relativistic electron annulus every day, even during geomagnetic calms, as far as the geometry of the relativistic electron belt goes, the threat seldom goes away. Having established that during a part of nearly every day ISS spends about 20 percent of each orbit in the relativistic electron annulus, we look next at the dose rate during this time.
The flux of relativistic electrons in the outer radiation belt varies over time by many orders of magnitude. The variation is marked by events, called highly relativistic electron (HRE) events, that have a characteristic life cycle, rising quickly (on the order of 1 day) and decaying slowly (on the order of 5 to 10 days), although some events decay anomalously even more slowly. The fact that the onset of HRE events is well correlated with changes in solar wind conditions offers an opportunity to develop a protocol for HRE-event alerts and warnings that could demonstrate sufficient predictive value to be of operational use. The decay rate, which is faster on L shells closer to Earth,1 seems to be stable enough to use to predict fluxes days ahead. To illustrate this point, we look at several HRE events.
Among the several satellites that currently record fluxes of relativistic electrons in the outer radiation belt are the Solar Anomalous Magnetospheric Particle Explorer (SAMPEX) and POLAR, two NASA research satellites. SAMPEX, which is in a low-altitude, 96-minute polar orbit, measures fluxes of electrons between 500 keV and 50 MeV in the low-altitude horns of the outer radiation belt. Plate 1 shows a color spectrogram from SAMPEX data of the average flux of electrons between 2 and 6 MeV as a function of L shell and day-of-year in 1997. During 1997, SAMPEX recorded 14 outer-belt, energetic-electron events during which the daily flux of energetic electrons jumped three orders of magnitude to values exceeding 104 electrons cm–2 s–1 sr–1. The bottom panel of Plate 1 shows similar data from the POLAR satellite, whose highly elliptical orbit, ranging from 2 Re at perigee to 9 Re at apogee, covers L values from 2 to 100. The same events are seen in both data sets. Note that the integral flux of four of the events exceeded 105 electrons cm–2 s–1 sr–1. Unfortunately for operational considerations, data from SAMPEX and POLAR satellites are not available in real time. Perhaps, however, a combination of LEO polar satellites that return data once per orbit and geostationary satellites that return data continuously in real time can assess electron flux in the outer belt completely enough to serve for critical operational purposes.
Two NOAA programs provide real-time satellite data on the space environment, including data on energetic electrons. One of these is the Polar-Orbiting Operational Environmental Satellites (POES) program, which collects data from satellites in low-altitude, polar orbits that cover the outer radiation belt, providing integral fluxes of electrons with energies >300 keV. POES data are taken at 840 km altitude, not far from the nominal 400 km altitude of ISS, and so are readily convertible to fluxes relevant to EVAs at ISS. The conversion must reduce (usually by a factor of less than 2) the >300 keV fluxes that POES provides to fluxes of >500 keV electrons relevant to EVAs, and it must compensate for the different altitudes of the two orbits (fluxes at ISS are about two-thirds of those at POES). The fluxes of >500 keV electrons at ISS are thus about 30 percent of >300 keV electron fluxes that POES measures on the same L shell.
Figure 3.1 illustrates POES energetic electron measurements. The first and third panels show 12-hour running means of fluxes in the outer belt (L from 4.5 to 4.8 Re) throughout 1997. The maximum average flux was often above 104 electrons cm–2 s–1 sr–1, and on one pass it exceeded 105 electrons cm–2 s–1 sr–1. It can be seen that the storm enhancements in Figure 3.1 decay at a rate that is about the same from event to event, forming a basis for the prediction capability described in Section 3.3.
The second NOAA program that provides real-time satellite data on outer belt energetic electrons is GOES. Satellites in this program also measure fluxes of SPE ions, solar X-ray flux, and the magnetic field at L = 6.6 (geosynchronous orbit). Daily averages of >2 MeV electrons recorded on GOES are shown in the second and fourth panels of Figure 3.1. Since L = 6.6 is beyond the core of the outer radiation belt, GOES fluxes are generally less than the core values that POES record. In 1997, the maximum daily value from GOES was about 103 electrons cm–2 s–1 sr–1, compared with 105 electrons cm–2 s–1 sr–1 from POES. Nonetheless, it can be seen that GOES fluxes track POES outer belt fluxes, which means that GOES can serve as a continuous proxy monitor of the intensity of outer belt electrons.