Harold Allen, W4MMC
HERE FOR INTRODUCTION
In Streaming Audio!
his is the second part of a two-part article discussing some of the amazing sounds that can sometimes be heard on a high gain audio amplifier and speaker when an antenna is connected to the input to the amplifier. This basic equipment combination can be called a "Natural ELF Whistler Radio". In essence it is transducing the existing Extremely Low Frequencies of electromagnetic (radio) energy existing between 0 to 10 kHz into audio that can be heard.
If you missed the first part of this article, please go to Archive III of antenneX and review the article. Once there, you will also be able to hear for yourself, on your own computer, some of these fascinating natural sounds that are provided, courtesy of NASA, via streaming audio.
I wish to thank Bryan Eyre, VK7KBE, from Tasmania, Australia, whose reader comment question on the antenneX Discussion Forum about Whistlers led to my writing of this article. Bryan is now experimenting with his own homemade portable ELF receiver and may provide an article about his experiences in next month's issue of antenneX.
Also, I wish to thank many of the readers who did provide very nice reader responses to the first part of this article. There is one point I need to expand on, though, in order to remove some more speculation about there being brilliantly colored red, blue, and green lightning strokes being observed on the tops of clouds, via TV cameras on various Shuttle Spacecraft. I should have added that common lightning strokes were at times observed simultaneously by more than one TV camera, with the resulting video pictures revealing highly saturated but different hues or colors! In other words, it is extremely unlikely that the lightning at the top of any cloud would provide a brilliant green stroke, to be immediately followed by a brilliant red or blue stroke. The facts should be taken into account.
Whistlers can be very useful in determining real-time size as well as other information about the magnetosphere medium they propagate through. By bouncing back and forth and echoing through it in both north to south and south to north magnetic directions between equal magnetic latitude areas on both hemispheres of the earth, the remaining whistler energy reveals those frequencies that were attenuated the least.
To learn more about whistlers, we should be knowledgeable of lightning characteristics since lightning is the source of whistler energy.
It has been estimated that lightning strikes the earth an average of 400 times per second. It is rare for a lightning strike to consist of only a single stroke or bolt, three or four strokes is common, and as many as twenty strokes per strike have been observed and documented on the "Lightning Cameras" I worked with at the Kennedy Space Center in Florida.
Each single stroke can initiate a whistler. Therefore, the more strokes occurring within a given lightning strike, the less pure sounding the resulting whistler will be and the wider it will appear on the time display of a spectrograph or waveform analyzer. (See the LINK reference at the end of this article to download a free FFT waveform analyzer program for your own computer. This will allow you to analyze the audio tape recording of any whistlers at a later time.)
Longer lightning strikes may also result in a slightly less pure whistler tone. Some horizontal strikes have been reported to have been 90-miles (140-km) long, while some vertical strikes were said to be nine miles (14-km) high. Lightning strikes with many multiple strokes may account for the "swish" sounds.
Normal parabolic radio navigation, such as used by LORAN and SHORAN, depend on comparing the timed reception by a vehicle receiver at an unknown location of the leading edge of r-f pulse trains transmitted from several precisely timed transmitters at known fixed locations to determine the location of the receiver. Using this "parabolic navigation system" the locations and speeds of aircraft, ships, and others can be determined. (During WW II, I devised the method of controlling autopilots with LORAN as an aircraft navigation option.)
In contrast, "upside-down" natural ELF radios can be used at combinations of multiple fixed receiving locations with an application of "reverse parabolic navigation" to provide the unknown location of the source of a static crash, namely where the lightning struck. By analyzing the time of the leading edge of lightning signal arrivals at some of the natural radio receivers, this technique not only permits determining the latitude and longitude of a single strike of lightning, but it will permit determining the elevation of the top of lightning strikes! This system was developed at the Kennedy Space Center in Florida (KSC).
Reverse parabolic navigation lightning detection systems can be used to provide valuable storm information to be relayed to pilots of small aircraft and ships that do not have weather radar installed. Several lightning strikes can then be used to also provide information as to the speed and direction of an electrical storm's movement, thus allowing valuable advance warning to be used in determining alternate navigation routes. This information has also been a factor in making missile launch "go-no go" decisions, too.
The rate and number of strikes is usually proportional to the severity of the electrical storm. This information can be automatically computer processed and displayed on color CRTs with altitude being displayed by assigned false colors. Storage CRT display retention time can be selected by the computer operator to easily provide observable storm tracking information. Strike information displayed over area maps is sometimes displayed by TV weathermen as a public service to TV viewers.
The earth is surrounded by a giant magnetic field called a magnetosphere. The earth spins within this giant magnetic field. The magnetosphere's theoretical doughnut shape surrounding the earth magnetic equator is greatly distorted by the pressure of corpuscular radiation and the solar wind pressing it down closer to the earth over the earth's "daylight" side. The greatest pressure is exerted on it over the earth's local "noon" position. At the other extreme, it is pushed the greatest distance from the earth over the earth's local "midnight" position resulting in a comet-tail like elongation. The final shape is sometimes called a "tear-drop" shape.
