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Sounds of VLF

Sounds of VLF. Prepared by Morris Cohen and Nader Moussa Stanford University, Stanford, CA IHY Workshop on Advancing VLF through the Global AWESOME Network. Very Low Frequency Radio. The history of VLF is joined with a history of `listening’ to VLF data.

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Sounds of VLF

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  1. Sounds of VLF Prepared by Morris Cohen and Nader Moussa Stanford University, Stanford, CA IHY Workshop on Advancing VLF through the Global AWESOME Network

  2. Very Low Frequency Radio 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 Audible to Human Ear

  3. 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…

  4. 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

  5. Clicks (type of sferic) • Impulsive noise • Frequency range limited by Earth-ionosphere waveguide • Usually from long, daytime sfreric path

  6. Pops (type of sferic) • Originates from nearby lightning activity • within a few hundred km • VLF energy at all frequencies

  7. Tweeks (type of sferic) • Impulsive noise • Frequency range limited by Earth-ionosphere waveguide

  8. Whistlers (magnetospheric) • Originates from lightning • Lightning energy escapes atmosphere, propagates to magnetic conjugate point • Frequency-energy signature caused by dispersion

  9. Chorus (magnetospheric) • More common at high latitudes • Often associated with high geomagnetic and solar activity

  10. Chorus (observed in situ) • Observation from Cluster spacecraft • Very structured and repetitive, rising tones

  11. Hiss (magnetospheric) • Impulsive noise • Frequency range limited by Earth-ionosphere waveguide • Whistlers can sometimes form hissband • Hiss may also be generated by chorus

  12. Whistlers forming hissband

  13. Power Line Hum Electric distribution networks generate VLF signals at 50 or 60 Hz, plus harmonics 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

  14. Power-line signals in space • Power line harmonics detected over land by DEMETER (Nemec et al. 2007, JGR)

  15. Power Hum Spectrum Multiples of 50/60 Hz Odd harmonics may be stronger

  16. 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

  17. Hum sniffing Students from Stanford use a portable antenna to listen for power line interference in Alaska, USA 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

  18. 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

  19. 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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