The polar cap absorption event of 19–21 March 1990: recombination coefficients, the twilight transition and the midday recovery.

For the polar cap event of 19–21 March 1990, effective recombination coefficients have been estimated at heights between 60 and 80 km by comparing the electron density, measured by the EISCAT VHF incoherent scatter radar, with production rates calculated from proton fluxes measured on the GOES-7 satellite. The estimated coefficients change considerably between day and night, the change being relatively greater at lower altitudes. During the midday recovery, for which there is clear evidence during this event, the edge of the polar cap moved over the radar, and it is thus possible to see the effect of the edge on the spectrum of precipitating protons. The spectrum hardened at this time, indicating an increase in the cut-off rigidity