E.G. Mullen

A double-peaked inner radiation belt: cause and effect as seen on CRRES

Data from the Combined Release and Radiation Effects Satellite (CRRES) show the formation of a second peak in the inner proton radiation belt during the Sudden Storm Commencement (SSC) at 03:42 UT on 24 March 1991. The authors believe that the injection of high energy protons into an L-shell of 2.55 R/sub F/ is directly related to the solar-initiated shock accompanying the SSC. Once injected, the greater than 20 MeV protons became stably trapped and produced the second peak in the proton belt that continues for months after the event. The secondary peak protons increased single event upset rates in microelectronic test devices on CRRES by over an order of magnitude in the region of the second peak, that is, for L-values of 1.8 R/sub F/ to 2.6 R/sub E/. This second belt has far-reaching effects for radiation belt modelers and for determining radiation degradation and single event upset (SEU) levels that must operate in this region of near-Earth space. >

Improved understanding of the Earth's radiation belts from the CRRES satellite

Energetic particle data gathered on the CRRES spacecraft have been used to produce new and more accurate models of high-energy electron and proton fluxes as well as total dose models out to geosynchronous altitude. In addition to providing the information necessary to improve designs and operations of near-Earth space systems, the models also give insight into the dynamic behavior of the radiation belts not considered in previous models. Sample orbit runs are compared to the earlier NASA models to elucidate their weaknesses. Areas of improved understanding in the radiation environment, gained from CRRES, and how they impact systems are summarized.

New Low-Altitude Dose Measurements

The Defense Meteorological Satellite Program (DMSP) F7 satellite, launched in November, 1983, carries a dosimeter that measures radiation dose behind four hemispherical aluminum domes of different thicknesses and distinguishes low (electron) and high (proton) thresholds of energy deposition. The dosimeter also returns accurate, high-time-resolution dose measurements. Short-term measurements of dose from three sources, inner radiation belt protons, outer radiation belt electrons and solar flares, are presented for periods in 1984 and 1985. Empirical models of dose rate are constructed for the 840 km altitude of the DMSP orbit and compared to predictions of the NASA models. The NASA model values for proton dose in the South Atlantic Anomaly (SAA) are approximately 50% higher than the DMSP average values. The NASA outer zone electron model prediction values are too high by an average factor of 6, and are not reliable for short-term predictions. Included in the analysis are two of the largest solar proton events of 1984 and 1985 that occurred on 16 February, 1984, and on 26 April, 1984. The February event was relatively short-lived and produced a hard energy spectrum. The April event was much softer but gave a total dose behind .55 gm/cm2 of aluminum shielding in excess of 25 rad(Si) for the first three days of the event.

Quasi-static model of outer zone electrons

With the Combined Release and Radiation Effects Satellite (CRRES) measurements, an extremely variable outer zone relativistic electron population from 25 July 1990 to 12 October 1991 was observed. Previously, this population has been modeled by the static NASA solar minimum and maximum models. To address the inadequacies of using a static model to describe this highly dynamic environment, the authors develop a quasi-static model of the outer zone electrons based on the Ap geomagnetic activity index. It is shown that certain quantities used to parameterize the electron belt morphology are moderately correlated with the logarithm of the 15-day running average of Ap (Ap/sub 15/). The authors therefore separate and average, as a function of Ap/sub 15/, the 438 daily average radiation belt profiles (electron flux versus L) for each of nine energy channels (1-8 MeV). The result is a set of average flux profiles which are keyed to geomagnetic activity. This quasi-static model provides a more accurate representation of the dynamic outer zone electron environment than could be expected from any static model. >

Preliminary comparison of dose measurements on CRRES to NASA model predictions

Measurements of proton and electron dose from the space radiation dosimeter on the CRRES satellite, in a 18.1 deg, 350 km by 33000km orbit, are compared to the NASA models for solar maximum conditions. Up to the time of the large, solar-initiated particle events near the end of March 1991, the results are similar to those previously reported for solar minimum at low altitudes. That is, prior to the March event, there is excellent agreement between model and measured values for protons and poor agreement for electrons. During the event period a second proton belt was formed at higher altitudes which is not contained in the proton models, and the electrons increased over an order of magnitude for the CRRES orbit. This resulted in poorer agreement between model and measured values for protons during and after the solar proton event and better agreement for electrons during the electron enhancement period. What the data show is that, depending on orbit, both the existing proton and electron models can give large errors in dose that can compromise space system performance and lifetime.

