ULTRA-SLOW TAILS OF SPRITE-ASSOCIATED LIGHTNING FLASHES

We describe the terrestrial excitation of horizontal magnetic field va riations in the Pc1 frequency range (0.2-5.0 Hz) by tropospheric, spri te-associated lightning flashes, measured similar to 1900 km west from the source. These variations, which we call ultra-slow tails, exhibit amplitudes on the order of tens of pT, they have a duration of simila r to 3 seconds, and they occur immediately following the initial pulse of the sprite-associated lightning flash. The ultra-slow tails exhibi t two peaks in the frequency domain at 0.67 Hz and 1.67 Hz. The mean p olarization ellipses at these two frequencies are oriented similar to 45 degrees clockwise from geographic north and exhibit right-hand and left-hand polarization respectively with a weak ellipticity of similar to 0.1. The horizontal magnetic intensity of the Initial pulse is rel ated to the horizontal magnetic intensity of the ultra-slow tail, in a greement with the interpretation of ultra-slow tails as ionospheric Al fven resonances.