Solar Energetic Particle Isotopic Composition

We discuss isotopic abundance measurements of heavy (6 ≤ Z ≤ 14) solar energetic particles with energies from ∼15 to 70 MeV/nucleon, focusing on new measurements made on SAMPEX during two large solar particle events in late 1992. These measurements are corrected for charge/mass dependent acceleration effects to obtain estimates of coronal isotopic abundances and are compared with terrestrial and solar wind isotope abundances. An example of new results from the Advanced Composition Explorer is included. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Cosmic-Ray Isotope Spectrometer for the Advanced Composition Explorer

The Cosmic-Ray Isotope Spectrometer is designed to cover the highest decade of the Advanced Composition Explorer's energy interval, from ∼50 to ∼500 MeV nucl−1, with isotopic resolution for elements from Z≃2 to Z≃30. The nuclei detected in this energy interval are predominantly cosmic rays originating in our Galaxy. This sample of galactic matter can be used to investigate the nucleosynthesis of the parent material, as well as fractionation, acceleration, and transport processes that these particles undergo in the Galaxy and in the interplanetary medium. Charge and mass identification with CRIS is based on multiple measurements of dE/dx and total energy in stacks of silicon detectors, and trajectory measurements in a scintillating optical fiber trajectory (SOFT) hodoscope. The instrument has a geometrical factor of ∼r250 cm2 sr for isotope measurements, and should accumulate ∼5×106 stopping heavy nuclei (Z>2) in two years of data collection under solar minimum conditions. This revised version was published online in June 2006 with corrections to the Cover Date.     

Solar Isotopic Composition as Determined Using Solar Energetic Particles

Solar energetic particles (SEPs) provide a sample of the Sun from which solar composition may be determined. Using high-resolution measurements from the Solar Isotope Spectrometer (SIS) onboard NASA’s Advanced Composition Explorer (ACE) spacecraft, we have studied the isotopic composition of SEPs at energies ≥20 MeV/nucleon in large SEP events. We present SEP isotope measurements of C, O, Ne, Mg, Si, S, Ar, Ca, Fe, and Ni made in 49 large events from late 1997 to the present. The isotopic composition is highly variable from one SEP event to another due to variations in seed particle composition or due to mass fractionation that occurs during the acceleration and/or transport of these particles. We show that various isotopic and elemental enhancements are correlated with each other, discuss the empirical corrections used to account for the compositional variability, and obtain estimated solar isotopic abundances. We compare the solar values and their uncertainties inferred from SEPs with solar wind and other solar system abundances and find generally good agreement.     

The Advanced Composition Explorer

The Advanced Composition Explorer was launched August 25, 1997 carrying six high-resolution spectrometers that measure the elemental, isotopic, and ionic charge-state composition of nuclei from H to Ni (1≤Z≤28) from solar wind energies (∼1 keV nucl−1) to galactic cosmic-ray energies (∼500 MeV nucl−1). Data from these instruments is being used to measure and compare the elemental and isotopic composition of the solar corona, the nearby interstellar medium, and the Galaxy, and to study particle acceleration processes that occur in a wide range of environments. ACE also carries three instruments that provide the heliospheric context for ion composition studies by monitoring the state of the interplanetary medium. From its orbit about the Sun-Earth libration point ∼1.5 million km sunward of Earth, ACE also provides real-time solar wind measurements to NOAA for use in forecasting space weather. This paper provides an introduction to the ACE mission, including overviews of the scientific goals and objectives, the instrument payload, and the spacecraft and ground systems. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Ultra-Low-Energy Isotope Spectrometer (ULEIS) for the ACE spacecraft

The Ultra Low Energy Isotope Spectrometer (ULEIS) on the ACE spacecraft is an ultra high resolution mass spectrometer designed to measure particle composition and energy spectra of elements He-Ni with energies from ∼45 keV nucl−1 to a few MeV nucl−1. ULEIS will investigate particles accelerated in solar energetic particle events, interplanetary shocks, and at the solar wind termination shock. By determining energy spectra, mass composition, and their temporal variations in conjunction with other ACE instruments, ULEIS will greatly improve our knowledge of solar abundances, as well as other reservoirs such as the local interstellar medium. ULEIS is designed to combine the high sensitivity required to measure low particle fluxes, along with the capability to operate in the largest solar particle or interplanetary shock events. In addition to detailed information for individual ions, ULEIS features a wide range of count rates for different ions and energies that will allow accurate determination of particle fluxes and anisotropies over short (∼few minutes) time scales. This revised version was published online in June 2006 with corrections to the Cover Date.     

