Response of low latitude geomagnetic field to solar proton events with energies >10MeV

It is shown that the low latitude geomagnetic field is depressed following solar proton events (SPEs) of energies>10 MeV but the effect is much weaker compared to that in association with the keV particles injected into the ring current. It appears that a minimum threshold change from background to peak level of the integral flux is necessary for any significant geomagnetic influence. Energetic particle injection into the ring current also causes electric field modulations of the equatorial ionosphere, which diminishes the amplitude of the quasi-bidaily variation observed there. A peculiar response in association with SPE of duration between 5 and 10 days is pointed out. (author). 13 refs., 4 figs