Long-Term Evolution of the Occurrence Rate of Magnetospheric Electron Precipitation into the Earth’s Atmosphere

Ionizing radiation fluxes in the Earth’s atmosphere have been measured at the Lebedev Physical Institute since 1957. They allow the recording of X-ray radiation generated in the atmosphere by precipitating magnetospheric electrons with energies above 100 keV. The precipitation events are associated mainly with high-velocity solar wind fluxes and are often observed during the decaying phase of the 11-year solar cycle. They exhibit 27-day and seasonal repeatability and correlate with geomagnetic disturbances and fluxes of relativistic electrons in the outer radiation belt. More than 500 precipitation events have been recorded in Murmansk oblast since 1961 (McIlwain parameter L ≈ 5.5). A long-term growing trend that does not correlate with the parameters of solar and geomagnetic activity has been observed in the occurrence rate of precipitation. Hypothetically, this trend could be due to the effect of ground-based VLF transmitters on the wave activity of the magnetosphere.