The areas above the earth normally encountering the greatest rate of change in its overhead fields can be expected to occur within a few hours of the local dawn and sunset positions. This is when "dawn chorus" and many other weird sounds are more likely to be heard.
The amount of charges accumulated by the Van Allen radiation belt and the many ionization layers floating above the earth within this magnetic field can be greatest during daylight hours with the greatest amount of charges being accumulated near local "noon".
To the surprise of many people including myself, the greatest aurora activity can be expected to occur then too, even though its not visible (unless there is a total eclipse of the sun). I have spent the greater portion of many nights looking upward at the dynamic horizon to horizon sky-filling displays of aurora when the northern lights were very active. Natural ELF radio is one of the ways of monitoring for this activity.
When some power line "hum" remains, even after attempting to find a receiving location free of it, a comb filter, with its primary frequency selected to match the fundamental 50 or 60 Hz power line "hum" frequency can help some by notching-out the "hum" and its "hum harmonics". However, if other signals have been modulated by this interference, the resulting sidebands will not be notched out. Therefore, receiver linearity is very important in a Natural ELF Whistler Radio to prevent generating modulation cross-products.
Seeing how many power line harmonic frequencies exist near faulty power lines, it would be ineffective to use a single frequency filter to notch out just the fundamental 60-Hz "hum" frequency. A band pass filter that would pass every thing from 0.5 to 7-kHz may be the most useful for monitoring most of the whistler frequencies. 2x60, 2x"hum", or "double hum" may be dominant because a faulty insulator breaks down twice during each cycle of the fundamental power line frequency.
However, some of the ELF radio spectrograms examined revealed a separation of 360-Hz between dominant harmonic horizontal lines which indicates that odd harmonics of the third harmonic of 120-Hz contain the most energy, as compared to the anticipated odd harmonics of 120-Hz being dominant.
Recorders should not include, or use, automatic volume controls (ALC), or automatic level controls (ALC), which negate accurate signal strength measurements.
Some spectrographs are difficult to use when the time and frequency are hard to decipher. The addition of a small amount of a 0.5 or a 1-kHz signal injected after the band pass filter, or on the tape recordings, as a frequency marker, would be valuable in speeding-up and improving the accuracy of data reduction. Similarly, injection of a small amount of timing signal into the data chain from an accurate wide band timing "tick-mark" generator would simplify data reduction, also. A combination of the two can be achieved by gating on the frequency markers only at the occurrence of the "time-ticks". Extremely accurate time can be obtained from hand held GPS receivers if you use one that has an external signal connector.
However, if real-time is not important to you, the timing ticks may be injected into the data when the tape recordings are played back, to provide accurate relative-time for simplified data reduction.
Remember, whistlers only come from lighting. Viewing the global weather reports on TV, or on your computer, may allow you to find when electrical storms are forecast for the area of the world existing at an equal latitude in the hemisphere opposite to yours and existing on the same magnetic longitude as yours. If lightning is not occurring there, such as in one of the great desserts of the world where it never storms, then do not expect any one-hop whistlers.
Of course there is still the possibility of receiving two-hop whistlers if an electrical storm is occurring in your region. Lightning will have to be occurring within approximately 500-miles (800 km) of you location for you to receive two-hop whistlers. (But, if it is within 5-miles (8 km)of you, turn off the receiver and head for cover!) I do not think there is any preferred time to listen for whistlers since they only occur when lightning originates them.
It is desirable to use two types of antennas; a vertical whip and a magnetic loop antenna. The best position for a fractional wavelength magnetic loop receiving antenna is vertical. Placing it horizontally is not good.
The plane of a receiving magnetic loop should be mounted vertically and should be aligned with the needle of a magnetic compass, i.e. the loop needs to be aligned with the earths magnetic north-south lines.
Remember, you are not trying to achieve "common mode rejection" of vertically polarized signals: you are trying to receive them, so avoid placing the feed point at the bottom center of the loop. Experiment with the magnetic loop antennas feed point being placed at various positions in the plane of the loop, such as on the corner toward the earths equator.
A vertical whip antenna should be available to allow frequent antenna trade-off, in order to compare which mode is best at a given time. I suggest nothing shorter than 3-feet (one meter) long whip.
Caution: Once I was using a 12" clip lead as a vertical antenna, I connected it directly to a MOS-FET preamplifier. The preamplifiers output was connected directly to the vertical input of an oscilloscope operating in d-c mode. When I moved my feet, even very slowly the scope displayed my motion from over ten feet away. Quick motions would abruptly saturate this simple sensitive natural ELF radio system. Because of this, d-c coupling of an electrostatic antenna to the system should be avoided, unless one is trying to make an electrostatic field mill to sense dangerously charged clouds in the area. (But, thats another subject area about instruments used at the Kennedy Space Center in Florida.)
