J. E. Mazur

3He Enhancements in Large Solar Energetic Particle Events

We have measured the 3He abundance from approximately 0.5 to 2 MeV nucleon-1 in 12 large solar energetic particle (SEP) events during the period 1997 November-1999 June. In five of the events, the 3He time-intensity profile is similar to the 4He time-intensity profile, indicating a common acceleration and transport origin for the two species. The average 3He/4He ratio during these events is &parl0;1.9+/-0.2&parr0;x10-3, a factor of approximately 5 enhancement over the solar wind value. During this same survey period, we have also measured the low-energy ion intensities during quieter periods in between the large-particle events. We find 3He and Fe remnants from impulsive events present on a majority of the days, implying that they fill a substantial volume (>50%) of the in-ecliptic interplanetary medium during our survey. We suggest that these suprathermal ions may therefore be a source population that is available for further acceleration by interplanetary shocks that accompany large SEP events, thereby leading to the 3He enhancements in a significant fraction of large SEP events. This impulsive SEP event material might also account for recent observations of large solar particle events with energetic particle ionization states that have a wide range of ionization states that encompass values expected for both gradual and impulsive solar SEP events.

Proton, helium, and electron spectra during the large solar particle events of October-November 2003

The extraordinary period from late October through early November 2003 was marked by more than 40 coronal mass ejections (CME), eight X-class flares, and five large solar energetic particle (SEP) events. Using data from instruments on the ACE, SAMPEX, and GOES-11 spacecraft, the fluences of H, He, O, and electrons have been measured in these five events over the energy interval from ∼0.1 to >100 MeV/nucleon for the ions and ∼0.04 to 8 MeV for electrons. The H, He, and O spectra are found to resemble double power laws, with a break in the spectral index between ∼5 and ∼50 MeV/nucleon which appears to depend on the charge-to-mass ratio of the species. Possible interpretations of the relative location of the H and He breaks are discussed. The electron spectra can also be characterized by double power laws, but incomplete energy coverage prevents an exact determination of where and how the spectra steepen. The proton and electron fluences in the 28 October 2003 SEP event are comparable to the largest observed during the previous solar maximum, and within a factor of 2 or 3 of the largest SEP events observed during the last 50 years. The 2-week period covered by these observations accounted for ∼20% of the high-energy solar-particle fluence over the years from 1997 to 2003. By integrating over the energy spectra, the total energy content of energetic protons, He, and electrons in the interplanetary medium can be estimated. After correcting for the location of the events, it is found that the kinetic energy in energetic particles amounts to a significant fraction of the estimated CME kinetic energy, implying that shock acceleration must be relatively efficient in these events.

Heavy-Ion Elemental Abundances in Large Solar Energetic Particle Events and Their Implications for the Seed Population

We have surveyed the ~0.1–10 MeV nucleon to the -1 abundances of heavy ions from 3He through Fe in 64 large solar energetic particle (LSEP) events observed on board the Advanced Composition Explorer from 1997 November through 2005 January. Our main results are (1) the 0.5–2.0 MeV nucleon to the -1 3He/ 4He ratio is enhanced between factors of ~2–150 over the solar wind value in 29 (~46%) events. (2) The Fe/O ratio in most LSEP events decreases with increasing energy up to ~60 MeV nucleon to the -1. (3) The Fe/O ratio is independent of CME speed, flare longitude, event size, the 3He/4He ratio, the pre-event Fe/O ratio, and solar activity. (4) The LSEP abundances exhibit unsystematic behavior as a function of M/Q ratio when compared with average solar wind values. (5) The survey-averaged abundances are enhanced with increasing M/Q ratio when compared with quiet coronal values and with average gradual SEP abundances obtained at 5–12 MeV nucleon to the -1. (6) The event-to-event variations in LSEP events are remarkably similar to those seen in CME-driven IP shocks and in 3He-rich SEP events. The above results cannot be explained by simply invoking the current paradigm for large gradual SEP events, i.e., that CME-driven shocks accelerate a seed population dominated by ambient coronal or solar wind ions. Instead, we suggest that the systematic M/Q-dependent enhancements in LSEP events are an inherent property of a highly variable suprathermal seed population, most of which is accelerated by mechanisms that produce heavy-ion abundances similar to those observed in impulsive SEP events. This heavy-ion-enriched material is subsequently accelerated at CME-driven shocks near the Sun by processes in which ions with higher M/Q ratios are accelerated less efficiently, thus causing the Fe/O ratios to decrease with increasing energy.