WIND Observations of Suprathermal Electrons in the Interplanetary Medium

We review some of the new results for suprathermal electrons obtained with the 3-D Plasma and Energetic Particle Instrument on the WIND spacecraft, which provides high sensitivity electron and ion measurements from solar wind thermal plasma up to ≳MeV energies. These results include: (1) the observation of solar impulsive electron events extending down to ∼0.5 keV energy; (2) the observation of a turnover at ∼12 keV for electrons in a gradual large solar energetic particle (LSEP) event; (3) the detection of a quiet-time population (the ‘superhalo’) of electrons extending up to ∼100 keV energy; and (4) the probing of the magnetic topology and source region for magnetic clouds, using electrons. These unique WIND measurements are highly complementary to the particle composition measurements which will be made by ACE. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Solar Energetic Particle Ionic Charge Analyzer (SEPICA) and the Data Processing Unit (S3DPU) for SWICS,SWIMS and SEPICA

The Solar Energetic Particle Ionic Charge Analyzer (SEPICA) is the main instrument on the Advanced Composition Explorer (ACE) to determine the ionic charge states of solar and interplanetary energetic particles in the energy range from ≈0.2 MeV nucl−1 to ≈5 MeV charge−1. The charge state of energetic ions contains key information to unravel source temperatures, acceleration, fractionation and transport processes for these particle populations. SEPICA will have the ability to resolve individual charge states and have a substantially larger geometric factor than its predecessor ULEZEQ on ISEE-1 and -3, on which SEPICA is based. To achieve these two requirements at the same time, SEPICA is composed of one high-charge resolution sensor section and two low- charge resolution, but large geometric factor sections. The charge resolution is achieved by the focusing of the incoming ions, through a multi-slit mechanical collimator, deflection in an electrostatic analyzer with a voltage up to 30 kV, and measurement of the impact position in the detector system. To determine the nuclear charge (element) and energy of the incoming ions, the combination of thin-window flow-through proportional counters with isobutane as counter gas and ion-implanted solid state detectors provide for 3 independent ΔE (energy loss) versus E (residual energy) telescopes. The multi-wire proportional counter simultaneously determines the energy loss ΔE and the impact position of the ions. Suppression of background from penetrating cosmic radiation is provided by an anti-coincidence system with a CsI scintillator and Si-photodiodes. The data are compressed and formatted in a data processing unit (S3DPU) that also handles the commanding and various automatted functions of the instrument. The S3DPU is shared with the Solar Wind Ion Charge Spectrometer (SWICS) and the Solar Wind Ion Mass Spectrometer (SWIMS) and thus provides the same services for three of the ACE instruments. It has evolved out of a long family of data processing units for particle spectrometers. This revised version was published online in June 2006 with corrections to the Cover Date.     

The composition of heavy ions in solar energetic particle events

We review recent advances in determining the elemental, charge-state, and isotopic composition of 1 to 20 MeV per nucleon ions in solar energetic particle (SEP) events and outline our current understanding of the nature of solar and interplanetary processes which may explain the observations.The composition within individual SEP events may vary both with time and energy, and will in general be different from that in other SEP events. Average values of relative abundances measured in a large number of SEP events, however, are found to be roughly energy independent in the 1 to 20 MeV per nucleon range, and show a systematic deviation from photospheric abundances which seems to be organized in terms of the first ionization potential of the ion.Direct measurements of the charge states of SEPs have revealed the surprisingly common presence of energetic He⁺ along with heavy ions with typically coronal ionization states. High-resolution measurements of isotopic abundance ratios in a small number of SEP events show these to be consistent with the universal composition except for the puzzling overabundance of the SEP ²²Ne/²⁰Ne relative to this isotopes ratio in the solar wind. The broad spectrum of observed elemental abundance variations, which in their extreme result in composition anomalies characteristic of ³He-rich, heavy-ion rich and carbon-poor SEP events, along with direct measurements of the ionization states of SEPs provide essential information on the physical characteristics of, and conditions in the source regions, as well as important constraints to possible models for SEP production.It is concluded that SEP acceleration is a two-step process, beginning with plasma-wave heating of the ambient plasma in the lower corona, which may include pockets of cold material, and followed by acceleration to the observed energies by either flare-generated coronal shocks or Fermi-type processes in the corona. Interplanetary propagation as well as acceleration by interplanetary propagating shock will often further modify the composition of SEP events, especially at lower energies.     