Several versions of natural ELF radios are available on the web as are schematics and instructions for making your own battery powered receiver (audio amplifier). Some very high gain audio amplifier IC chips, such as the LM386, are available that could serve as the basis of a natural ELF whistler receiver.
If one's interest in Natural ELF Whistler Radio is not scientific but only to hear the unusual music of the spheres and other weird non-manmade sounds in order for them to be combined into new exotic music, or sound effects to be combined with music, one may wish to mix (multiply) "echo chorus" signals with each other, or with other sounds. This may help introduce many interesting cross-product novel sounds, provided low-pass signal filtering is used to reject anything above 7-kHz prior to a mixing circuit. (Doing this would reject signals from the antiquated Russian ALPHA navigation system, signals radiating from poorly-designed TV horizontal sweep amplifiers, or any other manmade VLF signals. )
The OMEGA navigation system transmitters have now been turned off and the extremely useful LORAN navigation system is being phased out (prematurely, I think), in favor of the very accurate Global Positioning System (GPS) navigation system. (During WW II, I participated in the installation of the first LORAN system on the West Coast.) Of course, this and other signal processing can be done more conveniently at a later time using playbacks of tape recordings of the basic signal made out in the countryside far, far away from electric power lines.
If one does not wish to build a natural radio and make field trips to low "hum" regions, then it would be less expensive, less time consuming, and much simpler to purchase tape recordings or CDs from those who are prepared to provide impressive copies of their very best results which were obtained over years of collecting.
If obtaining ideas for music is the only purpose, vs. the awareness of dynamic real-time scientific activities, another great variety of natural sounds can also be heard from speeding-up recordings (about twenty times) of the huge number of and variety of earthquakes that can be detected daily. When I was a senior scientist on the staff of the Space Sciences Division of the Jet Propulsion Lab of the California Institute of Technology at Pasadena, California, Dr. Charles Richter, the creator of the method for classifying earthquake intensities called the Richter Scale, treated me with a play back of a tape recording of earth quakes that he had made. First he played it at normal speed. The normal speed play back was muffled, dull, and not interesting. Then he played it at 20X. It was fantastic! I will never forget it!
This experience motivated me to quickly build a seismometer using a 20-pound junk power transformer as the weight. The resulting instrument was much more sensitive than I ever expected. I was able to easily see deflections of several centimeters at times, and especially when a series of underground atomic bomb tests were being conducted hundreds of miles away in Nevada. By eaves-dropping on short wave radio to their count-down clock for detonation time reference, I found the predicted seismic propagation times even proved to be extremely accurate, too!
CLICK HERE and you can listen to some of the Natural ELF Radio sounds with streaming audio (Real Audio) courtesy of NASA.
A free Fast Fourier Transform (FFT) Waveform Analyzer program and an Audio Waveform Generator program can be downloaded for use on your own computer at: http://outpost.ietc.ca/home/bill/bbs.htm
You can view spectrograms of whistlers as well as hear excellent recordings of many more whistlers than we used in generating the audio clip for this article, courtesy of NASA: http://image.gsfc.nasa.gov/poetry/inspire/advanced.html
"The Advanced VLF Signals" page offers many options with sounds. All options are interesting and well worth seeing and hearing. Special attention is called to the "Whistler Echo Train"(893k) found under the option "More Whistlers".
Some US Government agencies doing Whistler research are:
NASA's Godard Space Flight center invites participation in a program called "INSPIRE" (an acronym for Interactive NASA Space Physics Ionosphere Radio Experiments) http://image.gsfc/poetry/inspire
Some of the university labs involved with Whistlers are:
University of Alaska Geological Institute
University of Iowa: http://www-w.physics.uiowa.edu/~/jsp/polar/sounds/whistlers.html
Stanford University: http://www.stanford.edu/~vlf/Science/Science.html
[An excellent site, but I could only access it via the "Google" search engine]
Kyoto University, Kyoto, Japan
An excellent search engine for "WHISTLERS" is: http://www.google.com (with 1465 matches; however, not all "whistlers" relate to radio, some are musicians.)
Also search for other subjects such as: VLF, ELF, Longwave, and Long Wave Club of America (LWCA)
Some other related references:
http://solar.uleth.ca [Solar Terrestrial Dispatch]
Source of Natural Radio Sound Recordings, plus other
S. P. McGreevy's ground-based ELF-VLF recordings:
S. P. McGreevy Productions/Society of Natural VLF Radio,
P. O. Box 928, Lone Pine, California 93545-0928, email@example.com
Download a "Whistler Handbook" from: http://www.triax.com/vlfradio/vlfhndbk.htm
The key to success in making good Natural ELF Whistler Radio recordings (and good earthquake recordings, too) is to be far removed from the many man-made noise sources that would otherwise mask the desired results. Both types of recordings can be signal processed in many ways at playback. They can even be played back backwards, too!
Enjoy your experimenting! 30-
Harold Allen, W4MMC,
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Last modified: March 23, 2005