<s>
Extremely	B-Language
low	I-Language
frequency	I-Language
(	O
ELF	O
)	O
is	O
the	O
ITU	O
designation	O
for	O
electromagnetic	O
radiation	O
(	O
radio	O
waves	O
)	O
with	O
frequencies	O
from	O
3to30Hz	O
,	O
and	O
corresponding	O
wavelengths	O
of	O
100,000	O
to	O
10,000	O
kilometers	O
,	O
respectively	O
.	O
</s>
<s>
In	O
the	O
related	O
magnetosphere	O
science	O
,	O
the	O
lower	O
frequency	O
electromagnetic	O
oscillations	O
(	O
pulsations	O
occurring	O
below	O
~	O
3Hz	O
)	O
are	O
considered	O
to	O
lie	O
in	O
the	O
ULF	B-Algorithm
range	O
,	O
which	O
is	O
thus	O
also	O
defined	O
differently	O
from	O
the	O
ITU	O
radio	O
bands	O
.	O
</s>
<s>
ELF	O
radio	O
waves	O
are	O
generated	O
by	O
lightning	B-Architecture
and	O
natural	O
disturbances	O
in	O
Earth	O
's	O
magnetic	O
field	O
,	O
so	O
they	O
are	O
a	O
subject	O
of	O
research	O
by	O
atmospheric	O
scientists	O
.	O
</s>
<s>
ELF	B-Language
waves	I-Language
can	O
penetrate	O
seawater	O
,	O
which	O
makes	O
them	O
useful	O
in	O
communication	O
with	O
submarines	O
,	O
and	O
a	O
few	O
nations	O
have	O
built	O
military	O
ELF	O
transmitters	O
to	O
transmit	O
signals	O
to	O
their	O
submerged	O
submarines	O
,	O
consisting	O
of	O
huge	O
grounded	O
wire	O
antennas	O
(	O
ground	O
dipoles	O
)	O
long	O
driven	O
by	O
transmitters	O
producing	O
megawatts	O
of	O
power	O
.	O
</s>
<s>
Some	O
medical	O
peer	O
reviewed	O
journal	O
articles	O
refer	O
to	O
ELF	O
in	O
the	O
context	O
of	O
"	O
extremely	B-Language
low	I-Language
frequency	I-Language
(	O
ELF	O
)	O
magnetic	O
fields	O
(	O
MF	O
)	O
"	O
with	O
frequencies	O
of	O
50Hz	O
and	O
50	O
–	O
80Hz	O
.	O
</s>
<s>
The	O
World	O
Health	O
Organization	O
(	O
WHO	O
)	O
have	O
used	O
ELF	O
to	O
refer	O
to	O
the	O
concept	O
of	O
"	O
extremely	B-Language
low	I-Language
frequency	I-Language
(	O
ELF	O
)	O
electric	O
and	O
magnetic	O
fields	O
(	O
EMF	O
)	O
"	O
The	O
WHO	O
also	O
stated	O
that	O
at	O
frequencies	O
between	O
0	O
and	O
300Hz	O
,	O
"	O
the	O
wavelengths	O
in	O
air	O
are	O
very	O
long	O
(	O
at	O
50	O
Hz	O
and	O
at	O
60	O
Hz	O
)	O
,	O
and	O
,	O
in	O
practical	O
situations	O
,	O
the	O
electric	O
and	O
magnetic	O
fields	O
act	O
independently	O
of	O
one	O
another	O
and	O
are	O
measured	O
separately.	O
"	O
</s>
<s>
Due	O
to	O
their	O
extremely	O
long	O
wavelength	O
,	O
ELF	B-Language
waves	I-Language
can	O
diffract	O
around	O
large	O
obstacles	O
,	O
are	O
not	O
blocked	O
by	O
mountain	O
ranges	O
or	O
the	O
horizon	O
,	O
and	O
can	O
travel	O
around	O
the	O
curvature	O
of	O
the	O
Earth	O
.	O
</s>
<s>
ELF	O
and	O
VLF	B-Architecture
waves	O
propagate	O
long	O
distances	O
by	O
an	O
Earth-ionosphere	O
waveguide	O
mechanism	O
.	O
</s>
<s>
The	O
Earth	O
is	O
surrounded	O
by	O
a	O
layer	O
of	O
charged	O
particles	O
(	O
ions	O
and	O
electrons	O
)	O
in	O
the	O
atmosphere	O
at	O
an	O
altitude	O
of	O
about	O
at	O
the	O
bottom	O
of	O
the	O
ionosphere	O
,	O
called	O
the	O
D	O
layer	O
which	O
reflects	O
ELF	B-Language
waves	I-Language
.	O
</s>
<s>
The	O
space	O
between	O
the	O
conductive	O
Earth	O
's	O
surface	O
and	O
the	O
conductive	O
D	O
layer	O
acts	O
as	O
a	O
parallel-plate	O
waveguide	O
which	O
confines	O
ELF	B-Language
waves	I-Language
,	O
allowing	O
them	O
to	O
propagate	O
long	O
distances	O
without	O
escaping	O
into	O
space	O
.	O
</s>
<s>
In	O
contrast	O
to	O
VLF	B-Architecture
waves	O
,	O
the	O
height	O
of	O
the	O
layer	O
is	O
much	O
less	O
than	O
one	O
wavelength	O
at	O
ELF	O
frequencies	O
,	O
so	O
the	O
only	O
mode	O
that	O
can	O
propagate	O
at	O
ELF	O
frequencies	O
