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.