3He-Rich Solar Energetic Particle Events

³He-rich solar energetic particle (SEP) events show huge enrichments of ³He and association with kilovolt electrons and Type-III radio bursts. Observations from a new generation of high resolution instruments launched on the Wind, ACE, Yohkoh, SOHO, TRACE, and RHESSI spacecraft have revealed many new properties of these events: the particle energy spectra are found to be either power-law or curved in shape, with the ³He spectrum often being distinctly different from other species. Ultra-heavy nuclei up to >200 amu are found to be routinely present at average enrichments of >200 times solar-system abundances. The high ionization states previously observed near ∼1 MeV/nucleon have been found to decrease towards normal solar coronal values in these events. The source regions have been identified for many events, and are associated with X-ray jets and EUV flares that are associated with magnetic reconnection sites near active regions. This paper reviews the current experimental picture and theoretical models, with emphasis on the new insights found in the last few years.     

An Update on Ultra-Heavy Elements in Solar Energetic Particles above 10 MeV/Nucleon

Measurements below several MeV/nucleon from Wind/LEMT and ACE/ULEIS show that elements heavier than Zn (Z=30) can be enhanced by factors of ∼100 to 1000, depending on species, in ³He-rich solar energetic particle (SEP) events. Using the Solar Isotope Spectrometer (SIS) on ACE we find that even large SEP (LSEP) shock-accelerated events at energies from ∼10 to >100 MeV/nucleon are often very iron rich and might contain admixtures of flare seed material. Studies of ultra-heavy (UH) SEPs (with Z>30) above 10 MeV/nucleon can be used to test models of acceleration and abundance enhancements in both LSEP and ³He-rich events. We find that the long-term average composition for elements from Z=30 to 40 is similar to standard solar system values, but there is considerable event-to-event variability. Although most of the UH fluence arrives during LSEP events, UH abundances are relatively more enhanced in ³He-rich events, with the (34<Z<40)/O ratio on average more than 50 times higher in ³He-rich events than in LSEP events. At energies >10 MeV/nucleon, the most extreme event in terms of UH composition detected so far took place on 23 July 2004 and had a (34<Z<40)/O enhancement of ∼250–300 times the standard solar value.     

Ionic Charge States of Solar Energetic Particles: A Clue to the Source

The ionic charge of solar energetic particles (SEP) as observed in interplanetary space is an important parameter for the diagnostic of the plasma conditions at the source region and provides fundamental information about the acceleration and propagation processes at the Sun and in interplanetary space. In this paper we review the new measurements of ionic charge states with advanced instrumentation onboard the SAMPEX, SOHO, and ACE spacecraft that provide for the first time ionic charge measurements over the wide energy range of ∼0.01 to 70 MeV/nuc (for Fe), and for many individual SEP events. These new measurements show a strong energy dependence of the mean ionic charge of heavy ions, most pronounced for iron, indicating that the previous interpretation of the mean ionic charge being solely related to the ambient plasma temperature was too simplistic. This energy dependence, in combination with models on acceleration, charge stripping, and solar and interplanetary propagation, provides constraints for the temperature, density, and acceleration time scales in the acceleration region. The comparison of the measurements with model calculations shows that for impulsive events with a large increase of Q _Fe(E) at energies ≤1 MeV/nuc the acceleration occurs low in the corona, typically at altitudes ≤0.2 R _S .     

Electron,Proton, and Alpha Monitor on the Advanced Composition Explorer spacecraft

The Electron, Proton, and Alpha Monitor (EPAM) is designed to make measurements of ions and electrons over a broad range of energy and intensity. Through five separate solid-state detector telescopes oriented so as to provide nearly full coverage of the unit-sphere, EPAM can uniquely distinguish ions (Ei≳50 keV) and electrons (Ee≳40 keV) providing the context for the measurements of the high sensitivity instruments on ACE. Using a ΔE×E telescope, the instrument can determine ion elemental abundances (E≳0.5 MeV nucl−1). The large angular coverage and high time resolution will serve to alert the other instruments on ACE of interesting anisotropic events. The experiment is controlled by a microprocessor-based data system, and the entire instrument has been reconfigured from the HI-SCALE instrument on the Ulysses spacecraft. Inflight calibration is achieved using a variety of radioactive sources mounted on the reclosable telescope covers. Besides the coarse (8 channel) ion and (4 channel) electron energy spectra, the instrument is also capable of providing energy spectra with 32 logarithmically spaced channels using a pulse-height-analyzer. The instrument, along with its mounting bracket and radiators weighs 11.8 kg and uses about 4.0 W of power. To demonstrate some of the capabilities of the instrument, some initial performance data are included from a solar energetic particle event in November 1997. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Plasma Ion and Electron Instruments for the Genesis Mission

