Abstract
In the past few years, dramatic new evidence has shown that cloud electrification and lightning discharges in the troposphere modify the overlying mesosphere and lower ionosphere through heating, ionization, gamma-ray bursts and optical emissions known as Sprites, blue jets and elves. Heating and acceleration of electrons to relativistic energies may have long-term chemical effects in the middle atmosphere. In addition, vertical currents in Sprites move significant charge from the thundercloud to the ionosphere and play a previously unconsidered role in maintenance of the global fair weather electric field of ~100 V/m at the Earth's surface. In order to assess the global effects of this electrodynamic coupling, a continuous indicator of Sprite occurrence is needed.

Sprites are intense, transient luminous events in the mesosphere and ionosphere above thunderstorm systems. They extend from ~40 to ~90 km in altitude, are primarily red in color, and develop to full brightness in a few ms. Sprites are nearly uniquely associated with positive cloud-to-ground lightning (+CG), yet they occur in association with only a small subset of +CG flashes. The peak current of each flash as measured by the National Lightning Detection Network (NLDN), is not a sufficient indicator of the likelihood of a +CG to produce a Sprite.

Radio atmospherics ('sferics') provide a unique signature of each lightning return stroke and propagate efficiently in the Earth-ionosphere waveguide. Broadband measurements of sferics performed near Ft. Collins, Colorado, ~500 km from the causative lightning, demonstrate that the ELF (extremely low frequency, here 15 Hz to 1.5 kHz) sferic energy is a proxy indicator for Sprite occurrence. Ultra-long range measurements at Palmer Station, Antarctica, show that Sprite-associated sferics have large ELF magnitudes in relation to non-Sprite producing sferics at a range of ~12,000 km. VLF (very low frequency, 3 to 30 kHz) direction finding allows lightning location to better than +/-100 km using two-station measurements at up to ~10,000 km range. These results suggest that six appropriately located ELF/VLF sferics receivers could provide a reliable assessment of global Sprite occurrence rates.