Observations of Anomalous Cosmic Rays at 1 AU

Anomalous cosmic rays (ACRs) provide a sensitive probe of the access of energetic particles to the inner heliosphere, varying in intensity by more than two orders of magnitude during the course of the solar cycle. New data which are becoming available from the Advanced Composition Explorer (ACE) can provide a detailed record of ACR intensity and spectral changes on short (~ 1 day) time scales during the approach to solar maximum, which will help address issues of ACR modulation and transport. The elemental and isotopic composition of ACRs provides important information on the source or sources of these particles, while their ionic charge state composition and its energy dependence serves as a diagnostic of their acceleration time scale. We review measurements of the ACR elemental, isotopic, and charge state composition and spectra as determined at 1 AU by SAMPEX, ACE, Wind, and other spacecraft. These results are important input to models of the acceleration, modulation, and transport of ACRs

The Solar Energetic Particle Event of 6 May 1998

The abundances of elements from helium to iron have been measured in more than a dozen moderate to large solar energetic particle (SEP) events using the Solar Isotope Spectrometer (SIS) on-board the Advanced Composition Explorer (ACE). Time variations within some of these events and from event to event have been reported previously. This paper presents an analysis of the event of 6 May 1998, for which relatively time-independent abundance ratios are found. This event has been considered to be an example of an impulsive event, a gradual event, and as a hybrid of the two. Difficulties with classifying this event are discussed

Variable fractionation of solar energetic particles according to first ionization potential

The average composition of solar energetic particles (SEPs), like the solar corona, is known to be depleted in elements with first ionization potential (FIP) more than ~10 eV by a factor of approximately four. We examine evidence for event to event variations in the FIP-related fractionation of SEPs, following up a 1994 study by Garrard and Stone. In a survey of 46 SEP events from 1974 to 1999 the deduced FIP-fractionation varies by a factor of ~2 from event to event, with no apparent relation to charge-to-mass dependent fractionation patterns in these same events. These results are compared to similar variations observed in the solar wind

Measurements of Heavy Elements in ^3He-rich SEP Events

Using the Solar Isotope Spectrometer (SIS) on the Advanced Composition Explorer (ACE), we have studied the properties of a selection of small ^3He-rich solar energetic particle (SEP) events with heavy ion enhancements in the energy range ~11–22 MeV/nucleon. These events contain significantly increased ^3He/^4He ratios over the solar wind value of 0.0004 in the energy range ~4.5–5.5 MeV/nucleon. In order to characterize the events, the following features have been investigated. First, the heavy element content has been measured and compared to that found in past studies of impulsive SEP events. Next, the simultaneous 38–53 keV electron flux, measured with the Electron, Proton, and Alpha Monitor (EPAM) on ACE, has been examined for possible activity near the ^3He-rich event onset times. Finally a list of measured solar X-ray flares, with corresponding H-alpha flares where possible, has been scrutinized for potential correlations with these events. The results show an apparent correlation between event onset and increased electron flux, and a possible association with X-ray flares

Solar Coronal Abundances of Rare Elements Based on Solar Energetic Particles

Although solar energetic particle (SEP) abundances vary from event to event, it has been shown that by accounting for these variations it is possible to use SEP data to obtain reliable estimates of elemental abundances for the solar corona. We analyze ~20 to 65 MeV/nucleon measurements from the Solar Isotope Spectrometer on ACE in large SEP events observed from November 1997 to January 2001 to obtain new values of the average SEP composition of rare species, P, Cl, K, Ti, Mn, Cr, Co, Cu, and Zn, which have had limited statistical accuracy in SEPs in the past. The measured SEP abundances are compared with other sources of solar-system composition data

The Isotopic Composition of Solar Energetic Particles

Since the launch of ACE in August 1997, the Solar Isotope Spectrometer (SIS) has observed 11 large solar particle events in which elemental and isotopic composition was determined over a large energy range. The composition of these events has raised many issues and challenged generally accepted characterizations of solar energetic particle (SEP) events. In particular, ^3He/^4He enhancements have been observed in several large events as well as enhancements of heavy ions typically associated with smaller impulsive events. The isotopic composition varies substantially from event to event (a factor of 3 for ^(22)Ne/^(20)Ne) with enhancements and depletions that are generally correlated with elemental composition. This correlation suggests that the isotopic enhancements may be related to the Q/M fractionation typically evident in the elemental composition of SEP events. However, there are also significant deviations from this pattern, which may imply that wave-particle resonances or other mass fractionation processes may be involved. We review the recent isotopic observations made with ACE and discuss their implications for particle acceleration and transport

