Paul Richard Boberg

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.

Geomagnetic transmission of solar energetic protons during the geomagnetic disturbances of October 1989

We present orbit-averaged geomagnetic transmission measurements during the large solar energetic particle events of October 1989 using proton data from the NOAA-10 and GOES-7 satellites. We compare the measurements to geomagnetic transmission calculations determined by tracing particle trajectories through the combination of the International Geomagnetic Reference Field (IGRF) model and the 1989 Tsyganenko magnetospheric magnetic field model. We modify the effective ‘ring current’ parameter in the 1989 Tsyganenko model based on the Dst data. We compare our results to calculations employing only the IGRF and to a parameterization of geomagnetically quiet-time cutoff rigidities derived from Cosmos/Intercosmos observations. Our 3-hour orbit-averaged results have ∼15% accuracy during the October 1989 events.

Flare- and Shock-accelerated Energetic Particles in the Solar Events of 2001 April 14 and 15

We report heavy-ion composition and spectra for the solar energetic particle (SEP) events of 2001 April 14 and 15, using the combined capabilities of the Advanced Composition Explorer (ACE), Wind, and the Interplanetary Monitoring Platform 8 (IMP-8) to cover the energy range from ∼30 keV nucleon^(-1) to ∼400 MeV nucleon^(-1). These two events are, respectively, the largest impulsive event and the largest ground-level event observed so far in solar cycle 23. These events arose from the same active region and launched into similar interplanetary conditions. Both were associated with large western flares and fast coronal mass ejections (CMEs). However, the two events are distinctly different, thereby providing useful reminders of the fundamental differences between flare- and shock-accelerated SEPs. The detailed observations present challenges for our theoretical understanding of SEP production. Of particular note is the fact that iron has a harder power-law energy spectrum than oxygen above ∼3 MeV nucleon^(-1) in the shock-dominated April 15 event. This spectral difference, which is seen in many other gradual events of various sizes and heliolongitudes, leads to enhanced Fe/O at high energies. Simple shock acceleration models predict the same power-law index for all species. Thus, understanding the origin of this spectral difference will significantly contribute to the resolution of the ongoing debate about the relative roles of CME-driven shocks and flares in producing high-energy solar heavy ions.

Temporal evolution in the spectra of gradual solar energetic particle events

We examine solar energetic particle (SEP) spectra in two very large “gradual” events (20 April 1998 and 25 August 1998), in which acceleration is caused by fast CME-driven shocks. By combining data from ACE/SIS, Wind/EPACT/LEMT, and IMP8/GME, we examine all major species from H to Fe, from ∼2 MeV/nuc to the highest energies measured. These events last for several days, so we have divided the events into 8-hour intervals in order to study the evolution of the spectra. The spectra reveal significant departures from simple power laws. Of particular note is the behavior at high energies, where the spectra exhibit exponential rollovers. We demonstrate that the fitted e-folding energies reflect both ionic charge states and a complex but orderly temporal evolution. We speculate that this behavior may be related to evolving rigidity dependence in the near-shock diffusion coefficient, which is of potentially great importance for models of SEP acceleration and transport.

LET spectra of trapped anomalous cosmic rays in low-Earth orbit.

Observations aboard Cosmos satellites discovered trapped anomalous cosmic rays (ACRs), tracked the variation in their intensity in 1986-1988, and measured their fluence, spectrum, and composition at solar minimum in the previous solar cycle. The MAST instrument aboard the SAMPEX satellite has observed trapped anomalous cosmic rays in the present solar cycle, confirmed the general features of the Cosmos data, and provided the first detailed observations of trapped ACRs. In this paper we apply theoretical modeling of trapped ACRs, which is shown to provide a reasonably good description of both the Cosmos and SAMPEX data, to calculate the integral linear-energy-transfer (LET) spectra due to trapped ACRs in typical low-Earth orbits. We compare these calculations with the LET spectra produced by galactic cosmic rays (GCRs) and non-trapped ACRs in order to assess the relative radiation hazard posed by trapped ACRs.

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.

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.

Variation in solar energetic particle elemental composition observed by ACE and Wind

We have used ACE and Wind data to study C/O and Fe/O ratios at ∼2.6–15 MeV/nuc in nine large solar energetic particle (SEP) events between November 1997 and November 1998. Six events have event-integrated Fe/O ratios that are larger than the nominal coronal value by a factor of two or more. However, the energetic storm particle event of 25 August 1998 single-handedly restores the fluence-weighted Fe/O average to nearly the coronal value. We suggest that this balancing indicates a common acceleration mechanism in all of these events. We also compare SEP results to a recent ∼300-day survey of slow solar wind by Ulysses. The average solar-wind C/O ratio is significantly larger (by ∼40%) than in SEPs and in the photosphere. The origin of this difference in C/O is not understood. Finally, we also use 1-hour and 2-hour averaged data to examine intra-event correlations of SEP Fe/O vs. C/O.