Advanced Composition Explorer (ACE or Explorer 71) is a NASA Explorer program satellite and space exploration mission to study matter comprising energetic particles from the solar wind, the interplanetary medium, and other sources.
Real-time data from ACE are used by the National Oceanic and Atmospheric Administration (NOAA) Space Weather Prediction Center (SWPC) to improve forecasts and warnings of solar storms. The ACE robotic spacecraft was launched on 25 August 1997, and entered a Lissajous orbit close to the Lagrange point (which lies between the Sun and the Earth at a distance of some from the latter) on 12 December 1997. The spacecraft is currently operating at that orbit. Because ACE is in a non-Keplerian orbit, and has regular station-keeping maneuvers, the orbital parameters in the adjacent information box are only approximate.
, the spacecraft is still in generally good condition. NASA Goddard Space Flight Center managed the development and integration of the ACE spacecraft.
History
The Advanced Composition Explorer (ACE) was proposed in 1986 as part of the Explorer Concept Study Program. ACE is designed to make coordinated measurements of the elemental and isotopic composition of accelerated nuclei from H (Hydrogen) to Zn (Zinc) spanning six decades in energy per nucleon, from solar wind to galactic cosmic ray energies, with sensitivity and with charge and mass resolution much better than heretofore possible. Following a Phase-A definition study, ACE was selected for development in 1989, and began construction in 1994. On 25 August 1997, ACE was successfully launched from Cape Canaveral Air Force Station by a Delta II launch vehicle. The August 1997 launch was originally scheduled back in 1993.
Science objectives
ACE observations allow the investigation of a wide range of fundamental problems in the following four major areas:
Elemental and isotopic composition of matter A major objective is the accurate and comprehensive determination of the elemental and isotopic composition of the various samples of "source material" from which nuclei are accelerated. These observations have been used to: Generate a set of solar isotopic abundances based on a direct sampling of solar material; Determine the coronal elemental and isotopic composition with greatly improved accuracy; Establish the pattern of isotopic differences between galactic cosmic ray and Solar System matter; Measure the elemental and isotopic abundances of interstellar and interplanetary "pick–up ions"; Determine the isotopic composition of the "anomalous cosmic ray component", which represents a sample of the local interstellar medium.
Origin of the elements and subsequent evolutionary processing Isotopic "anomalies" in meteorites indicate that the Solar System was not homogeneous when formed. Similarly, the Galaxy is neither uniform in space nor constant in time due to continuous stellar nucleosynthesis.
ACE measurements have been used to: Search for differences between the isotopic composition of solar and meteoritic material; Determine the contributions of solar wind and solar energetic particles to lunar and meteoritic material, and to planetary atmospheres and magnetospheres; Determine the dominant nucleosynthetic processes that contribute to cosmic ray source material; Determine whether cosmic rays are a sample of freshly synthesized material (e.g., from Supernova) or of the contemporary interstellar medium; Search for isotopic patterns in solar and galactic material as a test of galactic evolution models.
Formation of the solar corona and acceleration of the solar wind Solar energetic particles, solar wind, and spectroscopic observations show that the elemental composition of the solar corona is differentiated from that of the photosphere, although the processes by which this occurs, and by which the solar wind is subsequently accelerated, are poorly understood. The detailed composition and charge–state data provided by ACE are used to: Isolate the dominant coronal formation processes by comparing a broad range of coronal and photospheric abundances; Study plasma conditions at the source of solar wind and solar energetic particles by measuring and comparing the charge states of these two populations; Study solar wind acceleration processes and any charge or mass-dependent fractionation in various types of solar wind flows.
Particle acceleration and transport in nature Particle acceleration is ubiquitous in nature and understanding its nature is one of the fundamental problems of space plasma astrophysics. The unique data set obtained by ACE measurements has been used to: Make direct measurements of charge and/or mass-dependent fractionation during solar energetic particle and interplanetary acceleration events; Constrain solar flare, coronal shock, and interplanetary shock acceleration models with charge, mass, and spectral data spanning up to five decades in energy; Test theoretical models for 3He–rich solar flares and solar γ–ray events.
Instruments Cosmic-Ray Isotope Spectrometer (CRIS)
The Cosmic-Ray Isotope Spectrometer covers the highest range of the Advanced Composition Explorer's energy coverage, from 50 to 500 MeV/nucleon, with an isotopic resolution for elements from Z ≈ 2 to 30. The nuclei detected in this energy interval are predominantly cosmic rays originating in our Galaxy. This sample of galactic matter investigates 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 -sr for isotope measurements.
