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Sounds of VLF
Prepared by Morris Cohen and Nader Moussa Stanford University, Stanford, CA
IHY Workshop on Advancing VLF through the Global AWESOME
Network
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Very Low Frequency Radio
Audible to Human Ear
The history of VLF is joined with a history of `listening’ to VLF data.
Many common natural signals were described by how they sounded
Even today, you can learn a lot by listening to the ELF/VLF sound
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Early history of VLF
• Natural VLF first heard as coupling into long transmission lines, late 19th century
• Telegraph lines during WWI picked up whistlers…”you can hear the grenades falling”
• Natural VLF signals named after their sounds – tweek, click/pop, whistler, chorus, etc…
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Natural VLF Signals
• Impulsive radio atmospherics (“sferics”)– Clicks– Pops– Tweeks
• Whistlers– Sounds like a falling whistle
• Chorus– Sounds like birds chirping
• Hiss– Sounds like high pitched static noise
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Clicks (type of sferic)
• Impulsive noise• Frequency range
limited by Earth-ionosphere waveguide
• Usually from long, daytime sfreric path
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Pops (type of sferic)
• Originates from nearby lightning activity• within a few
hundred km• VLF energy at all
frequencies
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Tweeks (type of sferic)
• Impulsive noise• Frequency range
limited by Earth-ionosphere waveguide
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Whistlers (magnetospheric)
• Originates from lightning
• Lightning energy escapes atmosphere, propagates to magnetic conjugate point
• Frequency-energy signature caused by dispersion
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Chorus (magnetospheric)
• More common at high latitudes
• Often associated with high geomagnetic and solar activity
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Chorus (observed in situ)
• Observation from Cluster spacecraft
• Very structured and repetitive, rising tones
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Hiss (magnetospheric)
• Impulsive noise• Frequency range
limited by Earth-ionosphere waveguide
• Whistlers can sometimes form hissband
• Hiss may also be generated by chorus
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Whistlers forming hissband
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Power Line Hum
Power Line Types
High tension distribution lines – long distance, 10–100 kV
Residential distribution at 110-2400 volts
AC wiring inside buildings at 110 or 220 volts
Electric distribution networks generate VLF signals at 50 or 60 Hz, plus harmonics
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Power-line signals in space
• Power line harmonics detected over land by DEMETER (Nemec et al. 2007, JGR)
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Power Hum Spectrum
• Multiples of 50/60 Hz• Odd harmonics may be stronger
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Power Hum Frequencies
• Amplitude, phase, and instantaneous frequency are highly variable
• End-user electric demand and consumption affects radio emissions – Load on power grid
constantly changing
• Power-line harmonics have finite bandwidth
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Hum sniffing
• The best way to avoid power-line interference is to find several locations and check the noise at each one
• Locate antenna away from power lines, generators, and antennas
Students from Stanford use a portable antenna to listen for power line
interference in Alaska, USA
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Hum removal techniques
• Can process data to remove hum noise– High-pass filtering:
Removal of all power below 1.5kHz
– Notch Filtering and `comb’ filters at all 50/60Hz harmonics
– Frequency-tracking filter
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Recent References
• Bortnik, J. et al (2008) The unexpected origin of plasmaspheric hiss from discrete chorus emissions, Nature, Vol. 452
• Golden, D. I., M. Spasojevic, and U. S. Inan (2009), Diurnal dependence of ELF/VLF hiss and its relation to chorus at L = 2.4, J. Geophys. Res., Vol. 114.
• Nemec, F., et al. (2006), Power line harmonic radiation (PLHR) observed by the DEMETER spacecraft, J. Geophys. Res., Vol. 111
• Meredith, N. P., R. B. Horne, R. M. Thorne, D. Summers, and R. R. Anderson (2004), Substorm dependence of plasmaspheric hiss, J. Geophys. Res., Vol. 109
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