Craig became involved with the idea of locating lightning strikes when he was a student in the Space Physics group operating out of the University of Otago. Following on from the work of Professor Richard Dowden and now as a full-time staff member, Craig played a key role not only in developing and understanding the physics of a radio monitoring system but also in the setting up of a worldwide lightning location network.

Global lightning detection

The network that resulted from this work was given the acronym ‘WWLLN’, which stands for ‘world wide lightning location network’. Since the originators were New Zealand-based, an American collaborator suggested the acronym be pronounced ‘woollen’, given New Zealand’s close association with wool.

The network operates by receiving the very low-frequency (VLF) radiation produced by a lightning stroke in the frequency band 3–30 kHz. The dispersed waveform (sferic) of the lightning impulse is processed at each receiving site using a method known as time of group arrival (TOGA). The data obtained by each station is then sent to the University of Otago in Dunedin as well as the University of Washington in Seattle (USA) for further processing and loading onto the WWLLN website. Observations of the radio pulse produced by the same lightning and measured by at least five different receivers are required to pinpoint a location. Currently, it takes about 5 seconds for the system to work out the ‘where in the world and when’ of the lightning strike.

Valuable weather information

WWLLN is now a collective with more than 60 receivers scattered across the Earth, and more than 40 international institutions collaborate to make it work. It provides valuable weather information about thunderstorm location and activity to meteorological services, air travel operators, power line maintenance companies as well as the general public.

Probing the plasma surrounding the Earth

In addition to his involvement with WWLLN, Craig is currently looking in more detail at the radio wave radiation produced by lightning. Instead of being trapped between the Earth and the ionosphere, some of the radio waves escape into the plasma that surrounds the Earth. Once there, they tend to be guided back to the upper atmosphere by the Earth’s magnetic field.

By setting up widely spaced radio receivers, it is possible to pick up the dispersed radio wave. Instead of sounding like the ‘click’ of a sferic, produced about a second earlier by a lightning strike, it now has a whistling tone. This change from a click to a range of whistle tones provides information about the plasma surrounding the Earth. Since most satellites, such as GPS, weather and communications, orbit within this plasma, the more knowledge gained as to its density, variability and structure, the better the future design and operation of satellites will be.

High-energy particle precipitation

Another research area that Craig is involved in has to with the Van Allen radiation belts that surround the Earth like a doughnut. Trapped within the magnetic fields of these belts are very large numbers of protons and electrons. During the active stage of the Sun’s 11-year sun spot solar cycle, massive ejections of plasma from the upper atmosphere of the Sun occur. These are known as coronal mass ejections (CMEs).

If the Earth lies in the path of a CME, the Van Allen belt magnetic field can be distorted. When the distortion is large enough, some of the contained protons and electrons are released into the Earth’s upper atmosphere around the North and South Poles. These high-energy particles are now thought to interact with atmospheric gases such as nitrogen, converting them into ozone-destroying forms. This has important implications for high-latitude climate variability.

Useful links

The world wide lightning location network (WWLLN – pronounced ‘woollen’) website.

    Published 29 April 2014