Electron, Proton, and Alpha-particle Monitor (EPAM)
The Electron, Proton, and Alpha Monitor (EPAM) instrument on the ACE spacecraft is designed to measure a broad range of energetic particles over nearly the full unit-sphere at high time resolution. Such measurements of ions and electrons in the range of a few tens of keV to several MeV are essential to understand the dynamics of solar flares, co-rotating interaction regions (CIRs), interplanetary shock acceleration, and upstream terrestrial events. The large dynamic range of EPAM extends from about 50 keV to 5 MeV for ions, and 40 keV to about 350 keV for electrons. To complement its electron and ion measurements, EPAM is also equipped with a Composition Aperture (CA) which unambiguously identifies ion species reported as species group rates and/or individual pulse-height events. The instrument achieves its large spatial coverage through five telescopes oriented at various angles to the spacecraft spin axis. The low-energy particle measurements, obtained as time resolutions between 1.5 and 24 seconds, and the ability of the instrument to observe particle anisotropies in three dimensions make EPAM an excellent resource to provide the interplanetary context for studies using other instruments on the ACE spacecraft.
Magnetometer (MAG)
The magnetic field experiment on ACE provides continuous measurements of the local magnetic field in the interplanetary medium. These measurements are essential in the interpretation of simultaneous ACE observations of energetic and thermal particle distributions. The experiment consists of a pair of twin, boom-mounted, triaxial flux gate sensors which are located 165 inches (419 cm) from the center of the spacecraft on opposing solar panels. The two triaxial sensors provide a balanced, fully redundant vector instrument and permit some enhanced assessment of the spacecraft's magnetic field.
Real-Time Solar Wind (RTSW) The Real-Time Solar Wind (RTSW) system is continuously monitoring the solar wind and producing warnings of impending major geomagnetic activity, up to one hour in advance. Warnings and alerts issued by NOAA allow those with systems sensitive to such activity to take preventative action. The RTSW system gathers solar wind and energetic particle data at high time resolution from four ACE instruments (MAG, SWEPAM, EPAM, and SIS), packs the data into a low-rate bit stream, and broadcasts the data continuously. NASA sends real-time data to NOAA each day when downloading science data. With a combination of dedicated ground stations (CRL in Japan and RAL in Great Britain) and time on existing ground tracking networks (NASA DSN and the USAF's AFSCN), the RTSW system can receive data 24 hours per day throughout the year. The raw data are immediately sent from the ground station to the Space Weather Prediction Center in Boulder, Colorado, processed, and then delivered to its Space Weather Operations Center where they are used in daily operations; the data are also delivered to the CRL Regional Warning Center at Hiraiso Station, Japan, to the USAF 55th Space Weather Squadron, and placed on the World Wide Web. The data are downloaded, processed and dispersed within 5 minutes from the time they leave ACE. The RTSW system also uses the low-energy energetic particles to warn of approaching interplanetary shocks and to help monitor the flux of high-energy particles that can produce radiation damage in satellite systems.
Solar Energetic Particle Ionic Charge Analyzer (SEPICA)
The Solar Energetic Particle Ionic Charge Analyzer (SEPICA) was the instrument on the Advanced Composition Explorer (ACE) that determined 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 had the ability to resolve individual charge states with a substantially larger geometric factor than its predecessor ULEZEQ on ISEE-1 and ISEE-3, on which SEPICA was based. To achieve these two requirements at the same time, SEPICA was composed of one high-charge resolution sensor section and two low-charge resolution, but large geometric factor sections.
As of 2008, this instrument is no longer functioning due to failed gas valves.
Solar Isotope Spectrometer (SIS)
The Solar Isotope Spectrometer (SIS) provides high-resolution measurements of the isotopic composition of energetic nuclei from He to Zn (Z=2 to 30) over the energy range from ~10 to ~100 MeV/nucleon. During large solar events, SIS measures the isotopic abundances of solar energetic particles to determine directly the composition of the solar corona and to study particle acceleration processes. During solar quiet times, SIS measures the isotopes of low-energy cosmic rays from the Galaxy and isotopes of the anomalous cosmic ray component, which originates in the nearby interstellar medium. SIS has two telescopes composed of silicon solid-state detectors that provide measurements of the nuclear charge, mass, and kinetic energy of incident nuclei. Within each telescope, particle trajectories are measured with a pair of two-dimensional silicon strip detectors instrumented with custom very-large-scale integrated (VLSI) electronics to provide both position and energy-loss measurements. SIS was specially designed to achieve excellent mass resolution under the extreme, high flux conditions encountered in large solar particle events. It provides a geometry factor of 40 cm2 sr, significantly greater than earlier solar particle isotope spectrometers.
Solar Wind Electron, Proton and Alpha Monitor (SWEPAM)