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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