ACE Mission Data Overview

Volume 16, Number 1, March 2000

By John Cooper

Drs. John Cooper and Natasha Papitashvili discuss ACE data.

The Advanced Composition Explorer (ACE) satellite was launched from Cape Canaveral, Florida, on August 25, 1997, and began on-station science operations on December 13, 1997, in a halo orbit at the L1 libration point. At this location the ACE mission measures the interplanetary magnetic field, plasma, and energetic particle environment at about one million kilometers upstream of the Earth's magnetosphere in the solar wind. The key science objective is to measure the elemental and isotopic composition of ions from solar, interplanetary, interstellar, and galactic sources at energies from keV's for solar wind plasma to 600 MeV/nucleon for cosmic ray nuclei. Definitive measurements of isotopic abundance are made for hydrogen to zinc (Z = 1 30) with more exploratory work extending up to zirconium (Z = 40). Six high-resolution ion spectrometers provide measurements of elemental charge, mass, or ion charge state with collecting powers ten to 100 times greater than previous experiments. Additional instruments provide monitoring for space weather applications of the local magnetic field, light ions (H, He), and energetic electrons. Experiment names and acronyms are listed below in Table I.

Table I. ACE Instruments


Lead Institution

Magnetometer (MAG)

Bartol Research Institute / U. of Del.

Solar Wind Ion Mass Spectrometer (SWIMS)

University of Maryland

Solar Wind Ionic Composition Spectrometer (SWICS)

University of Maryland

Solar Wind Electron, Proton, and Alpha Monitor (SWEPAM)

Los Alamos National Laboratory

Ultra Low Energy Isotope Spectrometer (ULEIS)

Applied Physics Laboratory / JHU

Solar Energetic Particle Ionic Charge Analyzer (SEPICA)

University of New Hampshire

Electron, Proton, and Alpha Monitor (EPAM)

Applied Physics Laboratory / JHU

Solar Isotope Spectrometer (SIS)

California Institute of Technology

Cosmic Ray Isotope Spectrometer (CRIS)

California Institute of Technology

Real Time Solar Wind (RTSW)

NOAA Space Environment Center

The spacecraft has a design life of at least five years and returns recorded data in daily downlinks to NASA Jet Propulsion Laboratory's (JPL) Deep Space Network for initial telemetry processing at NASA-Goddard Space Flight Center (GSFC) and higher level processing at Caltech's ACE Science Center (ASC) before distribution to experimenter teams. The spacecraft's Real Time Solar Wind (RTSW) system continuously transmits a separate stream of data from selected experiment channels to a global network of ground stations operated by the Space Environment Center (SEC) of the National Oceanic and Atmospheric Administration (NOAA) for space weather monitoring.

The ACE mission is operating under a Project Data Management Plan (PDMP) approved by NASA in early 1994. The four categories of public access data covered by this plan and its further evolution are browse, key parameter, Level 1, and Level 2. The browse data are intended only for monitoring of large scale plasma, energetic particle, and magnetic field variations and are generated at ASC using simple algorithms provided by instrument scientists. Since no checking is done for accuracy, and revisions may occur at any time, browse data are considered non-citable and should therefore not be used for published work without approval from the responsible investigators. Key parameter data are formatted in NSSDC's Common Data Format (CDF) and submitted via NASA's International Solar-Terrestrial Physics (ISTP) project to NSSDC with usage restrictions similar to those for ASC browse data. Level 1 data products are produced at ASC in the Hierarchical Data Format (HDF) from Level 0 telemetry data provided by GSFC and from ancillary data for spacecraft position and attitude for distribution to experiment teams and for archiving at NSSDC. In that the experiment data are provided only in engineering units, Level 1 products are of little public interest with the possible exception of the ancillary data. The Level 2 products returned to ASC by the experiment teams provide the most useful science content for archiving and public access, since these data are appropriate averaged over various time intervals in physical units such as flow velocity, particle flux, compositional ratios, and directional magnetic field magnitude. The Level 2 data are also formatted in HDF by ASC for usage by ACE experiment teams and for eventual public archiving. Time resolutions by instrument and data type are listed in Table II below.

Table II. Time Resolutions for Accessible Data Types from ACE Instruments



Time Resolution

ASC Browse



ASC Level 2


Interplanetary Vector Magnetic Field, RTN & GSE

16s, 5m, 1h, 1d

16s, 5m, 1h


16s, 4m, 1h, 1&27d


Solar Wind Plasma Ion Mass, He - Ni

(1h, 1&27d)


Solar Wind Plasma Ion Composition, He - Ni

1h, 1d

(1h, 1&27d)


Solar Wind Plasma Ion & Electron Flux

5m,1h, 1d

64s, 5m, 1h


64&128s, 1h, 1&27d


Solar/Interplan. Energetic Ion Isotopes, H - Ni

5m, 1h, 1d

1h, 1&27d


Solar Energetic Ion Charge States, H - Fe

5m, 1h, 1d

120s, 1h, 1d


Energetic Particle Flux Anisotropy: e-, H, He

5m, 1h, 1d


1h, 1&27d


Solar High Energy Ion Isotopes, He - Ni

1h, 1d



256s, 1h, 1&27d


Galactic Cosmic Ray Ion Isotopes, Li - Ni

1h, 1d

1h, 1&27d



Public access to ACE data is provided by ASC, NOAA, and NSSDC. The browse data are posted in digital (HDF, ASCII) and graphic form by ASC immediately after processing from the daily telemetry data. The RTSW data are more immediately processed by NOAA within minutes of telemetry data receipt and made available for warnings of imminent space weather events affecting the Earth's magnetosphere and ionosphere. The ISTP-compliant key parameter data are accessible in digital and graphic formats through NSSDC's Coordinated Data Analysis Web (CDAWeb) service. The Level 1 HDF data files are provided to NSSDC for off-line archiving on compact disk, but the more useful ancillary data are available on line at NSSDC's Anonymous FTP site. Currently, the Level 2 data are available in digital (HDF, ASCII) and plot formats only from ASC, but archiving and public access will commence at NSSDC in the near future.

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Author:Miranda Beall
Curator:Lori Persichitti
Responsible Official: Dr. Joseph H. King, Code 633
Last Revised: 16 May 2000 [NAB]