Transient ULF pulsation decay rates observed by ground based magnetometers: The contribution of spatial integration

Abstract

Theoretical studies suggest that Joule dissipation in the ionosphere is the major source of damping for resonant ULF pulsations. The decay rates of transient pulsations (i.e. short-lived pulsations with latitude dependent periods) observed by ground based magnetometers are however generally larger than those predicted, and also larger than those observed in the magnetosphere. We have modelled the integration effects of ground based magnetometers on transient pulsations by considering empirical models of the associated ionospheric currents. The simulated ground magnetometer data show a smearing of the amplitude and period variations, which is more pronounced for smaller scale (specifically latitudinal) variations. The period increase with latitude is reduced, and may even be eliminated over appreciable latitude ranges. For all spatial scales the observed decay rates are typically 2–3 times larger than the true values, due to the additional decay resulting from spatial integration of the incoherent transient pulsations. Estimates of the ionospheric Pedersen conductance based on ground magnetometer observations of decay rates are correspondingly too small, and spurious gradients may be introduced. The present calculations reconcile observed decay rates on the ground with those predicted using the assumption that Joule dissipation is the dominant damping mechanism for toroidal mode resonant oscillations.