Electrical currents (both AC and DC) are generated by thunderstorms and other electrified shower clouds. The DC (direct current) is of the order of ~2 pA/m2 in fair weather regions, with observed surface potential gradients of ~130 V/m. However, the AC (alternating current) fields are more interesting (in my opinion). The Earth's surface and the atmosphere above 60 km (known as the ionosphere) are both highly conductive regions that allow for the reflection of electromagnetic waves (radio waves) back and forth off these "surfaces". This earth-ionospheric wave-guide allows us to receive radio waves from great distances, allowing us to hear radio stations from 1000's of kilometers away without any help from satellites. Depending on the frequency of these waves, they will travel different distances since the attenuation rate depends on frequency. The higher the frequency (or the smaller the wavelength), the shorter the travel distance.
In the atmosphere, lightning produces electromagnetic radio waves, called sferics, made up of many different frequencies. Most of the energy is in the very low frequency (VLF: 3-30 kHz) range, with detection of these sferics from 1000-10,000 km away from the thunderstorms (depending on your VLF antenna and the intensity of the lightning ). The extremely low frequency (ELF: 3-3000 Hz) waves actually manage to travel around the globe (within the wave-guide) a few times before they decay and dissipate. Since there is no other natural source of ELF radiation in the lower atmosphere, scientists have now shown theoretically and experimentally that if we measure the ELF radiation with the right instruments and at locations far from man-made "noise", we can actually monitor changes in global lightning activity from a single location. As these radio waves travel around the globe constructive interference occurs at specific wavelengths (multiples of the Earth's circumference) resulting in standing waves. These are known as the Schumann resonances and are found at approximately 8Hz, 14Hz, 20Hz, 26Hz,. ....<>Why am I interested in the Schumann resonances? Well, as mentioned above, the Schumann resonances can allow us to easily study global lightning activity in different regions of the globe. And it turns out that global lightning activity is very sensitive to changes in the earth's global temperatures. If the Earth's climate is warming, then it is very likely that global lightning activity will increase, and these increases could then be monitored in the ELF band from a single location on the earth's surface. The figure below shows the daily fluctuations in the South American lightning activity (blue curve) measured from Israel using our Schumann resonance data, together with the surface temperatures (red curve) obtained from the NCEP reanalysis. It should be pointed out that the ELF measurements are from a single station, while the temperature data are an average over the whole South American region. Could atmospheric electricity be used as a global thermometer to monitor climate change?
We have now also now found that
lightning activity around the globe
is closely linked to the amount of water vapor in the upper
The thunderstorms that produce the lightning also transport large
of water vapor into the upper atmosphere. Changes in the amount
upper tropospheric water vapor could have dramatic impacts on the
climate. As shown below, we may be able to use the Schumann
to track variations in upper tropospheric water vapor
concentrations. The water vapor data below is averaged over
(red curve), while the ELF lightning data is obtained at our one
station in the Negev Desert, Israel (black curve).
In the last few years we have built a few ELF monitoring sites in Israel for the primary purpose of monitoring changes in the earth's climate via changes in global lightning activity. This site is located at our university's astronomy observatory. Our vertical electric field sensor for studying the Schumann resonances is shown below.
References:Price, C., 1993: Global surface temperatures and the atmospheric electrical circuit, Geophys. Res. Lett.,20, 1363-1366.