Spherics-Whistlers
Spherics and whistlers
Lightning is a gigantic spark or electric discharge that occurs between the clouds and the ground. During a storm, there is accumulation of charges in clouds, with positive charges moving to the top and negative charges (electrons) to the bottom. Electrons are attracted by positive charges on the ground. Eventually, if the charge buildup increases significantly, becoming equivalent to a voltage that surpasses the breakdown voltage of the air, the electrons will flow towards the ground. As a result, electricity will flow through the air to the ground with a brief emission of light and sound.
The lightning discharge generates an electromagnetic pulse (EMP) of broadband nature, meaning consisting of a wide band of different frequencies, especially ELF and VLF. The most intense radiation occurs in the VLF range of 5-10 KHz. As the generated waves are being reflected by both the ground and by the ionosphere, they circulate in the cavity formed between the two, i.e. the corresponding waveguide.
There are two main categories of electromagnetic waves that are produced by lighting:
1) The "radio atmospherics" abbreviated as "sferics", which are broadband ELF-VLF waves that propagate in the Earth-ionosphere guide.
(https://en.wikipedia.org/wiki/Radio_atmospheric_signal)
(https://vlfstanford.ku.edu.tr/research_topic_inlin/wavelet-analysis-sferics)
2) The "whistler waves" or "whistlers", which are broadband mostly VLF waves which escape from the Earth-ionosphere guide and propagate in the magnetosphere along the magnetic field lines. As a result of their propagation in the magnetosphere, they are dispersed and therefore encompass a wide band of frequencies, hence corresponding to "wide-spectrum" or "broadband" VLF waves.
(https://en.wikipedia.org/wiki/Whistler_(radio))
(https://vlfstanford.ku.edu.tr/research_topic_inlin/introduction-whistler-waves-magnetosphere)
The described mode of propagation, which has been termed "whistler-mode" propagation, allows them to easily reach the opposite hemisphere and bounce back (from its ionosphere) to the hemisphere of origin.
The name "whistler" is due to the fact that when the corresponding electromagnetic wave frequency is converted to a sound wave frequency, it sounds similar to whistling of changing pitch. It is known that the human hearing range for sound frequencies starts from 20 Hz and ends at 20 kHz. The ELF and VLF electromagnetic waves span the 3 Hz to 30 kHz range. Therefore, if the whistler electromagnetic frequencies are converted to sound frequencies, they can be heard by humans.
It is noted that a subcategory of sferics called "tweeks" propagates for longer distances and may show some dispersion.
The following reference from the Georgia Tech College of Engineering LF group provides examples of the signal from sferics, tweeks and whistlers in spectrograms and in audio conversion: https://youtu.be/-jpe8EfGg48. In addition, signals from U.S. Navy transmitters and a Russian navigation beacon are included.
Other waves that are associated to lightning are ULF, as mentioned in this study:
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2014RS005566
Further to the waves that are produced by lightning, there is a category of waves that are excited due to it in the ionosphere-ground cavity. The dimensions of the cavity enable it to act as a resonant cavity in the ELF range. As a result, it excites and amplifies ELF waves. Scientist W. Schumann was the first to have predicted the fundamental frequency that is excited which is 7.83 Hz, as well as its harmonics i.e. 14.3, 20.8, 27.3, and 33.8 Hz. The generated ELF waves are called Schumann resonances and represent standing waves moving in the ionosphere-ground waveguide.
ELF/VLF electromagnetic wave phenomena
ELF and VLF waves are generated by both natural and anthropogenic processes. It is considered that on a global basis, by far the most significant source is ELF/VLF radiation is lightning propagating in the Earth-ionosphere waveguide .
(Reference: https://ieeexplore.ieee.org/document/6050782)
ELF/VLF electromagnetic wave phenomena are distinguished in the following categories:
0. Sferics - broadband ELF/VLF associated with lightning
I. Whistlers - broadband VLF associated with lightning.
II. ELF/VLF emissions not associated with lightning:
1. Continuous or noise-like. Cf. "hiss".
2. Discrete. Cf. "chorus"
III. Interactions Between Whistlers and ELF/VLF Emissions.
IV. Power Line Harmonic Radiation (PLHR)
"ELF-VLF harmonic radiation from power systems penetrates the ionosphere and propagates into the inner magnetosphere, where it is amplified by unstable ambient plasma and may take the form of a self-sustained ELF-VLF emission." https://www.sgo.fi/Publications/SGO/thesis/ManninenJyrki.pdf
A primer starting at e-page 15 of the reference https://ir.uiowa.edu/etd/4549/
Wikipedia references:
Sferics | ELF-VLF | https://en.wikipedia.org/wiki/Radio_atmospheric_signal
Whistler | VLF | 1 kHz to 30 kHz https://en.wikipedia.org/wiki/Whistler_(radio)
Hiss | ELF-VLF | 300 Hz – 10 kHz https://en.wikipedia.org/wiki/Hiss_(electromagnetic)
Chorus | https://en.wikipedia.org/wiki/Dawn_chorus_(electromagnetic)
Effect of (ULF) sferics on humans
Panagopoulos, D. J., & Balmori, A. (2017). On the biophysical mechanism of sensing atmospheric discharges by living organisms. Science of the Total Environment, 599-600, 2026–2034. https://doi.org/10.1016/j.scitotenv.2017.05.089
Sferics used for Positioning, Navigation and Timing (PNT)
LIGHTNING SFERICS (VLF) PNT - Lightning for Precision Timing
.@SPAWAR Systems Center Pacific uses lightning for precision timing to enable Signals Intelligence (SIGINT) and secure communications. http://www.secnav.navy.mil/innovation/Documents/2017/07/VLFPNT.pdf
Mechanism for wave amplification and stimulated emissions (triggered emissions)
https://vlfstanford.ku.edu.tr/research_topic_inlin/vlf-transmitter-siple-station-antarctica
When VLF waves are injected in the magnetosphere, they may be amplified or they may trigger other emissions. This may be due to interaction of the wave with energetic charged particles and specifically electrons which are gyrating along the magnetic field lines (Fig. 2 https://vlfstanford.ku.edu.tr/research_topic_inlin/vlf-transmitter-siple-station-antarctica). If the frequency of the wave is equal to the frequency at which the electron gyrates around the Earth's magnetic field, resonance may occur resulting in energy release which may induce amplification of the wave or triggering of emissions. Circularly polarized waves are favored in interactions with gyrating (circularly-moving) particles.
Figure: Spectrogram (frequency vs time plot) of a triggered emission, a constant frequency signal triggering a signal that rises in frequency (blue rectangle). From https://vlfstanford.ku.edu.tr/research_topic_inlin/vlf-transmitter-siple-station-antarctica/.