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