The Genesis Ion Monitor (GIM) and the Genesis Electron Monitor (GEM) provide 3-dimensional plasma measurements of the solar wind for the Genesis mission. These measurements are used onboard to determine the type of plasma that is flowing past the spacecraft and to configure the solar wind sample collection subsystems in real-time. Both GIM and GEM employ spherical-section electrostatic analyzers followed by channel electron multiplier (CEM) arrays for detection and angle and energy/charge analysis of incident ions and electrons. GIM is of a new design specific to Genesis mission requirements whereas the GEM sensor is an almost exact copy of the plasma electron sensors currently flying on the ACE and Ulysses spacecraft, albeit with new electronics and programming. Ions are detected at forty log-spaced energy levels between ∼ 1 eV and 14 keV by eight CEM detectors, while electrons with energies between ∼ 1 eV and 1.4 keV are measured at twenty log-spaced energy levels using seven CEMs. The spin of the spacecraft is used to sweep the fan-shaped fields-of-view of both instruments across all areas of the sky of interest, with ion measurements being taken forty times per spin and samples of the electron population being taken twenty four times per spin. Complete ion and electron energy spectra are measured every ∼ 2.5 min (four spins of the spacecraft) with adequate energy and angular resolution to determine fully 3-dimensional ion and electron distribution functions. The GIM and GEM plasma measurements are principally used to enable the operational solar wind sample collection goals of the Genesis mission but they also provide a potentially very useful data set for studies of solar wind phenomena, especially if combined with other solar wind data sets from ACE, WIND, SOHO and Ulysses for multi-spacecraft investigations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Solar Wind Composition at Solar Maximum

Although coronal mass ejections have traditionally been thought to contribute only a minor fraction to the total solar particle flux, and although such events mainly occur in lower heliographic latitudes, the impressive spectacle of eruptions - observed with SOHO/LASCO even at times of solar minimum - indicates that an important part of the low-latitude solar corona is fed with matter and magnetic fields in a highly transient manner. Elemental and isotopic abundances determined with the new generation of particle instruments with high sensitivity and strongly enhanced time resolution indicate that, apart from FIP/FIT-fractionation, mass-dependent fractionation can also influence the replenishment of the thermal ion population of the corona. Furthermore, selective enrichment of the thermal coronal plasma with rare species such as ³He can occur. Such compositional features have until recently only been found in energetic particles from impulsive flare events. This review will concentrate on this and other aspects of the present solar maximum and conclude with some outlook on future investigations of near-terrestrial space climate (the generalized counterpart of ‘space weather’). This revised version was published online in August 2006 with corrections to the Cover Date.     

Evidence for a Two-Stage Acceleration Process in Large Solar Energetic Particle Events

Using high-resolution mass spectrometers on board the Advanced Composition Explorer (ACE), we surveyed the event-averaged ∼0.1–60 MeV/nuc heavy ion elemental composition in 64 large solar energetic particle (LSEP) events of cycle 23. Our results show the following: (1) The Fe/O ratio decreases with increasing energy up to ∼10 MeV/nuc in ∼92% of the events and up to ∼60 MeV/nuc in ∼64% of the events. (2) The rare isotope ³He is greatly enhanced over the corona or the solar wind values in 46% of the events. (3) The heavy ion abundances are not systematically organized by the ion’s M/Q ratio when compared with the solar wind values. (4) Heavy ion abundances from C–Fe exhibit systematic M/Q-dependent enhancements that are remarkably similar to those seen in ³He-rich SEP events and CME-driven interplanetary (IP) shock events. Taken together, these results confirm the role of shocks in energizing particles up to ∼60 MeV/nuc in the majority of large SEP events of cycle 23, but also show that the seed population is not dominated by ions originating from the ambient corona or the thermal solar wind, as previously believed. Rather, it appears that the source material for CME-associated large SEP events originates predominantly from a suprathermal population with a heavy ion enrichment pattern that is organized according to the ion’s mass-per-charge ratio. These new results indicate that current LSEP models must include the routine production of this dynamic suprathermal seed population as a critical pre-cursor to the CME shock acceleration process.     

