VLF
Chorus,Sferics,Tweaks,and
Whistlers. Natural Radio Phenomena.
By Steve McGreevy,N6NKS
Welcome to very-low-frequency(VLF).Naturally occurring VLF radio emissions of
Earth will occur in the 0.2 to 11kHz (audio frequency) VLF electromagnetic
spectrum.
Planet Earth-along with several other planets in the Solar System including
Venus,Jupiter,Saturn,Uranus,and Neptune produce a variety of naturally occurring
radio emissions at the lowest end of the radio spectrum.These emissions are
primarily in the form of electromagnetic (radio) empulses generated by the
planets'ongoing lightning storms,and from the sun's solar wind interacting with
the magnetic envelope surrounding the Earth, called the
"Magnetosphere." Besides 50 or 60kHz (and harmonics)
alternating-current power line "hum" from electric-utility power
grids,the most noticeable sounds are going to be the snap,crackle, and pop of
lightning-stroke electromagnetic impulses (called "atmospherics" and
"sferics" for short) from lightning storms within a couple thousand
miles of the receiver-the more powerful the lightning stroke or the closer it is
to the VLF receiver's location, the louder the pops and crashes of sferics will
sound in the headphones. Several million lightning strokes occur daily from an
estimated 2000 storms worldwide,and the Earth is struck 100 times a second by
lightning.At night,many of the popping and crackling sounds of sferics take on a
pinging/dripping sound,called "tweaks are a result of the impulse path from
the lightning stroke to the receiver being influenced by the Earth surface-to-ionosphere(D
and E layers)region,which is about 45 to 75 miles in hight,measured vertically
during the nighttime hours.This region between the lower ionosphere and surface
of the Earth acts as a "duct" or "wave guide" at these VLF
radio frequencies,which have wavelengths ranging from 10 miles/29km. at 10kHz to
over 186 miles /289km. at 1 kHz,allowing lightning stroke impulse energy to
travel considerably farther than during the daytime.As the energy travels and is
reflected within this Earth-ionosphere wave guide,the energy undergoes a
"dispersion" effect whereby the higher frequencies of lightning
impulse arrive before the lower ones within a fraction of a second.The wave
guide dispersion effect abruptly cuts off below about 1.5 to 2 kHz(1,500 to
2,000 Hz) in frequency,resulting in the ringing/pinging "tweak" sound
which is also centered around 1.5 to 2 kHz. This "tweak" sound is the
lowest resonance frequency of the ionosphere-Earth surfice wave guide.This is
similar to what sound waves experience in a pipeline.In addition to the sounds
of lightning sferics and tweaks,you may be hearing downward falling musical
notes ranging from nearly pure to "swishy" or "breathly"
sounding tones from 1/2-to over four seconds in duration.These are
"whistlers," which sometimes happen a couple of seconds after the
static crashes and pops of sferics from lightning strokes. Whistlers generally
sweep downward in frequency from about 6 kHz to around 0.5 kHz,but the lower cut
off frequency does vary remarkably as conditions change,and the upper frequency
of whistler can start higher than 10 kHz.Whistlers sounds quite fascinating.Like
"science fiction" sounds effects,thay are of the more common Natural
Radio sounds you can hear.The Earth's magnetic field (the magnetosphere)
envelops the planet in elongated doughnut shape with its "hole" at the
north and south magnetic poles.The magnetosphere is compressed on the side
facing the sun and trails into a comet-like tail on the side away from the sun
because of the "Solar Wind," which consists of energy and
particles(plasma) emitted from the Sun "blown" toward Earth and the
other planets via the Solar Wind. Among the charged solar particles caught in
the magnetosphere and ions (electronically charged particles,which collect and
align along the magnetic filed "lines" stretching between the north
and south magnetic poles.These magnetic-field aligned ions bombarding Earth's
magnetosphere from "ducts" witch can channel lightning-stroke
electromagnetic impulse energy.Whistlers sound the way thay do because the
higher frequencies of lightning-stroke trade energy travel faster in the duct
and arrive before the lower frequncies in process researchers call
"dispersion." A person listening with a VLF receiver in the opposite
hemisphere to the lightning-stroke (at the far end of the magnetosphere duct
path) will hear this "one-hop" falling note whistler.One-hop whistlers
are about 1/3 of a second in duration.If the energy of the initial one-hop
whistler gets reflected back into the magnetoionic duct to return near the point
of the originating lightning impulse, a listener there with a VLF receiver will
hear a "pop" from the lightning-stroke impulse,then roughly on to two
seconds later,the falling note sound of the whistler,now called a two-hop
