On the Phenomena of Lightning

Thunderstorms are epic demonstrations of nature that can be quite fascinating when they aren't terrifying. The study of thunderstorms, in particular lightning, is of obvious practical interest, but also there is also a purely aesthetic and amusing aspect to them. I looked into the literature on lightning recently because lightning radio interference came up during my radio astronomy group meeting. The most visible form of thunderstorm lightning causes beautiful visible sprites and jets when a path of negatively charged ionized air reaches down from the clouds towards the ground where it meets the positively charged ground and following this low resistance channel of ionized air a luminous return stroke occurs, but beyond this visible display a full spectrum of electromagnetic waves may be induced in the ionosphere and magnetosphere. One of the most interesting phenomena are the reverberations around Earth in the radio spectrum called whistlers. With an appropriately tuned receiver and audio amplifier you can detect lightning's Earthly chorus. You can hear and see some of these sounds, including whistlers from lightning on Jupiter, over at professor Don Gurnett's selected sounds of space page. Read on to learn a little more about lightning, whistlers, and watch a video by the Kronos Quartet which incorporates some of these whistler sounds.

All about lightning

Observations of lightning from space show that there are about 2,000 thunderstorms active at any moment covering about 10% of the Earth's surface. Storms are not evenly distributed temporarily or spatially over the Earth; two out of three of lightning flashes occur over tropical regions and the peak of thunderstorm activity occurs through June to August. The simplest model of a thunderstorm is a vertically extended tripolar structure (though thunderstorms sometimes may involve up to six charge layers in the vertical direction) wherein there is a negative charge in the middle and positive charge at the upper and lower levels of the clouds. Thunderstorms produce various kinds of lightning strikes such as, cloud-to-cloud, cloud-to-ground and cloud-to-ionosphere electrical discharges. The common cloud-to-ground strike produces visible lightning that is the most familiar and transfers negative charges back to Earth. The intra-cloud process is a similar discharge between dipoles in a thunderstorm structure. Lightning can also create exotic processes like a Terrestrial Gama-ray Flashes which are most likely the result of bremsstrahlung radiation from relativistic electron beams generated in runaway lightning discharge processes. A bolt of lightning radiates an electromagnetic pulse with up 20 GW of peak power for 1 ms to 1 s in duration. This electromagnetic power jolt heats the partially ionized upper atmosphere with fields of up to 1 kV/m which may accelerate electrons to these relativistic energies.

Lightning plays an important role in the dynamics of Earth's atmosphere. Lightning can be viewed as the equalizer of electrons in the atmosphere just as rainfall is the equalizer of water in the atmosphere. The classic analogy of the flow of electrons to flow of water is appropriate in this situation because as thunderstorms return water to the Earth's surface they also return charge from the atmosphere to Earth's surface. The water which falls as precipitation during storms is equal to the water evaporated on other parts of the Earth such that the total flow of water onto the Earth's surface and into the Earth's atmosphere is roughly equal. Similarly, there is a fair weather current of 1,000 A (1 pA/m²) which flows from the ionosphere to the Earth's surface over the 90% of the Earth which at any given moment has clear weather; over the other 10% of the Earth covered by thunderstorms there is a roughly equal but opposite current. This current flow sustains a dynamic equilibrium because the global circuit is sensitive to the instantaneous number of storms, for example at 1800 UT there is a maximum global current and a minimum at 0300 UT. More technically the flow of charge is formulated in terms of the Maxwell current density J_M = J_E + J_C + J_L + dD/dt which has contributions from the direct charge transfer terms: J_E is the field dependent current, J_C is the convection current generated by mechanical transport of charge (rain, air motion, etc.), J_L is the impulsive and discontinuous lightning current. The final term is  dD/dt is displacement current necessary to take into account time varying electric fields such as the building of charged thunderstorm clouds. The result of the various current flows is an active atmosphere that allows for copious amounts of lightning.

Whistlers

Whistlers are electromagnetic waves generated by lightning in the frequency range 3 to 30 kHz which propagate globally along geomagnetic field lines without appreciable attenuation. They begin as a sharp pulse of radiation lasting a few milliseconds which is stretched out by the magnetosphere as the higher frequencies travel faster than the lower frequencies. There global nature allows one to use a radio antenna and an audio amplifier (since they occur in what would be the audible range were they sound waves) to study the dynamics of world wide lightning activity. Whistlers (and many other frequencies emitted by lightning in the range from a few Hz to several MHz) may reverberate around Earth several times because they are guided thorough the Earth-ionosphere waveguide like waves down a narrow water trough. If the wave has the right frequency it can propagate several times around the globe before being dissipated like water settling after bouncing back and forth from the end of a trough a few times. The image at right shows how a magnetic field line about the Earth is configured and how the electromagnetic wave propagates along that field line. The larger image above shows a whistler spectrogram which is the dynamic spectrum in the frequency-time domain of a whistler event (both images from professor Gurnett). From the spectrogram various patterns of spectra may be characterized as different signals from hiss to chorus; the characteristic modulation and stretching of the frequencies is a telltale sign of a periodic phenomena which is undergoing dispersion. Whistlers are  not only a great demonstration of universal physics (there are Jovian whistlers too!), but an opportunity to get creative with nature. Samples of some of professor Gurnett's sounds have been used by the Kronos Quartet to create a unique performance:





References:

Siingh, D., Singh, A., Patel, R., Singh, R., Singh, R., Veenadhari, B., & Mukherjee, M. (2009). Thunderstorms, Lightning, Sprites and Magnetospheric Whistler-Mode Radio Waves Surveys in Geophysics, 29 (6), 499-551 DOI: 10.1007/s10712-008-9053-z