is	O
the	O
TEM	O
mode	O
in	O
vertical	O
polarization	O
,	O
with	O
the	O
electric	O
field	O
vertical	O
and	O
the	O
magnetic	O
field	O
horizontal	O
.	O
</s>
<s>
ELF	B-Language
waves	I-Language
have	O
extremely	O
low	O
attenuation	O
of	O
1	O
–	O
2dB	O
per	O
,	O
giving	O
a	O
single	O
transmitter	O
the	O
potential	O
to	O
communicate	O
worldwide	O
.	O
</s>
<s>
ELF	B-Language
waves	I-Language
can	O
also	O
travel	O
considerable	O
distances	O
through	O
"	O
lossy	O
"	O
media	O
like	O
earth	O
and	O
seawater	O
,	O
which	O
would	O
absorb	O
or	O
reflect	O
higher	O
frequency	O
radio	O
waves	O
.	O
</s>
<s>
The	O
attenuation	O
of	O
ELF	B-Language
waves	I-Language
is	O
so	O
low	O
that	O
they	O
can	O
travel	O
completely	O
around	O
the	O
Earth	O
several	O
times	O
before	O
decaying	O
to	O
negligible	O
amplitude	O
,	O
and	O
thus	O
waves	O
radiated	O
from	O
a	O
source	O
in	O
opposite	O
directions	O
circumnavigating	O
the	O
Earth	O
on	O
a	O
great	O
circle	O
path	O
interfere	O
with	O
each	O
other	O
.	O
</s>
<s>
Lightning	B-Architecture
strikes	O
excite	O
these	O
resonances	O
,	O
causing	O
the	O
Earth-ionosphere	O
cavity	O
to	O
"	O
ring	O
"	O
like	O
a	O
bell	O
,	O
resulting	O
in	O
a	O
peak	O
in	O
the	O
noise	O
spectrum	O
at	O
these	O
frequencies	O
,	O
so	O
the	O
Schumann	O
resonances	O
can	O
be	O
used	O
to	O
monitor	O
global	O
thunderstorm	O
activity	O
.	O
</s>
<s>
The	O
United	O
States	O
Navy	O
in	O
1982	O
built	O
the	O
first	O
ELF	O
submarine	O
communications	O
facility	O
,	O
two	O
coupled	O
ELF	O
transmitters	O
at	O
Clam	O
Lake	O
,	O
Wisconsin	O
and	O
Republic	O
,	O
Michigan	B-Device
.	O
</s>
<s>
When	O
the	O
refractive	O
index	O
of	O
the	O
medium	O
is	O
greater	O
than	O
one	O
,	O
ELF	B-Language
waves	I-Language
propagate	O
slower	O
than	O
the	O
speed	O
of	O
light	O
in	O
vacuum	O
.	O
</s>
<s>
The	O
United	O
States	O
maintained	O
two	O
sites	O
,	O
in	O
the	O
Chequamegon-Nicolet	O
National	O
Forest	O
,	O
Wisconsin	O
and	O
in	O
the	O
Escanaba	O
River	O
State	O
Forest	O
,	O
Michigan	B-Device
(	O
originally	O
named	O
Project	O
Sanguine	O
,	O
then	O
downsized	O
and	O
rechristened	O
Project	O
ELF	O
prior	O
to	O
construction	O
)	O
,	O
until	O
they	O
were	O
dismantled	O
,	O
beginning	O
in	O
late	O
September	O
2004	O
.	O
</s>
<s>
Naturally	O
occurring	O
ELF	B-Language
waves	I-Language
are	O
present	O
on	O
Earth	O
,	O
resonating	O
in	O
the	O
region	O
between	O
ionosphere	O
and	O
surface	O
seen	O
in	O
lightning	B-Architecture
strikes	O
that	O
make	O
electrons	O
in	O
the	O
atmosphere	O
oscillate	O
.	O
</s>
<s>
Though	O
VLF	B-Architecture
signals	O
were	O
predominantly	O
generated	O
from	O
lightning	B-Architecture
discharges	O
,	O
it	O
was	O
found	O
that	O
an	O
observable	O
ELF	O
component	O
—	O
slow	O
tail	O
—	O
followed	O
the	O
VLF	B-Architecture
component	O
in	O
almost	O
all	O
cases	O
.	O
</s>
<s>
ELF	B-Language
waves	I-Language
have	O
also	O
been	O
tentatively	O
identified	O
on	O
Saturn	O
's	O
moon	O
Titan	O
.	O
</s>
<s>
Titan	O
's	O
surface	O
is	O
thought	O
to	O
be	O
a	O
poor	O
reflector	O
of	O
ELF	B-Language
waves	I-Language
,	O
so	O
the	O
waves	O
may	O
instead	O
be	O
reflecting	O
off	O
the	O
liquid-ice	O
boundary	O
of	O
a	O
subsurface	O
ocean	O
of	O
water	O
and	O
ammonia	O
,	O
the	O
existence	O
of	O
which	O
is	O
predicted	O
by	O
some	O
theoretical	O
models	O
.	O
</s>
<s>
The	O
source	O
of	O
natural	O
ELF	B-Language
waves	I-Language
on	O
Titan	O
is	O
unclear	O
as	O
there	O
does	O
not	O
appear	O
to	O
be	O
extensive	O
lightning	B-Architecture
activity	O
.	O
</s>
