Electric Skies - Thunderstorms and Lightning

EW: One can make a broad separation between lightning that contacts earth (cloud-to-ground lightning) and lightning that does not (intracloud lightning). Generally, the lightning type of concern for human safety, the initiation of forest fires and the destruction of electronic equipment is the cloud-to-ground flash, and as a consequence, this type of lightning is targeted by operational networks for lightning detection. Within these two broad categories, there are other subcategories. Cloud-to-ground lightning can transfer either negative or positive charge to earth. The great majority of ground flashes are negative, but the most powerful and most dangerous are the positive ground flashes that can also produce sprites in the mesosphere. Positive flashes occur in the decaying phases of large thunderstorms and in the very active stages of severe storms. Intracloud lightning is far more prevalent than cloud-to-ground lightning, and the mean peak currents in intracloud lightning tend to be smaller. Intracloud flashes that leave cloud base and approach ground, but do not reach ground, are called air discharges. Intracloud lightning that leaves cloud top and heads upwards are sometimes referred to as cloud-to-ionosphere flashes. Intracloud flashes that bridge two thunderstorms are called intercloud flashes. In the active stages of severe storms, intracloud flashes can outnumber ground flashes by 10-100 to one.

FS: What research are you currently carrying out?

EW: Two areas are receiving most of my attention at the moment: studies of the Earth's Schumann resonances and lightning in severe Florida thunderstorms.

The measurements of Schumann resonances provides access to many things on a global basis. These are electromagnetic standing waves trapped in the Earth-ionosphere cavity and maintained by global lightning. We record these signals in the remote woods of western Rhode Island. The fundamental resonant frequency is 8 Hz. Every lightning radiates a small amount of energy at 8 Hz and so contributes to the quasi-continuous background signal of Schumann resonances. Every few minutes or so, a single extraordinarily energetic lightning will single-handedly 'ring' the Earth-ionosphere cavity and such events can be located from the Rhode Island station on a global basis. The most energetic events of all are large ground flashes with positive polarity and accompanying horizontally extensive 'spider' lightning. Many of these events also trigger sprites high in the mesosphere (75 km altitude), a luminous phenomenon shaped like jellyfish but as large as a thunderstorm itself. The global mapping capability for large lightnings thereby provides indirectly a global map for sprites. The electromagnetic signals can be used to compute the dipole moment change of the lightning and from this one can determine whether the field increase over the storm is large enough to produce a sprite.