Abundances of Heavy and Ultraheavy Ions in 3He-rich Solar Flares

We have surveyed 3He-rich solar energetic particle (SEP) events over the period 1997 September-2003 April in order to characterize abundances of heavy ions near 400 keV nucleon-1. The first part of the study focuses on 20 distinct SEP events that show the previously observed pattern in which, relative to O, heavy ions through Fe are enriched, with the enrichment increasing with mass. We find that these enrichments are well correlated such that 3He-rich SEP events with high Fe/C also show larger enrichments in other heavy ions. Ultraheavy (UH; taken as 78-220 amu) ions are routinely seen in these events with abundance enhancements correlating with Fe/C but with even larger flare-to-flare variations. In one event with unusually little interplanetary scattering, we are able to estimate the time of heavy- and UH-ion injections at the Sun and find them to be simultaneous. The second part of the study sums up many impulsive-event time periods in order to construct a mass histogram of UH nuclei; this histogram shows broad mass peaks similar to those in compilations of solar system abundances. In this summed period, relative to O, the average enhancement of heavy nuclei increases with mass with values of ~7 for Fe, ~40 for mass 78-100 amu, ~120 for mass 125-150 amu, and ~215 for 180-220 amu. The maximum UH enhancements seen in the most-enriched events are at least a factor of 5 larger. The enhancements are approximately proportional to the particle charge-to-mass ratio raised to a power, as seen previously in large, shock-associated SEP events.

The energy spectra of solar flare hydrogen, helium, oxygen, and iron - Evidence for stochastic acceleration

The time-integrated differential energy spectra of H, He, O, and Fe measured in 10 large flare events observed at 1 AU over the energy range of 0.3-80 MeV/nucleon showed consistent patterns in their spectral shapes: particles with larger mean mass-to-charge ratios were generally less abundant at higher energies. A steady state model of stochastic particle acceleration with rigidity-dependent diffusion coefficients fit the spectra best; spectra representative of diffusive shock acceleration also described the spectra of some events with the same number of free parameters, but often fell off faster in energy above 30 MeV per nucleon than the observations. The two model predictions differed most at energies near 0.1 MeV per nucleon, below the lowest energies observed in this study. The stochastic model quantitatively described the observed spectral ordering with less efficient acceleration of species with larger mean mass-to-charge ratios.

Abundances and Energy Spectra of Corotating Interaction Region Heavy Ions Observed during Solar Cycle 23

Using instruments on the ACE spacecraft, we surveyed the heavy-ion spectra and composition over the range He-Fe for 41 corotating interaction regions (CIRs) during 1998-2007. Below ~1 MeV nucleon^(−1) the spectra are power laws in kinetic energy nucleon^(−1) with an average spectral index of 2.51 ± 0.10, rolling over above ~1 MeV nucleon^(−1) to power-law spectra with an average index of 4.47 ± 0.17. The spectral shapes for different species are similar, leading to relative abundances that are constant over our energy range, even though the intensities cover up to 8 orders of magnitude. Relative to oxygen, the measured abundances at 385 keV nucleon^(−1) for ^4He, C, N, Ne, Mg, Si, S, Ca, and Fe are 273 ± 72, 0.760 ± 0.023, 0.143 ± 0.005, 0.206 ± 0.009, 0.148 ± 0.006, 0.095 ± 0.005, 0.028 ± 0.002, 0.007 ± 0.001, and 0.088 ± 0.007, respectively. Except for an overabundance of ^4He and Ne, the abundances are quite close to that of the fast solar wind. We have found ^3He/^4He ratios to be enhanced over solar wind values in ~40% of the CIRs. The Fe/O ratio in individual CIRs is observed to vary over a factor of ~10 and is strongly correlated with the solar wind Fe/O ratio measured 2-4 days preceding each CIR. Taken together with previous studies showing the presence of pickup He^+ in CIRs, the observational data provide evidence that CIR energetic particles are accelerated out of a suprathermal ion pool that includes heated solar wind ions, pickup ions, and remnant suprathermals from impulsive solar energetic particle events.

Characteristics of energetic (≳30 keV/nucleon) ions observed by the Wind/STEP instrument upstream of the Earth's bow shock

We investigate here the characteristics of energetic ions (0.03-2.0 MeV/nucleon) during 1225 upstream events observed by the Energetic Particles: Anisotropy, Composition, and Transport/Suprathermal Energetic Particle (EPACT/STEP) instrument on board the Wind spacecraft from 1994 day 325 to 1999 day 92. We find that (1) the event occurrence rate showed significant variations with changes in the solar cycle, (2) the occurrence rate increased when both the solar wind speed and the geomagnetic activity index were enhanced, (3) most events were observed within ±80 R E in Y GSE and inside ∼100 R E in X GSE , although the events occurred at all locations of the Wind orbit, (4) ∼73% of the events were observed when the interplanetary magnetic field was radial and in the ecliptic plane, and when the spacecraft was most likely magnetically connected to the bow shock, (5) the events lasted typically between 10 min to 3 hours and exhibited strong sunward field-aligned flow, (6) ∼25% of the ion events were accompanied by 20-48 keV electrons as measured by the Wind/3DP instrument, (7) the energy spectra 100-300 keV protons and 30-300 keV/nucleon He-Fe during ∼70% of the events obeyed power laws with γ between 3 and 5, while the energy spectra of He and CNO for ∼30% of the events softened above ∼80 keV/nucleon obeyed power laws with γ ∼3-5, (8) the total energy ion spectrum above ∼0.5 MeV energy was dominated by heavier ions during the events, (9) a substantial fraction (≥40%) of the spectra for all species extended above ∼150 keV/e, and (10) the heavy ion composition of the events was similar to typical solar wind values. We compare the above findings with the main predictions of the magnetospheric leakage and the Fermi acceleration models and find that neither model can satisfactorily account for our results. We highlight the new challenges and requirements for both models.

