Sounds of VLFPrepared by Morris Cohen and Nader Moussa Stanford University, Stanford, CA
IHY Workshop on Advancing VLF through the Global AWESOME Network
Very Low Frequency RadioAudible to Human EarThe 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
Early history of VLFNatural VLF first heard as coupling into long transmission lines, late 19th centuryTelegraph lines during WWI picked up whistlersyou can hear the grenades fallingNatural VLF signals named after their sounds tweek, click/pop, whistler, chorus, etc
Natural VLF SignalsImpulsive radio atmospherics (sferics)ClicksPopsTweeksWhistlersSounds like a falling whistleChorusSounds like birds chirpingHissSounds like high pitched static noise
Clicks (type of sferic)Impulsive noiseFrequency range limited by Earth-ionosphere waveguideUsually from long, daytime sfreric path
Pops (type of sferic)Originates from nearby lightning activitywithin a few hundred kmVLF energy at all frequencies
Tweeks (type of sferic)Impulsive noiseFrequency range limited by Earth-ionosphere waveguide
Whistlers (magnetospheric)Originates from lightningLightning energy escapes atmosphere, propagates to magnetic conjugate pointFrequency-energy signature caused by dispersion
Chorus (magnetospheric)More common at high latitudesOften associated with high geomagnetic and solar activity
Chorus (observed in situ)Observation from Cluster spacecraftVery structured and repetitive, rising tones
Hiss (magnetospheric)Impulsive noiseFrequency range limited by Earth-ionosphere waveguideWhistlers can sometimes form hissbandHiss may also be generated by chorus
Whistlers forming hissband
Power Line HumPower Line TypesHigh tension distribution lines long distance, 10100 kVResidential distribution at 110-2400 voltsAC wiring inside buildings at 110 or 220 voltsElectric distribution networks generate VLF signals at 50 or 60 Hz, plus harmonics
Power-line signals in spacePower line harmonics detected over land by DEMETER (Nemec et al. 2007, JGR)
Power Hum SpectrumMultiples of 50/60 HzOdd harmonics may be stronger
Power Hum FrequenciesAmplitude, phase, and instantaneous frequency are highly variableEnd-user electric demand and consumption affects radio emissions Load on power grid constantly changingPower-line harmonics have finite bandwidth
Hum sniffingThe best way to avoid power-line interference is to find several locations and check the noise at each oneLocate antenna away from power lines, generators, and antennasStudents from Stanford use a portable antenna to listen for power line interference in Alaska, USA
Hum removal techniquesCan process data to remove hum noiseHigh-pass filtering: Removal of all power below 1.5kHzNotch Filtering and `comb filters at all 50/60Hz harmonicsFrequency-tracking filter
Recent ReferencesBortnik, 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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