Calculations of the response of F region plasma to rapid subauroral ion drifts (SAID) show that the maximum field-aligr].ed drift is a strong function of the time scale over which the SAID is applied. For SAID velocities of 2 km s '1 the peak field-aligned drifts can vary by almost a factor of 10 for differences in the onset time scale from a few seconds to a few minutes. Reasonable considerations of the convective configuration in a SAID event suggest that acceleration times between 10 and 20 rain might typically be expected. In such cases, much smaller field-aligned velocities should be observed than would be predicted assuming an essentially instantaneous turn on of the SAID event.determining the ion concentration that is observed in the trough where itself. Storm time ion outflows have also been observed at N i , ion concentration; A, magnetic flux tube cross section; v[', field-aligned ion velocity; Pi, ion production rate; Li , ion loss rate; 21,493 21,494 H•.•.T.IS ET AL.: FIELD-ALIGNED DRIF'IB IN SUBAURORAL ION DR1FT EVF2qTS •em, ion • x • drift velocity; s , magnetic flux tube arc length; t , time. The field-aligned drift velocity is obtained from the steady state momentum equation which enforces equilibrium between the forces due to gravity, the plasma pressure gradients, thermal diffusion and collisions between the charged and neutral species. The energy equation for the ions is expressed in the form 3 dT i 2 dt 3 kN.v.,, •T,

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S U M M A R YSecular changes in the solar geomagnetic variation at 11 observatories with a worldwide distribution have been studied. The secular changes at UK observatories were found to be large. Numerical calculations, details of which are described in this paper, agree in predicting the large changes at UK observatories. The numerical calculations also predict a large secular change in Sq at Hermanus in all elements; this would be expected as the main field at this station shows a large secular change. The secular change in Sq observed a t Hermanus is, however, found to be fairly small compared with that observed at the other stations.

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