R. E. McGuire

Two classes of solar energetic particle events associated with impulsive and long-duration soft X-ray flares

For the period 1978 September to 1983 December, 67 solar particle events have been identified for which the instruments detected electrons above 3 MeV and for which there are soft X-ray observations. The events are divided into two classes impulsive and long-duration - based on their signature in soft X-rays, and it is found that they have different properties. The events originating with impulsive flares are associated with intensed meter-wavelength type III bursts with associated type V continuum. The events associated with long-duration flares can originate anywhere on the solar disk, extend to much higher proton energies, and are well associated with coronal and interplanetary shocks; for about half of the long-duration events, the associated meter-wavelength events do not include type III bursts. The results discovered by Evenson et al. (1984) and by Kahler et al. (1984) are extended.

The composition of solar energetic particles

The present measurements of elemental abundances in 15 large solar energetic particle events confirm the existence of two major effects: systematic differences between solar energetic particle abundances and photospheric abundances that are approximately correlated with first ionization potential (implying that the solar corona is the likely source population for these particles), and an enhancement of heavy ion abundances relative to a baseline of solar energetic particle abundances whose magnitude increases with rising atomic number. These data suggest the possibility that the degree of heavy ion enhancement has some correlation with spectral slope, and confirm that the aforementioned effects are not due to time or energy variations within individual events. The maximum column density above the solar acceleration region is less than 0.1 g/sq cm, on the basis of these data.

Solar filament eruptions and energetic particle events

The 1981 December 5 solar filament eruption that is associated with an energetic (E greater than 50 MeV) particle event observed at 1 AU. The eruption was photographed in H-alpha and was observed by the Solwind whitelight coronagraph on P78-1. It occurred well away from any solar active region and was not associated with an impulsive microwave burst, indicating that magnetic complexity and a detectable impulsive phase are not required for the production of a solar energetic particle (SEP) event. No metric type II or IV emission was observed, but an associated interplanetary type II burst was detected by the low-frequency radio experiment on ISEE 3. The December 5 and two other SEP events lacking evidence for low coronal shocks had unusually steep energy spectra (gamma greater than 3.5). In terms of shock acceleration, this suggests that shocks formed relatively high in the corona may produce steeper energy spectra than those formed at lower altitudes. It is noted that the filament itself maybe one source of the ions accelerated to high energies, since it is the only plausible coronal source of the He(+) ions observed in SEP events.

The cosmic radiation in the heliosphere at successive solar minima

Comparative cosmic ray studies at the time of successive solar minima are of special importance in establishing the relative role of large-scale drift effects in the modulation process and in estimating the size of the modulation region and the local interstellar spectra of low- and medium-energy cosmic rays. In this paper the 1 AU cosmic ray observations are compared for the last three solar minimum periods along with the 1977/1978 and 1987 Pioneer 10 and Voyager 1 and 2 data from the outer heliosphere. There is good agreement between the 1965 and 1987 galactic cosmic ray H and He spectra at 1 AU. However, there are significant and complex differences between the 1977/1978 and 1987 measurements of the galactic and anomalous cosmic ray components at 1 and 15 AU. In the outer heliosphere there are large negative latitudinal gradients that reach their maximum magnitude when the inclination of the outer heliosphere current sheet is at a minimum. The radial gradients decrease with heliocentric distance as ∼1/r0.7 and do not differ significantly at the successive solar minima. While these effects, along with the shift in the intensity maximum of anomalous helium (He+), are in the direction expected from large-scale gradient and curvature drifts in the heliosphere, they are about an order of magnitude smaller than those calculated using the standard Parker model of the interplanetary magnetic field. The measured radial and latitudinal gradients are used to estimate the particle transport parameters in the outer heliosphere. The slope of the H spectra measured at Pioneer 10 for both solar minimum periods is given by γH ≈ 0.5 down to energies of 40 MeV. Using the local interstellar He spectrum of Webber et al. (1987), it is estimated that the modulation boundary is of the order of 160 AU.

Evidence for Remnant Flare Suprathermals in the Source Population of Solar Energetic Particles in the 2000 Bastille Day Event

The energy spectra of Fe in the very large solar energetic particle (SEP) event of 2000 July 14 are strikingly different from those of lighter species. We show that this difference can be explained by shock acceleration from a two-component source population, comprising solar wind suprathermals and a small (∼5%) admixture of remnant flare particles, as previously proposed to explain enhanced ^3He/^4He in some gradual SEP events. Flare remnants can also account for several previously unexplained features of high-energy solar heavy ions as well as important aspects of SEP event-to-event variability. These results offer a new perspective on the enduring controversy over the relative roles of flares and coronal mass ejections (CMEs) in producing SEPs. Flare activity clearly makes a unique and critical contribution to the source population. But the predominate accelerator in large gradual SEP events is the CME-driven shock, and many spectral, compositional, and charge state characteristics of highenergy heavy ions can be understood without invoking other acceleration mechanisms.

