M.S. Gussenhoven

Simulation of proton radiation belt formation during the March 24, 1991 SSC

The rapid formation of a new proton radiation belt at L ≃ 2.5 following the March 24, 1991 Storm Sudden Commencement (SSC) observed at the CRRES satellite is modelled using a relativistic guiding center test particle code. The SSC is modelled by a bipolar electric field and associated compression and relaxation in the magnetic field, superimposed on a dipole magnetic field. The source population consists of both solar and trapped inner zone protons. The simulations show that while both populations contribute to drift echoes in the 20–80 MeV range, primary contribution is from the solar protons. Proton acceleration by the SSC differs from relativistic electron acceleration in that different source populations contribute and nonrelativistic conservation of the first adiabatic invariant leads to greater energization of protons for a given decrease in L. Model drift echoes and flux distribution in L at the time of injection compare well with CRRES observations.

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.

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. >

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. >

Near-Earth radiation model deficiencies as seen on CRRES.

The Space Radiation (SPACERAD) experiments on the Combined Release and Radiation Effects Satellite (CRRES) gathered 14 months of radiation particle data in an 18 degrees inclination orbit between 350 km and 36000 km from July 1990 to October 1991. When compared to the NASA radiation belt models AP8 and AE8, the data show the proton model (AP8) does not take into account a second belt formed after major solar flare/shock injection events, and the electron model (AE8) is misleading, at best, in calculating dose in near-Earth orbits. The second proton belt, although softer in energy than the main proton belt, can produce upsets in proton sensitive chips and would produce significant dose in satellites orbiting in it. The MeV electrons observed on CRRES show a significant particle population above 5 MeV (not in the AE8 model) which must be included in any meaningful dose predictions for satellites operating between L-shells of 1.7 and 3.0 RE.

Solar particle events as seen on CRRES

High energy proton detectors on the Combined Release and Radiation Effects Satellite (CRRES) were used to measure near-Earth solar protons in an 18 degrees inclination orbit between 350 km and 36000 km from July 1990 to October 1991. CRRES data from the major solar particle event on 23-25 March 1991 show conclusively that MeV solar protons can penetrate deep inside the magnetosphere (to an L-shell of 2.5 RE) when a large shock-induced Sudden Storm Commencement (SSC) occurs and significant solar particle populations are present at geosynchronous altitudes. The penetration of solar particles well inside boundaries predicted by Stormer theory occurred during every large solar event of the CRRES mission, as well as many of the smaller ones. Often the deep penetrations occurred simultaneously with the formation of new trapped radiation populations which peak at L-values between 2.3 and 4 RE (depending on particle energy) and which last from days to months.

Radiation belt formation during storm sudden commencements and loss during main phase

Abstract Simulation of the March 24, 1991 storm sudden commencement (SSC) has illuminated the rapid formation of new radiation belts on the particle drift time scale. While this event was the most dramatic in terms of radiation belt effects of the last solar maximum, comparable signatures of such events were seen in 1962 by Explorer 15 and in 1986 by DMSP. Several smaller MeV proton events with comparable particle morphology, but less radial transport and energization, were observed during the lifetime of the CRRES satellite (July 1990 – October 1991), which was well instrumented for both particle and field measurements inside geosynchronous orbit. Typically a solar proton event is accompanied by an SSC in the CRRES data set, while the converse is not always true. An SSC accompanied by solar protons produces a trapped population which remains on closed drift orbits until ring current buildup disrupts trapping, either by violation of the adiabatic trapping criterion or generation of waves whose frequency is comparable to periodic particle motion. Thus, new radiation belts formed around L = 4 by SSC injection are short-lived compared to the March 24, 1991 storm, wherein solar protons were transported radially inward to L = 2.5, with greater energization corresponding to first adiabatic invariant conservation than for the weaker events.

Characterizing solar flare high energy particles in near-Earth orbits

The high-latitude radiation environment at 840 km for the solar minimum period from December 1983 to October 1987 was measured, using a dosimeter on the DMSP/F7 satellite to characterize solar proton events and compare them to events from earlier periods near solar maximum. The solar proton spectra agree well. A method of characterizing the high-energy particles in solar proton events is proposed. It uses a power spectrum index and dose number that can be useful in specifying polar radiation environments for the design of spacecraft. Latitudinal cutoff levels for higher-energy (>35 MeV, >55 MeV, and >95 MeV) particles are also given for the solar proton event periods and compared to calculated cosmic ray cutoffs. >

Modelling formation of new radiation belts and response to ULF oscillations following March 24, 1991 SSC

The rapid formation of a new proton radiation belt at L≂2.5 following the March 24, 1991 Storm Sudden Commencement (SSC) observed at the CRRES satellite is modelled using a relativistic guiding center test particle code. The new radiation belt formed on a time scale shorter than the drift period of eg. 20 MeV protons. The SSC is modelled by a bipolar electric field and associated compression and relaxation in the magnetic field, superimposed on a background dipole magnetic field. The source population consists of solar protons that populated the outer magnetosphere during the solar proton event that preceeded the SSC and trapped inner zone protons. The simulations show that both populations contribute to drift echoes in the 20–80 MeV range measured by the Aerospace instrument and in lower energy channels of the Protel instrument on CRRES, while primary contribution to the newly trapped population is from solar protons. Proton acceleration by the SSC differs from electron acceleration in two notable ways: ...

SEE relative probability maps for space operations

Normalized flux and dose data for protons with energies >50 MeV are used to produce contour maps of relative probabilities of experiencing Single Event Effects (SEEs) in the Earths inner radiation belt. The data were taken on the APEX and CRRES satellites. To make the maps, the data are averaged in 3/spl deg/ by 3/spl deg/ bins in geographic latitude and longitude, and in 50 km steps in altitude. All geographic longitudes, and latitudes between /spl les/70/spl deg/ are covered. The altitude range extends from 350 km to 14,000 km. This geographic range includes the complete region of inner belt >50 MeV protons, except in the area of the South Atlantic Anomaly below 350 km. The maps easily locate regions of high risk for SEEs and are designed primarily for use in space mission planning and operations. The data base includes the added proton peak following the March 1991 magnetic storm, but does not include SEE probabilities in the polar cap regions associated with high energy solar particle events.