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.








Comments

Post a Comment

Popular posts from this blog

Gigantomastia Breast hypertrophy

-
the first recorded case of gigantomastia, diagnosed in 23- or 24-year-old woman, circa 1670. in 1670, physician durston drew illustration of first recorded case of non-gravid gigantomastia; woman died of condition. a painting lam qua of lu-shi, age 42, on april 17, 1848, prior breast reduction surgery. one , extreme case study of gigantomastia dates 1670. patient died 4 months after onset of enlargement. 1 breast removed after woman s death weighed 29 kg (64 lb). on april 17, 1848, 42-year-old woman named lu-shi treated hypertrophy in chinese hospital. treated missionary physician. on december 24, 1849, left breast, measuring 67 cm (26.5 in) in circumference, , weighing 2.7 kg (6 lb), removed in procedure lasting 3 , half minutes. right breast removed 1 month later. measured 61 cm (24 in) in circumference , weighed 2.5 kg (5.5 lb). in 2005, woman reported breasts grew @ puberty nothing c cup in 1 month. when became pregnant first time, b...
Read more

History Conagra Brands

-
the previous conagra brands logo, used until june 2009. in 1971, nebraska consolidated mills changed name conagra , portmanteau of con consolidated , agra, short in english agriculture. 1970s brought company brink of ruin lost money expanding fertilizer, catfish, , pet product industries , commodity speculation wiped out conagra s margins on raw foods. in 1974, c. michael mike harper, experienced food industry executive, took on firm , brought brink of bankruptcy. nonetheless, conagra s business model left vulnerable volatile commodity prices. in response, company set off on two-decade-long buying spree, purchasing on 1 hundred prepared food brands, starting 1980 purchase of banquet foods. moved heavily frozen food business , packaged meat industry, , picked selection of other brands firms rjr nabisco , beatrice foods among others, leveraged buyouts of 1980s resulted in divestiture or breakup of many major american consumer product firms. in 1993 alone purchased $500 million in smal...
Read more

Xylok List of Doctor Who universe creatures and aliens (Q–Z)

-
xyloks crystalline race crashed earth meteorite 60 million years ago. consequently, xyloks survived crash trapped beneath surface of earth, regrowing on thousands of millennia. after eruption of mount krakatoa in 1883, 1 of xyloks found in post-volcanic aftermath. passed sarah jane smith geologist friend when researching volcanic activity, 18 months prior events of lost boy. during studies of crystalline structure, found use laptop communicate. xylok agreed sarah jane protect earth, , integrated supercomputer mr smith, built sarah jane under instruction of xylok. unbeknownst sarah jane, plot, in anticipation 1 day able release imprisoned xylok race. sarah jane , companions able thwart intentions; result, mr smith reprogrammed , became benefit human race once again.
Read more