Flux Enhancements of > 30 keV Electrons at Low Drift Shells L < 1.2 During Last Solar Cycles

Abstract

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 < 1.2, so‐called forbidden zone. We collected 1,750 days (~25% of the total time) when fluxes of the forbidden energetic electrons (FEE) exceeded 103 (cm2 s sr)−1. We found 530 days, when FEE fluxes reached high intensity from 104 up to 107 (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 > 450 km/s), high substorm activity (AL >150 nT), and enhanced interplanetary electric field perturbations (VδB > 1.5 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 the equinoxes. The May–September peak, persisting at different solar cycle phases, was assumed to originate 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.

Energetic electron enhancements under radiation belt (L

“…Note that enhancements in 93 the forbidden zone were first reported in 1960s (Krasovskii et al, 1961;Savenko et al, 1962;94 Heikilla, 1971), however, the conclusions were unconvincing due to the scarce information (see 95 Paulikas, 1975 for a review). The recent statistical study of electron enhancements in the 96 forbidden zone showed that the injections below the inner zone can also occur during 97 geomagnetically quiet conditions (Suvorova, 2017). This fact is consistent with the recent 98 finding of "quiet" injections in the inner magnetosphere Zhao et al, 2017a).…”

“…geomagnetic storms was found, the FEE enhancements appeared to be necessarily associated 140 with substorm activity in some events studied (Suvorova et al, 2014;. However, 141 statistically, such a casual relationship with substorms was not confirmed (Suvorova, 2017). 142…”

“…It is important to mention that one interesting feature was unexpectedly found from the statistical 147 study (Suvorova, 2017). It is that the most favorable conditions for the FEE enhancements arise 148 in the period from May to September independently on geomagnetic activity level.…”

“…From total statistics of ~530 days with FEE enhancements collected during two solar cycles 143 (Suvorova, 2017), we found more than three dozen days without essential substorm activity. 144…”

A Statistical Study on the Climatology of the Equatorial Plasma Depletions Occurrence at Topside Ionosphere During Geomagnetic Disturbed Periods

“…The same pattern of Ni enhancements is also noticeable at midnight and predawn, especially during equinoxes. Coincidently, energetic electrons, the flux of which could be significantly intensified during geomagnetic storms, are also preferred to be detected at longitudes from Pacific to South America (Figure in Suvorova et al, ), at a local time near midnight (Suvorova, ). These coincidences bring insights into the nighttime ionization resulted in the ionospheric positive storm, which further benefits the occurrence of EPDs especially at midnight and after.…”