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.

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The Advanced Composition Explorer

[1] The ionic charge states of He, C, O, Ne, Mg and Fe at ≈0.5 MeV/n have been obtained in several corotating interaction regions (CIRs) in 1999 and 2000 with the Solar Energetic Particle Ionic Charge Analyzer (SEPICA) on the Advanced Composition Explorer (ACE). A large fraction (on average 25%) of He+ is found relative to He2+, indicating a substantial contribution of interstellar pickup ions. The mean charge states of the heavy ions are consistent with those in CME related energetic particle events and in solar wind. A rather low upper limit of <1% is found for singly charged ions for C, O and Mg, while Ne shows a small, but noticeable, fraction (4.7%) of Ne+. These observations are consistent with a contribution from interstellar pickup ions, but seem to eliminate inner source pickup ions as a substantial source for CIRs at and near 1 AU.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001GL013410

Charge states of energetic (≈0.5 MeV/n) ions in corotating interaction regions at 1 AU and implications on source populations

Solar energetic particles, or SEPs, from suprathermal (few keV) up to relativistic (
 $$\sim $$
 few GeV) energies are accelerated near the Sun in at least two ways: (1) by magnetic reconnection-driven processes during solar flares resulting in impulsive SEPs, and (2) at fast coronal-mass-ejection-driven shock waves that produce large gradual SEP events. Large gradual SEP events are of particular interest because the accompanying high-energy (
 $${>}10$$
 s MeV) protons pose serious radiation threats to human explorers living and working beyond low-Earth orbit and to technological assets such as communications and scientific satellites in space. However, a complete understanding of these large SEP events has eluded us primarily because their properties, as observed in Earth orbit, are smeared due to mixing and contributions from many important physical effects. This paper provides a comprehensive review of the current state of knowledge of these important phenomena, and summarizes some of the key questions that will be addressed by two upcoming missions—NASA’s Solar Probe Plus and ESA’s Solar Orbiter. Both of these missions are designed to directly and repeatedly sample the near-Sun environments where interplanetary scattering and transport effects are significantly reduced, allowing us to discriminate between different acceleration sites and mechanisms and to isolate the contributions of numerous physical processes occurring during large SEP events.

Large gradual solar energetic particle events

The Solar Wind Ion Composition Spectrometer (SWICS) and the Solar Wind Ions Mass Spectrometer (SWIMS) on ACE are instruments optimized for measurements of the chemical and isotopic composition of solar and interstellar matter. SWICS determines uniquely the chemical and ionic-charge composition of the solar wind, the thermal and mean speeds of all major solar wind ions from H through Fe at all solar wind speeds above 300 kms-1 (protons) and 170 km s-1 (Fe+16), and resolves H and He isotopes of both solar and interstellar sources. SWICS will measure the distribution functions of both the interstellar cloud and dust cloud pickup ions up to energies of 100 keV e-1. SWIMS will measure the chemical, isotopic and charge state composition of the solar wind for every element between He and Ni. Each of the two instruments uses electrostatic analysis followed by a time-of-flight and, as required, an energy measurement. The observations made with SWICS and SWIMS will make valuable contributions to the ISTP objectives by providing information regarding the composition and energy distribution of matter entering the magnetosphere. In addition, SWICS and SWIMS results will have an impact on many areas of solar and heliospheric physics, in particular providing important and unique information on: (i) conditions and processes in the region of the corona where the solar wind is accelerated; (ii) the location of the source regions of the solar wind in the corona; (iii) coronal heating processes; (iv) the extent and causes of variations in the composition of the solar atmosphere; (v) plasma processes in the solar wind; (vi) the acceleration of particles in the solar wind; (vii) the physics of the pickup process of interstellar He in the solar wind; and (viii) the spatial distribution and characteristics of sources of neutral matter in the inner heliosphere.

Investigation of the Composition of Solar and Interstellar Matter Using Solar Wind and Pickup Ion Measurements with SWICS and SWIMS on the Ace Spacecraft

This paper reviews three important effects on energetic particles of corotating interaction regions (CIRs) in the solar wind that are formed at the leading edges of high-speed solar wind streams originating in coronal holes. A brief overview of CIRs and their important features is followed by a discussion of CIR-associated modulations in the galactic cosmic ray intensity, with an emphasis on observations made by spacecraft particle telescope 'anti-coincidence' guards. Such guards combine high counting rates (hundreds of counts/s) and a lower rigidity response than neutron monitors to provide detailed information on the relationship between cosmic ray modulations and CIR structure. The modulation of Jovian electrons by CIRs is then described. Finally, the acceleration of ions to energies of ∼ 20 MeV/n in the vicinity of CIRs is reviewed. Corotating interaction regions (CIRs) are regions of compressed plasma formed at the leading edges of corotating high-speed solar wind streams originating in coronal holes as they interact with the preceding slow solar wind. They are par- ticularly prominent features of the solar wind during the declining and minimum phases of the 11-year solar cycle, but may also be present at times of higher solar activity, interspersed with slow solar wind and transient flows associated with coronal mass ejections (CMEs) (e.g., Richardson, Cane, and Cliver, 2002). This paper reviews three important effects of CIRs on energetic particles in the heliosphere. The first effect is the tendency for the galactic cosmic ray intensity to be depressed temporarily ('modulated') during the passage of a CIR and high- speed stream. This phenomenon has been studied for many years, typically using ground-based neutron monitors. However, the emphasis in this review is on obser- vations made in the inner heliosphere and near the Earth by the 'anti-coincidence' guards of certain spacecraft particle telescopes. Such guards combine a large de- tector volume and integral energy response (> several tens of MeV) to give high counting rates (hundreds of counts/s) which can provide detailed information on the relationship between cosmic ray modulations and CIR structure. The observa-

Energetic particles and corotating interaction regions in the solar wind

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 ≈02 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 thinwindow flow-through proportional counters with isobutane as counter gas and ion-implanted solid state detectors provide for 3 independent △E (energy loss) versus E (residual energy) telescopes The multi-wire proportional counter simultaneously determines the energy loss △E and the impact position of the ions Suppression of background from penetrating cosmic radiation is provided by an anti-coincidence system with a Csl scintillator and Si-photodiodes The data are compressed and formatted in a data processing unit (S3DPU) that also handles the commanding and various automat-ted 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

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A. Anderson

Papers

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