ИСТИНА

We present results of statistical analysis of enhancements of >30 keV electrons observed by the NOAA/POES satellites during solar cycles 23 and 24 (1998 – 2016) at low drift shells L< 1.1, so-called forbidden zone. We collected 1750 days (~25% of the total time) when fluxes of the forbidden energetic electrons (FEE) exceeded 104 (cm2 s sr)-1. It was found that the FEE enhancements were observed mostly often at the declining phases and solar minimum. More than 85% of the events occurred under fast solar wind (V > 400 km/s), high substorm activity (AL >100 nT) and enhanced interplanetary electric field perturbations (VδB > 1 mV/m). The FEE occurrence rate peaks around the local midnight. We have also found a quite unexpected annual variation of the FEE occurrence rate with a pronounced maximum from May to September, a minor peak in December-January, and minima at equinoxes. The May-September peak, persisting at different solar cycle phases, was assumed to be originated from high conductivity in the auroral ionosphere, which is controlled by the dipole tilt angle and provides better conditions for penetration of electric field perturbations into the inner magnetosphere. This allows explanation of the shape and amplitude of annual variation in the FEE occurrence rate from the convolution of the solar wind driver with the penetration conditions.