Lorraine P. Beahm

The mean ionic charge state of solar energetic Fe ions above 200 MeV per nucleon

We have analyzed the geomagnetic transmission of solar energetic Fe ions at approximately 200-600 MeV per nucleon during the great solar energetic particle (SEP) events of 1989 September-October. By comparing fluences from the Chicago charged-particle telescope on IMP-8 in interplanetary space and from NRLs Heavy Ions in Space (HIIS) experiment aboard the Long Duration Exposure Facility (LDEF) in low-Earth orbit, we obtain a mean ionic charge (Q(sub 3)) = 14.2 +/- 1.4. This result is significantly lower than (Q) observed at approximately 1 MeV per nucleon in impulsive, He-3 rich SEP events, indicating that neither acceleration at the flare site nor flare-heated plasma significantly contributes to the high-energy Fe ions we observe. But it agrees well with the (Q) observed in gradual SEP events at approximately 1 MeV per nucleon, in which ions are accelerated by shocks driven by fast coronal mass ejections, and hence shows that particles are accelerated to very high energies in this way. We also note apparent differences between solar wind and SEP charge state distributions, which may favor a coronal (rather than solar wind) seed population or may suggest additional ionization in the ambient shock-region plasma.

The registration temperature effect for lightly ionizing particles in CR-39

Abstract Plastic track detectors have been used for years to measure the elemental composition of ultraheavy cosmic rays. Recently, it has been discovered that the response of these detectors can be drastically affected by detector temperature at the time particles are recorded. We have investigated this effect in CR-39 for relativistic Fe and Kr ions as well as for stopping He ions. The effect for these lightly ionizing particles is smaller and opposite in sign to that reported earlier for more intensely ionizing particles. The results reported here indicate that the detector temperature should be controlled to ±10°C in cosmic ray experiments designed to measure charge composition below Z=40. Thermal modeling of NRLs Heavy Ions in Space experiment indicates that this degree of temperature control is being achieved.

Geomagnetic transmission disturbances and heavy-ion fluences observed in low Earth orbit during the solar energetic particle events of October 1989

The large solar energetic particle (SEP) events and simultaneous large geomagnetic disturbances observed during October 1989 posed a significant, rapidly evolving space radiation hazard. Using data from the GOES-7, NOAA-10, IMP-8 and LDEF satellites, we determined the geomagnetic transmission, heavy ion fluences, mean Fe ionic charge state, and effective radiation hazard observed in low Earth orbit (LEO) for these SEPs. We modeled the geomagnetic transmission by tracing particles through the combination of the internal International Geomagnetic Reference Field (IGRF) and the Tsyganenko (1989) magnetospheric field models, extending the modeling to large geomagnetic disturbances. We used our results to assess the radiation hazard such very large SEP events would pose in the anticipated 52 degrees inclination space station orbit.

HIIS results on the mean ionic charge state of SEP Fe above 200 MeV per nucleon

We have analyzed the geomagnetic transmission of solar energetic Fe ions at ∼200–600 MeV/nuc during the great solar energetic particle (SEP) events of 1989 September–October. By comparing fluences from the Chicago charged‐particle telescope on IMP‐8 in interplanetary space and from NRL’s Heavy Ions in Space (HIIS) experiment aboard LDEF in low‐Earth orbit, we obtain a mean ionic charge 〈Q〉=14.2±1.4. This result is significantly lower than 〈Q〉 observed at ∼1 MeV/nuc in impulsive events, and suggests that neither acceleration at the flare site nor flare‐heated plasma significantly contributes to the high‐energy Fe ions we observe. But it agrees well with the 〈Q〉 observed in gradual SEP events at lower energies, demonstrating that acceleration by CME‐driven shocks is the primary SEP production mechanism in gradual events even at these very high energies.

Low energy ions in the heavy ions in space (HIIS) experiment on LDEF

We present data from the Lexan top stacks in the Heavy Ions In Space (HIIS) experiment which was flown for six years (April 1984-Jan 1990) onboard the LDEF spacecraft in 28.5 degrees orbit at about 476 km altitude. HIIS was built of passive (i.e. no timing resolution) plastic track detectors which collected particles continuously over the entire mission. In this paper we present data on low energy heavy ions (10 < or = Z, 20MeV/nuc < E < 200 MeV/nuc). These ions are far below the geomagnetic cutoff for fully ionized ions in the LDEF orbit even after taking into account the severe cutoff suppression caused by occasional large geomagnetic storms during the LDEF mission. Our preliminary results indicate an unusual elemental composition of trapped particles in the inner magnetosphere during the LDEF mission, including both trapped anomalous cosmic ray species (Ne, Ar) and other elements (such as Mg and Fe) which are not found in the anomalous component of cosmic rays. The origin of the non-anomalous species is not understood, but they may be associated with the solar energetic particle events and geomagnetic disturbances of 1989.

Below-cutoff ions in the heavy ions in space experiment

Abstract In January 1990, the LDEF spacecraft was retrieved after nearly 6 years in space in a 28.4° orbit at a mean altitude of 476 km. Onboard LDEF was NRLs Heavy Ions In Space (HIIS) experiment, comprising large thick stacks of plastic track detectors with a total collecting power of 2.0 m2-sr. We report preliminary results on the observation of stopping Fe-group ions at energies far below the geomagnetic cutoff for fully-ionized galactic cosmic rays.