whistler.Two-hop whistler are about one to four seconds in duration depending on
the distance the whistler energy has traveled within the magnetosphere.One-hop
whistler are usually higher pitched than two-hop whistler. The energy of the
originating lightning stroke may make several "hops" back and forth
between the northern and southern hemispheres during it's travel along the
Earth's magnetic field lines-of force. Researchers have observed that the
magnetosphere seems to amplify and sustain the initial lightning impulse energy,
enabling "multi-hop" whistlers to occur, creating long
"echo-trains" in the receiver output which sounds spectacular! Each
echo is proportionally longer and slower in it's downward seeping pitch and is
also progressively weaker.Conditions in the magnetosphere must be favorable for
multi-hop whistler echoes to be heard. Using special receiving equipment and
spectrographs,researchers have documented over 100 echoes from particularly
strong whistlers imagine how much distance the energy from the 100th echo has
traveled-certainly millions of miles/Km! Generally, only one to two echoes are
heard if thay are occurring,but under exceptional conditions,several echoes will
blend into a collage of slowly descending notes and can merge into coherent
tones on a single frequency quite unlike any sounds heard outside of a
science-fiction movie! It should be reiterated that strong two-hops (and echoes)
can occur from lightning that is within a couple of hundred miles from the
listener location,but perhaps from lightning over 100 miles distant. You may
notice that "louder" sferics (i.e.closer lightning strokes) often do
not trigger the loudest whistlers,of they so at all,but then a loud whistler may
come howling through from a relatively weak sferic from quite distant
lightning.This is because the lightning impulse sferic energy may propagate with
in the earth-ionosphere region for considerable distance before entering a
magnetospheric"duct." A majority of whistlers are heard ULC during
periods of locally fair weather.In fact,Many extremely loud"big
whistlers" are heard without any preceding lightning sferic audible
whatsoever,indicating the initiating lightning strokes of those strong whistlers
are far away-possibly over 3000 miles! Whistlers are best heard in 30deg and 55
deg. north latitude in North America,the prime latitude being 40deg and 50deg
north.Occasionally,shortly after sunrise and even extending into the mid-morning,a
phenomenon called "dawn Chorus" may occur.Dawn chorus can resemble the
sound of a flock of birds singing and squawking,dogs barking, or sound like
whistlers raining down by the hundreds per-minute (called a whistler
storm"). Dawn Chorus results from hundreds of overlapping,rapidly upward
rising tones that can be continuous or appear in bursts, called chorus
trains.Chorus trains sound fascinating-the bursts of chirps and squawks (risers)
seem to suddenly commence and over the course of two to five seconds,weaken and
fade away,then repeat over again,often in different pitches.During a chorus
train,the second sometimes seem to be echoing or reverberating back and forth
untill fewer risers happen,then there may be a brief pause before the next
chorus train commences.Chorus trains seem to be harmonically related-a chorus
train's center audio frequency may alternate randomly,first centered on about 1
kHz,then another chorus train will suddenly start up one octave higher at around
2 kHz,or maybe 4 kHz.Bursts of chorus trains happening at different octaves can
overlap in beautiful cacophony.Dawn chorus occurred several times a month during
years of high sunspot activity (1994-1997),coronal mass ejections from the Sun
can still cause magnetic storms once or twice a month.Chorus doesn't always only
occur at dawn,especially for listeners located at higher latitudes,particularly
in southern and central Canada (50 to 55 deg north latitude),Alaska,and in
northern Europe.This auroral zone is source to a vast amount of natural VLF
phenomena.When a solar disturbance on the Sun (such as the solar flare or
coronal hole mass ejection) sends highly charged and high-speed particles and
ions towards Earth via the solar wind,Auroral displays often occur,and are
visible to people near the auroral zone oval.Earth's magnetic field also
undergoes a "storming" process as well,called a "magnetic
storm." During auroral displays,chorus is often heard,as well as
"hiss" of various pitches,"sliding-tone emission" which
eerily and weirdly rise in pitch slowly over one to several seconds'duration.The
chorus which occurs during displays of Aurora is called "Auroral
Chorus." Both Auroral chorus and dawn chorus are related in that occur
during magnetic storms.The more severe the magnetic storm,the farther south away
