Ultra-slow tails of sprite-associated lightning flashes

We describe the terrestrial excitation of horizontal magnetic field variations in the Pc1 frequency range (0.2-5.0 Hz) by tropospheric, sprite-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 similar to 3 seconds, and they occur immediately following the initial pulse of the sprite-associated lightning flash. The ultra-slow tails exhibit two peaks in the frequency domain at 0.67 Hz and 1.67 Hz. The mean polarization 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 related to the horizontal magnetic intensity of the ultra-slow tail, in agreement with the interpretation of ultra-slow tails as ionospheric Alfven resonances.