Charge State Measurements of Solar Energetic Particles Observed with SAMPEX

We have measured the ionization states of major elements accelerated in two large solar energetic particle events by use of the geomagnetic cutoff technique. The observations were made in 1992 October/November using instrumentation on the SAMPEX satellite, which is in an 82° inclination low Earth orbit. We calibrate the geomagnetic cutoff rigidity by assuming that protons and He observed on SAMPEX are fully stripped. The mean charge states of the heavier elements are then determined from their measured cutoffs, along with this calibrated scale. Our results cover the energy range ~0.5-5 MeV nucleon^(-1) and are in good agreement with earlier studies by Luhn et al. (1985) near -1 MeV nucleon^(-1) in all cases except for Fe. We find a mean charge state of 11.04±0.22 for Fe, while the average from Luhn is 14.09±0.09. The lower ionization state of Fe reported here appears to be consistent with the general range of values recently measured in solar wind Fe on the Ulysses deep space probe. Although the observational picture is still incomplete, overall the present results confirm and extend previous evidence that the solar energetic particles in large events are accelerated from the coronal or solar wind material.

COMPOSITION AND SPECTRAL PROPERTIES OF THE 1 AU QUIET-TIME SUPRATHERMAL ION POPULATION DURING SOLAR CYCLE 23

We have surveyed the spectral and compositional properties of suprathermal heavy ions during quiet times from 1995 January 1 to 2007 December 31 using Wind/Energetic Particles: Anisotropy, Composition, and Transport/SupraThermal-through-Energetic Particle Telescope and Advanced Composition Explorer/Ultra-low Energy Isotope Spectrometer at energies between 0.04 and 2.56 MeV nucleon–1. We find the following. (1) Quiet-time Fe/O and C/O abundances are correlated with solar cycle activity, reflecting corresponding values measured in solar energetic particle and interplanetary (IP) shock events during solar maximum, and those measured in the solar wind and corotating interaction regions (CIRs) during solar minimum conditions. (2) The 3He/4He ratio lies in the 3%-8% range during the quiet times of 1998-2004 with finite 3He detected on ~27.4% of the days. This ratio drops to 0.3%-1.2% during 2005-2007 and finite 3He is detected on ~5% of the days. (3) All heavy-ion species exhibit suprathermal tails between 0.04 and 0.32 MeV nucleon–1 with spectral indices ranging from ~1.27 to 2.29. These tails sometimes extend above ~2 MeV nucleon–1 with Fe spectra rolling over at lower energies than those of CNO. (4) The suprathermal tail spectral indices of heavier species (i.e., Fe) are harder than those of the lighter ones (i.e., CNO). These indices do not exhibit a clear solar cycle dependence and for ~50% of the time, they deviate significantly from the 1.5 value. These compositional observations provide evidence that even during the quietest times in IP space, the suprathermal population (3He and C-through-Fe) consists of ions from different sources whose relative contributions vary with solar activity. The heavy-ion energy spectra exhibit suprathermal tails with variable spectral indices that do not exhibit the spectral index of 1.5 predicted by some recent models.

Charge state of anomalous cosmic-ray nitrogen, oxygen, and neon: SAMPEX observations

We report observations of the ionization state of anomalous cosmic-ray (ACR) nitrogen, oxygen, and neon during the period 1992 October to 1993 May, carried out with instrumentation on the Solar, Anomalous & Magnetospheric Particle Explorer (SAMPEX) spacecraft. The low-altitude (510 x 675 km) and high-inclination (82 deg) orbit enables SAMPEX to sample the interplanetary ACR fluxes on each polar pass and then to observe the cutoff of these fluxes by the geomagnetic field at lower latitudes. The arrival time and direction of each ion is recorded by the instruments, allowing detailed calculations of the particles trajectory through the Earths magnetic field and thereby placing upper limits on the ionization state of the particles. We find (a) that ACR nitrogen, oxygen, and neon each contain singly ionized particles and (b) that ACR oxygen is predominantly singly ionized with an upper limit of 10% for higher ionization states. These ionization states confirm theories of ACR origin as neutral interstellar material that is singly ionized near the Sun by UV or charge exchange with the solar wind, and is subsequently accelerated in the outer heliosphere.