Lightning location network
Professor Craig Rodger 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.
How WWLLN began
University of Otago’s space physics expert Associate Professor Craig Rodger describes how the lightning detection network known as WWLLN began. Its origins lie in a University of Otago science project set up by Professor Richard Dowden designed to monitor large thunderstorms for the appearance of red sprites high above the thundercloud.
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.
What is WWLLN?
Associate Professor Craig Rodger from the Space Physics group at the University of Otago describes his involvement in the setting up and running of the World Wide Lightning Location Network (WWLLN). Based on the production of a radio pulse produced by the lightning discharge, data collected from the network is analysed to accurately pinpoint the location of the lightning discharge.
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.
Lightning research
Associate Professor Craig Rodger describes his interest in tracking lightning produced radio pulses called sferics. As these pulses exit the Earth’s atmosphere and travel through the plasma surrounding the Earth, they are dispersed. On returning to Earth, the dispersed pulses, called ‘whistlers’, can be picked up by receivers and the degree of dispersion analysed to provide information about the plasma sphere.
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).
Van Allen radiation belts and CMEs
Associate Professor Craig Rodger, Space Physics group leader at the University of Otago, explains what the Van Allen belts are. He then describes the effect a coronal mass ejection (CME) can have on them as a result of the Earth’s magnetic field being squeezed. Craig’s research interest lies in the changing chemical balance of the upper atmosphere induced by such changes.
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 link
The world wide lightning location network (WWLLN – pronounced ‘woollen’) website.