CRRES high energy proton flux maps

The authors present proton flux maps of near-Earth space using the Proton Telescope (PROTEL) detector on CRRES (Combined Release and Radiation Effects Satellite). The proton energy range covered is 1-100 MeV. Contamination of PROTEL measurements due to >100-MeV protons is corrected using loss cone data, resulting in consistency with dosimeter measurements and a Monte Carlo computer model of PROTEL. Two states of the inner magnetosphere were found during the CRRES mission, a quite state having a single proton belt and an active state with a double proton belt. The properties of the new population in the second belt are presented. Comparisons with NASA proton codes are made. >

Identification of an unexpected space radiation hazard

A new radiation belt was formed on 24 March 1991 by the interaction of a strong shock in the solar wind with the Earths magnetosphere. The authors describe observations of the moment of creation by sensors aboard the CRRES (Combined Release and Radiation Effects Satellite). The electrons and protons injected into the magnetosphere in this event are highly penetrating and represent a threat to spacecraft orbiting in that region of near-Earth space. It is pointed out that the injection event was completely unexpected and may have been a unique occurrence in magnitude during 35 years of space research. However, the response of the early particle sensors to such an event is not obvious, and therefore a definitive conclusion is difficult to make. >

The effect of the March 1991 storm on accumulated dose for selected satellite orbits: CRRES dose models

Three dose models are constructed using direct measurements of dose on the CRRES (Combined Release and Radiation Effects Satellite) in a low-inclination, geosynchronous-transfer orbit. The average model uses data taken over the entire 14 months of the CRRES mission from July 1990 to October 1991. The quiet model uses data from July 1990 to March 1991. The active model uses data from March 1991 to October 1991. The separation of the quiet and active periods is based on the 24 March 1991 solar particle event and subsequent solar wind shock which rearranged the inner magnetosphere radiation populations. The dose models are dose rate averages in grids of L and B/B/sub 0/. A software program (CRRESRAD), developed for the models, allows the calculation of dose behind four shielding thicknesses for any satellite orbit. In the active period, dose acquired in a circular, low-inclination orbit in the slot region is greater than in the quiet period by up to two orders of magnitude, making this region, heretofore thought to be relatively benign, comparable in radiation harshness to the peak of the inner radiation belt. The suitability of the CRRES dose models for evaluating dose in high inclination orbits is also discussed. >

Radiation-induced insulator discharge pulses in the CRRES internal discharge monitor satellite experiment

The internal discharge monitor (IDM) is designed to observe electrical pulses from common electrical insulators in space service. The IDM is flying on the combined release and radiation effects satellite (CRRES). The sixteen insulator samples include G10 circuit boards, FR4 and PTFE fiberglass circuit boards, FEP Teflon, alumina, and wires with common insulations. The samples are fully enclosed, mutually isolated, and space radiation penetrates 0.02 cm of aluminum before striking the samples. The IDM results indicate the rate at which insulator pulses occur. Pulsing began on the seventh orbit. The maximum pulse rate occurred near orbit 600 when over 50 pulses occurred. The average pulse rate is approximately two per orbit, but nearly half of the first 600 orbits experienced no pulses. The pulse rate per unit flux of high energy electrons has not changed dramatically over the first ten months in space. These pulse rates are in agreement with laboratory experience on shorter time scales. Several of the samples have never pulsed. IDM pulses are the seeds of larger satellite electrical anomalies. The pulse rates are compared with space radiation intensities, L shell location, and spectral distributions from the radiation spectrometers on CRRES. >

Radiation belt dynamics during solar minimum

Two types of temporal variation in the radiation belts are studied using low-altitude data taken onboard the DMSP F7 satellite: those associated with the solar cycle and those associated with large magnetic storm effects. Over a three-year period from 1984 to 1987 and encompassing solar minimum, the protons in the heart of the inner belt increased at a rate of approximately 6% per hear. Over the same period, outer zone electron enhancements declined both in number and peak intensity. During the large magnetic storm of February 1986, following the period of peak ring current intensity, a second proton belt with energies up to 50 MeV was found at magnetic latitudes between 45 degrees and 55 degrees . The belt lasted for more than 100 days. The slot region between the inner and outer electron belts collapsed by the merging of the two populations and did not reform for 40 days. >