Solar flare nuclear gamma-rays and interplanetary proton events

Flare gamma-ray line (GRL) events and solar energetic proton (SEP) events are compared for the period from February 1980 - January 1985 and substantiated earlier results show a lack of correlation between gamma-ray-producing ions and interplanetary protons. This poor correlation results primarily from several large SEP events that originated in flares without detectable gamma-ray-emission. The converse case of GRL events unassociated with SEP events is rare. Evidence is presented which suggests that the ratio of trapped to escaping protons in GRL/SEP flares depends on the spatial scale size and the flare.

Role of drifts and global merged interaction regions in the long-term modulation of cosmic rays

It is shown that the long-term modulation of 0.2- to 3-GV galactic and anomalous cosmic rays over the 22-year heliomagnetic cycle is principally a combination of two solar related processes, the cumulative effect of long-lived global merged interaction regions (GMIRs) and large-scale particle gradient and curvature drifts in the interplanetary magnetic field. This paradigm for cosmic ray modulation is based on the observed changes in cosmic ray intensity from solar minimum to solar maximum over successive solar cycles (21 and 22) using data from 1 AU and from the outer heliosphere. For cycle 21 (when positive ions flow in over the solar pole and out along the heliospheric neutral current sheet; qA>0), the 1977–1980 modulation is dominated by GMIRs. While drifts may play a role in particle transport in the heliosphere at this time, the changing inclination of the heliospheric neutral sheet has a negligible effect on the intensity of cosmic ray nuclei. In cycle 22 when this flow pattern is reversed, it is shown that drifts are an important but not dominant factor for galactic cosmic ray modulation with the current sheet related drift effects decreasing with increasing rigidity R and heliocentric distance r. Anomalous cosmic rays are much more sensitive to changes in the current sheet inclination. The observed changes in the galactic cosmic ray intensity over the 1987–1988 period due to the increasing inclination of the current sheet, α, are a factor of 3–5 smaller than predicted by the time-dependent model of Potgieter and Le Roux (1992). The latitude variation at Voyager 1 (heliolatitude 32°) of the dependence of galactic cosmic ray intensity on α is consistent with theoretical expectations. The strong role of GMIRs and their episodic nature requires a long lifetime (1.5–1.8 years) and a magnetic structure that effectively extends over the solar poles. Such a long GMIR lifetime implies a modulation boundary of the order of ∼175 AU.

Energetic Charged Particles in the Uranian Magnetosphere

During the encounter with Uranus, the cosmic ray system on Voyager 2 measured significant fluxes of energetic electrons and protons in the regions of the planets magnetosphere where these particles could be stably trapped. The radial distribution of electrons with energies of megaelectron volts is strongly modulated by the sweeping effects ofthe three major inner satellites Miranda, Ariel, and Umbriel. The phase space density gradient of these electrons indicates that they are diffusing radially inward from a source in the outer magnetosphere or magnetotail. Differences in the energy spectra of protons having energies of approximately 1 to 8 megaelectron volts from two different directions indicate a strong dependence on pitch angle. From the locations of the absorption signatures observed in the electron flux, a centered dipole model for the magnetic field of Uranus with a tilt of 60.1 degrees has been derived, and a rotation period of the planet of 17.4 hours has also been calculated. This model provides independent confirmaton of more precise determinations made by other Voyager experiments.

A comparative study of cosmic ray radial and latitudinal gradients in the inner and outer heliosphere

The radial and latitudinal intensity gradients of 145–255 MeV/nucleon He, 34–50 MeV/nucleon He and 30–69 MeV H are studied over an extensive range of heliocentric distances and latitudes for the 1993.0–1996.0 time period using data from cosmic ray experiments on the Ulysses, IMP 8, Voyager 1 and 2, and Pioneer 10 spacecraft. The radial gradients are found to decrease rapidly with increasing heliocentric distance and agree with those measured 20 years earlier at a similar phase of the heliomagnetic cycle. The latitudinal gradients measured in the inner and outer heliosphere are in reasonable agreement and positive albeit exceedingly small. In agreement with other Ulysses energetic particle experiments it is found that a shift of heliolatitude by −7° to −10° is necessary to get reasonable symmetry in the measurements at midlatitudes. From the Ulysses data it appears there is a significantly reduced latitudinal variation in the intensity of the three energetic particle components at (magnetic) heliolatitudes above about 50° at this phase of the modulation cycle. Such a reduced entry of cosmic rays over such an extensive area above the solar poles implies a strong modification of the previously assumed cosmic ray transport processes at high latitudes, most probably a considerably increased rate of scattering combined with reduced particle gradient and curvature drifts. A significant higher intensity is observed over the north solar pole than over the south pole for the low-energy components after the corrections have been applied for the temporal changes at the 1-AU baseline.

The composition of corotating energetic particle streams

The relative abundances of 1.5--23 MeV per nucleon ions in corotating nucleon streams are compared with ion abundances in particle events associated with solar flares and with solar and solar wind abundances. He/O and C/O ratios are found to be a factor of the order 2--3 greater in corotating streams than in flare-associated events. The distribution of H/He ratios in corotating streams is found to be much narrower and of lower average value than in flare-associated events. H/He in corotating energetic particle streams compares favorably in both lack of variability and numerical value with H/He in high-speed solar wind plasma streams. The lack of variability suggests that the source population for the corotating energetic particles is the solar wind, a suggestion consistent with acceleration of the corotating particles in interplanetary space.