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Resonances of the earth-ionosphere cavity observed at Cambridge, England

1965; The National Institute of Standards and Technology; Volume: 69D; Issue: 8 Linguagem: Inglês

10.6028/jres.069d.115

2376-5771

M.J. Rycroft,

Geophysics and Sensor Technology

A progra m of obse rvation s of the na tural ve rti ca l e lec tri " fi e ld in th e atm osp he re in the ba nd Ito 30 Hz is di sc ussed .After reje ction of un wa nte d s ig nal s, th e ELF (extre me ly-low-frequ e nc y) s igna l is a mplifi e d, its wavefo rm be ing pres e rve d as a photographi c record whi c h c an be di giti zed a uto ma tica ll y.Th e pow e r s pec tral de n s it y of th e fi e ld is co mpu ted, a nd at all tim es s hows stati s ti ca ll y s ignifi c ant pea ks at 8 a nd 14 Hz , with evid e nce of o th e r peaks at 20 a nd 26 Hz.T hese a re the reso nant fr equ e nc ies of modes of th e e lec tro magne ti c fi eld in th e I"a rth -io nosp he re cav it y, exc ite d by radiat io n from li ghtnin g d isc hargf's .A co mp a ri so n is ma de be tween s uc h s pec t ra a nd th ose o bse rve d e lse whe re.The s pec tra a re al so co mpare d with th ose ca lc ul a ted theore t ica ll y.Th e ag ree me nt is o nl y fair for a s ha rpl y bo und ed , hom oge neo us, isotrop ic ionosp here co nce ntri c with the ea rth , bu t bl" tte r for Ga lejs' mode l ionos p he re with an exponf" nti a l co ndu c tivit y ve rs us he ig ht profi le.From th e de riv e d pro pagati o n cons ta nt , th e a tt e nu ati o n rat e in t he ba nd 7 to 30 Hz is fo und to be approx imat e ly 0.25 dB/Mm, i.e., 0.25 dB/ IOOO km , a nd th e ph ase velocit y 0.77 c.The ha lf widt h of the thund ers to rm be lt abou t an eq uator (with res pect to a po le at Ca mbrid ge) is fo und to be 44°± 12°, in agree me nt with thun de rsto rm s ta ti s ti cs.Di urn al and seaso na l vari a ti o ns of th e power in th e variou s modes are int e rpre ted in te rm s of nod es of th e fi eld a nd th e move me nt of thund er sto rm cen te rs .Th e fi ne s tru c ture of th e s pectrum is in ves ti gated.A typi c al meas ure d fi e ld s tre ngth of 0.34 mV/ m in a I -Hz b a ndwidth at 8 Hz is s ho wn to be reaso nabl e .T he e ne rgy in the bac kg round s pec trum , whi c h ov erwh e lm s th at in the ca vity reso nance s pectrum be lo w 5 Hz, may be du e to nea rb y li ghtning ac ti vit y, 10caJ e lec tros tati c e ffects, o r ext rat e rres t ria l phe no me na."The re is mu s ic in t he s pac ing of the s ph er es," P yth agoras.