Propagation Extremely low frequency

typical spectrum of elf electromagnetic waves in earth s atmosphere, showing peaks caused schumann resonances. schuman resonances resonant frequencies of spherical earth-ionosphere cavity. lighning strikes cause cavity ring bell, causing peaks in noise spectrum. sharp power peak @ 50 hz caused radiation global electric power grids. rise of noise @ low frequencies (left side) radio noise caused slow processes in earth s magnetosphere.

due extremely long wavelength, elf waves can diffract around large obstacles, , not blocked mountain ranges or horizon , can travel around curve of earth. elf , vlf waves propagate long distances earth-ionosphere waveguide mechanism., earth surrounded layer of charged particles (ions) in atmosphere @ altitude of 60 km @ bottom of ionosphere, called d layer reflects elf waves. space between conductive earth s surface , conductive d layer acts parallel-plate waveguide confines elf waves, allowing them propagate long distances without escaping space. in contrast vlf waves, height of layer less 1 wavelength @ elf frequencies, mode can propagate @ elf frequencies tem mode in vertical polarization, electric field vertical , magnetic field horizontal. elf waves have extremely low attenuation of 1 – 2 db per 1000 km,. giving single transmitter potential communicate worldwide.

elf waves can travel considerable distances through lossy media earth , seawater, absorb or reflect higher frequency radio waves.

schumann resonances

the attenuation of elf waves low can travel around earth several times before decaying negligible amplitude, , waves radiated source in opposite directions circumnavigating earth on great circle path interfere each other. @ frequencies these oppositely directed waves in phase , add (reinforce), causing standing waves. in other words, closed spherical earth-ionosphere cavity acts huge cavity resonator, enhancing elf radiation @ resonant frequencies. these called schumann resonances after german physicist winfried otto schumann predicted them in 1952, , detected in 1950s. modeling earth-ionosphere cavity conducting walls, schumann calculated resonances should occur @ frequencies of

f

n


=
7.49
[
n
(
n
+
1
)

]

1

/

2




hz



{\displaystyle f_{n}=7.49[n(n+1)]^{1/2}\;{\text{hz}}}

the actual frequencies differ due conduction properties of ionosphere. fundamental schumann resonance @ approximately 7.83 hz, frequency @ wavelength equals circumference of earth, , higher harmonics occur @ 14.1, 20.3, 26.4, , 32.4 hz, etc. lightning strikes excite these resonances, causing earth-ionosphere cavity ring bell, resulting in peak in noise spectrum @ these frequencies, schumann resonances can used monitor global thunderstorm activity.

interest in schumann resonances renewed in 1993 when e. r. williams showed correlation between resonance frequency , tropical air temperatures, suggesting resonance used monitor global warming.