Measurements of Heavy Elements and Isotopes in Small Solar Energetic Particle Events

Using the Solar Isotope Spectrometer on the Advanced Composition Explorer, we have examined the ~10–20 MeV/nucleon elemental and isotopic composition of heavy (Z≥6) energetic nuclei accelerated in 30 small solar energetic particle (SEP) events which occurred between 31 March 1998 and 2 January 2001. We have measured the average heavy element content, the ^(22)Ne/^(20)Ne ratio, and the ^(26)Mg/^(24)Mg ratio in these events, and find good agreement with past studies. We have categorized the events according to their 3He/4He ratios, and find significant enhancements in the neutron-rich heavy isotopes of Ne and Mg in the combined 3He-rich data set: ^(22)Ne/^(20)Ne = 0.17±0.05 and ^(26)Mg/^(24)Mg = 0.25±0.05. We discuss the implications of these measurements for the acceleration of energetic nuclei in SEP events

The Coronal Isotopic Composition as Determined Using Solar Energetic Particles

Solar energetic particles (SEPs), like the solar wind, provide a direct sample of the Sun. Although SEP abundances show a variable amount of mass fractionation, it is possible to develop methods of correcting for it in order to deduce the composition of the corona. Using high-resolution measurements from the Solar Isotope Spectrometer on the Advanced Composition Explorer, we have studied the isotopic composition of 10 abundant elements from C to Ni in 32 large SEP events from late 1997 to the end of 2001 at energies >15 MeV/nucleon. We show that various isotopic and elemental enhancements are correlated with each other, discuss the first order corrections used to account for the variability, and obtain estimated coronal abundances. We compare the coronal values and their uncertainties inferred from SEPs with those that are available from solar wind and meteoritic measurements and find generally good agreement. We include C and Ni isotopic abundances, for which no solar wind measurements have yet been reported

Time Variations in Elemental Abundances in Solar Energetic Particle Events

The Solar Isotope Spectrometer (SIS) on-board the Advanced Composition Explorer has a large collection power and high telemetry rate, making it possible to study elemental abundances in large solar energetic particle (SEP) events as a function of time. Results have now been obtained for more than 25 such events. Understanding the causes of these variations is key to obtaining reliable solar elemental abundances and to understanding solar acceleration processes. Such variations have been previously attributed to two models: (1) a mixture of an initial impulsive phase having enhanced heavy element abundances with a longer gradual phase with coronal abundances and (2) rigidity dependent escape from CME-driven shocks through plasma waves generated by wave-particle interactions. In this second model the injected abundances are assumed to be coronal. Both these models can be expected to depend upon solar longitude since impulsive events are associated with flares at longitudes well-connected magnetically to the observer, and shock properties and connection of the observer to the shock are also longitude dependent. We present results on temporal variations from event to event and within events and show that they appear to have a longitude dependence. We show that the events which have been well-explained by model (2) tend to be near central meridian or the west limb. In addition, we show that there are events with little time variation and heavy element enhancements similar to those of impulsive events. These events seem to be better explained by model (1) with only an impulsive phase

STEREO and ACE observations of CIR particles

In the present solar minimum, corotating interaction regions (CIRs) produce frequent particle enhancements at 1 AU as observed at STEREO and ACE. As the two STEREO spacecraft move apart, differences in CIR time profiles observed at each spacecraft are becoming large. The timing differences are often roughly similar to the corotation time lag between the two spacecraft, however many of the features seen at Ahead and Behind require more than just a time shift. Perhaps transient disturbances in the solar wind affect connection to or transport from the shock, or temporal changes occur in the CIR shock itself. Additional timing differences of >1 day result from the different heliographic latitudes of the two STEREO spacecraf