from the auroral zone and the louder the chorus will be heard.The Auroral Zone
"oval" surrounding the magnetic poles expands during magnetic storms
and reaches farther southward (and the southern Auroral Zone "oval" in
the southern hemisphere expands farther northward). Auroral is a daytime
phenomenon,but it is not visible to the naked eye due to daylight illumination
of the sky.Particularly intense events of nighttime and dawn chorus can get load
even for listeners below 40deg north latitude (in the U.S.),and point to the
evidence that aurora can reach southward into the middle latitudes despite it
not being visible. The maximum intensity region of chorus emissions,like
aurora,can spread southward during magnetically disturbed periods.Daytime aurora
can be more intense than nighttime aurora,and events of auroral and dawn chorus
reveals quite a bit about the nature of aurora. Even if geo-magnetic conditions
seem "quiet" and chorus events seem likely,conditions may still be
very good for whistlers to accur.However,determining when whistlers are going to
happen is still a rather unpredictable affair. The time between local midnight
and an hour after sunrise is when the greatest amounts of whistlers are
heard,although dusk to midnight may reveal substantial whistler activity,even
(though not very often) loud whistlers may be heard a couple of hours before
sunset. Over the long term,the period from two hours before sunrise untill an
hour after sunrise is the optimum time to listen for natural VLF phenomena of
all sorts,as the amount of sferics (lightning stroke pops and crackling) are
less-natural VLF phenomena are not as "buried" under the sferics as in
the evening when lightning storms are more numerous.Also,magnetospheric
conditions are optimum around morning twilight time. Intense whistler events of
a short duration can occur at any time between just before local sunset through
one to two hours after sunrise.A good whistler event that is happening at 10 p.m,
or even at sunset may not be occurring later on that night at the usual optimal
sunrise period,so don't rule out the evening hours to listen,especially during
geo-magnetic storms.On several mornings a month,one whistler a minute may be
heard on average,but as often the case,whistlers will not be heard at
all.Occasionally during a geo-magnetic storm caused by a solar flare,over 100
whistlers a minute or more may be heard-called a "whistler storm!"
Whistlers may or may not have echoes may be few and far between but occur
loud,or may occur often but quite weak.The sound characteristics,intensity,and
number of whistlers can change rapidly hour to hour. Everything depends upon the
sensitivity and conditions of the Earth's magnetosphere and location of
lightning storms and magnetospheric ducts in relation to the listener. Whistlers
are seldom heard mid-day, except during unusual conitions occurring with a
geo-magnetic storm and when lightning is within a few hundred miles of the
listener.Unfortunately,on a good number of days during the year,there will not
be any whistlers audible even though there is plenty of lightning activity and
sferics within a few hundred miles of the receiver. Often elusive,whistlers may
not be heard for days or weeks at a time.Again,it is hard to predict when
whistlers are going to occur based on the geo-magnetic indices,but thay are
generally more common in the spring and fall,surrounding the equinoxes. Auroral
chorus,like whistlers,is best heard between midnight and sunrise.Dawn chorus
tends to peak in intensity between sunrise and one hour later. Listeners to
natural VLF radio phenomena should't be discouraged after several listening
sessions,whistleres,chorus,or other VLF phenomena sounds are not heard.Soon,you
will be rewarded with a myriad of fascinating sounds from whatever VLF phenomena
is occurring at the time you listen. Remember,weather and outside temperature
permitting,the period around local sunrise will be the most rewarding time to
listen. Natural Radio sounds can sound eerie and awe-inspirring,especially when
one realizes it is all naturally occurring-not-man-made and that these radio
emissions have been occurring for millennia.
Steve McGreevy and his company manufactures the popular Wr-3 and deluxe WR-3E
specialized natural VLF phenomena receivers,also accessories,as well as
recordings of VLF phenomena. Check out his web site,home page on the link below.
Steve McGreevy Wr3guideIf
your interested in making your own VLF receiver,I have shcematics and info,has
been tested and used by me today.Just send me e-mail at bektrek@mnsi.net. I will
be more than happy to send them out to you.
E-mail click here bektrek@mnsi.net
Steve McGreevy BBB-4 receiver schematic
Here is a sound file that was made on April 2 2000.Sounds like a dog bark in an
almost human voice bark
Whistler wave recorded May 24 2000 near Detroit
River.Whistler