THE IONOSPHERIC RESPONSE TO FLUX-TRANSFER EVENTS - THE FIRST FEW MINUTES

We utilise high-time resolution measurements from the PACE HF radar at Halley, Antarctica to explore the evolution of the ionospheric respon se during the first few minutes after enhanced reconnection occurs at the magnetopause. We show that the plasma velocity increases associate d with flux transfer events (FTEs) occur first similar to 100-200 km e quatorward of the region to which magnetosheath (cusp) precipitation m aps to the ionosphere. We suggest that these velocity variations start near the ionospheric footprint of the boundary between open and close d magnetic field lines. We show that these velocity variations have ri se times similar to 100 s and fall times of similar to 10 s: When thes e velocity transients reach the latitude of the cusp precipitation, so metimes the equatorward boundary of the precipitation begins to move e quatorward, the expected and previously reported ionospheric signature of enhanced reconnection. A hypothesis is proposed to explain the vel ocity variations. It involves the rapid outflow of magnetospheric elec trons into the magnetosheath along the most recently reconnected field lines. Several predictions are made arising from the proposed explana tion which could be tested with ground-based and space-based observati ons.