii theo cac miia trong nam ciing cho thiy SqZ biln dii manh nhat vao thdi ky phan diem. Dilu nay hoan toan la quy luat bien d6i ciia trudng bien thien hi d vimg vT do thip va xich dao tir.
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Hinh 5. B i l n thien tu" SqZ trung binh nam tai cac tram dja tii' tren lanh tho Viet Nam.
212 T u y l n tap cac c o n g t r i n h n g h i e n CCFU Vat ly Dja cau 2008
4. Dac diem cua nhieu loan tu hinh vinh tai Sa Pa va Ha Noi
Trong so cac bien thien tir co chu ky tir vai chyc phut den vai gid thi nhieu loan tir hinh vinh dugc the hien ro net nhat. Loai nhieu loan nay c6 dang nhu dudng bao quanh cua vinh bien nen dugc ggi la "nhieu loan hinh vinh". Nhieu loan tir hinh vinh xuat hien dong thdi d tat ca cac tram quan sat dia tir tren the gidi, co cudng do cue dai d viing cue quang va giam dan theo vT do [1]. Nguon goc ciia loai nhieu loan tir nay la he thong cac vong d6ng xuat hien vao chieu toi va gan sang d khoang vT do 70° tren tang dien ly [1].
Miln Bac Viet Nam co hai tram Dia tir Sa Pa va Ha Ngi boat dgng tir vai chuc nam trudc day. Tren co sd so lieu ghi trudng tir bien thien ngay dem ciia hai tram quan sat Dia tir nay, chung toi da tien hanh nghien cim mgt so dac trung thong ke cua nhieu loan til hinh vinh trong v6ng 11 nam (tir 1987 din 1997) [14].
4. 1. Thai gian xudt hien cua nhiiu loan tir hinh vinh tai Sa Pa vd Hd Npi J So lieu thong ke nhieu loan tir hinh vinh tai Sa Pa va Ha Ngi trong vong 11 nam cho thay
chimg xuat hien vao thdi gian chieu toi, dem va gan sang theo thdi gian dia phuong. Phan Ion nhieu loan tir hinh vinh xuat hien vao chieu toi va dem; chi co 18 nhieu loan tir hmh vinh xuat hien vao thdi diem rang sang (chiem khoang 7% tong so nhieu loan tir hinh vinh thong ke dugc). Cac nhieu loan tir hmh vinh the hien ro net nhat doi vdi thanh phan nam ngang H va co bien do khoang vai chuc nT. Nhin chung trong mgt nam nhieu loan fir hinh vinh xuat hien nhieu nhat vao thdi ky phan diem va xuat hien it nliat vao miia he. Trong so 257 nhieu loan tir hinh vinh thong ke dugc hi 1987 - 1997 thi co 104 nhieu loan tir hinh vinh xuat hien d thdi ky phan diem, co 100 nhieu loan tir hinh vinh xuat hien vao miia dong va chi co 53 nhieu loan tir hinh vinh xuat hien vao miia he. Trong mgt nam, bien do ciia nhieu loan tir hinh vinh ciia ba thanh phan D, H va Z ciing thuong la Idn nhat vao thdi ky phan diem va nho nhat vao miia he.
Doi vdi cac nam mat Udi boat dgng manh, bien do ciia nhieu loan tir hinh vinh ciing Idn hon.
Tai hai tram Sa Pa va Ha Noi bien do ciia nliieu loan tir hinh vinh chi chenh lech nhau vai nT va thuong thi d Sa Pa bien do ciia nhieu loan tir hinh vinh Idn hon.
30 - 28 • 25 -^'^ —
22 _ M -18 - ^
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56 Lin xuit n i i n nhi<*u hUiti "Trih
<i{}{\^ il\a tiJ Ap
1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998
Sfl vtfi Jea mi l iriri
1 ' I ' 1
1966 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998
Hinh 6. Su x u i t hien n h i l u loan tip hinh vjnh, chi s6 hoat dong dja tir Ap (a) va s l v l t den mat tro-i (b) ti> nam 1987 - 1997.
Phan II: DjA TU" - DIEN LY
213
Hinh 6 bieu dien so nhieu loan hinh vinh ma chiing toi thong ke dugc trong vong 11 nam ciing vdi chi so boat dgng dja tir Ap va sl vlt den mat trdi. Chiing ta thiy ring dudng nhu so lan xuit- hien hinh vinh va chi sl boat dgng dia tix Ap co chu ky 2 nam, dieu nay ciing phii hgp vdi nghien cim gin day ciia cac nha khoa hgc Nga [2]. Nhin ting the thi quy luat bien doi ciia sl vlt den mat trdi va chi so Ap la tuong doi phii hgp, chi CO sy khac biet d thdi diem dat gia tri eye dai. Trong khi do dudng nhu su xuat hien cua nhieu loan tir hinh vinh khong tuan theo quy luat cua boat dgng mat trdi.
