An interplanetary shock interacted with the Earth on February 18, 1999, and excited an intense transient Pc5 pulsation. Analysis of the spatial variations and of the time variations of the electron drifts induced by the pulsation in the high-latitude ionosphere allows a deduction of the latitudinal variation of the height-integrated Pedersen conductivity. It is concluded that ionospheric Joule heating losses are sufficient to account for the attenuation of the pulsation. It is also shown that for a pulsation event of this type use of magnetometer observations lead to an overestimate of the attenuation, or, equivalently, to an underestimate of the ionospheric Pedersen conductivity. ¿ 2001 American Geophysical Union