Studies on VLF atmospherics during the tropical cyclone “AILA” and several monsoon period thunderstorms over north-east India
Authors: RAKESHROY RAKESH , ANIRBANGUHA ANIRBAN , BARINKUMARDE BARINKUMAR , CHOUDHURY ABHIJIT 
Published: January 2022

bl 'kks/k i= esa vizSy 2009 ls vDrwcj 2009 dh vof/k ds nkSjku HkkSfrdh foHkkx] f=iqjk fo’ofo|ky; }kjk oh- ,y- ,Q- ok;qeaMyh; vFkok LQsfjDl dh vfHkyk{kf.kd fofo/krk dks pkj vyx & vyx vko`fr;ksa esa ntZ fd;k x;k gSA bl  'kks/k i= esa mRrj iwoZ Hkkjr ds 76 lfØ; xtZ ds lkFk rwQku okys vf/kdk¡’k fnuksa ds vk¡dMs+ iz;qDr fd, x,A buls izkIr ifj.kkeksa esa ekulwu ds nkSjku LQsfjDl ifjorZu esa dbZ izdkj dh fo’ks"krk,¡ n’kkZbZ xbZ gSaA bu Øfed fxjkoV] Øfed o`f) ¼th- ,Q- th- vkj-½] Øfed o`f) vpkud fxjkoV ¼th- vkj- ,l- ,Q-½] Øfed o`f) Øfed fxjkoV ¼th- vkj th- ,Q-½] Øfed fxjkoV vpkud o`f) ¼th- ,Q-,l- vkj-½] vkdfLed o`f)  Øfed fxjkoV ¼,l- vkj- th- ,Q-½] vpkud fxjkoV vpkud o`f) ¼,l- vkj- ,l- ,Q-½ vpkud fxjkoV vpkud  o`f) ¼,l- ,Q- ,l- vkj-½ vpkud fxjkoV Øfed o`f) ¼,l- ,Q- th- vkj-½ rFkk dBksj ¼Spiky½ Lo:i esa n’kkZ;k x;k gSA ekulwuh lfØ; xtZ ds lkFk rwQkuksa okys vf/kdka’k fnuksa esa lHkh izdkj ds iSVuksaZ esa ¼th- vkj th- ,Q-½ dh vko`fr vf/kdrj izdj.kksa esa ntZ gqbZ gS  ¼izR;sd vko`fr esa vkSlru yxHkx 37 izfr’kr ?kVuk,¡ ntZ gqbZ gSa½ gekjh izs{k.kkRed vof/k ds nkSjku 23&26 ebZ dh vof/k esa caxky dh [kkM+h esa ‘vkbZyk’ uked ,d rhoz m".kdfVca/kh; pØokr vkj-,l-,e-lh- inuke% ch- vks- ch- vks-2] ts- Vh- MCY;w- lh- inuke% vks- Vw- ch- vk;k FkkA 25 ebZ 2009 ds m".k ds m".kdfVca/kh; pØokr ds nkSjku lHkh izdkj ds lkekU; ekulwuh vof/k dh vfHkyk{kf.kd fo’ks"krkvksa esa ,l-vkj-,l-,Q- ¼izR;sd vko`fr esa 86 izfr’kr ds vkSlru nj ls ?kfVr½ vko`fr vf/kdrj izdj.kksa esa ns[kh xbZ gSA tc pØokr caxky dh [kkM+h ds rVksa ls Vdjk;k ml fnu ds LQsfjDl ds vyx&vyx vko`fr forj.kksa ds vuqlkj o`f) vkSj fxjkoV dk xaHkhjrk ls fo’ys"k.k fd;k x;kA ekulwuh fnuksa vkSj ‘vkbyk’ pØokr lfØ; fnol ds lHkh ds lHkh izdkj ds iSVuksaZ dk rqyukRed v/;;u fd;k x;kA gekjs fu"d"kZ ds vuqlkj izpaM pØokr ‘vkbyk’ vkSj vU; xtZ Hkjs rwQku okys ekulwuh fnuksa ds nkSjku mRrj iwoZ Hkkjr esa cknyksa dh lw{e lajpuk esa dkQh varj fn[kkA pØokrh xfrfof/k ds nkSjku LQSfjDl ds ns[ks x, cnyko dh laHkkfor O;k[;k dks xtZ Hkjs rwQku okys cknyksa dh rfM+r xfrfof/k ds vk/kkj ij le>k;k x;k gSA

The present work reports the characteristic variations in VLF atmospherics or sferics at four discrete frequencies recorded at the Department of Physics, Tripura University, during the period from April 2009 to October 2009. Data from 76 active thunder days over North-East India is considered for the present investigation. Results show several types of features in the variation of sferics during the monsoon period. These are termed as gradual fall gradual rise (GFGR), gradual rise sudden fall (GRSF), gradual rise gradual fall (GRGF), gradual fall sudden rise (GFSR), sudden rise gradual fall (SRGF), sudden rise sudden fall (SRSF),  sudden fall sudden rise (SFSR), sudden fall gradual rise (SFGR) and spiky. During the Monsoon thunder active days, amongst all the patterns, GRGF occurred in most of the cases in all frequencies (average occurrence rate around 37% in each frequency). During our observational period, a severe tropical cyclonic storm named “AILA” (RSMC Designation BOB02, JTWC Designation 02B) occurred over the Bay of Bengal during 23-26 May 2009. Among several characteristic features during normal Monsoon period, SRSF (average occurrence rate around 86 % in each frequency) dominated the sferics on the 25 May, 2009, when the cyclone struck the coastal areas of the Bay of Bengal. The sferics of that day has been analyzed critically with respect to discrete frequency distribution of rise rate and fall rate of the intensity of the sferics. A comparison is made for all the patterns for the Monsoon days and the AILA cyclone active day. Our findings show substantial difference in the microstructure of clouds producing severe cyclonic storms like AILA and other thunderstorms during Monsoon seasons over North-East India. The possible interpretation of the observed variations in sferics is explained on the basis of the electrical activity that occurs inside a thunder-cloud especially during cyclonic activity.