lunar geomagnetic tides the low latitudes regionmetnet.imd.gov.in/mausamdocs/513112.pdfgeomagnet ic...

550'385: 523'3 Lunar Geomagnetic Tides in the low latitudes region K. S. RA.L\ RAO Jl el eorokJg ical Office, Poono (Rereit'Cd 1·) May 1961) AR.",,"HACl'. The IU Mr geomagnet ic tides at a few eepeeeentanv e 8ta tio DA in th e- low latiLudc region been synthesized to get. a uni fied picture of the L field in the equa torial region. The ...in the low latltu de e-chke the eq uatorial anomaly. SPMnoal variati on ll, jCPOma'tflMic yanahl"n. been tudied, The chemcterisrice of the low latitude vari ationll an- compared With th ose of high latitude 1. In trodu ction Our knowledge of the lunar tide."! is very meagre, compared to the solar counter part. Partly, it is due to the fact that amplitude of the lunar geomagnetic tide is very small in comparison with the sola r geomagn etic tide , being approximately one tenth of the solnr, and the determination of the lunar tides involves processing of R. large vol ume of da ta through a laho-. riou s method of computation, although th eoreti cal lv the lunar daily variation in the geOl uai,rnct.i c fie ld can be understood with more definitene... q t han the solar one, Th e lunar daily va r iat ion in the fiel d is caused by the movement. of ionised ma tter in the ionosphere (at a level which sti ll remains to be determined) across the lines of the earth's ma in magn eti c field. The movem ent of ionised air is caused hy the grav itational attra ct ion of the moon on the ionospheric layer . This, in substance, is the dynamo theory of the L field. 2. Importance of the study of L tleld in low lat.tude region The geomagnet ic var i ation s in the low latitude region display some interesting featurc. a. the studv of which has stimul ated very fUlUhlmellt 3'1 t heoretieal work in geomagnet ic and ionos pheric rese arch. For example, the opening of the Huancayo Obse rvatory soon revealed a l'urprising phenomenon hitherto lU l. ';;'US- pected : namely the quiet day variation 8'1 in horizontal intensity jl( twice a.. great as that at any oth er tropical magn eti c observatory existing at that time. St udy of thi s anomalous variation Jed to very important contributions by Hirono, Fejer, and on the conductivity in the ionosphere in the geomagnetic equatorial region modified by Hall currents, Polarisa- tion etc . Lat er it appenred that at Huancayo the lunar daily variation L in lJ is even more abnormally large. In addition to this abo normalit y, th e L sho ws peculia r seas ona l variation. For example at Ibadan (geomagnet ic Lat. 10 0 • /j N and geographic Lat. ; 0. fj N) amI Kodaikanal (geomagnet ic 0°'6 and geographic Lat. 10°· 2 N) the seasonal variation in L is exactly of the type noticed a t. Huuncayo, viz., larger amp litudes in the December solstice and smaller amplitudes in the Ju ne solstice. Therefore, to study the complete physical picture of the I, field in the low latitude region, the lunar tides hnve he en determined for a few representative stations for each of the three seaso ns. The co-ordinate. of the stat ions arc given in Table 1 (See Fig. I). The results obtained by earlier investi- Irators for Huancayo and Ihadan arc used and a synthesis of the L field in the low latitude region is made. The results for Kodaikanal, Alihag and Apia obtained by the auth or are being published sepa ra. telv. But they have been included here for purpose of synthesis.

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Page 1: Lunar Geomagnetic Tides the low latitudes regionmetnet.imd.gov.in/mausamdocs/513112.pdfgeomagnet ic and ionospheric research. For example, the opening of the Huancayo Jra~Jletjc Observatory

550 '385: 523' 3

Lunar Geomagnetic Tides in the low latitudes regionK. S. RA.L\ RAO

Jl eleorokJgical Office, Poono

(Rereit'Cd 1·) May 1961)

AR.",,"HACl'. The IUMr geomagnet ic tides at a few eepeeeentanve 8ta tio DA in the- low latiLudc region haT~

been synthesized to get. a unified pict ure of the L field in the equa torial region. The ~nl':&1 featu~...in the low latltu dee-chke the eq uatorial anomaly. SPMnoal variati on ll, jCPOma'tflMic ~ntrol.. latl t u~1D1L1 yanahl"n . ~" e beentudied, The chemcterisrice of the low lat itu de variationll an- compared With th ose of high latitude ~artahort!J.

1. Introduction

Our knowledge of t he lunar gcollla~netic

tide."! is very meagre, compared to the solarcounter part . Part ly, it is due to the factthat amplitude of the lunar geomagnet ictide is very sma ll in comparison with thesola r geomagnetic tide , being approximatelyone tenth of the solnr , and the determination of the lunar tides involves processingof R. large vol ume of data through a laho-.rious method of computation, althoughtheoretical lv the lunar daily variation inthe geOluai,rnct.ic field can be understoodwith more definiten e...q than the solar one,The luna r daily variat ion in the geomn.~net ic

field is caused by the movement. of ionisedmatter in the ionosphere (at a level whichsti ll remains to be det ermined) across thelines of the earth's main magn eti c field.The movem ent of ionised air is caused hythe gravitational attra ct ion of the moonon the ionospheric layer . This, in substance,is the dynamo theory of the L field.

2. Importance of the study of L tleld in low lat.tude region

The geomagnet ic variations in the lowlatitude region display some interest ingfeaturc.a. the studv of which has st imulatedvery fUlUhlmellt3'1 t heoretieal work ingeomagnet ic and ionos pheric research. Forexample, the opening of the HuancayoJra~Jle t jc Observatory soon revealed al'urprising phenomenon hitherto lU l.';;'US-

pected : namely the quiet day variat ion8'1 in horizontal intensity jl( twice a.. greatas that at any oth er tropical magnetic

observatory existing at that time. Studyof thi s anomalous variation Jed to veryimportant cont ribut ions by ~Iart}'Il , Hirono,Fejer, and ~[eida on the conductivity in theionosphere in the geomagnetic equatorialregion modified by Hall currents, Polarisation etc.

Later it appenred that at Huancayo thelunar daily variat ion L in lJ is even moreabnormally large. In addit ion to this abonormalit y, the L sho ws peculia r seasona lvariation. For exa mple at Ibadan (geomagnet ic Lat. 100

• /j N and geographic Lat .; 0.fj N) amI Kodaikanal (geomagnet icI~l t . 0° ' 6 and geographic Lat. 10°· 2 N) theseasonal variation in L is exactly of the typenoticed at. Huunca yo, viz., larger amp litudesin the December solstice and smaller amplitudes in the June solstice. Therefore, tostudy the complete physical picture of theI, field in the low latitude region, the lunart ides hnve heen determined for a few representative stations for each of the threeseasons. The co-ordinate. of the stationsarc given in Table 1 (See Fig. I ).

The results obtained by earlier investiIrators for Huancayo and Ihadan arc usedand a synthesis of the L field in the lowlatitude region is made. The results forKodaikanal, Alihag and Apia obtainedby the author are being published separa.telv. But they have been included here forth~ purpose of synthesis.