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 ~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 ~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 ~45¿ clockwise from geographic north and exhibit right-hand and left-hand polarization respectively with a weak ellipticity of ~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. ¿ 1998 American Geophysical Union