Editing Advanced Composition Explorer (section)

== Science objectives ==
[[File:ACE parts.png|thumb|Scientific instruments on ACE]]

ACE observations allow the investigation of a wide range of fundamental problems in the following four major areas:<ref name=stone98a>{{cite journal |last=Stone |first=E. C. |title=The Advanced Composition Explorer |journal=Space Science Reviews |date=July 1998 |volume=86 |pages=1–22 |doi=10.1023/A:1005082526237 |bibcode=1998SSRv...86....1S |s2cid=10744811 |display-authors=et al.}}</ref>

=== 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 [[Cosmic ray|galactic cosmic ray]] and [[Solar System]] matter;
* Measure the elemental and isotopic abundances of [[interstellar medium|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 [[Atmosphere|planetary atmosphere]]s and [[magnetosphere]]s;
* 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 (physics)|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 <sup>3</sup>He–rich solar flares and solar γ–ray events.