4. 2. Su khdc biet vi pha cua nhiiu loan tu- hinh vjnh thdnh phdn Z tgi Sa Pa vd Hd Npi Bang 3. Gia trj bien do trung binh nam cua nhilu loan tCi' hinh vjnh tai Sa Pa va Ha Noi.
Nam
1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997
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12 11 12 14 16 15 18 21 15 12 12
(nT) Ha Noi
11 11 12 13 14 16 16 21 20 11 11
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-6 -7 -10 -10 -12 -12 -10 -14 -14 -9 -10
(nT) Ha Npi
6 8 12 12 14 15 12 16 16 10 12
Hinh 7. Vi du v§ nhilu loan t u hinh vjnh tai Sa Pa va Ha Noi ngay 1 thang 10 nam 1997.
Tir s l lieu nhieu loan tir hinh vinh thong ke dugc, chiing toi da tien hanh danh gia bien do trung binh nam ciia nhieu loan tir hinh vinh d ca ba thanh phan D, H va Z, kit qua nay dugc trinh bay d bang 2. Qua bang nay chiing ta thay rang bien do cua nhieu loan tir hinh vinh thanh phan D va thanh phan H tai Sa Pa da so la Idn ban d Ha Ngi. Day la hai thanh phan narn ngang ciia trudng tir biln thien, chiing phan anh chii ylu anh cua trudng tir do nguon so cap d ben ngoai Trai dat gay ra, do la h?
vono dbno sinh ra nhilu loan tir hinh vjnh ma chiing toi da de cap d phan tren. Ve mat gia tri tuyet dii, bien do ciia nhieu loan tir hinh vinh thanh phan Z tai Sa Pa trong da sl trudng hgp la nho hon d Ha Ngi.
2 1 4 Tuy4n tap cac cong trinh nghien cu'u Vat ly D j a c l u 2008
Nhu chimg ta da bilt, sy ngugc nhau vl pha ciia nhieu loan hi hinh vinh thanh phan Z tai cac dierh nam khong each xa nhau la do d6ng dien cam ling xuat hien trong dai di thuong co do din dien cao nSm d dudi dit gay ra [7]. Khi quan sat nhieu loan tir hinh vinh tai cac tram dia tir Sa Pa va Ha Ngi, chiing toi thiy ring nhieu loan cua thanh phan Z tai Sa Pa luon luon ngugc pha vdi thanh phin H. Trong khi do d tram dia hi: Ha Ngi, cac nhieu loan tir hinh vinh cua thanh phan H va Z luon cimg pha vdi nhau. Mat khac cac Uam dia tir Sa Pa va Ha Ngi nam d hai phia cua Song Hing. Nhu vay sy ngugc pha ciia nhieu loan tir hinh vinh thanh phan Z tai Sa Pa va Ha Ngi chi c6 the dugc Iy giai la do co sy ton tai mgt dai dan dien tot nam d khoang giiia hai tram Sa Pa va Ha Ngi khi trudng bien thien tir bien ddi nhanh.
5. Ket luan
Viec nghien ciiu mgt so dac trung thong ke ciia hien tugng dong dien ngugc xich dao d Viet Nam tren co sd so lieu bien thien ngay dem ciia thanh phan nam ngang H tai cac tram dia hi Bac Lieu va Ha Ngi da khang dinh mgt so quy luat xuat hien ciia hien tugng nay va anh hudng ciia n6 doi vdi trudng bien tliien tir. Mgt so quy luat chung nhat ciia hien tugng CEJ tai Viet Nam ciing phii hgp vdi nhung nghien cim ve hien tugng nay d nhirng vimg xich dao tix khac Uen the gidi. Vdi viec nghien cim anh hudng cua CEJ doi vdi bien thien tir ngay.dem yen tinh ciia thanh phan thang dung Z tai tram dia tir Da Lat cho chiing ta mgt bang chung nu:a ve sy ton tai ciia no d vimg xich dao tir thuoc phan lanh tho Viet Nam. Anh hudng cua dong dien ngugc xich dao doi vdi bien thien tir ngay dem yen tinh ciia thanh phan thang dimg Z hiy khong ro ret nliu doi vdi bien tliien tir ngay dem yen tTnh cua thanh phan nam ngang H nhung tai Da Lat co mgt so ngay anh hudng ciia n6 ciing dat tdi 20 nT.
Qua viec nghien cim sy xuat hien ciia d6ng dien ngugc xich dao d Viet Nam va An Do tren co sd so lieu bien thien tir ngay dem yen tinh (nhirng ngay co chi so boat dgng tir trung binh Ap<7) ciia thanh phan nam ngang H tai cac Uam dia hi Bac Lieu, Phii Thuy, Trivandrum, Alibag trong nam Mat Trdi boat dgng manh (1990) va trong nam Mat Trdi boat dgng yeu (1996) da cho phep dua ra nhiing dac diem giong nhau va khac nhau vl su xuit hien ciia hien tugng CEJ doi vdi timg viing. Nhung dac diem giong nhau cang khang dinh them sy ton tai ciia d6ng dien ngugc xich dao d vimg xich dao hi. Nhiing dac dilm khac nhau c6 thi dugc giai thich hi: sy khac nliau ve do Idn ciia thanh phin nim ngang H cua hing vimg.
