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 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

3 Early history of VLF Natural VLF first heard as coupling into long transmission lines, late 19 th 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 Power Line Types High tension distribution lines – long distance, 10–100 kV Residential distribution at volts AC wiring inside buildings at 110 or 220 volts Electric distribution networks generate VLF signals at 50 or 60 Hz, plus harmonics

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 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

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 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