It shown theoretically and confirmed for the first time experimentally that the magnetic response of the lower auroral ionosphere to pulsed ionospheric HF heating depends critically on the parameter T/Th, where T is the pulse duration and Th is the time required for the electron temperature to reach its steady state value. Theoretical analysis shows that the strength and pulse shape of the near-zone magnetic field on the ground depends not only on the traditional quasi-static near-field term, but also on a term which depends on the derivative of the current (i.e., the impulse response of the ionosphere). Results from newly-conducted experiments using the HAARP transmitter in Gakona, Alaska are presented that verify our model, and it is shown that for T ≤ Th the magnetic flux on the ground due to the impulse response is 10--15 dB larger than the flux due to the quasi-static term.