Cling nhu sy khac nhau ve cudng do ciia dong dien xich dao tai mli khu vyc.
Biln thien hi ngay dem yen tinh ciia thanh plian thang dimg Z Uen lanh thi Viet Nam mang dac tnmg co ban ciia quy luat bien doi ciia trudng bien thien tir vimg vT do thip va xich dao tix, do la biln thien manh nhat vao thdi ky phan dilm va biln thien ciia SqZ tai Da Lat (Uam nim d vimg ria xich dao hi:) la Idn nhat. Trong thdi gian tdi chiing ta cin nghien cuu ca quy luat bien doi ciia bien thien tix ngay dem thanh phan thing diing Z trong nhirng ngay nhieu loan va bao tir vdi khoang thdi gian dai.
Vdi viec nghien cim mgt so dac trung ciia nhieu loan hi hmh vinh cua cac thanh phan thang truong dia tix D, H va Z tai hai tram Dia tir Sa Pa va Ha Ngi trong khoang thdi gian 11 nam (1987 - 1997) da cho phep chimg ta khing dinh sy tin tai mgt dai di thuong cd do dan dien tot nam d khoang giira hai tram Sa Pa va Ha Ngi khi Uudng bien thien tir biln dii nhanh.
p|i4n II; DjA T i r - DIEN LY
Tdc gid xin bdy to long cdm on GS. TSKH. Nguyin Thi Kim Thoa da giiip da vd tgo nhiiu diiu kiin thuan lai cho tdc gid trong nghien cim ndy. Tdc gid ciing xin chdn thdnh cdm an PGS. TS. Nguyin Vdn Gidng, TS. Li Huy Minh vd PGS. TS. Hd Duyen Chdu da ddng gdp nhicng y kiin quy bciu trong nghien cim nciy.
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ABSTRACT
SOME CHARACTERISTICS OF DAILY VARIATIONS OF THE GEOMAGNETIC FIELD IN THE VIETNAM
The characteristics of CEJ m Vietaam were studied basuig on the daily magnetograms of the H component of magnetic field during the quiet days (Ap < 7) at the equatorial magnetic station Baclieu (inside the magnetic equatorial region) and at the observatory Phuthuy (outside the magnetic equatorial region) during fifteen years (1989 - 2003). Accordmg to this analysis 422 CEJ with two types: the morning CEJ was observed around 06.00 - 07.00 LT and the aftemoon CEJ was observed at 15.00 - 17.00 LT. During the Sunspot's cucle the apperance of the CEJ was gettuig maximum during low-simspot years and minimum during high-simspot years. The apperance of morning CEJ is maximum during high-sunspot years, but that for aftemoon CEJ is maximum during low-sunspot years. The seasonal dependence of CEJ is: morning CEJ is maxunum during the winter and aftemoon CEJ is maximum
during the summer. -.
The equatorial counter-electrojet (CEJ) was studied basing on the hourly mean data of the horizontal 7/component of magnetic field during the quiet days (Ap < 7) at two equatorial magnetic sectors (Vietaames sector and Indian sector) during two years (1990: maxunum solar activity; and 1996: minimum solar activity). The apperance of the CEJ during low- sunspot year (1996) was more than during high-sunspot year (1990) on both sectors. The apperance of the CEJ at Indian sector was more than at Vietnamese sector. The apperance of morning CEJ at Vietaames sector is minimum diudng the summer, but that at Indian sector is minimum during the equinox. The aftemoon CEJ is maxunum during the summer on both sectors.
The characteristics of CEJ in Vietnam was studied basing on the daily magnetograms of the Z component of magnetic field during the quiet days (Ap < 7) at the magnetic station Dalat (fi-om 1989 to 2003). The influence of the equatorial counter-electrojet on the variation of the horizontal component H in the Vietaam is greater than that on the variation of the
vertical component Z. j The variation of the vertical component during the quiet days (SqZ) was studied basing
on the hourly mean data at four magnetic stations in the Vietaam during fifteen years (1989 - 2003). The amplitade of SqZ at the magnetic station Dalat (at die edge of the magnetic equatorial region) was greater than that at the anothers magnetic stations at Vietaam. The variation of SqZ during high-sunspot year was greater than during low-simspot year. In the
year, the ampIiUide of SqZ is maximum during the equinox. m The geomagnetic bays were stadied at two magnetic stations (Phuthuy and Sapa) during
eleven years (1987-1997). The geomagnetic bays of the vertical component Z are alv/ays the reversals of fliose of the horizontal component H at the magnetic station Sapa.