The Low-Energy Telescope (LET) and SEP Central Electronics for the STEREO Mission

The Low-Energy Telescope (LET) is one of four sensors that make up the Solar Energetic Particle (SEP) instrument of the IMPACT investigation for NASA’s STEREO mission. The LET is designed to measure the elemental composition, energy spectra, angular distributions, and arrival times of H to Ni ions over the energy range from ～3 to ～30 MeV/nucleon. It will also identify the rare isotope ³He and trans-iron nuclei with 30≤Z≤83. The SEP measurements from the two STEREO spacecraft will be combined with data from ACE and other 1-AU spacecraft to provide multipoint investigations of the energetic particles that result from interplanetary shocks driven by coronal mass ejections (CMEs) and from solar flare events. The multipoint in situ observations of SEPs and solar-wind plasma will complement STEREO images of CMEs in order to investigate their role in space weather. Each LET instrument includes a sensor system made up of an array of 14 solid-state detectors composed of 54 segments that are individually analyzed by custom Pulse Height Analysis System Integrated Circuits (PHASICs). The signals from four PHASIC chips in each LET are used by a Minimal Instruction Set Computer (MISC) to provide onboard particle identification of a dozen species in ～12 energy intervals at event rates of ～1,000 events/sec. An additional control unit, called SEP Central, gathers data from the four SEP sensors, controls the SEP bias supply, and manages the interfaces to the sensors and the SEP interface to the Instrument Data Processing Unit (IDPU). This article outlines the scientific objectives that LET will address, describes the design and operation of LET and the SEP Central electronics, and discusses the data products that will result.     

The Galileo Heavy element monitor

The Heavy Ion Counter on the Galileo spacecraft will monitor energetic heavy nuclei of the elements from C to Ni, with energies from 6 to 200 MeV nucl^-1. The instrument will provide measurements of trapped heavy ions in the Jovian magnetosphere, including those high-energy heavy ions with the potential for affecting the operation of the spacecraft electronic circuitry. We describe the instrument, which is a modified version of the Voyager CRS instrument.     

Fractionation of SI, NE, and MG Isotopes in the Solar Wind as Measured by Soho/Celias/MTOF

Using the high-resolution mass spectrometer CELIAS/MTOF on board SOHO we have measured the solar wind isotope abundance ratios of Si, Ne, and Mg and their variations in different solar wind regimes with bulk velocities ranging from 330 km/s to 650 km/s. Data indicate a small systematic depletion of the heavier isotopes in the slow solar wind on the order of (1.4±1.3)% per amu (2σ-error) compared to their abundances in the fast solar wind from coronal holes. These variations in the solar wind isotopic composition represent a pure mass-dependent effect because the different isotopes of an element pass the inner corona with the same charge state distribution. The influence of particle mass on the acceleration of minor solar wind ions is discussed in the context of theoretical models and recent optical observations with other SOHO instruments. This revised version was published online in June 2006 with corrections to the Cover Date.     

Energetic Particles Investigation (EPI)

The Energetic Particles Investigation (EPI) instrument operates during the pre-entry phase of the Galileo Probe. The major science objective is to study the energetic particle population in the innermost regions of the Jovian magnetosphere — within 4 radii of the cloud tops — and into the upper atmosphere. To achieve these objectives the EPI instrument will make omnidirectional measurements of four different particle species — electrons, protons, alpha-particles, and heavy ions (Z > 2). Intensity profiles with a spatial resolution of about 0.02 Jupiter radii will be recorded. Three different energy range channels are allocated to both electrons and protons to provide a rough estimate of the spectral index of the energy spectra. In addition to the omnidirectional measurements, sectored data will be obtained for certain energy range electrons, protons, and alpha-particles to determine directional anisotropies and particle pitch angle distributions. The detector assembly is a two-element telescope using totally depleted, circular silicon surfacebarrier detectors surrounded by a cylindrical tungsten shielding with a wall thickness of 4.86 g cm^-2. The telescope axis is oriented normal to the spherical surface of the Probe's rear heat shield which is needed for heat protection of the scientific payload during the Probe's entry into the Jovian atmosphere. The material thickness of the heat shield determines the lower energy threshold of the particle species investigated during the Probe's pre-entry phase. The EPI instrument is combined with the Lightning and Radio Emission Detector (LRD) such that the EPI sensor is connected to the LRD/EPI electronic box. In this way, both instruments together only have one interface of the Probe's power, command, and data unit.