Relativistic electrons in the Earth's outer radiation belt are a significant space weather hazard. Satellites in GPS-type orbits pass through the heart of the outer radiation belt where they may be exposed to large fluxes of relativistic electrons. In this study we conduct an extreme value analysis of the daily average relativistic electron flux in GPS orbit as a function of energy and L using data from the US NS41 satellite from 10 December 2000 to 25 July 2020. The 1 in 10 year flux at L=4.5, in the heart of the outer radiation belt, decreases with increasing energy ranging from 8.2x10^6 cm^-2s^-1sr^-1MeV^-1 at E = 0.6 MeV to 33 cm^-2s^-1sr^-1MeV^-1 at E = 8.0 MeV. The 1 in 100 year is a factor of 1.1 to 1.7 larger than the corresponding 1 in 10 year event. The 1 in 10 year flux at L=6.5, on field lines which map to the vicinity of geostationary orbit, decrease with increasing energy ranging from 6.2x10^5 cm^-2s^-1sr^-1MeV^-1 at E = 0.6 MeV to 0.48 cm^-2s^-1sr^-1MeV^-1 at E = 8.0 MeV. Here, the 1 in 100 year event is a factor of 1.1 to 13 times larger than the corresponding 1 in 10 year event, with the value of the factor increasing with increasing energy. Our analysis suggests that the fluxes of relativistic electrons with energies in the range 0.6 <= E <= 2.0 MeV in the region 4.25 <= L <= 4.75 have an upper bound. In contrast, further out and at higher energies the fluxes of relativistic electrons are largely unbounded. The research leading to these results has received funding from the Natural Environment Research Council (NERC) grants NE/V00249X/1 (Sat-Risk) and NE/R016038/1.

Relativistic electrons cause internal charging on satellites and are a significant space weather hazard. In this study we analyse approximately 20 years of data from the US Global Positioning System (GPS) satellite NS41 to determine the conditions associated with the largest daily averaged fluxes of E = 2.0 MeV relativistic electrons. The largest flux events at L = 4.5 and L = 6.5 were associated with moderate to strong coronal mass ejection (CME)-driven geomagnetic storms. However, the majority of the fifty largest flux events at L = 4.5 (30 out of 50) and L = 6.5 (37 out of 50) were associated with high speed solar wind streams from coronal holes. Both solar drivers are thus very important for relativistic electron flux enhancements in GPS orbit. The 1 in 3 year flux level was not exceeded following any of the fifteen largest geomagnetic storms as monitored by the Dst index (Disturbance storm time index), showing that the largest geomagnetic storms, most often associated with extreme space weather, do not result in significantly larger relativistic electron flux events in GPS orbit. The datasets include a summary plot of the month associated with the largest flux of 2.0 MeV electrons in GPS orbit during the study period (Figure 1) and a summary plot of the month associated with the largest geomagnetic storm during the study period (Figure 2). The fifty largest 2.0 MeV flux events at L = 4.5 as a function of the minimum Dst of the associated storm are provided in Figure 3.csv, the peak 2.0 MeV electron fluxes associated with the fifteen largest geomagnetic storms at L = 4.5 as a function of the minimum Dst of each of the storms are provided in Figure 4.csv, and the fifty largest 2.0 MeV flux events at L = 4.5 and the sunspot number are provided as a function of time in Figure 5_events.csv and Figure_5_sunspots.csv respectively. The characteristic widths of the fifty largest flux enhancements at L = 4.5 and L = 6.5 are provided in Figure 6.csv. Finally, the fifty largest 2.0 MeV flux events at L = 6.5 as a function of the minimum Dst of the associated storm are provided in Figure 7.csv, the peak 2.0 MeV electron fluxes associated with the fifteen largest geomagnetic storms at L = 6.5 as a function of the minimum Dst of each of the storms are provided in Figure 8.csv, and the fifty largest 2.0 MeV flux events at L = 6.5 and the sunspot number as a function of time are provided in in Figure 9_events.csv and Figure_9_sunspots.csv respectively. The research leading to these results has received funding from the Natural Environment Research Council (NERC) grants NE/V00249X/1 (Sat-Risk), NE/X000389/1 and NE/R016038/1.